1

1.0 Introduction

1.1 The trade in crocodilians

For over a century the hides of crocodilians have been used for the manufacture of exotic

leather products. The resulting commercial hunting has led to drastic population declines

and the designation of the majority of these modern-day archosaurs as endangered

species, especially in conjunction with the more recent threat of extensive habitat loss

(Plotkin et al., 1983; Ross, 1998; Thorbjarnarson, 1999). By the late 1800s exotic leather

fashions had spread to Europe and to meet this new demand the skin hunting business

expanded from the United States into Mexico, Central America and the Caribbean

(Thorbjarnarson, 1999). By the 1930s many of the hides tanned in Europe originated

from northern South America.

It became recognized in the 1960s that rates of exploitation could result in, at least, the

commercial extinction of several crocodilian species, leading to protective legislation that

applied mostly to commercial hunting (in the form of liscensing, closed seasons and a

minimum size under which crocodilians could not be taken or their hides sold) (Klemm

and Navid, 1989). In many countries, complete protection became the rule, since mere

exploitation regulations were deemed to be insufficient to prevent further population

declines (Klemm and Navid, 1989). With the adoption of the Convention on International

Trade in Endangered Species of Flora and Fauna in 1975, a comprehensive international

system for the trade in wildlife and wildlife products was finally established

(Thorbjarnarson, 1999). The species covered by CITES are listed in three Appendices,

according to the degree of protection they need. Appendix I includes species threatened

2

with extinction and prohibits international commerce. Appendix II includes species not

necessarily threatened with extinction, but in which trade must be controlled in order to

avoid utilization incompatible with their survival. Trade is only permitted with CITES

documentation issued by government authorities in the country of origin. Appendix III

contains species that are protected in at least one country and may be traded without such

stringent requirements (Brazaitis et al., 1998, CITES, 2006).

Particularly for those crocodilian species for which habitat loss was not a significant

factor, a reduction of commercial hunting due to improved legislation initiated a phase of

recovery, with the dramatic recovery of the American alligator Alligator mississippiensis

being the most well-known and documented case (Thorbjarnarson, 1999). With growing

crocodilian populations, the United States, Zimbabwe and Papua New Guinea launched

managed-harvest programs and demonstrated that crocodilians could be managed on a

sustainable-use basis (Child, 1987; Joanen et al., 1997; Hollands, 1987). Developing

nations in Africa, however, had only limited funds available for wildlife management,

restricting their ability to document the recovery and status of wild populations. As a

result of this, a series of CITES resolutions were established that loosened the

requirements on legal trade, and working with the Crocodile Specialist Group of the

World Conservation Union – Species Survival Commision (IUCN-SSC), the CITES

secretariat played an important role in providing funds and technical assistance that

allowed African nations to develop proposals for managed use under the CITES

guidelines (Thorbjarnarson, 1999). By the 1990s many African countries had taken

advantage of these resolutions to legally export crocodile skins.

3

In the New World, however, a different pattern of trade and managed use resulted from

the ongoing trade in crocodilian skins. In the Amazonian region of northern South

America, five crocodilian species are found: Caiman crocodilus, Paleosuchus

palpebrosus, Paleosuchus trigonatus, Crocodylus intermedius and Melanosuchus niger

(Plotkin et al., 1983). With the exception of Crocodylus intermedius, these species belong

to the family of caimans (Brazaitis et al., 1998). Due to the extensive development of

ventral osteoderms (boney inclusions), caiman belly skins are of an inferior commercial

value in comparison to those of the American alligator (Alligator mississippiensis) and

crocodiles (Ross, 1998). Owing to the low value hide, the commercial exploitation of

caiman did not flourish until stocks of more valuable hides dwindled, yet by the 1950s

enormous numbers of caiman were being exported from Amazonia (Ross, 1998, Plotkin

et al., 1983). Smith (1980) estimated that between 1950 and 1965, 7.5 million caiman

skins were exported from Amazonas State in Brazil, and by 1980 Medem (1981)

calculated that a minimum of 11.65 million caiman skins had been exported from

Columbia. During the 1980s the number of caiman exploited in South America was

estimated to be in excess of 1 million per year (Jenkins and Broad, 1994).

The more rigid CITES controls on classic, more valuable skins provided increased

incentive to trade caiman hides. The resulting high demand, the scarcity of legal sources,

the low prices of illegal skins, and the inability of countries to adequately regulate

exports and imports, led to a complex web of illegal trade in caiman skins in South

America (Medem, 1980; Thorbjarnarson, 1999). Although large-scale managed harvest

4

programs have been established in some parts of South America (Gorzula, 1987;

Thorbjarnarson and Velasco, 1998; King et al., 2003), the illegal caiman trade has

remained a problem. In 1992 and 1994 CITES adopted the Universal Tagging

Resolutions, an important tool for identifying the origin of species and regulating trade

(Collins and Luxmoore, 1996; CITES, 2000). Evidence suggests that this has reduced

illegal caiman trade to some extent (Thorbjarnarson, 1999).

1.2 Caiman and the bushmeat trade

Meat from wildlife species is an important source of animal protein for rural populations

(Robinson and Bodmer, 1999; Cowlishaw et al., 2005). Ungulates, primates, and rodents

provide most of the biomass consumed in northern South America, but a wide variety of

wildlife species are hunted for both subsistence and commerce, including caiman

(Bodmer, 1994; Bodmer et al., 1994; Da Silveira and Thorbjarnarson, 1998). The

demand for bushmeat has increased in recent years as tropical forests become more

accessible to hunters, effective human population densities increase, traditional hunting

practices change, the meat trade becomes more commercial, demand increases for wild

meat from urban centers and there is a scarcity of alternative protein sources (Robinson

and Bodmer, 1999; Robinson and Bennett, 2000). Ojasti (1996) collaborated surveys

from across the Amazonia region and found that caiman were hunted for food by the

Sharanahua in Peru, the Yanomamo in Venezuela, Siona-Secoya in Ecuador and the

Ache people of Paraguay, but was considered a secondary food source. The Yékwana and

Yanomamo communities of Venezuela, however, classed caiman as a particularly

important source of food (Ojasti, 1996). Da Silveira and Thorbjarnarson (1998)

documented widespread hunting for caiman meat in the Mamiraua Sustainable

5

Development Reserve in Brazil. Illegal commercial hunting for caiman meat in the region

began not long after the end of legal skin hunting. The caiman meat trade in Brazil

appears to have had two destinations: upstream to Columbia and Peru where it is

fraudulently sold as catfish, and downstream to Para state, Brazil where it is sold as

caiman meat (Da Silveira and Thorbjarnarson, 1999).

1.3 Impacts of exploitation

Both the commercial and subsistence use of caiman species in South America places

enormous pressure on populations. Despite survey evidence of stable or recovering

populations of certain species, notably Caiman crocodilus, the population status of many

species of caiman is inconsistent across the Amazonia region, with overexploitation

occurring at the level of local catchments (Rebelo and Magnusson, 1983; Da Silveira and

Thorbjarnarson, 1999; Britton, 2001). The effects of such large-scale past exploitation

has affected species‟ ranges, and impacted community structure, and the consequences of

such impacts are often site specific, depending on influencing factors such as current

hunting pressure and interspecific competition (Rebelo and Magnusson, 1983; Ross,

1998; Da Silveira and Thorbjarnarson, 1999; Britton, 2001). One high-profile example is

that of the black caiman (Melanosuchus niger) in Peru, which was once described as

being on the verge of extinction (Plotkin et al., 1983). More recently, however, viable

populations can be found in some areas of Peru, with a stronghold population in Cocha

Cashu in Manu National Park (Herron, 1985; Herron et al., 1990). Vasquez (1982-3)

found a recovering population in the Jenaro Herrera region, and there is also evidence of

a recovering population in the Pacaya-Samiria National Reserve, located in the

6

headwaters of the Amazon River in Loreto, northeastern Peru (Verdi et al., 1980; Street,

2004). However, in other parts of Loreto, notably along the Yavari River, the population

status of the black caiman is less optimistic. A recent biological inventory did not observe

any black caiman in the Yavari region (Pitman et al., 2003), and other studies that have

found the species in the area have documented very low densities (Newell, 2002; Pask,

2005). Pitman et al. (2003) places priority on documenting the population status of the

black caiman and the other caiman species present in the Yavari, for a better

understanding of historical trends and the impacts of historical and current (if any)

harvest of caiman. Should populations be found to be in decline, appropriate conservation

or management plans may need to be put into place (Pitman et al., 2003).

