51 CHAPTER 5 Ecological factors limiting the distributions and abundances of Odonata Mark A. McPeek Overview Many ecological processes contribute to regulating the distributions and abundances of odonate species. In local populations, mortality imposed by predators (including cannibalism and predation by other odonates) on larvae appears to be the dominant factor limiting abundances of many odonate species, although lower growth rates due to food limitation and stress responses to the presence of predators also contribute to lim- iting population sizes in most species that have been studied. Little is known about such processes in the adult stage of the life cycle, but parasites have been shown to limit adult survival and fecundity. Predation also causes many species to segregate among different water bodies with different top predators in eastern North America: different assemblages of odonate species are found at ponds and lakes that support cen- trarchid fishes than at fishless ponds and lakes. However, this pattern of species segregation between fish and fishless water bodies is not apparent in other parts of the world. Stream-dwelling odonates also show analogous types of segregation to different types of stream (e.g. small creeks compared with large streams and rivers), but the ecological processes that enforce this segregation is not known. Many unanswered ques- tions about the ecological regulation of odonates makes them a continually fascinating group for study. 5.1 Introduction Every budding amateur odonatologist quickly learns the type of habitats to search if he or she wants to find a particular species. If one is after a Calopteryx, then a slow-flowing stream with woody structures is needed. If an Epitheca is sought, then one goes to a lake with good macrophyte beds. Gomphus can be found around sandy-bottomed waters. This predictability in species distributions results from the fact that different species have dif- ferent ecological requirements to maintain popula- tion abundances greater than zero. Although individuals of species can sometimes be found in places where they cannot sustain a pop- ulation (e.g. migrant individuals passing through an area, or a so-called sink population that is only maintained at a site by continual immigration from nearby thriving populations), the distribu- tion of a species in the environment is determined largely by the distribution of suitable habitats to maintain source populations (i.e. populations that can be maintained without continual immigra- tion) (Pulliam 1988). Local abiotic factors such as the temperature and water chemistry as well as biotic factors such as the abundances of various food resources, predators, and parasites all affect the survival, growth, and fecundity individuals at a particular site. At some sites, local ecological conditions will allow a species to have an adequate combination of survival, growth, and fecundity to maintain a source population. However, at other Book 1.indb 51 Book 1.indb 51 6/27/2008 8:40:11 PM 6/27/2008 8:40:11 PM