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Risk to prey imposed by intraguild predation (IGP) can be influenced by a number of factors, yet to date, few studies have measured the contributions of these factors to overall risk. A three-species IGP system with larvae of the dragonfly Anax junius as IG (top) predators, larvae of the dragonfly Plathemis lydia as IG prey (intermediate predators), and fathead minnow hatchlings (Pimephales promelas) as shared prey was used to estimate the contribution of the following three factors to shared-prey mortality rate in combined predator treatments: (1) the trophic effect of the IG predator on IG prey density; (2) the effect of reduced shared prey consumption by the IG prey in the presence of the IG predator; and (3) the effect of alternative prey for the IG predator. These factors were integrated into a model of multiple predator effects. To quantify minnow mortality, P. promelas were exposed to A. junius only, P. lydia only, A. junius and P. lydia, or neither in a two-by-two factorial design. Additional trea...

Despite the importance of understanding persistence, there are few direct tests of the ability of models to predict predator and prey population persistence. We tested whether an individual-based model could forecast the dynamics and time to extinction in aquatic microcosms of a protist predator and prey: predatory Didinium nasutum and bacterivorous Colpidium striatum. By addressing both persistence and dynamics, the model increases the testability of mechanisms of extinction. Population-level equations modeled the functional response and prey growth. For individual predators, we simulated time since dividing and feeding, and number of prey consumed; these influenced the timing of division and death. We tested the model by comparing simulated dynamics to data from three experiments: (1) an experiment initiated with low predator–prey ratios in 30-mL bottles; (2) an experiment similar to Experiment 1, but in which immigrant predators, prey, or both were added during the first density cycle; (3) an experiment in 30-mL bottles, initiated with various predator–prey ratios. Using only nine parameters measured in independent experiments, simulations gave satisfactory predictions of the period and amplitude of cycles of predator and prey densities, and predator and prey densities through time for Experiment 1. Adding stochasticity to the model also allowed it to reproduce observed prey and predator persistence and the proportion of replicates with prey extinctions. We used the improved model to forecast the results of Experiments 2 and 3. In Experiment 2, persistence changed with immigration. The model qualitatively reproduced these changes but underestimated their magnitude. Increasing the initial predator–prey ratio reduced persistence in Experiment 3. Simulations failed to qualitatively reproduce these results for 30-mL microcosms, unless we raised initial prey density. This study demonstrates the use of an individual-based model to help identify and test mechanisms of extinction in predator–prey interactions. The combination of individual-based and population-level formalisms can maintain both model tractability and a close working relationship between models and accessible data.

Under the influence of natural and sexual selection, males and females will often differ in the costs and benefits of achieving some particular body size; we might therefore expect to find them growing at different rates for different times, and dying on different schedules. The analysis of sexual dimorphism presented here assumes that life histories are optimized over three key variables: activity level (or “effort”) during the juvenile stage, duration of the juvenile stage, and size at maturation. Reproduction is taken to be semelparous (or monocarpic), reproductive success is size dependent, and females are demographically dominant. Juvenile male and female life histories are assumed to be “structurally identical” (i.e., governed by identical functions of activity, size, and development time). Under these conditions, sexually dimorphic life histories readily arise when features of juvenile behavior and development are adjusted to maximize r in response to gender-specific selection pressures on adults. ...

To capture their prey, larval antlions invest energy in building and maintaining conical pit traps in fine-particulate substrate. The resident antlions (Myrmeleon immaculatus) at Sleeping Bear Dunes National Lakeshore, Michigan, USA, seldom relocated their pits, and we wondered whether this site fidelity could be understood as an optimal (or near-optimal) response to observed spatial and temporal variation in prey availability. To determine this, we considered a large number of compound foraging strategies, each composed of the number of days over which the antlion evaluates foraging success at a site; the weighted-average foraging success during this interval, below which threshold the antlion moves to a new site; and the length of the random walk taken by the antlion to a new pit location. Using Monte Carlo simulation, we determined the expected net energy gain from each of these strategies by antlions rewarded according to the field data set. The overall highest gain strategy generally agreed with our a priori expectations for the observed pattern of patchiness in prey availability over space and time. Moreover, the corresponding optimal frequency of pit relocation, 1.65 moves over the observation period of ∼8 wk, is in rough agreement with field observations. However, the gain surface was relatively flat: 60% of the investigated strategies yielded within 8% of the maximum gain. When costs of pit relocation were reduced, maximal gain strategies shifted to generate frequent movement, suggesting that the magnitude of such sampling costs may control the foraging strategy in environments with high spatiotemporal variability.

We present two laboratory experiments designed to show how cannibalism and its effects on population numbers and size structure vary in response to density, size, and food manipulations in larvae of the dragonfly Epitheca cynosura. Because young larvae of this species interact at very high densities after hatching asynchronously from clumps of egg masses, our efforts focused on these early stages. In one experiment, we varied the combination of sizes (instars) and the presence of food for 233 pairs of early-instar larvae. Cannibalism was uncommon if larvae were of the same instar (only 2.4% of such pairs exhibited cannibalism), frequent (53%) if larvae differed by one instar, and certain (100%) if a two-instar size difference was present. The rate at which cannibalism increased with size differences between larvae was greater when food was absent than it was when food was provided. In a secQnd experiment, we subjected replicate cohorts of newly hatched dragonflies to manipulations of hatching synchrony, initial density, and food availability. Asynchronous hatching over 25 d produced a broad size distribution apparently conducive to cannibalism, whereas synchronous hatching over 3 d initially precluded cannibalism. As a result, strong density-dependent mortality only appeared following the asynchronous hatch. For high-density, asynchronous treatments, cannibalism significantly reduced size variation and tightened size distributions over the course of development. Survivors from high-density treatments were significantly larger than were low-density survivors. More abundant food allowed larvae to reach larger sizes, but did not improve survival. Survival was affected mainly by the density and size distribution of larvae. These results suggest that cannibalism was more important than exploitative competition for food in determining the size and survival of dragonflies in the laboratory. We conclude that when juveniles hatch asynchronously in close proximity, cannibalism can: (1) contribute to population regulation by imposing greater per capita mortality at high densities, and (2) increase population synchrony by exerting size-specific mortality on smaller individuals throughout development.

A bst, act. Damselfly naiads were fed Dapliniia iniagnia in laboratory predation studies. A.lnomalagrion liastatittin predation rates were highest during the first 2 days of the penultimate instar and then declined until ecdysis. The functional response of final instar Isclinura 1rainlb)11/ii was most sensitive to changes in Daplmnia densities of 5-50/liter. Many more prey were killed than eaten in both studies; this wastefulness increased significantly as densities r-ose from 5 to 10/liter. Observations of feeding behavior revealed that damselfly naiads freqLuently strike at and capture prey and then discard them uneaten or only partially eaten. This is presumed to be the proximate cause of wasteful killing. Photographic documentation of gut contents showed that during feeding the foregut often becomes full before the midgut, leading us to hypothesize that wasteful killing occurs when hunger in the midgut motivates capture, but fullness of the foregut precludes eating. When wasteful killing is a normal component of hunger-motivated feeding behavior, it has sometimes been explained by hypothesizing the existence of a hunger threshold which is higher for eating than for capture (Holling 1966). We propose a two-gut-compartment folrmuLlation of Holling's simulation model of invertebrate predation which accounts for dynamic characteristics of wasteful killing without hypothesizing an elevated hunger threshold for eating.

