Dynamic disequilibrium: a long-term, large-scale perspective on the equilibrium model of island biogeography

1The equilibrium model of island biogeography developed in the 1960s by MacArthur and Wilson has provided an excellent framework in which to investigate the dynamics of species richness in island and island-like systems. It is comparable in many respects to the Hardy–Weinberg equilibrium model used in genetics as the basis for defining a point of reference, thus allowing one to discover the factors that prevent equilibrium from being achieved. Hundreds of studies have used the model effectively, especially those dealing with brief spans of time and limited geographical areas. In spite of this utility, however, there are important limitations to the MacArthur–Wilson model, especially when we consider long-term and large-scale circumstances. 2Although their general theory is more complex, the MacArthur–Wilson equilibrium model treats colonization and extinction as the only two processes that are relevant to determining species richness. However, it is likely that phylogenetic diversification (phylogenesis) often takes place on the same time-scale as colonization and extinction; for example, colonization, extinction, and phylogenesis among mammals on oceanic and/or old land-bridge islands in South-east Asia are all measured in units of time in the range of 10 000–1 million years, most often in units of 100 000 years. 3Phylogenesis is not a process that can be treated simply as ‘another form of colonization’, as it behaves differently than colonization. It interacts in a complex manner with both colonization and extinction, and can generate patterns of species richness almost independently of the other two processes. In addition, contrary to the implication of the MacArthur–Wilson model, extinction does not drive species richness in highly isolated archipelagoes (those that receive very few colonists) to progressively lower values; rather, phylogenesis is a common outcome in such archipelagoes, and species richness rises over time. In some specific instances, phylogenesis may have produced an average of 14 times as many species as direct colonization, and perhaps 36 species from one such colonization event. Old, stable, large archipelagoes should typically support not just endemic species but endemic clades, and the total number of species and the size of the endemic clades should increase with age of the archipelago. 4The existence of long-term equilibrium in actual island archipelagoes is unlikely. The land masses that make up island archipelagoes are intrinsically unstable because the geological processes that cause their formation are dynamic, and substantial changes can occur (under some circumstances) on a time-scale comparable to the processes of colonization, phylogenesis, and extinction. Large-scale island-like archipelagoes on continents also are unstable, in the medium term because of global climatic fluctuations, and in the long term because of the geologically ephemeral existence of, for example, individual mountain ranges. 5Examples of these phenomena using the mammals of South-east Asia, especially the Philippines, make it clear that the best conceptual model of the long-term dynamics of species richness in island archipelagoes would be one in which colonization, extinction, and phylogenesis are recognized to be of equivalent conceptual importance. Furthermore, we should expect species richness to be always in a dynamic state of disequilibrium due to the constantly changing geological/geographical circumstances in which that diversity exists, always a step or two out of phase with the constantly changing equilibrium point for species richness.