Based on the recommendations of Pitman et al. (2003), this study is an attempt to

document the population status of the caiman species present in the Yavari region, and

explore possible explanations as to the observed low black caiman densities, in

comparison to healthy populations in the relatively nearby Pacaya-Samiria National

Reserve.

1.4 The caiman species

There are three caiman species that have been previously documented in the study site:

Caiman crocodilus, Paleosuchus trigonatus, and Melanosuchus niger. The following

gives a brief overview of the basic ecology and relevant natural history of each of the

species.

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1.4.1 Caiman crocodilus (common caiman, spectacled caiman)

As its name suggests, the common caiman is the most widely distributed of the New

World crocodilians. Its range includes Brazil, Costa Rica, Ecuador, Colombia, Guyana,

French Guiana, El Salvador, Honduras, Guatemala, Nicaragua, Panama, Mexico,

Suriname, Trinidad and Tobago, Venezuela, and of course, Peru (Ross, 1998). It has also

been introduced into Cuba, Puerto Rico and the United States.

The common caiman is a small to medium sized crocodilian, with males generally

reaching 2.0 m to 2.5 m and females reaching a mean maximum size of 1.4 m (Britton,

2001). It is thought that this species may have been restricted to a much smaller

ecological niche in the past, but has benefited from commercial utilization and over-

hunting of other larger species within its range (Crocodylus acutus, C. intermedius and

Melanosuchus niger), allowing it to take over habitat which it would otherwise have been

out-competed by healthy populations (Ross, 1998; Britton, 2001). C. crocodilus now

inhabits virtually every type of low altitude wetland habitat in the Neotropics. It is also

the most geographically variable species with generally four or five subspecies being

recognized (Ross, 1998; Brazaitis et al., 1998, Britton, 2001). At present, common

caimans continue to supply the vast majority of skins on the market, and yet appear to be

relatively resilient to this largely illegal commercial hunting (Glastra, 1983; Ross, 1998,

Britton, 2001). This seems to reflect the reproductive potential and adaptability of the

species (Britton, 2001). However, illegal hunting is still a threat, and Britton (2001) notes

that in some areas of El Salvador surveys reveal severe depletion. Caiman crocodilus is

noted as Appendix II in CITES (CITES, 2005) and is not listed on the 1996 IUCN Red

List (IUCN, 2004) or the Peruvian Red List (Perú Ecologico, 2005).

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1.4.2 Paleosuchus trigonatus (Smooth-fronted caiman, Schneider‟s dwarf caiman)

The smooth-fronted caiman is the smallest species in this study, with males measuring a

maximum length of 2.3m. It has a smaller distribution than that of C. crocodilus, and is

found in Brazil, Colombia, Bolivia, Ecuador, Venezuela, French Guiana, Guyana,

Surinam and Peru (Britton, 2001). Although more research on this cryptic predator has

been done in recent years (Magnusson and Lima, 1991; Magnusson, 1992), many aspects

of the smooth-fronted caiman‟s life history remain to be investigated (Ross, 1998). In

terms of conservation issues, populations of smooth-fronted caiman appear to remain

healthy throughout the species‟ range, however quantitative data is lacking (Ross, 1998;

Britton, 2001). The limited potential for commercial exploitation due to its small size and

ventral ossification has meant that the smooth-fronted caiman has been hunted mostly on

a subsistence basis and has been of sufficiently low intensity to avoid damaging

populations (Britton, 2001). CITES lists the species as Appendix II (CITES, 2005) and,

like the common caiman, it is not found on the 1996 IUCN Red List (IUCN, 2004) or the

Peruvian Red List (Perú Ecologico, 2005). One management issue that may be important

for the species‟ conservation is the effect of environmental pollution associated with

gold-mining occurring in Brazil and Venezuela, and increasingly in Bolivia and Peru

(Ross, 1998; Brazaitis et al., 1998).

1.4.3 Melanosuchus niger (Black Caiman)

The black caiman is the largest species in this study and is in fact the largest member of

the Alligatoridae family. Adult males surpass 4m in total length, and the mean adult

female length is 280cm (Ross, 1998). The species is distributed throughout the Amazon

9

basin including Brazil, Colombia, Ecuador, Bolivia, Guyana, French Guiana and Peru

(Britton, 2001). As with the smooth-fronted caiman, little research on the black caiman

had been done until relatively recently, and many aspects of this species‟ ecology, in

particular its reproductive ecology, are still poorly known (Plotkin et al., 1983; Herron et

al., 1990; Ross, 1998; Britton, 2001). Commercial hunting of the black caiman

intensified in the 1940s, when South American crocodile numbers became very low.

Hunting declined in some areas in the 1960s when trade in C. crocodilus increased,

however in other areas significant trade in black caiman extended into the 1970s (Plotkin

et al. 1983, Britton, 2001). Subsequently, although widely distributed, past overhunting

and continued poaching of this species has dramatically reduced populations. It is

estimated that numbers have reduced by 99% in the last century (Britton, 2002). Black

caiman populations are classed as severely depleted in four of the seven countries in

which it occurs, and are still depleted in the others (Ross, 1998). However, as previously

mentioned, recent studies have shown evidence of recovery in many populations in

different parts of Peru and the Amazon region (Ross, 1998; Da Silveira and

Thorbjarnarson, 1998). Black caiman are still listed under Appendix I in all countries

with the exception of Ecuador, where it is under Appendix II subject to a quota from

1997 (Ross, 1998; CITES, 2005). The 1996 IUCN Red List classes the species as

Endangered with exploitation occurring over much of its range, but points out that the

observed current recovery could place black caiman closer to Vulnerable (IUCN, 2004).

It is classed as vulnerable on the Peruvian Red List (Perú Ecologico, 2005).

10

2.0 Aims of the study

To assess the population status of the three present caiman species in the Yavari

region

To explore the ecological relationships between species, including habitat use and

diet composition

To examine and compare previous studies carried out in the study site to establish

any population trends, with a particular focus on the recovery of the population of

M. niger

3.0 Methodology

3.1 Study Site

The study was carried out between the 18th

June and the 8th

July 2005 in the study site

Lago Preto and was based from the research vessel the „Lobo de Rio‟. The Lago Preto

study site is situated on the Peruvian side of the Yavari River which also borders Brazil,

close to the mouth of the Yavari Miri river (4°30‟S, 71°43‟W) (Bowler, 2005). Due to its

diverse habitats and rich wildlife, particularly the high density of the endangered red

uakari monkey (Cacajao calvus ucayalii) and the presence of giant river otters

(Pteronura brasiliensis) (a flagship species for the Greater Yavari Valley), the site is now

a conservation concession, created as part of a proposed reserved zone of northeastern

Peru (Pitman et al., 2003; Bowler, 2005). Furthermore, there are plans to extend the area

covered by the nearby Tamshiyacu-Tahuayo Community Reserve to include the area of

11

forest between the Yavari and Yavari Miri rivers, close to the Lago Preto study site

(Bowler, 2005).

