Equilibrium Theory of Island Biogeography

NRE 220 Lecture 15

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 7: pages 163-174

Species-Area Relationship

We are now moving from a discussion of genetics, populations, and species to communities and ecosystems. The next few lectures will describe concepts of major importance to conservation in terms of the effects of habitat fragmentation and maintenance of species diversity.

A great deal of conservation research has been done on islands, because they are small, replicated units of area, isolated from other habitat. They are very useful for species, community, and ecosystem studies.

Early observations of biogeography involved the examination of the geography of biodiversity around the globe. This was followed by recognition of the species-area relationship - as area increases, the number of species present (diversity) also increases. This can be represented by one of two graphs, depending on the axes used:

1) a concave, upward slope (# of species vs. area)

2) a straight, upward sloping line (log(# of species) vs. log(area)).

If we use the second form of the graph, we find that the equation describing the line is

log (S) = log (c) + z log(A)

where z represents the slope.

What factors influence z?

- Climate, e.g. latitudinal gradient factors

- High average r across the community or group of species

- Habitat complexity

- Isolation, e.g. distance from the mainland

- Type of species represented, e.g. mammals vs. birds

Data collected by Harris for mountaintop islands in the Great Basin show that mammals have a higher z (steeper slope on the species-area graph) than birds.

Equilibrium Theory of Island Biogeography (ETIB)

The ETIB describes the theoretical relationship between immigration and extinction of species to islands, depending on their size and distance from the mainland or other species source.

Consider the degree of isolation of the area under study:

Isolate (oceanic and continental islands) vs. Sample (e.g. Amazon)

Oceanic islands are usually created by volcanic activity.

Continental islands are formed when the water level rises (e.g. glaciers melt).

How do species access these islands over time?

1) On oceanic islands, the number of species present increases over time until it reaches the level of the nearest mainland (theoretically the source of the species which immigrate to the island).

2) On continental islands, the number of species present decreases over time. Species richness "relaxes" to a new equilibrium depending on the degree of isolation and the size of the island.

According to ETIB, the number of species present on an island is determined by a balance between immigration and extinction. Generally, as the number of species present increases, the immigration rate decreases and the extinction rate increases.

There are two general relationships to remember:

1) Immigration is higher on near islands than on distant islands (in relation to the mainland), hence the equilibrium number of species present will be greater on near islands.

2) Extinction is higher on small islands than on larger islands, hence the equilibrium number of species present will be greater on large islands.

Therefore,

The number of species on near, large islands > The number of species on distant, small islands

Work by Simberloff and Wilson on mangrove islands in Florida has validated the ETIB:

They killed all of the organisms on various sizes of mangrove islands and different distances from the "mainland" source of species and measured recolonization rates. They found that near, large islands experienced faster recolonization than distant, small islands.

Much of ETIB, which was founded on the study of true islands, can be extended to islands in fragmented habitat. Island biogeography has become an essential component of conservation biology, particularly in the analysis of preserve design, which will be covered in the next lecture.

Links:

Spotlight on Island Biogeorgraphy: a good, concise summary of Island Biogeography.

Edge Effects and the Extinction of Populations Inside Protected Areas

Rain Forest Fragments Fare Poorly

Heeding the Warning in Biodiversity’s Basic Law

Biomass Collapse in Amazonian Forest Fragments