COMPARATIVE LIFE HISTORIES AND LARVAL POPULATION INTERACTIONS IN A DIVERSE ASSEMBLAGE OF DRAGONFLIES (ODONATA: ANISOPTERA) (COMPETITION, PREDATION, INDIANA)

SCOTT ALAN WISSINGER, Purdue University

Abstract

In this study I first compared the life histories of 14 dragonflies inhabiting a small Indiana pond. Data on emergence, adult activity, and larval development were used to characterize the phenology, voltinism, and synchrony of each species. Although a wide variety of life histories were observed, the assemblage was dominated by asynchronous, univoltine species whose populations contained numerous sizes of co-occurring larvae. To predict the opportunity for competition and predation between species I modified Hurlbert's overlap index to include data on the frequency that similar and disparate size combinations of larvae co-occurred. The opportunity for pairwise and diffuse competition was highest for 5 medium-sized libellulids. These and other species were potentially most vulnerable to predation by 3 larger species that immigrate to the pond as adults. The indices predicted numerous patterns of biotic interactions that would go undetected in conventional overlap analyses. In the second part of the study I focused on Libellula lydia and Libellula luctousa, which were the numerical dominants at my study site. Both similar and disparate sizes of larvae of these species frequently co-occurred. To separate the intensities of competition and predation among these larvae, I conducted two (fall and spring) manipulative field experiments in artificial ponds. The design of these experiments relied on laboratory data which showed that food availability affected growth, but not mortality; and that larvae similar in size larvae do not eat each other. Thus, I could assay competition in the field by comparing the effect of density on growth rates in treatments containing only larvae similar in size. I measured inter-odonate predation by comparing the survivorship of larvae in combination with larger instars to that in the competition treatments. In spring, when resources were low, larvae at high densities grew significantly slower than those at densities. In fall, when resources were more abundant, no density-dependent growth responses were observed. In contrast, inter-odonate predation was more intense in fall when it accounted for 25-45% of larval mortality. The results of my study indicate that the relative abundance of these and other dragonflies should be determined by a complex interplay between competition and predation.

Degree

Ph.D.

Subject Area

Ecology

COinS