Philip H. Crowley

MATE DENSITY, PREDATION RISK, AND THE SEASONAL SEQUENCE OF MATE CHOICES: A DYNAMIC GAME

We describe and analyze a computer-simulation model of mate choice, featuring two different quality groups (based on offspring per mating) in each sex. Mating between quality groups results from two-dimensional random encounter and mutual assent, where assent reflects an attempt to maximize expected lifetime reproductive success, E(LRS). Premating predation (via random encounter with predators) and other mortality also influence E(LRS). Given potentially conflicting optimal choices, the model finds the evolutionarily stable patterns of choosiness for the four quality groups. When there are multiple mating episodes by individuals through the season, the resulting dynamic game is solved to obtain a seasonal pattern of mate choice and reproduction. The model generates seven different mating patterns among quality groups. These patterns imply different opportunities for selection, as indicated by the variance components of normalized lifetime reproductive success, var(LRS). The changes in E(LRS), var(LRS), and mating patterns in response to different densities of predators and of potential mates are explored in detail. Decreasing predation risk or increasing mate availability tends to increase E(LRS), choosiness, and assortative mating. Var(LRS) and thus the opportunity for selection for mate quality is highest at intermediate densities of predators and of potential mates. When density remains constant throughout the mating season, choosiness increases late in the season, a time at which less of the potential E(LRS) is jeopardized by the greater predation risk associated with choosiness. Reproductive success of the low-quality group of the less choosy sex is particularly sensitive to changes in density and other parameters. When seasonal density patterns of predators and potential mates are predictable, these low-quality individuals should do better and may thus be more numerous when mate densities are high and predator densities are low, or when predator densities are high and mate densities are low, than for other combinations of relative densities.

Dispersal and the Stability of Predator-Prey Interactions

A predator-prey interaction between spatially dispersed populations can be considered as many local interactions connected by dispersal. Even when the local subsystems (cells) quickly become extinct in isolation, an ensemble of interconnected cells can, under certain conditions, persist much longer. These conditions include: (1) sufficiently low exchange rates among cells; or (2) a sufficiently high level of environmental stochasticity (noise), either of which can prevent local populations from becoming synchronized; or (3) a large number of cells within the system. Computer simulations based on a broadly applicable predator-prey model demonstrate the efficacy of the conditions 1-3; neither spatial structure (texture) nor density-dependent dispersal are additionally required to obtain long-term persistence. The critical problem of avoiding synchrony in a spatially contiguous, locally unstable system can apparently be solved by size alone. If the system is large enough, it should tend to fragment into two or more hypercell subsystems with the low exchange rates that permit these subsystems to remain out of phase with each other. Environmental noise that is spatially correlated on scales somewhat smaller than the size of the system may also generate or help maintain the hypercells. These features create a mosaic of fluctuating subpopulations that can yield remarkably constant population densities for the system as a whole.

LETHAL AND SUBLETHAL EFFECTS OF ATRAZINE, CARBARYL, ENDOSULFAN, AND OCTYLPHENOL ON THE STREAMSIDE SALAMANDER (AMBYSTOMA BARBOURI )

Agricultural contaminants may be contributing to worldwide amphibian declines, but little is known about which agrichemicals pose the greatest threat to particular species. One reason for this is that tests of multiple contaminants under ecologically relevant conditions are rarely conducted concurrently. In this study, we examined the effects of 37-d exposure to the agrichemicals atrazine (4, 40, and 400 micrograms/L), carbaryl (0.5, 5, and 50 micrograms/L), endosulfan (0.1, 1, and 10 micrograms/L for 31 d and 0.1, 10, and 100 micrograms/L for the last 6 d), and octylphenol (5, 50, and 500 micrograms/L) and to a solvent control on streamside salamanders (Ambystoma barbouri) in the presence and absence of food. We found that none of the agrichemicals significantly affected embryo survival, but that hatching was delayed by the highest concentration of octylphenol. In contrast to embryos, larval survival was reduced by the highest concentrations of carbaryl, endosulfan, and octylphenol. Growth rates were lower in the highest concentrations of endosulfan and octylphenol than in all other treatments, and the highest concentration of endosulfan caused respiratory distress. Significantly more carbaryl, endosulfan, and octylphenol tanks had larvae with limb deformities than did control tanks. Refuge use was independent of chemical exposure, but 10 micrograms/L of endosulfan and 500 micrograms/L of octylphenol decreased larval activity. Systematically tapping tanks caused a greater activity increase in larvae exposed to 400 micrograms/L of atrazine and 10 micrograms/L of endosulfan relative to solvent controls, suggesting underlying nervous system malfunction. Hunger stimulated a decrease in refuge use and an increase in activity, but this response was least pronounced in larvae exposed to the highest concentration of any of the four agrichemicals, possibly because these larvae were the most lethargic. More studies are needed that concurrently examine the effect of multiple contaminants on amphibians so we can better identify effective mitigating measures.

DENSITY DEPENDENCE, HATCHING SYNCHRONY, AND WITHIN-COHORT CANNIBALISM IN YOUNG DRAGONFLY LARVAE'

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.

Behavior and Ecological Interactions of Larval Odonata

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.