This area of Peru – the extensive interfluvium bordered by the Amazon, Ucayali, and

Yavarí rivers – is relatively homogeneous in climate and geology but is a complex

combination of topography, soils, and forest types (Pitman et al., 2003). The site is

comprised of three distinct vegetation types; high 'terra firme' forests, white water

flooded forests known as varzea, and 'aguajal' swamp forest dominated by Mauritia

flexuosa palms (Bowler, 2005). The region has two distinct seasons; a dry season lasting

from May to September, and a wet season from November through to April.

Legend

Area of conservation

concession at Lago Preto

Area used by Ribereño

community of Carolina

Figure 1: Map of the conservation concession of Lago Preto (Bodmer, 2005, Pers. comm.)

12

The study included both a general census and a capture method, and was carried out

along sections of the Yavari and Yavari Miri rivers, and in two nearby lakes: Ipiranga and

Tipischca (Figure 1). Both the Yavari and the Yavari Miri are white water rivers, which

are distinguished by high quantities of recent geological material and high levels of

nutrients that are deposited in varzea areas during high water (Huston, 1996). Lake

Tipishca is a black water lake characterized by poor soils, increased leeching and high

levels of tannants that give it its distinguishing dark appearance (Huston, 1996). Lake

Ipiranga is a mix of black water and white water. The study was undertaken during the

dry season, and subsequently rivers and lakes were bordered by vegetation interspersed

with occasional banks of mud and sand.

3.2 Census

Two distinct caiman habitats were surveyed: river and lake habitat, which were

subsequently divided into transects (Table 2). Four river transects were surveyed

totalling 37.41km and two lake transects totalling 38.26 km. Total censuses within all

areas during the study amounted to 75.67km. All transects were accurately measured

using a Global Positioning System (GPS).

Table 1: Length of transects and total length of censuses

Length of transect (km) Total censused (km)

Transect 1: River

Yavari Upriver 6 11.62

Transect 2: River

Yavari Downriver 1 5.33 10.66

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Transect 3:River

Yavari Downriver 2 5.3 9.4

Transect 4: River

Yavari Miri 3.06 5.73

Transect 5: Lake

Tipishca 6.33 25.28

Transect 6: Lake

Ipiranga 8.55 12.98

Work began at approximately 1900 with a four or five person research team, including

two biologists, one experienced caiman catcher and handler, and one person to navigate

the canoe. The team began night counts along one transect, locating caiman by their

eyeshine (light reflected from the tepetum lucidun) using a one million-candle power

spotlight powered by a 12-volt car battery. The boat traveled at less than 1km per hour,

approximately 20 m from the vegetation or shore line. Individuals were then approached

in order for species and size estimates to be noted. This is a commonly-used technique for

crocodilian census (Glastra, 1983; Ouboter & Nanhoe, 1988; Herron 1994; Da Silveira &

Thorbjarnarson, 1999, Platt & Thorbjarnarson, 2000). Where species could not be

identified they were classed as „Eyes only‟ which can be used as an index of wariness

when taken as a proportion of total animals observed (Ron et al., 1998; Pacheco, 1996a).

3.3 Species Identification

Species were, in most cases, easily identified using the following species characteristics:

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3.3.1 C. crocodilus

Its snout is slightly elongate, longer that its width at the base (Brazaitis, 1973). A bony

ridge is present between the front of the eyes (infra-orbital bridge), appearing to join the

eyes like a pair of spectacles, hence its common name (spectacled caiman) (Figure 2). A

triangular ridge is present on the heavily-ossified upper eyelids (Britton, 2001).

3.3.2 P. trigonatus

Ossification is more extensive with significant sideways projection on the double row of

sharp scutes on the tail, which is more dorso-ventrally flattened that in other crocodilian

species (Britton, 2001). Its snout is elongate and pointed and also lacks the infra-orbital

Adults are a dull olive-green. Juveniles are

yellow in colour with black spots and bands

on the body and tail. As they mature, they

lose this yellow colour and the markings

become less distinct (Britton, 2001).

Figure 2: C. crocodilus (Source: Britton, 2001)

This species is dark brown in colour, with

dark brown or black crossbands on the

back and tail. Some individuals are

laterally tinged with yellow (Brazatis,

1973)

Figure 3: P. trigonatus (Source: Britton, 2001)

15

bony ridge (which serves to strengthen the skull) found in Caiman crocodilus (Brazaitis,

1973; Britton, 2001).

3.3.3 M. niger

jaws (Brazaitis, 1973). It has a massive head that is structurally dissimilar in shape to

other caiman species, with distinctly larger eyes and a relatively narrow snout. A bony

ridge extending from above the eyes down the snout is also present (Britton, 2001)

(Figure 4).

3.4 Captures

Captures were carried out using an iron-wire noose attached to a long wooden pole,

although juveniles could sometimes be caught by hand. Once captured the caiman would

be secured using heavy-duty string, beginning with the jaws. Measurements were then

taken, including head length, head to cloaca length, total length and weight using a

measuring tape and a spring balance. Individuals were also sexed.

As its common name suggests, the black

caiman has jet black colouration, with

narrow yellow crossbands on the body

and tail. It is one of the few species that

retains most of the juvenile colouration

throughout life. Three to five large dark

blotches are present on the sides of the Figure 4: M. niger (Source: Britton, 2001)

16

3.5 Sex Determination

Males and females can be difficult to tell apart visually. One indication may be size as

males grow larger than females in all crocodilian species and consequently, a very large

individual is likely to be a male (Britton, 2006). But such traits are unreliable indicators

of sex. To reliably sex a crocodilian one must either feel or visually identify the penis of a

male or the clitoris of a female. In the larger animals a clean finger was inserted into the

cloaca in order to feel the copulatory organ. Juveniles are more difficult to sex, however,

and in this study a pair of blunt forceps was used. The forceps were used with extreme

care to spread the vent apart in order to see down into the cloaca looking for what is

called the “cliteropenis”. It is so-named because the penis and clitoris are relatively

similar in appearance in hatchlings (Britton, 2006) and it therefore requires experience to

distinguish between the two sexes.

Figure 5: The sexing of a sub-adult male C. crocodilus

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Stomach Samples

3.6 Stomach samples

Should the animal caught be over 50cm in total length, stomach contents would be

obtained. This involved placing an appropriately sized piece of robust plastic tubing into

the caiman‟s jaws (using pliers for safety) and securing with the string, so that the mouth

is held open (Figure 7). Once secure, a longer, more pliable tube can be inserted into the

mouth, past the esophagus, down into the stomach. The tube was lubricated with water to

make insertion easier and less likely to cause harm to the animal. Using a funnel, filtered

river water was then poured into the tube and into the caiman‟s stomach. After gently

massaging its stomach, the caiman was then turned headfirst into a bucket allowing the

water and accompanying stomach contents to be collected. This process was repeated

until the water coming out of the tube was clear, and the contents were stored in plastic

bottles. This method of collecting stomach contents from crocodilian species has been

commonly used in the past and seems to be the most appropriate and effective method,

causing only mild discomfort to the caiman (Magnusson et al., 1987; Santos et al., 1996;

Figure 6: The sexing of a juvenile male C. crocodilus

18

Da Silveira & Magnusson, 1999). When all the applicable data had been collected the

animal was also marked (following Glastra, 1983) by cutting triangles out of one or more

dorsal scutes of the tail crest with a pair of small sharp scissors. The caiman was then

checked over for any injuries or specific markings before being released back into the

water.

3.6.1 Stomach Sample Analysis

Stomach contents were analyzed the following day after collection. The contents were

poured through a filter and rinsed, before being placed on a suitable surface. Using

tweezers the stomach contents were identified and split into the following groups:

Figure 7: Capture method with a sub-adult M. niger

19

crustacean, gastrapod, fish, insect, aves, mammalia, vegetation, parasitic species and

other (table 2).

Table 2: Food type categories with descriptions

Food Type Description/Examples

Crustacean Freshwater crabs, claws, carapace etc

Gastropod Freshwater snails, fragments of shell etc

Fish Whole fish, flesh/meat, bones, scales etc

Insect Whole insects, exoskeletons, wing cases

Aves Feathers, flesh/meat, claws, bones etc

Mammalia Flesh/meat, bones, fur etc

Vegetation Leaves, sticks, bark etc

Parasitic species Nematode spp., other stomach parasites

Other Other food and non-food items

4.0 Statistical Analysis

Statistics were carried out in Microsoft Excel.

4.1 Measuring the average:

The arithmetic mean was used throughout the study to provide average values of the

sample observations and measurements:

20

Where x is each observation/measurement, n equals the number of observations in the

sample, and ∑ (sigma) means the „sum of‟ (Fowler et al., 1998)

4.2 Measuring Variability:

Standard deviation and Analysis of Variance (ANOVA) were used for the quantitative

analysis of variability within, and between, samples.

4.2.1Standard Deviation:

This is the most widely applied measure of variability and is calculated directly from all

the observations of a particular variable. The standard deviation of a population ( ) is

estimated from the sample data (s):

Where x is the sample mean and N is the number of observations in the sample.

4.2.2 ANOVA:

This flexible technique allows the comparison of two or more means to distinguish

whether differences between sample means are significant. Throughout this study a

confidence limit of 95% was used.

4.3 Measuring Abundance:

Abundance of each transect was calculated using the following equation:

Abundance = n/L

21

Where n = the total number of caiman observed and L represents the total length of

transect.

4.4 Schoener‟s Method

This estimates the amount of resource overlap between species (0-1, where 0 = no

overlap and 1 = complete overlap).

Qjk = 1- ½ Σ I pij – pik I

Where Qjk = resource overlap between species j and k, and pi is the proportional use of

the resource by species j (pij) or k (pik).

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5.0 Results

5.1 Census

During the study a total of 540 caiman were observed and identified along a total of

75.67km of river and lake transects. 505 were identified as C. crocodilus, making this

species the most abundant in this study. 24 were identified as M. niger and 11 were

identified as P. trigonatus. Table 3 gives the average abundance per transect for each

species.

Table 3: Abundance per transect in individuals per km

C. crocodilus M. niger P. trigonatus

Transect 1 5.92 0.26 0.38

Transect 2 3.1 0.47 0.21

Transect 3 3.3 0.21 0.17

Transect 4 8.55 0.17 0.34

Transect 5 6.21 0.44 0

Transect 6 12.79 0.15 0

5.1.1Abundance between habitats

As shown in figure 8, C. crocodilus was found to be particularly abundant in both river

and lake habitat (5.22 and 9.2 individuals per km, respectively). P. trigonatus on the

other hand, was absent from the lake habitat and at only a relatively low abundance in

river habitat (0.28 individuals per km), whereas M. niger had similar low abundances in

both river and lake habitat (0.28 and 3.0 individuals per km).

23

Figure 8: Mean Abundance Between

Habitats

0

1

2

3

4

5

6

7

8

9

10

c.croc p.trig m.niger

Species

Mean

ab

un

dan

ce (

ind

ivid

uals

/km

)

River

Lake

There was no significant difference in abundances between the two habitats in C.

crocodilus or M. niger (one-way ANOVA: F1,4 = 2.362167, P = 0.199122; F1,4 =

0.017098, P = 0.902277 respectively). The lack of P. trigonatus in the lake habitat,

however, was found to be statistically significant, indicating a particular avoidance of

lake habitat (one way ANOVA: F1,4 = 13.22404, P = 0.022033)

5.1.2 Size class:

Observations were placed into size class categories based on maturation sizes for C.

crocodilus (Ross, 1998; Britton, 2002) and size classes used by Pacheco (1996b) for M.

niger: <60 cm, 60-119 cm, 120-179 cm, >180 cm.

24

Size class distribution between habitats:

Figures 9-11 depict the percentage of each size class observed in both the river and lake

habitat for each species (with the exception of P. trigonatus that was only documented

during river censuses).

Figure 9: Size class distribution of C. crocodilus

0

10

20

30

40

50

60

70

<60 60-119 120-179 >180

Size Class

Perc

en

tag

e o

f o

ccu

ren

ce

River

Lake

The size class distribution of C. crocodilus is comparable in both surveyed habitats with a

majority of individuals of size class 60-119 cm (river: 64.92%, lake: 50.89%). The C.

crocodilus population was also found to have relatively high numbers of the smallest size

class (river: 22.82%, lake: 34.10%) and generally demonstrates a reasonably healthy

population structure.

25

Figure 10: Size class distribution of P.trigonatus

(river habitat)

0

10

20

30

40

50

60

70

80

<60 60-119 120-179 >180

Size class

Perc

en

tag

e o

f o

ccu

ren

ce

Comparable to C. crocodilus, the size distribution of P. trigonatus shows a high

proportion of individuals in the lowest size classes, particularly the 60-119 cm class

(68.75%). The largest individual observed was 120 cm, just placing it into the larger size

category. Despite this being the smallest species in the study, male P. trigonatus

generally reach 1.7 to 2.3 m (Britton, 2001), yet no individuals were recorded in the

largest size class.

Figure 11: Size class distribution of M. niger

0

10

20

30

40

50

60

<60 60-119 120-179 >180

Size class

Perc

en

tag

e o

f o

ccu

ren

ce

River

Lake

26

The population structure of M. niger is very irregular, particularly in the river habitat.

The complete lack of juveniles in the lowest size class in river habitat, and very low

numbers in the lake habitat may be an indication of population decline and is a cause for

concern. Nevertheless, the lake habitat shows high proportions of M. niger in the 60-119

cm size class (53.84%), and demonstrates a potentially more stable population structure.

5.2 Sex ratios

Sex ratios can have a significant effect on a population‟s ability to increase from low

numbers, enhancing that ability when females predominate and depressing it when males

dominate (Sinclair et al., 2006).

Figure 12: Sex ratio of each species in river habitat

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

C.crocodilus P. trigonatus M. niger

Species

Perc

en

tag

e o

f ccu

rren

ce

Female

Male

Figure 12 demonstrates that all three species have an apparently healthy sex ratio. C.

crocodilus demonstrates a 50:50 ratio, while both P.trigonatus and M. niger have slightly

higher numbers of females than males.

27

Figure 13: Sex ratio of each species in lake habitat

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

C.crocodilus M. niger

Species

Perc

en

tag

e o

f o

ccu

err

en

ce

Female

Male

Figure 13 shows a particular difference in sex ratio of C. crocodilus in comparison to

river habitat (Figure 12) demonstrating a heavy bias towards males. M. niger has a sex

ratio in lake habitat relatively similar to that of river habitat with slightly higher numbers

of females.

5.3 Diet analysis

In total 66 caiman were captured during the study; 51 C. crocodilus, 9 P. trigonatus and 6

M. niger. However, stomach contents could not be obtained with individuals less than 50

cm in total length. Therefore, stomach samples amounted to 35 C. crocodilus samples

(including one empty stomach), 9 P. trigonatus samples and 5 samples of M. niger. The

largest individual caught in this study, a 2.44 m female M. niger, was too heavy to obtain

a stomach sample (estimated to be at least 30 kg).

