In this chapter, I evaluate the concept of equilibrium in insular systems. I focus on species numbers rather than population abundances, because many studies of island biotas have been motivated by the ideas of MacArthur and Wilson (1967), who focused on species numbers. A useful system for thinking about equilibrium on islands was presented by Rey (1984) and further expanded and developed by Whittaker (1998). In this system, an island is cross-classified according to two characteristics: (1) whether it is in equilibrium or not, and (2) whether the species composition is static or dynamic. In an equilibrium condition, immigration and extinction rates are balanced, and species are neither gained nor lost. In a nonequilibrium condition, immigrations exceed extinctions, or vice versa, and species number increases or decreases over time. A static condition implies no change in the identities of species present, whereas a dynamic condition describes changes in species composition, due to turnover. I make reference to this system, with some modifications, in describing a number of different island scenarios. I agree with Whittaker that it is better to think of these characteristics in terms of continua rather than discrete categories, although the use of categories facilitates the comparison of various studies.

Equilibrium vs. nonequilibrium states of islands

Stasis

An island in stasis (“static equilibrium” in Whittaker, 1998) experiences neither immigrations nor extinctions; the species composition remains unchanged over time (Figure 9.1a). The classic example of this scenario derives from the work of Lack (1969, 1976) on birds. Lack assumed dispersal was not limiting, and the main factor determining whether a given species would be present on an island was whether the appropriate habitat type existed. Once all niche spaces were occupied, the island would be closed to further colonization. Other studies, many with birds, have found a relative stasis in community composition, with very few immigrations or extinctions (e.g., Walter, 1998; Foufopoulos & Mayer, 2007). In this relatively deterministic view, observed turnover of species has often been attributed to anthropogenic effects, which Lack acknowledged could lead to changes in the fauna.