PARENT SESSION
Oral Session #4: Animal Population Ecology.
Presiding: L. Mitchell
Monday, August 5. 8:00 AM to 11:30 AM. Gila Meeting Room, TCC.
Allee effects and random catastrophes in the population dynamics of a social spider.
Salazar, Patricio*,1, Avilés, Leticia2, 1 Pontificia Universidad Católica del Ecuador, Quito, Ecuador2 University of Arizona, Tucson, Arizona
ABSTRACT- The study of density-dependent and density-independent effects on the dynamics of populations is made difficult by the large spatial and temporal scales at which such processes usually take place. The social spiders provide an opportunity for such a study since their colonies, which may contain from one to several thousand individuals and occur by the dozens in any local area, constitute not only social groups, but also relatively closed and self-sustaining populations. By tracking the growth and dynamics of 39 colonies of the social spider Anelosimus domingo Levi (Araneae: Theridiidae) in Amazonian Ecuador, we investigated the interaction between density-dependence and density-independence in local population growth. We found that colony size had both positive and negative effects on colony growth - the per capita rate of growth increased vertiginously at low colony sizes, creating an Allee effect, reached its maximum at intermediate colony sizes, but decreased drastically at higher sizes. It also appears that the intrinsic density-dependent dynamics would result in complex fluctuations of colony size (cycles or chaos), not expressed frequently because the occurrence of random catastrophes often prevented colonies from reaching large enough sizes for negative density dependent effects to occur. We discuss how an Allee effect and complex density-dependent fluctuations might be a consequence of the spider's sociality. We also discuss how high local instability might be sustainable in species with a metapopulation structure where local extinction may be balanced by recolonization. Finally, we discuss the implications of this study to the problem of the relative importance of density-dependence and density-independence in the regulation of natural populations.
KEY WORDS: Allee effects, density dependence, random chatastrophes, metapopulations