MULTIPLE STRESSORS AND SALAMANDERS: EFFECTS OF AN HERBICIDE, FOOD LIMITATION, AND HYDROPERIOD

Amphibian populations can be affected adversely by multiple biotic and abiotic stressors that together can contribute to their local and global decline. We focused on the combined effects of food limitation, drying conditions, and exposure to possibly the most abundant and widely used herbicide in the world, atrazine. We used a factorial design to evaluate the effects of exposure to four ecologically relevant doses of atrazine (approximate measured doses: 0, 4, 40, and 400 μg/L), two food levels (limited and unlimited food), and two hydroperiods (presence or absence of a dry down) on the survival, life history, and behavior of the streamside salamander, Ambystoma barbouri, from the embryo stage through metamorphosis. In general, food and atrazine levels did not interact statistically, and atrazine affected dependent variables in a standard, dose-dependent manner. Exposure to 400 μg/L of atrazine decreased embryo survival and increased time to hatching. Drying conditions and food limitation decreased larva...

Population Regulation in Animals with Complex Life-histories: Formulation and Analysis of a Damselfly Model

Publisher Summary This chapter discusses the population regulation in animals with complex life-histories of taxa such as damselflies obscure the mechanisms of population regulation. The information suggests four plausible mechanisms of damselfly population regulation: food availability; feeding-related intraspecific interference; mortality-related intraspecific interference; and density-dependent predation. The chapter demonstrates the model represents six damselfly life-stages and their interactions with a population of aquatic prey, using coupled ordinary and delay-differential equations, which are solved numerically. Analyzing the models behavior both in steady state and dynamically with the literature-derived parameter values, and performs sensitivity analyses. The resulting larval densities, larval stage durations, emergence rates, and general emergence pattern for the standard parameter values are in good agreement with those in the literature: the generation time slightly exceeds one year, and the emergence pattern is strongly bimodal, as observed for some I. elegans populations in the British Midlands. The chapter concludes that emergence patterns produced by the model seem to reflect the balance between forces promoting and opposing the coexistence of the asynchronous subpopulations that produce separate emergence peaks; promoting coexistence are density-dependent predation and intra-stage, mortality-related larval interference, and opposing it is interstage interference.

Competition among Larval Dragonflies: A Field Enclosure Experiment

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.

The effects of density and relative size on the aggressive behaviour, movement and feeding of damselfly larvae (Odonata: Coenagrionidae)

Abstract How the aggressive behaviour of the larvae of a damselfly species, Ischnura verticalis, changed when density and size combinations were manipulated, and whether changes in feeding and movement patterns accompanied changes in aggressive behaviour were investigated in the laboratory. During aggressive encounters larvae struck with their mouthparts at larvae of the same size as themselves more frequently as density increased. However, proportionately fewer encounters occurred between larvae of the same size as density increased and as the size of individuals present, but not involved in the encounter, increased within the highest density. Density did not affect the behaviour used to initiate encounters by larger or smaller larvae, but both larger and smaller larvae initiated proportionately fewer encounters as density increased. All instars decreased their movement and responsiveness toward prey as larval density was increased and when paired with successively larger instars within each density. However, only the two larger instars decreased the amount of prey they consumed in response to the manipulations. Larvae appeared to reduce their involvement in aggressive encounters by increasing their vigilance of other larvae. The potential population consequences of these alterations in individual behaviour are discussed.

Direct and indirect effects of predators on the dominant invertebrates of two freshwater littoral communities

SummaryTwo congeneric damselfly species, Enallagma traviatum and E. aspersum, dominate the littoral macroinvertebrates of Bays Mountain Lake and of the adjacent fish-free Ecology Pond, respectively (northeastern Tennessee, USA). Extending previous experimental studies, we test seven hypotheses concerning the role of fish (bluegill sunfish, Lepomis macrochirus) and larvaldragonfly (Anax junius) predation, competitive effects on damselflies, and the interaction between competition and predation, in determining invertebrate dominance in these communities. Three types of experiments were conducted: an enclosure experiment within Ecology Pond, an outdoor replicated tub experiment, and a laboratory behavior experiment. The in-situ enclosure experiment showed that E. traviatum larvae were more susceptible to Anax predation than were E. aspersum larvae; a tendency toward greater vulnerability to fish of E. aspersum compared with E. traviatum was not statistically significant. The outdoor tub experiment confirmed both of these trends with statistically significant results. In the tubs, both predators inhibited feeding of both zygopterans (as indicated by reduced fecal mass), particularly for E. aspersum in the presence of fish. This effect appears to have been primarily indirect, mediated through exploitation of the zooplankton. We also detected competitive effects of E. traviatum on E. aspersum: E. traviatum reduced the emergence and increased the exposure above the substrate of E. aspersum. In the absence of predators, E. traviatum inhibited feeding of E. aspersum via interference. In the laboratory behavior experiment, predators inhibited crawling by E. aspersum. E. aspersum was more exposed than was E. traviatum; it swam and crawled more than did E. traviatum, considerably increasing these movements at night. Over all, E. traviatum consistently appeared to be the more cryptic of the two species, and E. aspersum appeared to be much more active. Our results suggest an explanation for the clear difference in structure between communities like Bays Mountain Lake and Ecology Pond: predaceous fish eliminate large invertebrate predators and shift the community toward cryptic forms at relatively low densities, reflecting the effects of both predation and exploitation competition. In the absence of fish, large invertebrate predators are less able to deplete littoral invertebrates but may favor the more active forms, perhaps because these are better able to avoid invertebrate predators.