28

5.3.1 Diet composition

Figures 14-16 give the percentage occurrence of each food category based on mean mass

data for each of the three caiman species (see Appendix 1 for the recorded masses).

Figure 14: Percentage of each food category found in stomach samples

of C. crocodilus

0

5

10

15

20

25

30

crus

tace

an

gastra

poda fis

h

inse

ctav

es

mam

malia

vege

tatio

n

para

site

othe

r

Food category

Perc

en

tag

e o

f o

ccu

rren

ce

The highest diet proportions consumed by C. crocodilus were insects (21.73%) fish

(19.70%), and particularly „other‟ food components (27.73%). The most notable

components of the „other‟ category were found on transect 5 (Lake Tipisca) and consisted

of species of worm (in three individuals), and even the foreleg bone of a caiman (species

unidentified), possibly indicating cannibalism.

29

Figure 15: Percentage of each food category found in stomach samples

of P. trigonatus

05

10152025303540

crus

tace

an

gastra

poda fis

h

inse

ctav

es

mam

malia

vege

tatio

n

para

site

othe

r

Food category

Perc

en

tag

e o

f o

ccu

rren

ce

The stomach samples obtained from P. trigonatus show that crustacean (34.43%), insect

(13.36%), fish (10.28%) and vegetation (7.71%) constitute the main part of the species‟

diet. The category „other‟ in this case did not include any food items, and interestingly,

only comprised of small stones, found in four of the nine individuals studied.

Figure 16: Percentage of each food category found in stomach samples

of M. niger

0

5

10

15

20

25

30

35

40

crus

tace

an

gastra

poda fis

h

inse

ctav

es

mam

malia

vege

tatio

n

para

site

othe

r

Food cateogory

Perc

en

tag

e o

f o

ccu

rren

ce

30

The diet of M. niger was found to comprise mainly of insects (37.17%) and vegetation

(31.05%), also with relatively high amounts of fish (13.15%) and crustacean (11.11%)

(Figure 16).

ANOVA

In all cases, the differences in the masses in each food category were not statistically

significant between species, with the exception of vegetation in the diet of M. niger. This

indicates that the diets between all three species are very similar, experiencing significant

overlap. Vegetation was found to be statistically significantly higher in M. niger than in

C. crocodilus (one way ANOVA : F1,37 = 4.14851, P = 0.048874).

5.3.2 Niche overlap

To investigate competition for food resources Schoener‟s method of niche overlap can be

used to determine the degree of dietary overlap between species.

The method uses proportional indices utilising the percentage occurrence and the

associated proportion of the percentage occurrence of each food category. Table 4 shows

the levels of dietary overlap between all three species in this study (1 = complete

overlap).

Table 4: Dietary niche overlap between species

C.crocodilus M.niger P.trigonatus

C.crocodilus x x x

M.niger 0.780728477 x x

P.trigonatus 0.846877206 0.846820016 x

31

Table 4 indicates significantly high dietary niche overlap, and thus potential competition,

between all the species surveyed. Surprisingly, P. trigonatus experiences the highest

overlap with the two other species.

5.4 Microhabitat

A total of six microhabitats were identified in the river habitat, five of which were also

found in the lake habitat. Table 5 gives the definitions of each identified microhabitat.

Table 5: Microhabitat definitions

Open water Submerged in water, not close to vegetation or other

shelter.

Between vegetation Submerged in water, at the boundary of land and water,

within vegetation providing cover.

Bare bank On the land, exposed, with no vegetation or other

shelter.

Ground vegetation On land, with vegetation providing shelter.

Pond in river bank In a shallow water body formed on land next to larger

water body.

Overhanging vegetation In shallow water with overhanging vegetation providing

shelter

32

Figure 17: Microhabitat use in river habitat

0%

20%

40%

60%

80%

100%

C.crocodilus P. trigonatus M.niger

Species

Perc

en

tag

e o

f o

ccu

rren

ce

overhanging vegetation

pond in river bank

between vegetation

ground vegetation

open water

bare bank

C. crocodilus was found in the most diverse number of microhabitats (figure 17). Chi-

squared analysis of microhabitat use in river transects found that C. crocodilus is

significantly positively associated with „open water‟, and negatively associated with

„ground vegetation‟, „ponds in river bank‟ and „overhanging vegetation‟ (χ2

5 = 225.8, P <

0.05). P. trigonatus was found in four microhabitats, however a statistical test did not

find any significant association with any particular microhabitat (χ2

5 = 10.30, P < 0.05).

M. niger was only found in three microhabitats and the chi-square analysis found a

significant preference for „between vegetation‟ (χ2

5 = 15.76, P < 0.05).

33

Figure 18: Microhabitat use in lake habitat

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

C. crocodilus M.niger

Species

Perc

en

tag

e o

f o

ccu

rren

ce

overhanging vegetation

between vegetation

ground vegetation

open water

bare bank

Both C. crocodilus and M. niger identified less microhabitats in lake habitat compared to

river habitat (figure 18). Again, there are statistically significant differences in

microhabitat use for both species, particularly C. crocodilus. C. crocodilus was found to

be positively associated with „bare bank‟, „open water‟ and „between vegetation‟, and

negatively associated with „ground vegetation‟ and „overhanging vegetation‟ (χ2

4 =

322.52, P = <0.05). M.niger is significantly associated with „between vegetation‟ (χ2

4 =

21.21, P = <0.05).

6.0 Discussion

This study reports the presence of C. crocodilus, P. trigonatus and M. niger in the Lago

Preto study site. In terms of abundance C. crocodilus densities in this study are

considered average and represent healthy populations. In other countries surveyed,

densities of 5–50+ individuals per kilometer of standard survey are observed, however, it

should be noted that densities and inferred numbers are highly variable due to seasonal

34

aggregation at times of low water, and dispersal at high water (Montague, 1983; Ouboter

& Nanhoe, 1988, Ross, 1998). A great deal of biological investigation has been carried

out on this species, particularly in seasonal savanna habitats, however, relatively less is

known about its behavior and ecology in forested or swamp habitats (Ouboter and

Nanhoe, 1988; Ouboter, 1996; Ross, 1998). C. crocodilus is noted for its extreme

adaptability after it quickly filled the niches left vacant by sympatric competitors that

suffered diminished ranges due to over-hunting (Plotkin et al., 1983; Ross, 1998; Britton,

2001). This adaptability, that has allowed it to inhabit virtually all lowland wetland and

riverine habitat types throughout its range, was evidenced in this study by similar high

abundances in both river and lake habitat, in conjunction with its presence in a variety of

microhabitats.

The population dynamics of C. crocodilus in this study revealed a stable population with

higher numbers of smaller individuals, i.e. juveniles and sub-adults, than larger size

classes. The sex ratio, however, indicated less conclusive results. Sex determination is

thought to be temperature-dependant in all crocodilian species (Lang and Andrews,

2005). In C. crocodilus cold or extremely hot nest temperatures tend to produce females,

while intermediate temperatures favour males (Lang and Andrew, 2005), and in many

populations females outnumber males (as is the case of most crocodilians) (Webb et al.,

1983; Outboter and Nanhoe, 1989). In most Venezuelan savanna populations studied,

however, sex ratio does not deviate from 50:50 (Outboter and Nanhoe, 1989), as was the

case for the population found in river habitat during this study. In the lake habitat, a bias

towards males was found, which potentially limits the population‟s ability to increase

35

(Sinclair et al., 2006). A study by Magnusson (1985) also found a male biased sex ratio

of the species in Amazonian forest streams, however, in Outboter and Nanhoe‟s study of

C. c. crocodilus in Suriname (1989) females were found to be more difficult to catch than

males. Should this be the case in the lakes of the Yavari region, this would have

influenced the results, and catchability may need to be accounted for in future studies.

P. trigonatus was found at relatively low densities along the Yavari and Yavari Miri

rivers, and was apparently absent from lakes. However, rivers and particularly lakes, are

not usually described as this species‟ habitat, with most studies placing P. trigonatus

principally in shallower forest streams (Magnusson and Lima, 1991; Britton, 2001)

Extensive use of burrows has been recorded in adults, where they spend much of the day,

only emerging at night to patrol their territories and to feed (Magnusson and Lima, 1991).

The observed population in this study could, therefore, be a small portion of a larger

population that is utilizing rivers to feed, and does not represent actual numbers in the

region. Furthermore, P. trigonatus is noted for its cryptic nature (Magnusson and Lima,

1991), so that densities observed may be a result of the species‟ elusiveness.

The population structure observed in P. trigonatus does not yield conclusive evidence of

the status of the population. The lack of ecological, biological and behavioural data on

the species makes it difficult to interpret observed patterns. Low numbers of the larger

size classes, relatively high proportion of juveniles and high numbers of what are likely

to be sub-adults indicate a stable population. A slight bias towards females supports a

36

healthy population, giving improved ability to increase from low numbers (Sinclair et al.,

2006).

The mean abundance for M. niger was found to be 0.28/km in rivers, and 0.30/km in

lakes. In other parts of its range, relatively stable populations of M. niger abundances

have been documented at approximately 5.68/km in lakes and 3.15/km in rivers, with

high densities of 23.5/km in some lakes (Ross, 1998). This indicates that the population

in the Yavari region is particularly low. Concurrent to low numbers, this study also

suggests a pronounced irregular population structure in the river habitat. No individuals

of the smallest size class were found along any river transects and increasing proportions

of larger individuals implies that the population could be in decline. The lake habitat also

contained significantly low numbers of juveniles; however the remaining size classes

demonstrated a more stable structure. Nonetheless, slightly more optimistic data was

found with M. niger sex ratios. In both the lake and the river habitats surveyed, the sex

ratio was biased towards females which may increase potential recovery rates.

The significantly similar diets found in all three species of caiman may signify

competition over food resources. Competition between species occurs when individuals

of different species utilize common resources that are in short supply, such as food and

space, yet it can also occur when the resources are not in short supply, when the

organisms seeking that resource nevertheless harm one or other in the process (Sinclair et

al., 2006). Interspecific competition has the potential to influence population numbers

and distribution and could, therefore, help to explain the observed abundances in this

37

study. Schoener‟s niche overlap method gave evidence for high levels of overlap between

all three species, with P. trigonatus demonstrating the highest degree of overlap between

the two other species. The results of significant dietary overlap may indicate different

conclusions; firstly, if overlap denotes high levels of competition of food resources, the

population of one species could influence another, especially when there are significant

differences in population size. Secondly, there is also likely to be further factors that may

separate each species into a slightly different ecological niche, thereby allowing

coexistence. This is known as the competitive exclusion principle that states that species

cannot coexist if they are in complete competition (Ricklefs and Miller, 2000).

However, although it is necessary for species to require common resources, we cannot

conclude there is competition based on resource overlap unless it is also known that the

resource is in short supply, or that the species affect each other (Sinclair et al., 2006). A

study by Da Silveira and Magnusson (1999) addressed frequent claims by popular press

that caiman consume large quantities of commercial fish and compete intensively with

fishermen. Small volumes of food found in the stomachs of C. crocodilus and M. niger

led to conclusions that this was unlikely to be the case (Da Silveira and Magnusson,

1999). The main components of diet in all the species in this study were fish, and

particularly invertebrates, which are, perhaps unlikely to be at low enough abundance to

affect caiman densities, although this may vary seasonally.

Furthermore, evidence from the diet analysis of the M. niger stomach samples in relation

to microhabitat use may support the concept of species niche separation through other

ecological factors. The significantly higher amounts of vegetation found in M. niger

38

samples is likely to be due to accidental consumption whilst hunting for prey. This may

occur due to microhabitat preference, as in both river and lake censuses M. niger

demonstrated a significant association with the „between vegetation‟ microhabitat. This

could also indicate differences in foraging strategies that further separate species into

niches that allow coexistence.

Evidence of the ecological niche of P. trigonatus provides an explanation as to how,

despite demonstrating the highest levels of dietary overlap, this species can persist in the

same areas as the larger C. crocodilus and M. niger. According to Medem (1958) P.

trigonatus are found in a well-defined niche which, in general terms, is the swift running

water in the tropical rainforest. The shape of the snout has also been said to indicate

increased preference for faster-flowing water (Britton, 2001). The results of this study

contradict this, as the species was found in the relatively slow flowing Yavari and Yavari

Miri rivers. However, this may just strengthen the idea that the individuals found in this

study do not represent the total population, and it is likely that a better representation

would be found by surveying more typical habitats, such as forest streams.

Should this be the case, then the overlap of dietary resources may be more significant

between C. crocodilus and M. niger, as previous studies have investigated (Ross, 1999;

Britton, 2001). Even if food sources are abundant enough to support both species,

competition is likely to be linked to the low densities of M. niger. The results of this

study show differences in microhabitat use that may be a factor. C. crocodilus was found

to inhabit a much more diverse range of microhabitats compared to M. niger. This

39

increases the carrying capacity of the area for C. crocodilus, leading to the exclusion of

M. niger to limited microhabitats.

The observation of a caiman bone found in the stomach of a C. crocodilus (Figure 14)

may be evidence of more direct impacts of one species on another, but without species

identification of the bone this is only speculation. The species could also have been a

conspecific, evidencing cannibalism.

6.1 Comparative studies:

The need to assess the status of caiman populations in the Yavari region stems from

previous studies in the area indicating potentially worrying low densities of M. niger

(Pitman et al., 2003). Of particular relevance to the issue of this species‟ status is the

apparent recovery of M. niger in nearby areas, notably in the Pacaya-Samiria National

Reserve. The study and comparison of the Yavari region and Pacaya-Samiria may yield

explanations as to why current patterns in the Lago Preto site are being observed. Only

through improved knowledge and understanding of the diverse influencing factors on

caiman population status and community structure in the Yavari region, can appropriate

and effective conservation measures be put into place.

6.1.1 Yavari

Three studies on caiman populations in the Yavari region have taken place in previous

years. The first, undertaken in 2001, surveyed two sections of the Yavari River with

different levels of human activity and the lake Lago Preto (Newell, 2002). The study

found M. niger to be present only at the lake site at densities of 0.30/km. Densities of C.

40

crocodilus were found to be particularly high at Lago Preto (16.92/km). Statistical

analysis in the study was limited to data on C. crocodilus, as data was insufficient for M.

niger.

In 2002, Boulton (2003) investigated the diet of caiman species along the Yavari River at

the Lago Preto study site. This study, however, found no evidence of the presence of M.

niger, and analysis was based on data collected from C. crocodilus and P. trigonatus.

Interestingly, higher numbers of P. trigonatus were documented than C. crocodilus,

however, it should be noted that only a total of 26 caiman contributed to the study, and

data may reflect discrepancies of a small sample size.

Pask (2005) investigated the abundance of C. crocodilus, P. trigonatus and M. niger. The

study found C. crocodilus at an abundance of 1.94/km and P. trigonatus at 0.25/km.

Unfortunately, due to high water levels, the abundance of M. niger along lake transects

could not be obtained due to logistical problems. Along the river transects an abundance

of only 0.01/km was observed (Pask, 2005)

The studies that have been carried out do not allow for any decent quantitative

monitoring of the area for the population of M. niger, due to differences in sample size,

transects, data analysis, seasonality and length of study. What can be concluded however,

is that the population of M. niger has been significantly low in all studies carried out in

the region, including a biological inventory (Pitman et al., 2003). With the exception

perhaps of a female biased sex ratio found in this study, there have been no signs of

recovery within the population.

