Equilibrium Theory of Island Biogeography | IB 453, Study notes of Biology

Download Equilibrium Theory of Island Biogeography | IB 453 and more Study notes Biology in PDF only on Docsity! Island Biogeography Species-area relationship - well described by a power law S = cAz where c and z are constants Typically works very well for islands. For oceanic islands Darlington (1957) proposed that a ten-fold increase in island area results in a two-fold increase in species richness Species-island relationship for land-bird species richness in the West Indies LogS = 0.94 + 0.11 log(A) After Darlington, it was recognized that distance to the mainland also affects species richness (‘distance effect’) Diamond (1972) compared species richness on islands with that expected for an island ‘near’ (<500 km) from a mainland source Mainland = New Guinea Islands = Tropical Pacific Maximum immigration rate (I) occurs when island is empty and decreases as more species are added (so that fewer species remain in the source pool as potential colonists) Once all the potential colonists are on the island then S (species richness on the island) = P (mainland source pool) and immigration rate must = 0 Immigration rate λs = Intercept - slope(S) = I -(I/P)S Determinants of immigration rate: Determinants of extinction rate (µS ) Should increase with S (more species greater the rate than they can disappear…Maximum extinction rate (E) occurs when all species from the source pool are on the island (when S = P), and must be zero when no species are present Intercept = 0, therefore extinction rate = slope * S µS = (E/P)*S Substitute in linear terms for immigration and extinction into formula for rate of change in species richness: dS/dt = I-(I/P)S - (E/P)S Solve for equilibrium species richness: S* = IP/(I+E) determined by size of source pool and max. immigration and extinction rates... S* is point at which rate of arrival of species is exactly matched by rate of extinction S* has a characteristic T* - the rate of turnover of species per unit time at equilib. Can also account for lower S* on more distant islands of the same size by changing immigration rate: In = immigration rate for islands close to species pool > rate for far island If Turnover rate of species lower for far island No biology in this theory!! Species richness determined solely by area of the island (extinction) and distance from mainland (immigration)... Species are ecologically equivalent 1. Species have similar colonization and extinction probabilities 2. Population sizes scale with island size (is this reasonable?) 3. Immigration rate inversely proportional to distance (any arguments?) 4. Probability of extinction is inversely proportional to population size 5. Probability of immigration and extinction is independent of species composition on the island (is this reasonable?). What are the assumptions of the theory? Model may be robust to some assumptions (e.g. assumptions 1 and 4. Why? Model predictions are fairly robust to non- linear extinction and immigration functions and were incorporated into the original model Target effect: Island size also likely to influence immigration rate. Large islands present larger targets to which immigration can successfully occur. Results in prediction of greater turnover rate on large islands. Immigration rate with target effect Immigration rate no target effect Increase in S* due to Target effect Coleman (1982) “passive sampling model” - relates probability of occupancy of a species to the relative area of a given island in an archipelago of islands. Species disperse to islands and accumulate (no extinction) Unlike M-W, also predicts which species are abundant on large islands: Those that are common the mainland. Species that are rare on the mainland would be rare or absent on small islands Target effect raises question of whether one might expect to see higher species richness on large islands simply because of higher colonization (with no need to invoke lower extinction rates). How well supported are the assumptions and predictions of the MacArthur-Wilson theory? 1. Variation in immigration and extinction rates: Even apart from the consideration whether extinction rates and immigration rates vary linearly or non-linearly with species richness, what evidence is there that they vary at all with S? Does extinction rate increase with S? Are immigration rates higher when island S is low? 2. Variation in species population size with island size: Island size may not be the primary determinant of population size in many cases… Some populations may be larger on small islands. Ecological release from predators or competitors: some taxa maintain higher population densities on small vs large islands/mainland when interacting species are absent? Crowell (1983) introduced deer mice and onto rodent-free islands in the Deer Island Archipelago, Maine. Populations grew to greatly exceed mainland densities Case (1975) Lizard densities on islands in Gulf of California highest on small islands. Lizard density declined with increasing island area and island species richness. MW’s equilibrium model is distinguished from passive sampling colonization models and community assembly based on species interaction by virtue of predicting substantial species turnover. Turnover however is difficult to assess (Friday discussion). High presumed rates of species turnover tend to be largely the result of cycling of ‘transient’ peripheral species. If ‘transient’ species are excluded then turnover often very low. Departures from predictions of the MW island biogeography ‘null model’ may be the most important contribution of this theory… Looking at how islands ‘work’ compared to the mainland provides an important investigative tool in community ecology. 3. Substantial species turnover at equilibrium (T*) Summary 1. MacArthur-Wilson Island Biogeography is a non-equilibrium theory that predicts species richness and species turnover based on island size and isolation alone. 2. MWIB is essentially a neutral theory. Species are ecologically equivalent. There are no species interactions. 3. Passive sampling model provides an alternative to MWIB, and is driven by the ‘target effect’. Still neutral. Predicts species abundance on islands from abundance on the mainland. Differs from MWIB in prediction of species turnover.