This study focuses on the coexistence of the dominant populations among 46 odonate species found in Bays Mountain Park, Sullivan County, Tennessee, USA. By monthly sampling of six aquatic habitats within the park, we establish the larval biomass distributions of the 12 dominant populations across habitats and seasons and from these derive estimates of distributions along cor- responding niche axes. We use an index proposed by Hurlbert (1978) to estimate niche specialization (breadth-') and overlap on habitat and seasonal axes, both separately and simultaneously (two-di- mensional analysis). This enables us to evaluate two possible mechanisms of coexistence in this community: resource partitioning (i.e., differing time-averaged patterns of resource utilization among competitors) and ecological shift (i.e., nonevolutionary change in resource utilization in response to competition). Three results support the resource-partitioning mechanism: 1) Competition coefficients, obtained by dividing niche overlap values by the appropriate niche spe- cialization values, are all less than one; this indicates that each population should inhibit its own access to a limited food supply more than it inhibits the access of the other populations. 2) Though little or no complementarity between the habitat and seasonality axes is apparent when single-axis index values are examined, the two-dimensional competition coefficients are slightly smaller on average than the product of single-axis means, suggesting two-dimensional comple- mentarity. 3) There are about the same number of consistent 3-yr trends in specailization and overlap as would be expected by chance, suggesting a relatively persistent arrangement of odonate niches in niche space. But the ecological shift mechanism may be operating within or between those populations that do exhibit consistent 3-yr trends in specialization or overlap; more of these values consistently decrease than would be expected by chance. The six populations that comprise the detritus-submersed macrophyte guild account for most of the larval biomass, and the dominant population within the guild (and the community as a whole) is the semivoltine anisopteran Tetragoneuria cynosura. A particularly intense interaction, as indicated by the competition coefficients, is between the abundant zygopterans Enallagma traviatum and E. signatum; unusually high specializations and overlap were observed for the rush-dwelling zygopterans Ischnura verticalis and I. posita.

In this paper we re—analyze previously published data regarding the response of several prey populations to manipulation of predaceous larval dragonfly (Insecta: Odonata) densities in four separate field enclosure experiments. Using a computer—intensive "rerandomization" approach to testing hypotheses, we show that the individual experiments were not sufficiently powerful to consistently reject false null hypothesis. Combining the data from three comparable experiments, we can enhance the power associated with such tests. Three prey categories (Trichoptera., Oligochaeta, and large Cladocera), constituting less than one—third of the typical odonate diet, were found to be consistently depleted in enclosures with odonate larvae; but the extent of their depletion was not increased at high (ambient) compared with low (half—ambient) odonate densities. These results support our previously published conclusions that exploitation competition was not an important phenomenon for odonate larvae in these experiments.

Tetragoneuria cynosura and Celithemis elisa dominate the larval dragonfly assemblage of Bays Mountain Lake, Tennessee, USA, where they coexist in the extensive submersed macrophyte and allochthonous detritus habitats despite relatively high overlap in both seasonal occurrence and diet. Field enclosure experiments, designed to determine the intensity of intraspecific and interspecific competition at approximately natural densities, were conducted during September 1981 and April 1982. Survival rate for both species was dependent on intraspecific density in September, and that for C. elisa was also affected by the presence of T. cynosura. These effects are attributed to interference (encounter) competition rather than to exploitation (consumption) competition. The mechanism of competition seems to be predation by larger larvae on smaller larvae. No evidence of either exploitation or interference competition was found in the April experiment.

Enallagma aspersum and E. traviatum (Odonata: Coenagrionidae) are the most abundant larval odonates in Bays Mountain Park (Sullivan County, Tennessee, USA), although their spatial distributions are essentially nonoverlapping. E. traviatum coexists with insectivorous fish in a small lake, whereas E. aspersum is restricted to a small fishless pond nearby. Behavioral observations revealed that E. aspersum larvae were more active than E. traviatum, and tended to occupy more conspicuous positions. E. aspersum also enganged in more confrontations than E. traviatum, especially at higher density. In laboratory experiments with juvenile bluegills (Lepomis macrochirus) as predators, E. aspersum larvae were more vulnerable to predation than E. traviatum. Red—spotted newts (Notophthalmus viridescens) also preyed on E. aspersum disproportionately. Field enclosure experiments revealed that dry mass of individual E. aspersum larvae was density dependent, and that increased density of E. aspersum or addition of E. traviatum produced similar reductions. Competition was asymmetrical, as E. aspersum appeared to have no significant effect on E. traviatum. The substantial increase in confrontations among E. aspersum larvae at higher density, and the lack of evidence for prey depletion, suggest that interference may be the mechanism of competition. Our results suggest that the distribution of E. aspersum larvae may be limited by fish predation, but although competitive interactions were detected, we have no evidence that larval competition influences the distribution of either species.