41

6.1.2 Pacaya-Samiria National Reserve

Street (2004) studied the diet and abundance of the three caiman species in Pacaya-

Samiria National Reserve. As previously stated, the reserve is home to a stable

recovering population of M. niger, in concurrence with stable populations of C.

crocodilus and P. trigonatus. Of particular significance to this study is the conclusion that

in areas where M. niger are more abundant, the two other species are found at lower

densities, with the same pattern occurring with high C. crocodilus abundances. This is

more conclusive evidence of competition, rather than niche overlap, and demonstrates

that the abundance of one species may directly affect the abundance of another. The

study enabled the use of the Lotke-Volterra model which describes the competitive effect

of one species on another (Sinclair et al., 2006). Unfortunately, due to insufficient data on

M. niger, the analysis could not be carried out in this study. The implications of the

Lotke-Volterra equations can be examined graphically and figures 19 and 20 show the

graph representations generated in Street‟s study (2004).

Figure 19: Lotke-Volterra for C. crocodilus and M. niger (Street, 2004)

42

The model relies on a number of assumptions, such as that populations are at carrying

capacity and competition does not change with age, size and density of the species

(Sinclair et al., 2006). However, Street‟s study may be an indicator that the population in

the Lago Preto study site also shows a Lotke-Volterra competition relationship signifying

that low levels of M. niger are due to the presence of high numbers of C. crocodilus, and

the resulting competition between the species. The introduced population of C. crocodilus

in Cuba is thought to have been primarily responsibly for the dramatic decline and

probable disappearance of Crocodylus rhombifer from the Isle of Pines, demonstrating

the genuine effects of interspecific competition (Britton, 2001). Nevertheless, the most

direct approach to actually demonstrate that interspecific competition does take place is

to carry out a removal experiment, whereby one of the species is removed or reduced in

number, and the responses of the other species are then recorded (Sinclair et al., 2006);

an unlikely prospect.

Figure 20: Competition between C. crocodilus and M. niger (Street, 2004)

43

Other factors may also be impacting the population of M. niger at Lago Preto and the

Yavari region. The following explores different possibilities that may need to considered,

especially in the planning of management programs.

6.2 Hunting

The threat of hunting could be a potential factor that restricts the M. niger population

from recovering. Current hunting pressure in the study area has not been quantitatively

assessed and is currently unknown (Pitman et al., 2003). Data and knowledge of the scale

and nature of any hunting pressure in the Yavari region is vital in order to fully

understand the population status of all three caiman species (Da Silveira and

Thorbjarnarson, 1998). If hunting is occurring, it could be focusing particularly on M.

niger, especially if hunting is for the illegal skin trade, however this seems unlikely. The

relatively high numbers M. niger of the large size-class in river habitat in this study,

could be construed as evidence against the occurrence of hunting, since selective pressure

focuses on the more valuable larger size classes (Da Silveira and Thorbjarnarson, 1999).

Hunting is more probable on a subsistence basis, and would perhaps see the taking of all

three species found in this study. C. crocodilus‟ resilience to hunting is evidenced by its

wide range and relatively stable populations despite huge-scale harvesting of the species

for the skin trade (Brazaitis et al., 1998; Ross, 1998)

44

The reasons behind the resilience of C. crocodilus to hunting compared to M. niger has

been previously investigated (Rebelo and Magnusson, 1983; Da Silveira and

Thorbjarnarson, 1999; Britton, 2001). Part of the answer seems to lie in the size at sexual

maturity (Rebelo and Magnusson, 1983; Ross, 1998; Britton, 2001). The size at which

female C. crocodilus become sexually mature is around 130 cm based on data from

Staton and Dixon (1977). Information concerning reproductive ecology of M. niger is

scarce (Britton, 2001), however it is thought that females become sexually mature at 2 m

(Brazaitis, 1974). An analysis by Rebelo and Magnusson (1983) suggested that due to

selectivity of hunters, C. crocodilus is more likely to be large enough to have bred before

they were killed. M. niger on the other hand, is more likely to have been cropped before

reaching sexual maturity (Rebelo and Magnusson, 1983), affecting rates of recruitment in

the populations of both species. Using growth rates for the two species Rebelo and

Magnusson (1983) predicted that a population of C. crocodilus hunted during one year

would require eight months to one year before recruitment to the breeding population

occurred. M. niger, however, would require approximately three years before there was

new recruitment to the breeding population. Most effective hunting is done during the

season of low water each year, allowing recruitment of C. crocodilus‟ breeding

population, but not for M. niger (Rebelo and Magnusson, 1983). Thus, it seems that

populations of C. crocodilus are resilient to hunting pressure due to regular recruitment to

the breeding stock. M. niger populations, on the other hand, may be maintained by a few

inaccessible adults. Recovery of M. niger is therefore limited due to a lack of recruitment

in the breeding population (Rebelo and Magnusson, 1983).

45

6.3 Genetic Issues

The severe decline of M. niger in abundance and range due to previously high levels of

hunting is evidence of a species bottleneck. Distributions of both C. crocodilus and M.

niger have been fragmented by habitat loss causing separation into smaller, isolated

populations (Ross, 1998; Britton, 2001; Frankham et al. 2004). Small population size is a

pervasive concern in conservation biology, as small or declining populations of

threatened and endangered species are more prone to extinction than large stable

populations (Frankham et al. 2004). Genetic factors are increasingly recognized as an

important component to be considered in conservation and population management

(Frankel and Soule, 1981; Frankham et al., 2004). Bottlenecks and sustained small

populations can lead to reduced genetic diversity, higher levels of inbreeding, lower

reproductive fitness and compromised ability to evolve (Milligan et al., 1994; Flagstad et

al., 2003; Randi et al., 2003; Frankham et al., 2004). However, with the exception of a

recent study by Farias et al. (2004), almost nothing is known about the genetic diversity,

metapopulation structure, or other population genetic indicators of either C. crocodilus or

M. niger (Farias et al., 2004). This information is vital for both the conservation of wild

crocodilian populations, and for the management of captive breeding efforts, as well as

for mitigating and preventing fitness losses associated with the isolation and decline of

populations (Hedrick and Miller, 1992; Farias et al., 2004; Frankham et al., 2004). The

study by Farias et al., (2004) tested to see whether M. niger and C. crocodilus are

genetically structured and whether they are genetically depleted as a result of past over-

exploitation in study sites across Brazil and French Guiana. Using the mitochondrial

cytochrome b gene, the study surprisingly found that genetic diversity was, in general,

46

slightly higher in M. niger than in C. crocodilus (Farias et al., 2004). However, the

French Guianan populations of both species showed low levels of gene diversity

compared to the Brazilian Amazon populations (Farias et al., 2004), demonstrating that

levels of genetic diversity loss vary across regions and could possibly be higher in Lago

Preto. The molecular data also suggested that isolation-by-distance is a significant

population structuring force acting on M. niger, which is a potential factor that places M.

niger at a disadvantage to C. cocodilus. In addition, Farias et al. (2004) notes that M.

niger is a habitat specialist, while C. crocodilus is a habitat generalist (Herron, 1994).

Habitat loss therefore, coupled with hunting pressure, will affect the ability of M. niger to

disperse into suitable habitat limiting its rate of recovery, in comparison to C. crocodilus

that has demonstrated an aptitude for colonizing a variety of habitats (Ross, 1998;

Britton, 2001; Farias et al., 2004).

