GEOLOGICALLY RAPID BIOTIC RESPONSE TIMES TO CHANGES IN HABITAT AREA: EVIDENCE FROM A NEUTRAL ECOLOGICAL MODEL

One hypothesis for the lack of elevated marine extinction during Pleistocene sea-level changes is that the loss of shelf area was so rapid and so short-lived that biotas did not have sufficient time to respond. This hypothesis and the more general question of the response time of ecosystems to changes in habitat area is tested with Hubbell's neutral model of metacommunities. These modeled metacommunities consist of a fixed number of competitively equal individuals of multiple species. At each time step, a given number of individuals die, with a small proportion of these replaced with new species and the remaining vacancies replaced with extant species, with the probability of replacement equal to the abundance of those species in the metacommunity.

When habitat area is fixed, modeled metacommunities fluctuate around a constant equilibrial diversity that increases with Hubbell's theta, which is proportional to the number of individuals in the metacommunity and the per-individual probability of speciation. When habitat area is lost in a single time step, extinction of species occurs, with an initial instantaneous loss of species that were confined to the area of lost habitat, followed by a longer asymptotic decline to the new equilibrial diversity. Equilibrial diversity is typically reached in less than 1000-2000 years, and often in less than 500 years. When habitat area is increased instantaneously, there is no instantaneous diversity gain, and equilibrial diversity climbs asymptotically to its new level, but over 2-3x the time span of an equivalent habitat loss.

These response times of biotas to changes in habitat area are geologically rapid, particularly so during habitat loss. These models suggest that biotas are essentially in equilibrium with habitat area on geological time scales, and that the preserved response to brief and rapid sea-level changes is not likely to differ from long and slow sea-level changes of comparable magnitude.