摘要:

Connor and Simberloff (1979) have argued that in many archipelagoes bird species are randomly distributed subject only to constraints of a minimum island size and a give number of occupied islands for each species and a given number of species per island. Thus, interspecific competition would not seem to be important in determining the distribution of birds. However, the Galapagos finches (Geospiza) are not distributed as predicted by a random model with contraints of minimum island size and each species inhabiting a given number of islands. Interspecific competition is a likely explanation for the dissimilarity of distributions in this genus, as availability of suitable habitat or isolation alone are not satisfactory as alternative explanations. For the birds and bats of the West Indies many cases of competitive exclusion may be included in the seemingly small difference that Connor and Simberloff found between the numbers of exclusive distribution and those expected under the constraints of their random model. However, factors other than interspecific competition may be responsible for the dissimilarity of these distributions. Random models are a weak method for studying the role of interspecific competition in animal distribution.

ISSN:0012-9658    

DOI:10.2307/1937226

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Discriminant functions have been used to identify axes of niche separation, and dispersion of locations on these axes have, in turn, been interpreted as representing niche width and specialization. We discuss the influence of sampling scheme and choice of niche measures on the results of an analysis conducted in canonical space. Specifically, habitat variables measured at random locations should provide a more representative measure of habitat available than would recording habitat only where animals are observed. Species presence information alone tends to bias the mean habitat toward the one where the most common species is found. If niche specialization is to be measured by distance from the overall mean habitat, each sampling station should receive equal weighting. The most straightforward measure of habitat breadth seems to be the variance or standard deviation of canonical scores. The mean squared distance from the species centroid can then be used as a comparable multidimensional breadth measure. Finally, we suggest that niche overlap be measured via a joint probability density function rather than by area of overlap of concentration ellipses.

ISSN:0012-9658    

DOI:10.2307/1937227

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The nonoverlapping vertical distributions of sessile species in the marine intertidal are usually considered to result from two interacting factors: (1) broadly restricted settlement of juveniles in the vertical horizon, and (2) postsettlement mortality which affects species differently. Investigated here is the role of larval behavior in molding the zonation of the acorn barnacles Balanus glandula and Balanus crenatus. I document the vertical distributions of: (a) adults, (b) newly settled spat, and (c) planktonic cyprid larvae. The data show that the vertical distribution of adults is a reflection of the vertical zonation of cyprids in the plankton and settlement patterns of spat. Although postsettlement mortality and substratum selection occur, presettlement behavior can strongly influence vertical zonation of sessile organisms.

ISSN:0012-9658    

DOI:10.2307/1937228

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The response of carabid beetles to differences in tillage and chemical use was studied using four matched pairs of sites. Abundance and species richness were significantly different between treatments, but four commonly used measures of diversity were relatively insensitive indicators of change. It is suggested that the use of diversity indices in impact assessment is redundant or, when used with ground beetles, misleading.

ISSN:0012-9658    

DOI:10.2307/1937229

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Density—dependent aspects of growth, metamorphosis, and survivorship of Scaphiopus holbrooki tadpoles were examined in the laboratory under two experimental regimes. In the first density experiment, the growth index (W) of tadpoles decreased exponentially with density. Mean growth rate varied from 0.023 mL/d at the lowest density to 0.006 mL/d at the highest density. The mean number of days to metamorphic climax was positively associated with the initial density treatment: 27 d at the lowest density to 86 d at the highest density. The body size of tadpoles at metamorphosis showed a concave curvilinear relationship to initial density, indicating tadpoles at the highest densities are apparently capable of growth recovery once released from density stress. The survival of tadpoles decreased exponentially with initial density, from 90% at the lowest density to 20% at the highest initial density. In the second experiment a cross—classified design was used to examine the effects of density and duration of treatment (time) on growth and metamorphosis. Density and time had significant effects on body size at metamorphosis and days to metamorphosis. There was no significant interaction between density and time. These results indicate that the inhibitory effect of density stress varies with the duration of the stress. Scaphiopus holbrooki tadpoles exhibit developmental traits (rapid growth, short larval period, small body size at metamorphosis) that should be favored by natural selection in high density habitats. "Dispersability" may be a mechanism whereby S. holbrooki can minimize the detrimental effects of density stress

ISSN:0012-9658    

DOI:10.2307/1937230

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The orb—weaving spiders Argiope trifasciata and Araneus trifolium were tested for specific behavioral responses to spatial variation in the net rate of energy return. In laboratory feeding experiments with both species, no changes in attack time, capture success, or web design were caused by increased experience with particular prey types. Separate field experiments in enclosures of old—field vegetation were conducted with Argiope trifasciata. Spiders were free to move between four contiguous quadrats which differed only in the ration fed to spiders. Length of stay at a site was positively correlated but probability of moving and distance moved were negatively correlated with ration level. Spiders moved more often than predicted by random expectation when current rations were less than previous rations. These responses produced an aggregation of spiders in quadrats with high ration levels over an 11—d—period. This increase in search intensity which increasing return rate is qualitatively similar to that observed in searching predators in previous studies.

