Tramea

Abstract: Size-limited predator-prey systems can be the ecological background for natural selection on prey growth rate and body size. In a laboratory experiment on one such system, individual predatory Tramea lacerata (Odonata: Anisoptera: Libellulidae) naiads of varying sizes were exposed to three densities of Rana areolata (Anura: Ranidae) tadpoles in each of five size classes. Predation rate decreased with increases in tadpole body size. Larger naiads consumed slightly more tadpoles than did smaller naiads when tadpoles were very small, but at larger tadpole sizes, naiad size did not affect the predation rate. Although the number of tadpoles consumed increased with increasing tadpole density, the proportion of the total number that were eaten decreased with increasing density. When tadpoles were very small, naiads that ate more prey grew larger. When tadpoles were larger, there was little variation in the number eaten and no detectable relationship between the number eaten and subsequent naiad growth. These data resolve several paradoxical reports in the literature.

Abstract: One goal of ecology is to generate dynamic models that predict the species composition of complex natural communities. The general applicability of such models will depend on how often unpredictable higher order interactions occur between competitors and predators. I experimentally tested for interactions in a two predator—multiple prey community by manipulating the presence or absence of two species of predatory dragonfly larvae (Anax junius and Tramea carolina) in replicated outdoor artificial ponds containing identical number of four species of anuran tadpoles and a diverse zooplankton assemblage. The impact of predation on prey communities was measured by the relative abundances and larval performances (size at metamorphosis, length of larval period, and growth rate) of the anurans, and by the densities of zooplankton sampled at 10—d intervals. Zooplankton reached high densities in the experimental ponds (up to 2700 animals/L). Dragonfly predation had a marginally significant impact on zooplankton species composition, and the effects on zooplankton size structure were not significant. Predation by Anax and Tramea profoundly modified the species composition of anurans: the relative abundance of toads (Bufo americanus) increased in the presence of predators, as two hylid species (Hyla crucifer and Pseudacris triseriata) decreased. The interaction between Anax and Tramea on anuran species composition was additive. The performance of surviving larvae of two anurans (Bufo and Pseudacris) was significantly improved by predation (growth rate increased and larval period shortened), whereas the performance of two others (Hyla and Rana utricularia) showed no response to dragonflies. The absence of strong interactions between the predatory odonates implies that in some cases it may be valid to build models of complex ecological communities by additively combining results from more simple subsets of species.

Abstract: Odonate larvae are important components of aquatic environments in tropical areas. They also develop in aquaculture ponds, where they can cause economic losses. In this study, we have tried to describe the general community patterns in aquaculture ponds in Vicosa, south-eastern Brazil. Our aim is to evaluate how environmental and bionomic factors can determine their composition and species richness. We identified two groups of species (plants and bottom-dwellers) based on larval microhabitat preferences. Vegetation determines the occurrence of some species whose adults select certain plants for oviposition. The ponds with more extreme conditions (extensive cover of plants or vegetation absent) showed lower species richness than those with intermediate conditions. Coryphaeshna adnexa and Brachmesia furcata were of larger size but had low abundance or were only collected accidentally. Species of intermediate size (Tramea cophysa, Micrathyria spp., Orthemis discolor and Erytrodiplax fusca) were more abundant and are considered as potential predators of fish fry.

Abstract: High larval mortality has plagued efforts to raise razorback suckers (Xyrauchen texanus) in a Lake Mohave, Arizona-Nevada backwater. Observations indicate odonate nymph densities may be high enough to impact larval survival. In laboratory tests conducted in aquaria, damselfly (Coenagrionidae: Enallagma sp.) and dragonfly (Libellulidae: Tramea sp.) nymphs consumed 81% and 76% respectively of 11.8 ± 0.7 mm total length larval razorbacks in 7 days compared to 12% mortality in controls. Larger razorback larvae (14 to 15 mm TL) were less susceptible than smaller fish, showing 53% mortality versus 18% in controls. Extensive growth of sago pondweed (Potamogeton pectinatus) may exacerbate predation effects in the backwater, by allowing odonates access to more of the water