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191.
192.
Modeling the Effects of Habitat Fragmentation on Source and Sink Demography of Neotropical Migrant Birds 总被引:5,自引:0,他引:5
Therese M. Donovan Roland H. Lamberson Allison Kimber Frank R. ThompsonIII John Faaborg 《Conservation biology》1995,9(6):1396-1407
Many songbird populations in the midwestem United States are structured as a network of sources and sinks that are linked by dispersal. We used a modeling approach to examine explicitly how populations respond to incremental fragmentation of source habitat and how this response may vary depending upon two life-history attributes: fidelity to natal habitat type and reproductive strength of the source. Fragmentation of source habitat led to a predictable decline in population for both attributes examined, but the manner in which populations declined varied depending upon the reproductive strength of the source and the level of fidelity. When the source was weak and produced few excess individuals, fragmentation of source habitats resulted in a predictable and parallel population decline of adults in both the source and the sink. In this situation high fidelity to natal habitats was important for maintenance of population size and structure. Low fidelity to weak sources resulted in population extinction; populations experienced a demographic cost by dispersing from high quality source habitat to low quality sink habitat. In contrast, when the source was strong and produced many excess individuals, fragmentation of the source led to population declines in both the source and the sink, but this decline was more abrupt in sink habitats. When the source was strong and produced a large excess of individuals, nonfidelity to natal habitats had little effect on metapopulation size and structure. 相似文献
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Summary In many bees and wasps, solitary females produce offspring without help from other females. The transition from lone mothers producing offspring to situations in which females often help to rear siblings is an important step in the origins of complex sociality and nonreproductive castes. Recent work on Hymenoptera has stressed the role of sex ratio variation in this transition; when a mother's brood is more female biased than average, older daughters are favored to help rear their younger siblings because they are more closely related to sisters than to their own offspring. Here the direction of causality is from biased sex ratios, which arise by some extrinsic mechanism, to the origins of sib-rearing (eusociality). We present a model in which there is a synergism between sib-rearing and female-biased sex ratios, which may either complement the sex ratio variation idea by increasing the rate at which helping spreads or be an alternative hypothesis about the origins of eusociality. The synergism in our model depends on three conditions. 1) Daughters that help cause more food to be provisioned per offspring, which in turn causes larger offspring. 2) Females gain more than males by being large, which favors mothers with helpers to produce a higher proportion of daughters. 3) A helper's inclusive fitness rises as her mother's brood becomes increasingly female biased because a female helper is more closely related to her sisters than to her brothers. A female helper may also be more closely related to her sisters than to her own offspring, but this particular sibling-offspring relatedness asymmetry is not required by the synergism model. These three conditions create a synergism which favors a rapid transition from solitary (subsocial) to eusocial. Demographic and ecological factors that facilitate the evolution of eusociality reduce the stringency of the relatedness asymmetry condition (3) required by our idea. The synergism model therefore complements factors other than relatedness that may have been important during the evolution of eusociality. 相似文献
195.
The spectral characteristics of single identified epipelagic sources of bioluminescence from the western Sargasso Sea were measured with an optical multichannel analyzer (OMA) system during the April, 1985, Biowatt cruise. The emission spectra of specimens representing 45 species from 8 phyla were measured. Peak bioluminescence emissions typically occurred between 440 and 500 nm, in the blue region of the visible spectrum. Three exceptions involved emission in the green, yellow, and red spectral regions. Intraspectific variability in spectra, was noted in several species. One shrimp species exhibited two modes of light emission, each with different emission spectra. Other cases involved dynamic color shifts of 10 to 14 nm; the source of the spectral variability is unknown, but may involve optical filtering or differences in the color of luminescence from multiple sites of light emission. Measurements from independent samples of unsorted plankton revealed different spectral distributions. This suggests that the spectral emissions of bioluminescence in the upper water column will vary, based on species assemblage. 相似文献
196.
The main issues of the studies described were the validation of a test design for the assessment of the bioaccumulation potential of environmental pollutants and the applicability ofEisenia fetida as a model-organism and artificial soil (OECD) as a standard test substrate. The test organisms used wereE. fetida of our own breeding stock andAllolobophora caliginosa, Allolobophora chlorotica, Allolobophora longa and Lumbricus rubellus sampled from a field site. Test soils used were the artificial soil (OECD) and a BBA field soil. Soils were each contaminated with 10 mg HCB and Pyrene per soil dry-weight. Within the test period of 4 weeks, samples were taken weekly for residue analysis in the worms and soils. Bioaccumulation factors (AF) calculated forE. fetida and the free-living species were in the range of 10–17 (HCB) and 0.9–1.7 (Pyrene) depending on the soil used. By re-calculation of the concentrations in soil to concentrations in soil water, the resulting bioconcentration factors are compared with published BCF values determined from QSAR’s of other worm species and fresh water fish. It could be concluded that the existing earthworm tests (OECD 1984, ISO 1998) represent an appropriate design for testing the bioconcentration potential of chemicals in soil. 相似文献
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Steven A. Frank 《Behavioral ecology and sociobiology》1987,20(3):195-201
Summary Small colonies of ants often produce mostly male alates, while large colonies produce mostly female alates. I present a simple model consistent with this pattern in which males that compete for mates are related (Local Mate Competition). The model explains the observed trend even when relatedness among competing males is low, so that there is only a negligible effect on the predicted sex allocation ratio in the population. The reverse trend is expected when there is competition among related females for a limited resource, such as nest sites (Local Resource Competition); small broods are predicted to be mostly female and large broods are predicted to be mostly male. 相似文献
200.
Many studies have examined differences in avian community composition between urban and rural habitats, but few, if any, have looked at nesting success of urban shrubland birds in a replicated fashion while controlling for habitat. We tested factors affecting nest survival, parasitism by the Brown-headed Cowbird (Molothrus ater), and species abundance in shrubland habitat in rural and urban landscapes. We found no support for our hypothesis that nest survival was lower in urban landscapes, but strong support for the hypothesis that survival increased with nest height. We found strong support for our hypothesis that cowbird parasitism was greater in urban than rural landscapes; parasitism in urban sites was at least twice that of rural sites. We found strong support for an urban landscape effect on abundance for several species; Northern Cardinal (Cardinalis cardinalis) and Brown-headed Cowbirds were more abundant in urban landscapes, whereas Field Sparrow (Spizella pusilla) and Blue-winged Warbler (Vermivora pinus) were more abundant in rural sites. There was support for lower abundances of Blue-gray Gnatcatcher (Polioptila caerulea) and Indigo Bunting (Passerina cyanea) with increased housing density. For six other species, edge and trail density or vegetation parameters best explained abundance. Lower abundances and greater parasitism in habitat patches in urban landscapes are evidence that, for some species, these urban landscapes do not fulfill the same role as comparable habitats in rural landscapes. Regional bird conservation planning and local habitat management in urban landscapes may need to consider these effects in efforts to sustain bird populations at regional and local scales. 相似文献