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

Abstract

The 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. Although 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. Phylogenesis 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. The 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. Examples of these phenomena using the mammals of South‐east Asia, especial...

Origination and extinction patterns of mammals in three central Western Mediterranean islands from the Late Miocene to Quaternary

“…One of the most important characteristic of the Gargano fauna is its in situ radiation. Gargano is the only case which therefore fits well into Heaney's (2000) theoretical definition. In the other islands here considered, on the contrary, speciation is very rare, perhaps with the only exception of the genus Leithia.…”

“…First, the contribution of in situ speciation events to biodiversity in island archipelagos. This process, which has been exhaustively discussed by Heaney (2000) mainly on theoretical and actualistic basis finds in the Gargano an evident fossil example. Secondly, the population history of Gargano gives a fairly convincing example of so-called ''advantage of resident taxa''.…”

“…The former process is obviously not very realistic for large mammals but it could be relevant for small mammals, which, in spite of their very high metabolism and great body surface heat loss, might survive for a while on tangles of vegetation and/or lumps of soil with attached plants. Last but not least, the number of taxa on islands can increase also by in situ speciation events, phylogenesis sensu Heaney (2000) (evolutionary radiation in De Giuli et al, 1986Giuli et al, , 1990. This is most likely expected in long-lasting isolated systems as stressed by Heaney (2000).…”

“…Last but not least, the number of taxa on islands can increase also by in situ speciation events, phylogenesis sensu Heaney (2000) (evolutionary radiation in De Giuli et al, 1986Giuli et al, , 1990. This is most likely expected in long-lasting isolated systems as stressed by Heaney (2000).…”

Influences of Island Characteristics on Plant Community Structure of Farasan Archipelago, Saudi Arabia: Island Biogeography and Nested Pattern

“…Although classical island biogeographical theory has been questioned (Gilbert, 1980;Whittaker, 2000) and a call for a new paradigm of island biogeography has been issued 4 (Lomolino, 2000a), area and distance still play primary roles in alternative theories (Heaney, 2000;Lomolino, 2000b). In general island area, and to a lesser degree isolation, can hardly be disputed as important determinants of insular species richness.…”