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Ecologists have long studied the effect of predators on prey population abundance while evolutionary biologists have measured prey trait evolution in response to predation. Ecological and evolutionary processes were generally thought to occur on different time scales, but recent evidence suggests that evolution may alter the ecological effects of predation over the course of ecological experiments. We used a protozoan and its mosquito-larvae predator, naturally found in the water-filled leaves of pitcher plants, to examine the effect of prey evolution on predator--prey interactions. In experiments conducted over 12 days (approximately 50 prey generations, but less than one predator generation), we measured a decrease in the effect of mosquito larvae predators on protozoa prey populations. In a separate set of experiments, we found that the presence of predators corresponded with evolution of smaller cell size and increased population growth rate. In ecological experiments, two situations commonly occur: strong selection pressure applied by the treatment itself and discrepancies in generation times of associate species. Our results suggest that in either situation, the resulting evolutionary patterns may lead to dramatic and important changes in ecological effect size. Key words: Colpoda; effect size; mosquito larvae; predator; prey; protozoa; rapid evolution; resistance; Sarracenia purpurea; tolerance.

Competition is predicted to affect the expression of inducible defenses, but because costs of behavioral and morphological antipredator defenses differ along resource gradients, its effects on defenses may depend on the traits considered. We tested the predictions from different defense models in tadpoles of the common frog Rana temporaria, which exhibit both types of defenses. In an outdoor experiment, we exposed the tadpoles to nonlethal predators (Aeshna dragonfly larvae) and to a gradient of intraspecific competition. Morphological responses did not follow any of the expected patterns, since investment in defense was not affected by resource level. Instead, tail depth decreased in the absence of predators. Behavioral defenses followed a state-dependent model. Overall, the defense strategy of the tadpoles revealed a shift from morphological and behavioral defenses at low tadpole density to morphological defense only at high density. This difference probably reflects the different efficiency of the defenses. Hiding is an effective means of defense, but it is unsustainable when resources are scarce. Morphological responses become more important with increasing density to compensate for the increase in behavioral risk-taking. Our results indicate that competition can strongly affect reaction norms of inducible defenses and highlight the importance of integrating ecological parameters that affect the cost benefit balance of phenotypic plasticity. Key words: competition; costs; inducible defenses; phenotypic plasticity; predation; tadpole.

Larvae of Phratora vitellinae (Coleoptera: Chrysomelidae) convert salicyl glucosides from the host plant into a larval defensive secretion with salicylaldehyde. This secretion repels generalist predators. Willows vary greatly in the concentrations of salicyl glucosides in their leaves. One may predict that P. vitellinae prefers and survives better on plants that contain more salicyl glucosides. We determined the amount of larval secretion, host preference, larval growth, and larval survival of P. vitellinae on Salix myrsinifolia, S. pentandra, and S. phylicifolia. We also measured feeding rates of three natural predators on P. vitellinae larvae feeding on different hosts. Salix pentandra and S. myrsinifolia contained substantial amounts of salicyl glucosides, but S. phylicifolia contained very little of them. Phratora vitellinae larvae produced more secretion on S. pentandra than on S. myrsinifolia. They produced little secretion on S. phylicifolia. Adult beetles preferred S. myrsinifolia over S. pentandra and S. pentandra over S. phylicifolia. Larvae grew most rapidly on S. myrsinifolia and S. pentandra. Their growth was slowest on S. phylicifolia. The larval survival was similar on S. myrsinifolia and S. phylicifolia, but it was significantly lower on S. pentandra. The natural predators fed equally well on P. vitellinae feeding on S. myrsinifolia and S. phylicifolia. Thus, the host preference of P. vitellinae did not correspond to larval survival on these hosts, but rather to larval growth. Larval survival of P. vitellinae was not related to the amount of defensive secretion. Natural predators were not repelled by the host-derived defensive secretion. We discuss the implications of these findings for the evolution of host plant use in this herbivore. Key words: host suitability; larval defensive secretion; larval performance; Phratora vitellinae; Salix; trophic level interactions., INTRODUCTION Herbivorous insects have diversified greatly during the last 60 x [10.sup.6] yr. They now constitute a large proportion of terrestrial arthropods. However, most of them feed on only a [...]

What factors permit the coexistence of competing species? In central Oklahoma, the predaceous ant lions Myrmeleon crudelis and M. immaculatus live in dense aggregations at the sheltered bases of cliff ledges. Three larval instars of each species act as predators and competitors of one another. In controlled field experiments, mortality of second and third instars increased with density, although intra- and interspecific effects were indistinguishable. The presence of third-instar larvae did not affect recruitment or survivorship of first-instar larvae. In all experiments, mortality was predictable on the basis of body mass and larval density, but not species identity. Increased food supply shortened development time and increased adult body mass but did not affect mortality. Larvae near the front of the ant lion aggregation grew faster due to greater food availability but suffered greater mortality in the pupal stage. Although food and space were limiting, neither species was excluded because (1) intra- and interspecific effects were similar; (2) third-instar larvae could not suppress the recruitment of first-instar larvae; and (3) recruitment was patchy in time and space. Oviposition behavior and interactions among adult ant lions may also contribute to larval coexistence. Key words: ant lion; cannibalism; coexistence; competition; food and space limitation; insect; intraguild predation; larvae; life history; predation; recruitment., INTRODUCTION The coexistence of species remains a central, unsolved problem in community ecology (Case and Diamond 1986, Gotelli and Graves 1996). If a pair of species consumes shared resources, why [...]