Studies assessing levels of heterozygosity in both C. crocodilus and M. niger should be

carried out in the study site in order to give an indicator of fitness, and the levels of

inbreeding and possible impacts (i.e. reduced fecundity, offspring size, growth or

survivorship) should be explored. Genetic issues stemming from such a low population of

M. niger is potentially a factor that is influencing the recovery rate of the species. To

alleviate the problems associated with inbreeding, gene flow may need to be increased.

This can be achieved through the translocation of individuals from stable populations

elsewhere in Peru (Frankham et al., 2004).

47

6.4 Effects of pollution

Gold mining activities are associated with loss of habitats. Environmental pollution

linked to gold-mining occurrs in Brazil and Venezuela, and more increasingly in Bolivia

and Peru (Ross, 1998; Brazaitis et al., 1998). Gold mining operations take place mostly in

the riverbanks and sediments at the headwaters of virtually all river systems (Brazaitis et

al., 1996). Due to large numbers of immigrant miners attracted to otherwise inaccessible

regions for the stability of a gold-based economy, the extirpation or depletion of caiman

populations are to be expected (Brazaitis et al., 1996). Yet the impacts of resulting

pollution are likely to be much more far-reaching. The use of mercury is one of the most

efficient, inexpensive and widespread means of extracting even poor concentrations of

gold (Brazaitis et al., 1996, 1998). Environmental contamination with mercury takes

place as mercury-contaminated ore is discarded to erode and leach back into the

watershed and Malm et al. (1990) identified contamination up to 200km downstream of

the nearest gold-mining operations (Brazaitis et al., 1996). Brazilian biologists have

discovered high levels of mercury in virtually all species of fish eaten by caimans and

humans throughout the Amazon (Brazaitis et al., 1996). Caiman, as top predators, are

likely to be particularly affected by toxic pollution due to biological magnification which

is the tendency of pollutants to become concentrated in successive trophic levels

(McShaffrey, 2006). Furthermore, studies have found that some caiman tissues are

contaminated with dangerous levels of lead (Brazaitis et al., 1998). Walsh et al. found

lead levels higher than 500 p.p.b. in fish, indicating a threat to aquatic environments and

constituting a serious level of concern. The source of lead contamination in caiman in

gold-mining regions is unknown but could stem from associated ores during excavation

48

or a significant increase in motor boat use, pumps and leaded fuels (Brazaitis et al.,

1996). Cook et al. (1989) documented elevated lead levels in captive populations

appearing to affect fecundity and general health. The toxic metal contamination of

caimans also has implications for people, as contaminated meat can be consumed through

the bushmeat trade. Although the full extent or long-term impacts of toxic metal

contamination and toxicosis on wild populations of crocodilians is not known, it is

reasonable to presume that a serious threat exists for affected species, especially when

contamination coincides with hunting pressure and environmental degradation (Brazaitis

et al., 1996). Brazaitis et al. (1996) also notes that depleted populations of M. niger are

particularly susceptible to the threat of contamination, and could therefore be a relevant

factor in the population in the Yavari region. Research into levels of potentially harmful

concentrations of toxic metal should be assessed in the region, in conjunction with

investigations to assess and quantify the long-term effects on caiman populations and on

people who consume them (Brazaitis et al., 1996).

7.0 Limitations of the study

A particular limitation of this study resides in the diet analysis. The analysis assumed a

constant digestion rate for all food categories, when in fact this is not the case. Garnett

(1985) noted that differential rates of digestion must cause bias in the analysis of

crocodilian stomach contents, due to the refractory nature of chitin digestion. This

inability to digest food items containing chitin may have led to overrepresentation of food

types such as crustacean, gastropoda and insects. Magnusson et al. (1987) also found that

fish digest particularly rapidly and perhaps fish/meat was under-represented in this study.

49

Faecal samples in conjunction with stomach sample analysis may give a more complete

picture of diet (Garcia, 2006).

This study also did not take into account lunar phases. Some studies suggest that lunar

phase affects caiman counts and captures, as with a fuller moon the animal can see

observers more easily, and this may increase wariness (Ouboter & Nanhoe, 1989).

Undertaken during the dry season, this study may be complemented by surveys

completed in the wet season to allow for comparisons, and to give further insights into

the ecology of the caiman species found in the area. It therefore becomes important to

standardize methods of survey techniques and data analysis.

8.0 The future in the Yavari region

Priority should now be given to further research that quantitatively investigates the

underlying causes of low M. niger populations. Without this information, any

management or conservation proposals may prove to be ineffective. There is a growing

recognition among conservation managers and scholars that successful project

management is integrally linked to well-designed monitoring and evaluation systems

(Stem et al., 2004). Thus, appropriate monitoring and evaluation of the populations in the

area and of any management implemented is vital.

50

Further studies into the population status of P. trigonatus should also be considered.

Detailed surveys of more suitable habitat, such as forest streams should give a better

estimation of population size and structure.

The potential of culling of C. crocodilus as part of a sustainable use program should be

investigated in the region. Sustainable use programs are well developed in several

countries, notably Venezuala (Gorzula, 1987; Ross, 1998; Thorbjarnarson, 1999). Most

of these rely upon regular cropping of wild populations. Although the long-term effects

of cropping needs to be investigated, the reproductive potential of C. crocodilus makes

properly controlled sustainable yield programs look promising (Ross, 1998). If viable, the

culling of C. crocodilus could allow for the recovery of M. niger through decreased

competition as predicted by the Lotke-Volterra model. The monitoring of such a program

would be particularly important to ensure that M. niger is not culled. Community-based

programs that focus on conservation education may alleviate this potential problem. A

community-based approach to a culling program may be particularly beneficial for

caiman populations in the area. Such projects provide tangible economic benefits for

local inhabitants through sustainable use and have been successful in other parts of Peru,

notably in the Reserva Comunal Tamshiyacu-Tahuayo (RCTT) (Bodmer and Puertas,

2000). Detailed research into whether such a program would be economically viable

should be undertaken before being instigated.

51

8.0 The role of caiman in the ecosystem

Keystone species play an important part in maintaining ecosystem balance and their

removal can cause the reduction or degradation in diversity of the ecosystem (Ricklefs

and Miller, 2000). First presented by Fittkau (1970), there has been evidence to suggest

that caiman significantly impact nutrient cycling, and consequently fish populations, in

Amazonian water systems. They are also implicated in maintaining ecosystem structure

and function by activities such as selective predation on fish species and maintenance of

wet refugia in droughts (Craighead, 1968; King 1988; Ross, 1998), and have

consequently been described as keystone species. This concept was originally supported

by anecdotal evidence of local people noticing a decrease in fish stocks concurrent to a

decline in caiman numbers. Fittkau‟s (1970) hypothesis suggests that a decrease in

caiman leads to changes in the original biocenosis of a lake, indicating a reciprocal

relationship between the fish and caiman. Some lakes in central Amazonia are

particularly nutrient-poor and harbor very little primary production. However, during

periods of high water, fish from the eutrophic varzea lakes or from the channel itself

migrate into these lakes to breed. As top predators, caiman consume many of these

migratory fish leading to an increase in the autochthonous biomass of the biocenosis, and

as a result of their metabolism, bring allochthonous nutrients into the cycles of these

nutrient-poor lakes (Fittkau, 1970). This increases primary production and allows for a

greater amount of fish fry to exist, which in turn supplies a better supply of nutrients to

higher links in the food chain. The removal or severe reduction of caiman populations in

the nutrient-poor lakes of the Amazon could potentially disrupt the entire ecosystem

structure and balance, as the effects could reach all levels of the food chain. This concept

52

is particularly important in areas where local inhabitants rely on fish stocks for

subsistence or livelihood, and may provide further support for sustainable use programs

of caiman.

53

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