摘要:

Fertilization success was measured in the bluehead wrasse, Thalassoma bifasciatum, a tropical reef fish with external fertilization of pelagic eggs. This species exhibits intraspecific variation in its spawning behavior; females either spawn with single males (pair spawning) or spawn with a group of at least three and often>20 males (group spawning). Fertilization success averaged °75% and did not differ between pair and group spawning, despite an estimated 80—fold increase in sperm release in group spawns. There was also no evidence that pair—spawning males suffered sperm depletion over the course of the spawning period. Thalassoma bifasciatum occurs in a variety of habitats and is exposed to varying levels of water turbulence. Fertilization success varied among days, and decreased with rougher water conditions. Within a reef, the calmer spawning sites behind the reef relative to the current had higher fertilization success than those along the side of the reef. These data suggest that while the type of spawning occurring at the site does not affect selection for fertilization success, females may gain fertilizations by selecting particular locations or periods of calmer water conditions to spawn. This is an alternative hypothesis to explain temporal and spatial patterns of mating. We provide detailed methods on how to accurately obtain data on fertilization success. Our technique can be used to study natural spawning in a wide variety of reef fishes and other marine organisms with pelagic eggs, external fertilization, and predictable spawning.

ISSN:0012-9658    

DOI:10.2307/1940747

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The existence, causes, and consequences of dietary specialization were explored for a temperate reef fish, striped surfperch (Embiotoca lateralis). Analyses of diets of adults from Santa Cruz Island, California, revealed three dietary regimes within the same population: specialization on caprellid amphipods, specialization on gammarid amphipods, or a mixed (generalized) diet composed of both caprellids and gammarids. Individuals that had specialized on caprellids at the time of capture had obtained a greater intake of prey than did generalists or specialists on gammarids. Different diets were associated with marked differences on body size within most adult year classes; specialists on caprellids were the largest and specialists on gammarids were the smallest for a given age. Generalists were intermediate in size. Females in the different dietary categories were projected to experience different expected lifetime reproductive outputs. Fitness was calculated to be °10% lower for generalists and °20% lower for specialists on gammarids compared with caprellid specialists. The different diets did not arise from active choice of prey taxa, but from selective use of available foraging patches (i.e., types of benthic algae from which prey are harvested). The degree to which an individual fed from the red alga Gelidium robustum was an excellent indicator of the taxonomic composition of its diet. Gelidium was the only alga harboring substantial members of caprellid amphipods; more importantly, it was the most profitable foraging microhabitat. Access to Gelidium was unequal among striped surfperch due to strong interference, and only competitively dominant individuals appeared able to consistently harvest crustaceans from this microhabitat. The least aggressive individual fed from other, less profitable patches that contained mostly gammarid amphipods. "Dietary" specialization was a passive consequence of patch selection dictated by a strong competitive hierarchy, and was not the result of active choice among available types of prey items.

ISSN:0012-9658    

DOI:10.2307/1940748

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Sea Otter (Enhydra lutris), well documented as "keystone" predators in rocky marine communities, were found to exert a strong influence on infaunal prey communities in soft—sediment habitats. Direct and indirect effects of sea otter predation on subtidal soft—bottom prey communities were evaluated along a temporal gradient of sea otter occupancy around the Kodiak Archipelago. The results indicate that Kodiak otters forage primarily on bivalve prey and dramatically reduce infaunal bivalve and green sea urchin (Strongylocentrotus droebachiensis) prey populations. Bivalve prey abundance, biomass, and size were inversely related to duration of sea otter occupancy. The relative conditions of shells discarded by otters in shallow (20 m) water at the same sites indicate that otters first exploited Saxidomus in shallow—water feeding areas, and later switched to Macoma spp. in deeper water. Otter—cracked shells of the deep—burrowing clam Tresus capax were rarely found, even at otter foraging sites where the clam accounted for the majority of available prey biomass, suggesting that it has a partial depth refuge from otter predation. The indirect effects of otter predation included substratum disturbance and the facilitation of sea star predation on infaunal prey. Sea stars, Pycnopodia helianthoides, were attracted to experimentally dug excavations as well as natural sea otter foraging pits, where the sea stars foraged on smaller size classes of infaunal bivalves than those eaten by otters. Otters also discard clam shells on the sediment surface and expose old, buried shells during excavation. Surface shells were found to provide attachment sites for large anemones and kelp. Our study shows that sea otters can affect soft—sediment communities, not only through predation, as in rocky habitats, but also through disturbance, and thus retain a high degree of influence in two very different habitat types.