The role of plant allelochemicals on insect predatory behavior under thermal regimes representing summer and spring was analyzed in tomatoes and Podiscus maculiventris stink bugs. Controlled thermal regimes and tomato allelochemicals such as rutin and chlorogenic acid affected the stadium duration of the predatory stink bug second-instar nymphs. On the other hand, the developmental time of fourth-instar predatory nymphs was affected only by rutin and tomatine.

Net-spinning caddisfly larvae of the family Hydropsychidae are known to prefer microhabitats with large, stable substrate and high water flow velocity. It is often assumed that net spinners in high-velocity microhabitats have higher feeding or growth rates than larvae in less preferred sites, but there is no direct evidence to support this assumption. We hypothesized that net-spinning caddisflies would select microhabitats that offered the greatest feeding rates. This hypothesis was tested by field experiments in which we determined if net-spinning caddisfly larvae preferred high-velocity sites even when substrate size and type were held constant. We then measured feeding rates of net spinners in microhabitats with different flow characteristics. High-flow positions were selected by 96% of hydropsychid larvae colonizing artificial moss substrates. Artemia nauplii released into the water column were captured by individual larvae in high-flow sites at a rate of 0.016%/m, significantly higher than the capture rate in low-flow sites. Combining this rate of prey capture with mean hydropsychid densities of 1125 individuals/[m.sup.2], we estimate that hydropsychid larvae in riffles remove drifting invertebrate prey at a rate of [is nearly equal] 18% m. Assuming exponential prey removal, a prey item in the drift would travel an average of only 5.5 m before being consumed. This study is one of the first to show that the distribution of a stream filter feeder is related to the feeding rates obtainable in different microhabitats. Key words: artificial substrate; colonization; current velocity; feeding rates; invertebrate drift; microhabitat; net-spinning caddisflies; seston.

Optimal values for life history traits are expected to depend upon environmental conditions during development and the period within which development is constrained (e.g., biotic factors and time constraints, respectively). Theory predicts that life history responses to both biotic factors and time constraints may be both direct and behaviorally mediated. Few experimental studies of life histories have considered the joint effects of biotic factors and time constraints, and fewer still have been able to disentangle direct from behaviorally mediated effects. We studied such interactions by manipulating the perceived time to the onset of winter, predation risk, and food resources level in larvae of the damselfly Lestes sponsa. In the first experiment (predation X time constraint), the presence of a predator caused an overall reduction in foraging activity, development rate, and mass at emergence. However, larvae that had less time available before the end of the season, increased foraging activity and development rate, while mass at emergence decreased. These results suggest that the observed changes in life history characters were behaviorally mediated in the presence of predators. In contrast, life history responses of time-constrained larvae occurred independently of the behavioral changes and, therefore, were direct. In the second experiment (food level X time constraint) larvae under high food levels had a higher foraging activity, increased development rate, and higher growth rates, compared to low food-level treatments. Time-constrained larvae accelerated development and had a smaller mass at emergence at high food levels than larvae that were not time constrained. In contrast, and opposite to predictions, time-constrained larvae at low food levels had the slowest development rate and the largest mass at emergence. We suggest that larvae in the latter group were aiming to delay emergence to the next season (cohort splitting). Our results suggest that both behaviorally mediated and direct responses to biotic factors and time constraints are a feature of the life history of this damselfly. Key words: damselflies; development; food resource; growth rate; Lestes sponsa; life history; mass at metamorphosis; phenotypic plasticity; predation risk; time constraints; trade-offs.

MATTHEW P. AYRES [1,5] RICHARD T. WILKENS [2] JONATHAN J. RUEL [3] MARIA J. LOMBARDERO [1] ERICH VALLERY [4] Abstract. The nitrogen content of plant tissue is low relative to [...]

MIKKO KUUSSAARI [13] MICHAEL SINGER [2] ILKKA HANSKI [1] Abstract. We studied host plant abundance, host use, and oviposition preference in metapopulations of the butterfly Melitaea cinxia within an area [...]

CHARLES W. Fox [1] Louis Calder Center, Biological Field Station and Department of Biological Sciences, Fordham University, 53 Whippoorwill Road, Box K, Armonk, New York 10504 USA Abstract. Texas ebony [...]

DAVID A. BOUGHTON [1] Department of Zoology, University of Texas, Austin, Texas 78712 USA Abstract. In heterogeneous landscapes, a species' habitat may be partitioned into sources and sinks. Conceptually, three [...]