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What Drives Communities On Islands?

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What drives communities on islands?

A community can broadly be defined as a congregation of various species occurring simultaneously in time and space. A more detailed description comes from Whittaker (1975): An assemblage of populations of plants, animals, bacteria and fungi that live in an environment and interact with one another, forming together a distinct living system with its own composition, structure, environmental relationships, development and function.

From this definition it is clear that the interactions that occur in communities are complex, numerous and operate on a multitude of levels. Communities vary extensively in their framework and are scale dependent, so anything from a tree or puddle to a whole forest or desert can be regarded as a community. Despite these huge differences certain processes drive and operate in all communities.

This essay will venture to explain how these processes function when the communities exist on islands and are presumably, under the influence of a greater degree of isolation and limited area.

Islands are discrete, insular land masses. They are simpler than whole continents and oceans and are therefore an appealing study object (MacArthur and Wilson 1967). An island is not necessarily always surrounded by water and can also include isolated areas within continents. For example fragmented landscapes are considered as being islands, as is a single freshwater pool that is detached from all other freshwater sources. Ecologists may also classify individual plants as islands provided other plants are difficult to reach by the organisms in question. This essay however will focus mainly on oceanic islands.

The fundamental difference between island communities and those of continents is that island communities are a lot simpler and contain fewer species. An obvious reason for this is size restriction. Islands are invariably smaller than mainland continents and the species area relationship is a well documented fact in ecology (e.g. Gleason, Willis). Smaller areas support fewer organisms than larger ones because there are fewer habitats, and interactions between organisms such as competition and predation are more vigorous.

Another reason for this is due to problems with distance and dispersal. Species can arrive at islands by the wind or sea, or if they are carried there by animals.

Many organisms, such as most flightless mammals find it impossible to cross oceans and will never be able to get to an island unaided. Seeds of numerous trees also cannot reach islands easily as their seeds are heavy and not transported in the wind for long distances.

Plants that have light seeds which are designed to be conveyed by the wind, such as members of the Compositae family (whose seeds have a pappus of feathery hairs), and many insects, will often be able to reach islands without too much difficulty. Also some seeds are carried in the digestive system of flying birds, and are then excreted onto the island where they may have a chance to grow.

As the species which can access an island is already limited, the ones that are able to have an advantage in colonizing the area. This formed the basis for a famous theory in biogeography - MacArthur and Wilson's Theory of Island Biogeography (1967).

In brief, they suggested that on an island that is newly available for colonization, the rate of immigration will at first be high because organisms which are good at dispersal will reach the island and become established, as no other organisms are present. As time progresses however, the new immigrants will steadily belong to species that are already present on the island and the arrival of new taxa will decrease. They also propose that the rate of immigration depends on the proximity to the source of the potential colonists. Contrastingly, extinction rates on the island will begin low but as the number of species increases, extinctions will rise accordingly. This happens because the more species there are present, the more likely it is that any one of them will go extinct. Also population sizes decrease with the input of more species due to increased competition, and smaller populations makes extinction more probable (Cox 1973). Mac Arthur and Wilson stated that eventually equilibrium will be reached where the number of new arrivals on the island is equal to the number going into extinction.

Although immigrants may have successfully accessed the island, they must now survive there long enough to establish themselves. This is often problematic as the island environment surely differs from that of the organism's origin. If they are not able to adapt they will soon die. Both biogeographic and environmental factors affect patterns of diversity (Benayas et al 2002).

Often immigrants to an island are few in number and therefore do not have a varied enough gene pool to endure the environmental differences. This is known as the 'founder principle' (Cox 1973).

Natural disasters such as fires and volcanic eruptions have more persistent effects on the biota of islands. This is because if any species is wiped out, the chances that it will successfully reinvade are relatively low compared to continental land where fast re-colonization can occur.

Exotic species introduced to an island can also quickly out-compete established species into extinction for the same reason as given above.

These reasons point to why biodiversity is deficient on islands. Even though 20% of the world's species and subspecies of birds are found on islands, they contributed to 155 (90%) of the 171 taxa that are known to have become extinct since 1600 - an extinction rate about 50 times as great as on the continents. The influence of area on extinction rate is underlined by the fact that 75% of the island extinctions took place on small islands (Diamond 1984).

The absence of large carnivorous mammals on islands often leads to increased herbivory, as the herbivores are released from predation. In mainland communities there is nearly always a complex food web consisting of few trophic levels. The average food chain length is three to four, but may be up to six or seven (James 2004). On many islands the herbivore is at the top of the food chain.

Rao et al (2001) found that on small land bridge islands in Lago Guri, Venezuela, saplings of both preferred and less preferred species eaten by leaf cutter ants Atta cephalotes were highly underrepresented. This was because there was nothing to predate the ants. Strong intra-specific between the ants caused them to consume less favorite plant species and also regeneration of the plants could not occur.

Another study (Donlan et al 2003) on the Pacific Islands



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