Island Biogeography

Why study Islands?• First biologists and geographers studied them like

Wallace (East Indies), Darwin (Galapagos Islands) and Hooker (Southern Ocean).

• Islands are natural experimental plots which offer differences in sizes, number of species, isolation, number of predators.

• With fewer species interactions are much less complex than in mainland habitats.

• Due to their isolation evolutionary processes work at different rates

• There is little or no gene flow dilute the effect of selection and mutation causing a very high level of endemism in island species

• Depending on scale and dispersal ability many habitats can be ‘Islands’ (lakes, mountaintops, etc.)

• Islands can serve as natural field laboratories to study the relationship between area and species diversity

• Part of unintentional experiments are habitat loss and introductions of invasive species by humans, often detrimental consequences

• Only with a better understanding of species-area relationships can we design optimum conservation areas

What types of islands are there?• Oceanic islands; which are located over oceanic

plates and have never been connected to the continental shelf

• Continental shelf islands: which are part of the continental shelf and can be connected to the mainland during periods of lower sea level

• Habitat islands: distinct patches of terrestrial habitat surrounded by very different habitats but not water

• Non-marine islands: which are somewhere between habitat and continental shelf islands in their level of isolation

Natural disturbances of islands• Any relative discrete event in time that removes

organisms and opens up space which can be colonized by individuals of the same or a different species

• Disturbances can be short term and frequently reoccurring like high winds or high rainfall

• Some disturbances like ENSO events and hurricanes occurring every decade or more with larger impacts on islands

• Other events occur only between 100 -1000 years for example volcanic eruptions, tsunamis or earthquakes

Implications of small founding populations

• Typically the number of organisms arriving by a chance event on a remote island is small

• Small founding populations containing only a subset of the source population’s biodiversity can cause a genetic bottleneck

• Studies on Hawaiian fruit flies suggest that following the arrival of a single female with eggs on one of the islands, strong selection for females with less strict mate selection genes were more successful

• That led to a significant shift in gene frequencies allowing better adaptation to the new environment (Carson 2002)

Implications of small founding populations

• The reduced genetic diversity in the founder population can also give rise to random genetic drift

• Genetic drift by can lead to significant changes in a species genetic makeup even without further adaptation

The result of founder effects, drift, and selection on islands is the occurrence of a number of extreme forms that occur only on islands…

Giants and dwarfs

The Galapagos and Indian Ocean tortoises were long regarded as typical island giants, but there have been large mainland species, only many are extinct due to humans.

But a study on insular species of mammals found that 85% of island rodents are larger, possibly due to the absence of predators (Foster 1964, Arnold 1979)

On several islands in the Mediterranean dwarf hippopotami, elephants and deer existed several thousand years ago (Reyment 1983). The record is the Maltan elephant which stood 1.5m shoulder height (Lister 1993).

The untested hypothesis is that on small islands there are less resources available for large herbivores and often no predators. Size reduction is an advantage.

Maybe even human dwarf species Homo florensis on the Island of Flores (Brown et al 2004).

Three hypothesis have been proposed for gigantism of island species (Schwaner & Sarre 1988):

1.Predation hypothesis: either a) there is selective release if no predation occurs or b) there is selective advantage to escape a window of vulnerability

2.Social-sexual hypothesis: due to high densities that occur among island populations, intraspecific competition (sexual selection) among males and females selects for larger body size

3.Food availability hypothesis: increase in the mean and variance in food supply/demand ratio selects for giants

Loss of disperseability

• An interesting aspect of many species which dispersed to islands is that in many cases they lost their dispersal ability afterwards.

• Many birds became flightless, e.g. Aldabran rails, Dodo’s, Kakapo.

• Plants lost their ability of wind dispersal on near shore islands in BC (Cody and Overton 1996) and on Juan Fernandez Island.

Centauria (A-C)and Senecio (D-F)seeds. A-Argentina,B,C-Juan Fernandez

• Flies lost their wings on Tristan da Cunha and Gough islands.

The original theory was that this occurred (was selected) to prevent wind loss in insects and seeds, but Roff (1990,1994) found no clear relationship.

balancers

‘wings’

Ecological release on islandsDue to reduced competition or from other interactions

including predation, two main changes in newly arrived species are commonly observed.

1. The loss of now unnecessary features (defensive traits, bold pattering, flight loss in many birds).

Examples are the Solomon Island rails which lost bold patterning and the ability to fly (Diamond 1991).

Many birds also reverted to simpler song patterns (Otte 1989).

Unfortunately many species also lost all fear of humans.

2. The second form of release is from close competitors, allowing the colonists to occupy not only different niches but also a wider array than its ancestral form (Cox & Ricklefs 1977).

It’s an important part of many scenarios of island evolution (e.g. adaptive radiation).

Examples are Fijian fruit bats, that are more diurnal on islands without predatory eagles (Lomolino 1984).

The meadow vole uses habitat types indiscriminately on islands without predators (Lomolino 1984).

Nesting sites of several bird species on the Orkney Islands shifted from cliffs and trees to shrubs and flat ground.

Adaptive radiation• The most well known examples are the Galapagos finches

and the Hawaiian honey-creepers

• The availability of empty niches is very important to adaptive radiation, allowing the diversification which sometimes leads to new species

• There are also cases of non-adaptive radiation like the land snail genus Albinaria on the Island of Crete, which diversified without occupying different niches (Gittenberger 1991)

Island endemics

• Many endemics to islands used to have a much wider distribution, but were replaced in other habitats, hence not all endemics have evolved in situ (paleo-endemics)

• One example is the St Helena Ebony; it originated from a more widespread species 9 million years ago. Since then the family on the mainland has developed away from this species (Cronk 1987)

• Species evolved on islands are called neo-endemics.

• The issue: whether paleo-endemics are more important for conservation due to a higher contribution to global biodiversity.

• The number of plant species endemic to islands (below) (36,500) contribute 13.8% of the worlds higher plant species.

• About 7,000 of these are only found within a single island or island archipelago

• The percentages of endemics are the highest for ancient continental islands like Madagascar and New Zealand.

• Islands contribute a disproportionate amount for their land area to global plant biodiversity.

Land snails: only 8 archipelagos account between 7.7-9.0% of the world land snail species. In particular larger islands with a larger range of elevations harbour many species (Groombridge 1992)

Insects: in Hawaii’ alone there are about 1000 species of fruit flies (Wagner & Funk, 1995).

Lizards: Caribbean anoles are small arboreal insectivores and one of the larger and better studied vertebrate taxa. Out of 300 known Anolis species half occur on Caribbean islands (Losos 1994, 2004).

Birds: e.g. Galapagos finches and Hawaiian honeycreepers. 1750 species of birds are confined to islands; that represents 17% of described species.

Species-isolation relationships

• Another key factor determining the number of species on an island is the level of isolation

• Islands of comparable sizes have a lower number of species if they are more isolated than habitat islands which are on continents (Wilson 1961)

Species-isolation relationships

• Williams (1981) found a decrease in the number of mainland bird species with increased distance from the mainland

• The reasons for decline of species diversity with distance relate to species’ dispersal capacity and pathway

• terrestrial mammals (except bats) can only disperse very limited distances (Lomolino, 1982)

• Bird species can disperse over larger distances, as seen in the example of resident land birds (Diamond 1972)

Dispersal abilities are also dependant on the type of reproduction a organism uses.

Estimates for ocean dispersal without human assistance are:

• freshwater fish 5 km,

• elephants and other large mammals 50 km,

• tortoises, snakes and rodents reached the Galapagos 1100 km,

• bats and land birds reached Hawaii’ 3600km (Menard 1986)

Therefore the further an island is from the mainland the less species can disperse to it.

• Isolation from the mainland can also change over time

• Example of lizard species on Islands in the Gulf of California (Wilcox 1978)

Species-area relationships

• One of the most obvious traits of Islands are a limited number of species, more countable than on the mainland

• The area available for species is also easier defined than on continents

• Darlington (1957) found an empirical relationship between Island area and number of reptile and amphibian species in the West Indies

As a log-log plot, it is not a curve but a straight line.