ISSN:0012-9658    

DOI:10.2307/1937231

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The "biennial" plant series Verbascum thapsus, Oenothera biennis, Daucas carota, and Tragopogon dubius are all found in abandoned agricultural fields, but they differ in their time of first appearance, peak abundance, and length of time populations persist. In experimental field studies these species showed differences in their abilities to colonize various types of ground cover. Seedling emergence, survival, and growth were highest for all four species in patches of bare ground. The small—seeded species, Verbascum and Oenothera, survived only in bare ground. In contrast, the large—seeded species, Daucus and Tragopogon, survived in several ground cover types. Relative coverage of three ground cover types and differences in seedling performance in these cover types were compared in a 1—yr and 15—yr old field. Verbascum and Oenothera became established only in the 1—yr old field where bare ground was abundant and not in the 15—yr old field where these patches were rare (°1% of area). When bare—ground patches were created experimentally in the 15—yr old field, both species were able to establish, supporting the conclusion that it is availability of bare ground rather than age of field per se that confines these two species to recently disturbed areas. In contrast, Daucus and Tragopogon became established in both fields, although seedling emergence, survival, growth, and reproductive output were higher in the 1—yr old field. Under nonexperimental conditions, however, these two species are not generally found in newly abandoned fields. Their absence is due to the fact that they produce seeds that, although broadly dispersed, do not survive in the soil for>1—2 yr and are not often present in a newly distributed area. Once dispersed to and established in an area, they can persist for several generations because their seedlings can survive in several types of ground cover, including those with thick vegetation.

ISSN:0012-9658    

DOI:10.2307/1937232

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The species richness and composition of ant assemblages at the extrafloral nectaries of four Costus spp. (Zingiberaceae) in Panama were quantified to determine (1) the relative specificity of these ant—plant mutualisms, and (2) the ecological factors that influence ant assemblages. Although inflorescences of Costus spp. commonly produce extrafloral nectar for several months (species means 1—2 mo, range to 4 mo), there was little turnover in ant—species composition on individual inflorescences. A total of 34 ant species in five subfamilies were observed, but plant species varied significantly in ant species richness. The number of ant species expected from equal sampling (40 inflorescences per Costus sp.) was 9 for C. pulverulentus, 13 for C. scaber, 15 for C. allenii, and 20 for C. laevis. Among—species differences in ant species richness were not a function of plant distribution, the volume or composition of extrafloral nectar, or the duration of nectar flow. The number of ant species was positively correlated with inflorescence height, which varied from 0.6 to 2.0 m. The differences in ant species richness with inflorescence height were due primarily to the addition of arboreal ants, which increased from 3 species representing 3% of all ant species observations on the shortest plant, C. pulverulentus, to 12 species representing 62% of all ant observations on the tallest plant, C. laevis. The number of terrestrial ant species varied only from six to eight. Cumulative ant—species curves as a function of number of inflorescences sampled (N = 41—80) did not plateau for any Costus sp., indicating that a large number of ant species can utilize extrafloral nectar in this system. The high ant species richness and the similarity in nectar characteristics among Costus spp. suggest these are relatively generalized mutualisms with limited coevolution between the plants and their attendant ants. The brief period of extrafloral nectar production (° = 36—55 d), coupled with spatial variation in ant species distributions, probably limit the degree of specialization among mutualists. This is in contrast to more specialized ant—plant mutualisms where plants are relatively predictable sources of food and/or nest sites for ants.

ISSN:0012-9658    

DOI:10.2307/1937233

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

An example of the association of evergreen plants with nutrient—poor soils is described from the Sierra Occidental of northwestern Mexico. At elevations from 600 to 1100 m in this mountain range, distinct patches of evergreen—oak woodland are surrounded by subtropical deciduous forest in a mosaic pattern. To quantify the vegetation pattern and examine its environmental basis, vegetation and soil samples were taken throughout the elevational range of the mosaic. Analysis of the samples indicated that the patches of evergreen vegetation occur on extremely acid, infertile soils (derived from hydrothermally altered rock), while deciduous vegetation dominates on less acid, more fertile soil (derived from unaltered volcanic rock). The two community types have very little floristic overlap, and the deciduous forest has much higher density of woody plants and greater species richness. The actual location of boundaries between patches of the two community types can be more abrupt than can be accounted for by soil change. At higher elevations (above 1100 m) evergreen woodland is not restricted to acid soils and occurs on all soil types. These data are discussed in terms of a general model to account for the association of evergreen plants with nutrient—poor soils in the Sierra Madre as well as in other areas with similar growth form soil fertility patterns. I propose that evergreen plants would grow and survive at least as well on fertile soils as on infertile soils in isolation, but are unable to establish in dense, productive vegetation because of low growth rates associated with high carbon costs and low photosynthetic rates of evergreen leaves.

ISSN:0012-9658    

DOI:10.2307/1937234

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Avain community structure was studied along a meadow—aspen—fir—spruce subalpine sere in northern Utah and southern Idaho during the breeding seasons of 1976, 1977, and 1978. During 1977 one of the worst droughts of this century occurred in the western United States, and comparative changes in avian community structure were documented during and after drought in the four seral stages. The number of breeding species declining most markedly during drought in aspen. Breeding communities on the forested plots during drought were least similar to predrought communities; pre and postdrought communities were consistently most similar. Densities of singing males declined more markedly on all plots during drought than did number of species, and remained below predrought levels in 1978. Changes were also noted in avian trophic structure during drought: insectivores generally declined in number, nectivorous hummingbirds completely disappeared, and the number of granivorous cardueline finch species increased. These changes can be related to changes in food resources. During drought, insect species composition and densities changed, flower phenology was 5 wk early, and one of the best coniferous cone crops in 30 yr occurred in northern Utah. Direct effects of lack of water were observed, and many bird species probably experienced water stress. Although difficult to document, indirect effects of drought on food resources are suggested for insectivores, nectivores, and possibly carnivores. The result indicate that a drought can represent an "ecological crunch" to and affect the structure of, temperature avain communities. However, separating the variation due to drought from inherent variation is difficult without comparative long—term studies. Avian communities in deciduous vegetation may be greatly affected by drought than those in coniferous vegetation.

ISSN:0012-9658    

DOI:10.2307/1937235

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY