摘要:

In a previous study it was demonstrated that a major selection pressure responsible for seasonal changes in the searching and oviposition preferences of the pipevine swallowtail butterfly, Battus philenor (Lepidoptera: Papilionidae), is a seasonal increase in the disappearance rate of early—instar larvae from one of their two host plants in east Texas, Aristolochia reticulata (Aristolochiaceae). The primary objective of this study was to determine the causes, both proximate and ultimate, of that increased disappearance rate. The results may be summarized as follows: 1) The seasonal increase in disappearance rates of larvae feeding on A. reticulata plants is due primarily to a decrease in the size at which larvae disperse from their original host plant. No seasonal increase in the intensity of predation on larvae was detected. 2) Larvae readily consume the young foliage of A. reticulata plants but seldom accept mature foliage. 3) The amount of A. reticulata foliage that is acceptable to larvae decreases between early April and late May. This decrease explains the seasonal decrease in the size at which larvae disperse from their original host plant. 4) Growth rates, corrected for variation in consumption rates, are much lower for larvae fed mature leaves than for larvae fed young leaves. 5) Poor larval growth on mature leaves appears to be due to two effects of sclerophyllization: low nitrogen content and increased toughness. No effects of digestibility—reducing plant secondary compounds were detected. 6) For A. reticulata plants in the field, the probability that a leaf will be eaten once it is discovered by a larva is lower for mature leaves than for young leaves. 7) For A. serpentaria plants, which grow sympatrically with A. reticulata, mature leaves and young leaves are equally acceptable to larvae. Mature A. serpentaria leaves are not markedly sclerophyllous. These results indicate that the causes of the observed seasonal increase in disappearance rates of larvae on A. reticulata can be traced ultimately to changes in the nutritional content of the foliage as it ages. They also suggest that a seasonal deterioration in the nutritional quality of A. reticulata foliage is a major selection pressure leading to the observed seasonal shift in the searching and oviposition preferences of females from A. reticulata in the early spring to A. serpentaria late in the spring. Finally, the results provide evidence supporting the contention that an important ecological function of sclerophylly is to defend plants from herbivores.

ISSN:0012-9615    

DOI:10.2307/2937304

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

We studied the relations between bird distribution and abundance and habitat characteristics at a regional scale of investigation, using surveys conducted over three consecutive years on 14 plots at nine locations in the shrubsteppe of the northwestern Great Basin of North America. Bivariate and multivariate analyses revealed no large suites of bird species that were correlated in their distribution and abundance, and few associations existed between pairs of species, suggesting that bird populations in this system vary largely independently of one another. Both bivariate and multivariate correlational analyses between birds and habitat physiognomy indicated that the bird species that were widely distributed in this shrubsteppe system had few significant associations with habitat features, while species with more localized distributions did exhibit habitat affinities, most notably with a suite of characters associated with the occurrence of rocky outcrops. Bird species whose primary distributions and habitat affinities lie in grassland regions to the east demonstrated the greatest degree of correlation with features of habitat physiognomy in this shrubsteppe region, increasing in abundance as vegetation coverage and stature increased and horizontal heterogeneity of habitats decreased. Some bird species, however, exhibited no correlations with the habitat features we measured, and multivariate analyses comparing variation in the bird abundances with variation in features of habitat physiognomy accounted for<17% of the overall variation in the bird abundance matrix. Consideration of coverages of different species of shrubs, however, generally produced more significant correlations with variations in bird abundances, especially for the widespread shrubsteppe species. Attributes of avian community structure varied among the locations surveyed. Variations in the abundance of Brewer's Sparrows, the most abundant species at most sites, accounted for 86% of the variation in total avian density. Species diversity was negatively correlated with total density (presumably because of the overwhelming influence of one species on density) but positively correlated with species richness. Richness variations, in turn, were a consequence of variations in the abundances of several locally distributed shrubsteppe species or grassland species. Richness decreased with increasing horizontal habitat heterogeneity and general vegetation sparseness, but increased with increasing structural diversity of the habitat. Variations in avian community biomass were largely a function of abundances of the widespread or peripheral species; none of the local shrubsteppe forms that contributed so much to species richness was correlated with total biomass variations. We compare the findings of these regional—scale analyses with those of a continental—scale study that included a habitat spectrum ranging from shrubsteppe through tallgrass prairies. Bird species exhibited different patterns of habitat correlations on the two spatial scales. In particular, the characteristic shrubsteppe species showed strong correlations with features of habitat physiognomy in the continental analysis, but in the regional study such associations were generally lacking and these birds instead were correlated with the coverages of various shrub species. This suggests that at a large scale, between—habitat level of analysis these birds may respond to some elements of general habitat configuration, but their within—habitat responses may be more strongly associated with details of habitat floristics. These results complicate studies of avian community/habitat relationships: complete understanding of ecological patterns apparently requires knowledge of vegetational floristics as well as physiognomy; the response of birds to habitat characteristics, and the habitat features that are important, may differ at different scales of spatial resolution. At both spatial scales, however, a substantial portion of the variation in avian abundance remains unexplained after consideration of habitat features. Relatively few significant correlations thus emerge, increasing the probability that those that are revealed may well be spurious, and reinforcing the view that biotic interactions such as competition probably play a minor role in structuring these communities.