As a rule of thumb with every 10 fold increase in size double the number of species are present.

S = CAz

S is number of Species

C is a constant which varies with the taxonomic group under study (taxa which consist of good dispersers (these species also typically have rapid population growth) will logically accumulate more species on an isolated island, all else being equal).

A is the area of the island, and the exponent z has been shown to be fairly constant for most island situations.

z represents a parameter for the slope of the relationship between S and A on a log scale.

Geographic variation in C has been observed and 'loosely' reflects the isolation of island groups typically studied.

The presence (or absence) of major air or water circulation pathways nearby increases (decreases) C.

There are also effects of gross climatic difference; C is higher in the tropics than for islands at high arctic latitudes.

C is also regarded as the scaling factor.

z, in an all out treatment, is related to the distribution of abundances of species.

Therefore the number of species expected increases if the total number of individuals increases, as it would on a larger island. We expect, from basic theory, that those species follow a Preston log-normal distribution of abundance (see May 1975).

According to that theory, the value of z should be .263.

Many studies have looked at and compared z-values for different habitats.

An early comparison (MacArthur and Wilson, 1967) found islands to have z between 0.20-0.35 whereas non-isolated samples on continents or within large islands had a z of 0.12-0.17.

This suggests that any reduction in island area lowers the diversity more than a similar reduction of sample area in a contiguous mainland habitat.

Other studies (Williamson 1988) have found a less clearly marked difference in z between mainland habitats and islands.

Why might there be a difference in the species-area relationship between islands and isolated habitat areas on larger islands or continents?

The answer is the inclusion of transients in species counts from small 'islands‘ on continents. Those species may have large home ranges, larger than the area of whole islands, but still be occasionally present within continental ‘islands’. They could not survive on the isolated island.

Species-area relationships

Species-area curveshave been generated for a large variety of places and taxa, and the range of z values is remarkably small (Preston 1957, Williams 1953).

Normally the relative abundance of species within a local biota fita log normal distribution.

The curves indicate the presence of a few common species (the right hand end of the curve) and a larger number of species of intermediate abundance.

The left hand end of the curve (the very rare species) are rarely included in studies as they require a very high sampling effort.

The truncation of the distribution at the left hand end is called a “veil line”.

Species turnoverThere is a good record of recolonization, particularly by bird species for the Krakatau Islands after the volcanic eruption in 1883 formed the current island system.

Bird species numbers rapidly increased until 1920; after that the number of species remained constant, but there was replacement (turnover).

That general pattern – of turnover, but relative constancy in species richness, resulted in the …

Equilibrium theory of island biogeography

• It’s based on the combination of species-area relationship, species-isolation relationship and species turnover (MacArthur and Wilson 1967).

• It proposes that the number of species inhabiting an island is based on the dynamic equilibrium between immigration and extinction.

• The model is one of a dynamic equilibrium between immigration of new species onto islands and the extinction of species previously established.

The immigration rate decreases as there are fewer and fewer potential immigrant species remaining in the species pool P. This decrease is non-linear as the rate at which different species can disperse is different (e.g. tortoise vs bat).

The extinction rate increases non-linearly as factors like competition, predation, and parasitism become more important at higher species densities.

Tests of the equilibrium theory

• In an experiment Simberloff (1976) censused terrestrial insect species on mangrove islands, and then cut the islands into smaller ones by creating 1m divides. This was sufficient to require jump dispersal from many insects

• The smaller islands maintained a lower species number according with the equilibrium theory

• Therefore, in this study, since known ‘islands’ were fragmented and diversity on the ‘half-islands’ was lower, area was the only variable key in determining the number of species.

Is the world that simple?

The theory ignores autoecology - species are not exchangeable units (Armstrong 1982, Sauer 1969).

Data is rarely adequate for testing turnover, and most turnover involves transients (Simberloff 1976).

Observed turnover may depend on the interval between censuses (Diamond 1969, Diamond and May 1977, Gilbert 1980)

Immigration, extinction, and species pool are usually poorly defined (Williamson 1981,1989). Census interval has been shown to be important.

The theory ignores all events involving species interactions (e,g, succession) and evolutionary change on islands.