Niche overlap and the potential for competition and intraguild predation between size-structured populations.

Abstract

Many populations are heterogeneous collections of different sizes or stages of conspecifics. Existing overlap indices do not account for the size—/stage—structured nature of these populations. In this paper I present new overlap indices that use information about the sizes of individuals that co—occur in time and space to predict the potential for interactions in size—structured guilds. An index of the opportunity for competition (IOC) calculates the frequency with which similar size classes of two species encounter each other, whereas an index of the opportunity for intraguild predation (IOP) calculates the frequency of encounters among disparate size classes of the same two species. To illustrate that these indices are more appropriate for size—structured populations than conventional indices, I calculated overlap, IOP, and IOC for all species pairs in a diverse assemblage of dragonfly larvae. The new indices revealed size—specific patterns of overlap that were not detected by the conventional index, including that (1) some species with high overlap values should interact mainly as competitors, others mainly as intraguild predators, and many as both competitors and predators, (2) subtle differences in phenology and/or size—specific shifts in habitat distribution can lead to the potential for asymmetric interspecific interactions, (3) some species with low pairwise IOP and IOC values are nonetheless vulnerable to the effects of diffuse competition or intraguild predation, (4) seasonal segregation reduces competitive overlap but at the same time increases the opportunity for intraguild predation. The indices are general in form and should be useful for analyzing distributional data for any size—structured assemblage in which the type and intensity of interaction varies as a function of relative size.