摘要:

This paper analyzes the foraging behaviors of a community of insectivorous birds breeding in a high—altitude willow floodplain. The eight species belong to two guilds. Active searchers are represented by the gleaning guild (warblers): Wilsonia pusilla, dendroica petechia, and Geothlypis trichas; passive searchers are represented by the fly—catching guild (flycatchers): Empidonax oberholseri, Empidonax tralli, Empidonax difficilis, Contopus sordidulus, and Nuttallornis borealis. The paper focuses on the following general questions: (1) What are the different components, and their correlations, of foraging behavior in the eight species? (2) What are the overall adaptive syndromes associated with predatory tactics of active and passive searchers? (3) Are the predictions of several models of optimal foraging supported by the results of the present study? Adaptive syndromes (coordinated sets of characteristics, including the specific manner of resource utilization, and an array of other, related adaptations) are described for each species. Warblers forage primarily by gleaning, concentrating on stationary prey. Their velocity (average number of perches per minute) is>10.0, their search intensity (perches visited prior to an attack) is>5.0, and their foraging intensity (number of attacks per minute) is>1.7. They have an attack radius (mean distance from perch to prey) of<0.6m. Members of the gleaning guild may thus be characterized as active searchers (they look for prey difficult to find) but passive pursuers (they attack prey easy to catch). In addition, gleaners exhibit a narrow range of preferences in habitat structure. Flycatchers forage primarily by hovering or sallying, concentrating on prey available via aerial attack. They have both a velocity and search intensity of<4.0, and a foraging intensity of7.5 m. Members of the fly—catching guild may thus be characterized as passive searchers (they look for prey easy to find) but active pursuers (they attack prey difficult to capture). Flycatchers show a broad range of preferences in habitat structure. For all species considered together there is an inverse correlation between body size and velocity, and a positive correlation between body size and attack radius. Three core adaptations identify the adaptive syndromes of actively and passively searching predators. Active searchers have high velocities, high search intensities, and a foraging repertoire associated with passive prey. Passive searchers have low velocities, low search intensities, and a foraging repertoire associated with active prey. Additional characteristics also differentiate the two adaptive syndromes. Search/pursuit ratios critical to numerous optimal foraging models are impossible to measure reliably in the field; thus the concept of active and passive searchers is proposed as a meaningful and readily quantifiable substitute. The data presented here exhibit poor fit with the predictions of several selected models. Predictions concerning differences in diet specialization, the extent of territorial and foraging overlap, and the prevalence of sexual dimorphism are not confirmed. In general, predictions concerning morphological size gradients are upheld; those concerning habitat specialization are confirmed if habitat is measured in structural terms, but are not supported when measured by vegetation type.

ISSN:0012-9615    

DOI:10.2307/1942510

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

There is little research on processes limiting macroconsumer species in salt marshes. We investigated the significance of competition for space and food resources in three North Carolina salt marshes by altering densities of the molluscs Littorina irrorata and Geukensia demissa, and by manipulating Spartina litter. Morality and weight gain were the two principal responses measured. Enclosures (7.5 m2) were used to contain Littorina and marsh grass. Molluscan densities and Spartina litter levels were one—half, control, and double control levels. The field experiments ran from February 1975 through March 1977. The overall mortality and growth patterns of both species were similar within each marsh, but differed among marshes. The molluscs exhibited high mortality and slow growth in Tar Landing marsh, high mortality and rapid growth in Causeway marsh, and low mortality and slow growth in Calico marsh (enriched by sewage effluent). In Tar Landing survival and weight gain in both species were reduced by crowding. Increasing Spartina litter produced significant weight gain of medium—sized Littorina and small Geukensia. In Causeway marsh crowding decreased Littorina and Geukensia growth rates, but increased mortality only in the mussel population. Litter manipulations primarily affected weight gain in Littorina and mortality in Geukensia. In the enriched Calico marsh crowding depressed Geukensia survivorship and Littorina weight gain. Only in the mussel population was weight gain enhanced (1st yr only) by the addition of Spartina litter. We conclude that competition for food is an important limiting process, particularly for larger Littorina in Tar Landing marsh. In Causeway marsh competition for space appears more significant. In Calico marsh space limitation in Littorina populations and competition for food resources in Geukensia appear to be the principal processes. Variation in these competition—limiting mechanisms between two distant marsh consumers, a grazer and a filter feeder, may represent different procedures for dealing with the variable resource levels existing among salt marshes.

ISSN:0012-9615    

DOI:10.2307/1942511

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

A streambank salamander community has proved to be an ideal system for investigating resource allocation and the evolution of community organization. Four aspects of microhabitat were investigated: substrate, substrate moisture, proximity to surface water, and size/type of cover utilized. Selection of substrate particle size was strongly positively correlated intra— and interspecifically (particularly the latter) with salamander body size, appearing to be a highly adaptive feature. The breadth or diversity of utilization of microhabitat components was interspecifically inversely related to body size. Definite shifts in microhabitat utilization and narrowing of niche (resource) breadths were observed in the presence of larger congeners. The mechanism of the shifts as well as the selective force that molded the organization of this community appear to be interspecific interference competition. Therefore, body sizes and species compositions in streambank salamander communities are determined by substrate size, its patchiness, and the presence of interference competitors. Observed macrohabitat distribution patterns have been analytically described and can be predicted by a knowledge of microhabitat preferences and niche breadths. The ratios of snout—vent lengths or head widths between adjacent species on a body size gradient for both adults and juveniles were remarkably constant, generally between 1.2 and 1.3. Mean prey size (width, length, and volume) also formed a geometric spacing which paralleled body sizes. The coefficient of variation (standard deviation/mean) was constant within a given measure of prey size. Morphological ratios for sympatric competitors in the range of 1.2 to 1.3 have been interpreted by investigators as character displacement to allocate prey sizes and therefore minimize competition. Such interpretations must be made carefully because in this study observed body size differences primarily reflect microhabitat allocation (the principal niche dimension of this study) and only secondarily allocation of prey sizes. Also, species packing (d/w) along the prey size resource gradient is much greater than anticipated by the theoretical May—MacArthur model and is demonstrated to be high in a community of opportunistic generalist insectivores. Species packing also requires careful interpretation because in reality spacings and widths of utilization functions are not uniform and separation of community members along a single niche dimension is rarely if ever encountered. Total niche overlap among community members (species and age classes) can accurately be determined because the relationships among the four microhabitat resources and the prey size dimension are known. The use of prey taxa as a niche dimension for opportunistic generalistic insectivores either overestimates or underestimates total niche overlap, since the utilization of prey items is a function of their size, the microhabitat preference of the predator, and stochastic events of prey occurrences. The community appears to be well coadapted since the following have been demonstrated: (1) total niche overlap is low, therefore minimizing competitive interactions, (2) interspecific predation could not be documented and must be minimal, and (3) demographic strategies are consistent in all macrohabitats regardless of congeneric composition.

ISSN:0012-9615    

DOI:10.2307/1942512

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY

摘要:

The North Pacific central gyre is a large, monotonous, and geologically old system. It (and other such gyres) is probably the nearest oceanic equivalent to an essentially self—regulating, climax ecosystem. We have taken a series of replicated, vertically stratified net tows for macrozooplankton and have replicated measures of a number of habitat variables. Copepods are the most numerous members of the macrozooplankton fraction of the community of the central gyre. It is likely that they are also the largest part of the biomass. There are at least 125 species regularly present and although we do not know what many of them eat, certain species can almost certainly be identified as herbivores or carnivores, and others as omnivores. We show that some species are significantly more frequent parts of each other's biotic environment than other species. These recurrent groups of species tend to occupy different depth zones. The members of the groups show strong concordance of abundance at the depth of high group coherence. Thus there is a dividing up of the water column and a clear tendency for vertical spatial structure in the copepod fauna. Within groups there is a significant constancy of dominance at the group's preferred depth over time. Thus there appears to be a large amount of species structure as well as spatial structure. A highly regulated species equilibrium is implied. There are congeners occurring within the same recurrent groups. The physical and biotic environment of each group differs with respect to both concentration (e.g., chlorophyll α) or magnitude (e.g., temperature) of habitat variables and with respect to the spatial variability of these. Our most diverse group tends to occupy a depth zone ranking high in primary productivity but does not have the greatest total environmentalrichness or greatest or least heterogeneity, and this seems contrary to what some current community theory predicts. Our evidence indicates that within groups many similar species coexist spatially and temporally. We speculate about the relative roles of competition and predation influencing the structure we see. Aspects of modern community theory, based on competitive equilibrium, seem inadequate to explain our results. Predator regulation of structure seems a more likely explanation. However, existing information indicates a lack of sufficient specialization of what we believe to be the main predators on copepods to account for the observed constancy of copepod species structure. We suggest that a more intensive study of the role of predation is in order.

ISSN:0012-9615    

DOI:10.2307/1942513

出版商:Ecological Society of America    

年代:1979

数据来源: WILEY