ISSN:0012-9658    

DOI:10.2307/1940749

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Diet selectivity by the planktivorous bluegill sunfish (Lepomis macrochirus) is correlated with the fishes' visual resolution. Poor visual capabilities are thought to impose a constraint on the ability of small sunfish to chose prey. Li et al. (1985) suggested that sunfish subjected to conditions of poor visual resolution select prey in accordance with a model in which prey choice is determined by apparent prey size. Further, enhanced visual resolution permitted large sunfish to assess absolute prey size more accurately than could smaller conspecifics and, consequently, large—sized sunfish consumed relatively more large prey than was predicted by the apparent size model. Here, we examine the visual capabilities and prey selection of very small sunfish, extend the analyses of Li et al. by determining the optimal diet sets for the fish used in their study, and compare prey selection by all sunfish size categories to that predicted by the apparent size and optimal foraging models. Contrary to the conclusions of Li et al., small sunfish (<20 mm standard length [SL]) were more selective than was predicted by the apparent size model and preferentially attacked small—sized Daphnia. The optimal foraging "prey" model predicted correctly the size—related changes in mean size of prey attacked by sunfish, whereas the apparent size model predicted a preference for large Daphnia for all sizes of fish. Yet, the observed diet breadths were always larger than those predicted by the optimal foraging model. The visual system any energy considerations constrain prey selection in small sunfish but operate in opposite fashions.

ISSN:0012-9658    

DOI:10.2307/1940750

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Drift activity of stream invertebrates typically is greatest during the nighttime hours in running waters throughout the world. Such diel periodicity may be an adaptive response that minimizes exposure to visually hunting, drift—feeding fishes. I tested this risk—of—predation hypothesis by examining drift behavior of mayflies in a series of Andean mountain and piedmont streams that vary in the abundance of drift—feeding fishes. Drift was primarily nocturnal in piedmont streams with natural populations of visually hunting predators. In contrast, mayfly drift activity did not differ between day and night in mountain streams that historically lack drift—feeding fishes. However, in naturally fishless Andean streams containing introduced rainbow trout, nocturnal peaks in drift were observed for the mayfly Baetis, suggesting a rapid evolutionary change in behavior in response to an exotic predator. When drift periodicity was examined along a gradient of predation regimes, activity was found to be increasingly restricted to the nighttime hours as predation risk became more intense. Diel periodicity was observed even when fish were experimentally excluded, suggesting that nocturnal activity has evolved as a fixed behavioral response to predation, and is not a direct ecological consequence of diurnal feeding by fishes. These observations support the hypothesis that predation risk is important in determining the timing of prey drift behavior.

ISSN:0012-9658    

DOI:10.2307/1940751

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Male territorial behavior and female oviposition behavior of Megaloprepus coerulatus, a damselfly that develops in water—filled tree holes, is interpreted in light of its larval ecology. Sexual selection favors large males because they win more territorial contests than do smaller males, and females mate only in the vicinity of defended tree holes (i.e.,passive female choice). Small males behave as territory holders until displaced, and/or act as subordinate satellites at large territories. Females are more predictably found in forest light gaps, even though they oviposit in both gap and understory tree holes. Males defended only 14% of the sites used by ovipositing females. Large tree holes are relatively rare, but support a greater number of emerging adults per season and produce larger adults than tree holes containing ≤1 L of water, which rarely produce more than one individual per season. By defending only large tree holes in gap areas, a male increases his chances of mating, and of producing multiple offspring and reproductively successful sons. Results from a field experiment suggest that both mate acquisition and procurement of superior larval habitats have been selective pressures in the evolution of male territory choice. Because even large holes are inadequate to support a female's entire clutch, and the presence of conspecific and heterospecific odonate larvae makes offspring survival uncertain, selection favors a female that partitions her clutch among multiple tree holes.

ISSN:0012-9658    

DOI:10.2307/1940752

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Wide—ranging but ecologically variable species and their close relatives provide the best opportunity to analyze the role of trade—offs in specialization. Poecilochirus carabi Can. is a holarctically distributed mite that rides between patches of carrion on the bodies of carrion beetles (Silphidae, Nicropohorus). Using behavioral tests, we demonstrated differences in local host specialization of P. carabi at two sites in Michigan, the Kellogg Biological Station (KBS) and the University of Michigan Biological Station (UMBS). At KBS, Nicrophorus orbicollis and N. tomentosus each supported a strict host specialist. These KBS "orbicollis— and tomentosus—specialists" differed significantly in both body size and a measure of dorsal setal lengths. Laboratory crosses between specialists were not successful, suggesting that the specialists are distinct species. At UMBS, in contrast, such strong specialists were not found. Mites preferred two beetle species, N. tomentosus or N. defodiens (a species not found at KBS), over all others, but moved between these two species readily. UMBS mites all resembled the KBS tomentosus—specialist in morphology. No orbicollis—specialist mites were found at UMBS, despite the high relative abundance of N. orbicollis in the beetle community. We raised mites from each site on all available hosts to test whether preference for a host species was associated with higher fitness in association with that host species. Although P. carabi were able to reproduce in association with any Nicrophorus species, mites at UMBS had the highest reproductive success on the two species that they preferred. Each KBS specialist mite was raised in association with N. tomentosus and N. orbicollis. While the tomentosus—specialist had significantly higher reproductive success in association with its preferred host species, the orbicollis—specialist showed no difference in reproductive success between the two host species. However, when the pattern of dispersal on beetles was considered, a difference in fitness between the two hosts was apparent. This difference in dispersal pattern was due to an interaction between the parental behavior of the two host species and mite generation times. We discuss the evidence for trade—offs in this system and the use of interspecific comparisons in the analysis of specialization.

ISSN:0012-9658    

DOI:10.2307/1940753

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

The mathematical theory of animal diseases has seen explosive growth in the past decade, yet most of the existing theory examines only temporal disease spread, ignoring the effects of patchy host or pathogen spatial distributions. Here I present a model for the within—season spatial spread of insect pathogens that incorporates host movement in an otherwise conventional insect host—pathogen model. Mathematical analysis of the model reveals that the pathogen will spread through the host population in a moving wave front of disease, known as a "travelling wave." This analysis shows how the spatial rate of spread of the pathogen depends upon the transmission rate of the disease, the rate of production of the pathogen by infected hosts, the initial population of the host, the decay rate of the pathogen, and the death rate of infected hosts. To test the predictions of the model, I performed a series of field experiments with the nuclear polyhedrosis virus (NPV) of Douglas—fir tussock moth, Orgyia pseudotsugata. First, I estimated each of the parameters of the model in the field with a series of small—scale experiments, and used the parameter estimates to predict the spatial rate of spread of the NPV through a population of tussock moth larvae (NPV diseases, like many insect pathogens, do not infect adults). To test this prediction, I then performed an experiment in which I measured the rate of spread of the NPV in an experimental population of tussock moth larvae on linear arrays of Douglas—fir seedlings. The model predicts the rate of spread of tussock moth NPV fairly accurately, suggesting that one can use this type of model to extrapolate individual behavior and localized transmission patterns to broader—scale spatial dynamics.

ISSN:0012-9658    

DOI:10.2307/1940754

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

Temporal density—dependent parasitism and a host threshold density are important features of disease induced by infectious parasites in populations of aboveground, macroscopic organisms. We determined whether these features also occur in soil microcosms containing a microscopic host (the nematode Heterodera schachtii) and its parasite (the nematophagous fungus Hirsutella rhossiliensis). Soil microcosms are especially interesting because (1) the environment and scale are quite different from conventional host—parasite systems and may result in considerably different disease dynamics, (2) the small size of the soil microcosms, although biologically appropriate, facilitates experimentation and parameter estimation, and (3) some soil—borne, microscopic organisms (such as H. schachtii) are important agricultural pests. Temporal density—dependent parasitism was directly assessed with laboratory experiments in which host density and environment were controlled. A theory, which complements and extends the experiments, was developed to enable direct comparison of observed and predicted dynamics and to provide a stringent test of our understanding of processes underlying the dynamics. The theory was simple, yet explicitly described the essential biology. Parameters for the theory were measured with short—term experiments. We found that the disease dynamics in soil microcosms exhibited both temporal density—dependent parasitism and a host threshold density. However, epidemics were slow to develop. Observed and predicted dynamics were quite similar, indicating that our understanding of the underlying biology was correct.

ISSN:0012-9658    

DOI:10.2307/1940755

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY

摘要:

I investigated the interactions of intra— and interspecific competition with parasitism for populations of two terrestrial isopods, Porcellio scaber and Porcellio laevis, infected with a lethal virus. In field experiments, I varied densities of one or both isopod species in enclosures in which food and virus presence were also manipulated. I estimated the effects of intra— and interspecific competition, additional food, and virus infection on both the survival of P. scaber and the prevalence of the virus. In uninfected populations, intraspecific and interspecific competition negatively affected isopod survival, although food resources had no effect. In infected treatments, interspecific competition produced higher levels of virus prevalence than did intraspecific competition. Virus infection and reduced food interacted to produce higher overall mortality in interspecific competition treatments than in intraspecific competition treatments. The lack of significant effect of food on competition suggests that interference interactions rather than resource competition predominated. This mechanism is supported by a laboratory study in which total food availability was controlled. These results indicate that competition and parasitism interacted to produce the observed mortality patterns, and that ignoring mortality due to the virus would produce apparent interspecific competition. The data suggest that the impact of P. laevis on P. scaber involves both direct and indirect effects.

ISSN:0012-9658    

DOI:10.2307/1940756

出版商:Ecological Society of America    

年代:1992

数据来源: WILEY