ISSN:0012-9615    

DOI:10.2307/2937305

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Pollen diagrams from the Prairie Peninsula in Illinois record the climatically related vegetation shifts that have occurred since the late Pleistocene. They indicate that the major period of prairie development started between 8500 and 7900 BP, apparently occurring first along the southern margin. These events mark the beginning of the Hypsithermal interval and are correlated with the final disintegration of the Laurentide ice sheet and the establishment of the postglacial system of atmospheric circulation over North America. The late—glacial vegetation was spruce woodland in the north and open spruce woodland/tundra in central Illinois. These vegetation communities disappeared from the state in a northward direction between 14 000 and 10 900 BP. They were followed in northern Illinois by Pinus, Abies, Betula, and Fraxinus; in central Illinois Fraxinus expanded in the lowlands while the uplands remained relatively treeless. These assemblages, which persisted longer in the south than in the north, were subsequently replaced first by a mixture of cool temperate, then warm temperate deciduous trees as climatic amelioration continued. By 9000 BP, Illinois was dominated by deciduous forest. In response to Hypsithermal climatic stress, prairie vegetation began replacing deciduous forest about 8300 BP on the uplands in central Illinois and by 7900 BP xeric oak—hickory forest became dominant in northern Illinois. This is the beginning of the Prairie Peninsula as a discrete floristic area in Illinois. About 5000 BP the southern border of the Prairie Peninsula experienced increased moisture and renewed forest development suggesting a western contraction of the prairie margins. Central Illinois, however, remained grassland to the present. Between 900 and at least 400 yr ago an indicated trend toward cooler temperatures is correlated with late stages of the Neoglacial. Pollen evidence from northern Illinois and adjacent Lake Michigan indicates the reappearance or increase in Betula, Pinus, Picea and Larix. This short cool episode appears to end prior to the beginning of large—scale historic land clearance, about 140 yr ago, as the evidence for a vegetational readjustment to warmer climate is truncated by the logging of the forests, the plowing of the prairies and the resultant dramatic increase in Ambrosia pollen.

ISSN:0012-9615    

DOI:10.2307/2937306

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Efficiencies of food use, growth rates, and consumption rates were measured for larvae of eight tropical insect species, reared upon normal and fertilized Heliconia species (Zingiberales: Heliconiaceae) and Musa sp. (Zingiberales: Musaceae) in Costa Rica. Cephaloleia consanguinea and Chelobasis perplexa (Coleptera: Chrysomelidae, Hispinae) feed only on Heliconia. Caligo memnon and Opsiphanes tamarindi (Lepidoptera: Brassolidae) are oligophagous, with known hosts in two families of Zingiberales. Sibine apicalis, Sibine sp., and Metraga sp. (Lepidoptera: Limacodidae) and Megalopyge sp. (Lepidoptera: Megalopygidae) are polyphagous, with dicot and monocot hosts. Foliar nitrogen content of natural Heliconia imbricata (° = 1.76%) was lower than that of H. latispatha (° = 3.01%) and Musa sp. (° = .30%). The oligophagous brassolids generally had lower growth rates, lower efficiencies of food use, and lower nitrogen accumulation rates on H. imbricata than on Musa sp. The hispines had lower relative consumption and growth rates than any of the lepidopterans examined on any host or treatment. Growth rates and nitrogen accumulation rates for the hispine species were not affected by host plant fertilization. Efficiencies of nitrogen use declined with increasing foliar nitrogen content for the hispines. Several parameters of food use efficiency were correlated with larval feeding specialization. The specialized hispines and oligophagous brassolids had higher efficiencies of food assimilation and nitrogen use on H. imbricata than did the polyphagous limacodids and megalopygid. Other efficiency indices, such as net growth efficiency, were not correlated with feeding specialization. The hispine Ch. perplexa has one of the longest larval developmental times known for a nondiapausing chrysomelid (°200 d). Its slow growth is correlated with a low metabolic rate, which we interpret as a physiological adaptation for starvation resistance. Larvae feed only on rolled Heliconia leaves, which are produced infrequently by host plants, and long fasts are a predictable feature of development. Ch. perplexa's eight larval instars are about equal in duration, and closely match the duration of a rolled leaf's suitability as food for these larvae. After a rolled leaf unfurls, the larvae must either wait for another on the same plant, or search haphazardly for one on nearby plants. The feeding of Ce. consanguinea larvae is apparently moisture limited in this tropical rain forest, and these larvae also have a high resistance to starvation. The oligophagous brassolids have become pests of banana, which is introduced into the Americas. These species increase growth rates with increased nitrogen content of host tissue. They are infrequent on H. imbricata, which has few chemical defenses against herbivores. Thus, the low nitrogen content of H. imbricata may protect it from some herbivory.

ISSN:0012-9615    

DOI:10.2307/2937307

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Laboratory experiments have suggested that ingestion rates should increase with food concentration, that these functional responses should be depressed by increases in the availability of alternate foods, and that feeding preference should be shown for the most abundant of available foods. To test these hypotheses, simultaneous measures of the foraging of three species of littoral Cladocera, on two spatially differentiated food sources, were made in situ. The species investigated were Sida crystallina (O. F. Muller), Chydorus sphaericus (O. F. Muller), and Alona affinis (Leydig). The uptake rates of suspension and periphyton were measured with a double isotope technique. In situ foraging responses were determined through regression analysis. Ingestion rates were related to concentration of food, concentration of alternate food, and body size. Functional responses in situ ranged from Holling's type 2 to feeding inhibition, and the increased abundance of alternate food depressed functional responses in some cases and elevated them in other cases. Variations in feeding preference were examined in two species of cladocerans. One species (Sida) responded in a complex manner, showing increased preference for the most abundant food in one circumstance and increased preference for the least abundant food in another circumstance. The other species (Chydorus) responded to the inhibitory effect of one food source by increasing feeding preference for the alternate food. Due to the complexity of the behaviors, a multivariate in situ approach is suggested for future studies.

ISSN:0012-9615    

DOI:10.2307/2937308

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

A guild of ground—foraging, insectivorous birds which seasonally occupy montane temperate forests was investigated. By examining the roles of interspecific competition and specific habitat selection, I have dissected the factors controlling the distribution patterns of the guild members along extensive elevational gradients. The guild was composed of Hylocichla mustelina (Wood Thrush), Catharus fuscescens (Veery), C. guttatus (Hermit thrush), C. ustulatus (Swainson's Thrush), and C. minimus (Gray—cheeked Thrush). The five thrush species are sympatric on high mountains in the northeastern United States, but as one proceeds southward down the Appalachian Mountain chain, the species drop out one at a time. Anecdotal information indicated that the species showed elevational amptitude expansion as members of the guild dropped out with decreasing latitude. As a consequence of the opportunity for ecological release in southern latitudes, the thrush guild was ideally suited for testing the role of competition in determining distributional limits of the species when sympatric. I studied the behavioral interactions and habitat selection of the five species in sympatry on high mountains in the northeastern United States. The species have distinct distributions along these elevational gradients, but large spatial overlaps between adjacent species were observed. To test for significant differences in habitat selection, the breeding territories of these species were quantified by measuring 55 structural habitat variables. Using stepwise discriminant function analysis, I detected statistically significant differences in the species' breeding habitats. By employing a subset of 9 of the initial 55 variables, over 77% of the Northeastern habitat quantifications were assigned to the correct species group. Additional information extracted from the structural niche analysis indicated the habitat niche breadth of each species and the degree of habitat overlap between adjacent species, and provided insight into the limiting similarity allowed between these coexisting species. Song playback was used to test the contribution of behavioral interactions to the observed distribution patterns. All species showed greater response to conspecific song. The results indicated that over interspecific behavioral interactions contributed little or nothing to the observed distributional and specific habitat selection patterns of these species. To test the contribution of interspecific competition to the habitat selection patterns detected on Northeastern mountains, a subset of the five—member guild was studied on Southeastern mountains. In the Smoky Mountain range only H. mustrelina and C. fuscescens breed. By employing identical quantification techniques, I tested for expansion by these two species into habitat types not occupied when sympatric with the other three species. Evidence of extensive ecological release was obtained, implying a strong role for interspecific competition in Northeastern mountains. Thus, my results further implicate interspecific competition as an integral force contributing to the organization of temperate bird communities.

ISSN:0012-9615    

DOI:10.2307/2937309

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY