摘要:

The underlying causes of habitat use in an assemblage of Ozark minnows were investigated in an observational field study. Six habitat variables were used: depth of water, current speed, substrate size, lateral distance from stream edge, vertical position in the water column, and presence in pool, riffle, or raceway. Analysis focused on the six numerically dominant species that used a common set of pool microhabitats and thereby formed a natural habitat guild. Observations on habitat use were conducted so as to record simultaneous habitat use by all species at each sample point. This technique made possible an analysis of interspecific interactions over habitat use. The six species were segregated primarily by the vertical position variable (X overlap = 0.160). Subdivision of pool habitat data into two depth zones yielded subsets that were homogeneous with respect to all habitat variables except vertical position. Within these data subsets, interactions between species pairs over use of vertical position were examined. Species whose distributions shifted significantly toward those of heterospecifics were considered associations while distributions that shifted away were considered dissociations. Of 60 possible vertical interactions, 33 were significant, and of these approximately two—thirds were associations and one—third were dissociations. The subdivision of data also permitted an analysis of horizontal interactions between species pairs over the sample points. This analysis showed that guild members were significantly clumped into multispecies groupings. Analysis of species—pair interactions within these groupings showed that of 60 possible interactions, 37 were significant and of these approximately half were associations and half were dissociations. A pattern of vertical and horizontal interactions was evident between the two depth zones. In the shallower zone, associations were the predominant interaction in the vertical dimension, but the number of associations and dissociations was nearly equal in the horizonal dimension. In the deeper zone, vertical associations still predominated, though to a lesser extent, and associations predominated in the horizontal dimension. Several explanations for these patterns were proposed. Some associative interactions may facilitate opportunistic feeding behaviors, and others may promote interspecific schooling as an antipredation tactic. Dissociations probably promote habitat segregation among guild members. A balance of these two types of interactions may be critical to guild maintenance. The frequent interactions over habitat use indicate that habitat use among guild members is highly variable; the overall pattern of segregation results from a combination of habitat selection as modified by interspecific interactions over time and space. This view contrasts with the more conventional interpretation of segregational patterns in which shifts or differences in resource use are assumed to be relatively static and interactions and competition among species are reduced or eliminated. the dynamic nature of habitat partitioning observed in this minnow assemblage is consistent with the requirement of ecological flexibility in the characteristically unstable and unpredictable environments of streams.

ISSN:0012-9615    

DOI:10.2307/1942631

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

In the Australian arid zone, the species richness of ants is greater and that of mammalian grainvores is less than in North American deserts. This study aimed to determine if the structure of harvester ant communities differs from that seen in North American deserts, focussing on differences related to the paucity of rodents. We tested three hypotheses: (1) because there are fewer rodents, Australian harvester ants should be more abundant and diverse in local habitats than in North American deserts of similar productivity; (2) because the absence of rodents would allow ants to use larger seeds that are preferred by rodents in North America, Australian ant communities should include a larger size range and contain larger workers; and (3) that apart from differences resulting from a paucity of rodents, Australian and North American communities would be convergent in characteristics of community structure resulting from competition. We sampled 19 communities across a climatic gradient in the Australian arid zone and compared the results with data previously obtained for 10 North American communities. Australian harvester ants exhibited similar alpha (within—habitat) diversity but higher beta (between—habitat) diversity between communities. Australian and North American communities were similar in species richness, species diversity, numbers of common species, and abundance of ants, although Australian communities tended to be richer and more diverse at sites with lower precipitation. North American communities increased in species richness, diversity, numbers of common species, and proportions of column—foraging species with increasing precipitation. In contrast, Australian communities did not change regularly across a gradient in precipitation. Thus, the first hypothesis was rejected. The second hypothesis was also rejected because Australian harvester ants were smaller, covered a smaller size range, and tended to be more tightly distributed along the size gradient. Dietary data indicated a lack of correspondence between resource use and availability, suggesting that Australian communities may not generally be in equilibrium with their resource environment. However, neither of the first two hypotheses could be tested unequivocally because it remains possible that evolutionary interaction between seeds and ants in the absence of rodents has allowed seeds to adopt defenses minimizing consumption by ants, or that other granivores (particularly birds) compensate in part for the paucity of rodents. Tests of the third hypothesis were ambiguous. Some results suggest basic similarities between the effects of competition on the communities, such as the similar maximum values for abundance, richness, and species diversity. However, several attributes of Australian communities differed from those in North America: foraging occurred over a wider range of soil temperatures; temporal displacement of foraging among coexisting species was prominent; and individually foraging species occurred with equal frequency across climatic gradients. We discuss various biotic and abiotic features that may explain differences in community structure between continents. We conclude that the paucity of rodents, a potentially competing group of granivores, has not led to predictable changes in harvester ant communities of arid Australia.

ISSN:0012-9615    

DOI:10.2307/1942632

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Our purpose in this paper is to determine how the degree of polyphagy of different herbivorous insect species affects their yearly population variability. We assembled data from three studies on herbivorous insects: on British aphids, British moths, and Canadian Macrolepidoptera. Within each data set, we compared estimates of population variability across species, and related these differences to estimates of the degree of polyphagy. The degree of polyphagy was negatively correlated or uncorrelated with population variability, i.e., highly polyphagous species have a weak tendency to be less variable than host specialists. This result lends some support to MacArthur's (1955) argument that polyphagous species may be less susceptible to fluctuating resource levels. Population variability in monophagous or oligophagous herbivorous insects may, in part, reflect variation in resource levels. However, we have not yet evaluated the possibility that the levels of predation and parasitism suffered may affect variability even more strongly.

ISSN:0012-9615    

DOI:10.2307/1942633

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY