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).
8
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:
14
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
17
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).
22
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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