摘要:

Pine Warblers and Blue—gray Gnatcatchers forage extensively on bark in the pine forests on Andros Island, while on neighboring Grand Bahama Island they forage largely on foilage. The two islands are very similar in physiography, climate, and vegetation, but differ in the number of insect—gleaning species sharing the arboreal foraging substrates. Andros has only the two species mentioned above; Grand Bahama has five, and to of the additional species are bark specialists. Summed densities and consuming biomasses (masses adjusted for size variation in metabolic rate) of the birds in these arboreal insectivore communities were found to be roughly equal on the two islands, but the distribution of total foraging activity as measured in these same units was clearly unequal; foraging biomass on bark was 2.6 times as great on Andros as on Grand Bahama, on foliage it was 1.7 times as great on Grand Bahama as on Andros. When food (insect) abundance and distribution between bark and foliage was found to be similar on the two islands, I concluded that food abundance was probably not the deciding proximal factor behind the dissimilar bird distributions. Alternatively I speculated that some form of interference competition from the bark specialists on Grand Bahama could be inhibiting Pine Warbelers and gnatcatchers from visiting the bark on that island, while in the absence of bark specialists on Andros the birds were free to forage on both substrates. Since bark avoidance is characteristics of Pine Warblers and gnatcatchers throughout Grand Bahama, however, even where the presumptive competitors are scare or absent, I tentatively ruled out interference as the overall proximal factor. With the two most obvious extrinsic factors essentially eliminated, I propose that the proximal or immediate determinant of substrate distribution in these birds is substrate preference–an innate response tendency to select foliage and avoid bark in the one case, and to accept a balance of foilage and bark in the other. Further, since bypassing a rich and readily accessible food source would be clearly maladaptive under ordinary circumstances, I suggest that populations are currently maintained below carrying capacity and that the foliage preference trait of the Grand Bahama population arose under special circumstances of intensified interference competition during an environmental crisis or series of crises in the past. Individuals which avoid the competition from bark specialists at such times by preferring substrates other than bark should theoretically be favored in natural section and emerge numerically dominant from the evolutionary bottleneck created by the crisis. Similar food crises on Andros would, in the absence of bark—gleaning specialists, produce no comparable selective pressures and permit the resident populations to retain broad general responses to all profitable substrates.

ISSN:0012-9658    

DOI:10.2307/1936702

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

To identify the responses of a Predator to resource variation, age—specific fecundity and survivorship were measured at several food levels for the sessile, predatory ciliate protozoan Tokophrya lemnarum Stein. The major life history characteristics identified were: (1) a direct relationship exists between net reproductive rate and food level, (2) age—specific fecundity closely follows food intake rate (3) each embryo is endowed with a nutrient supply upon birth and (4) an inverse relationship exists between net reproductive rate—food level and life span. Totally starved individuals had the longest mean life spans. Increased survival time at low food levels probably results from low metabolic rate between meals and a slowed accumulation of toxic waste products. Fecundity is directly related to prey number and prey size. It is postulated that this set of adaptations acts to decrease variability of population size in a predator with cannot control its hunting rate and in which prey density must vary greatly during the lifetime of an individual.

ISSN:0012-9658    

DOI:10.2307/1936703

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Data on body size, age/size at sexual maturity, and adult survival were obtained between 1975 and 1978 for a population of bullfrogs, Rana catesbeiana, at the E. S. George Reserve of the University of Michigan. Both males and females continued to grow when adults; older individuals were often 50% larger than newly matured individuals. No sexual differences in adult growth rate were observed, but data on average and maximum adult body size indicated that females were significantly larger than males during each year of the investigation. The discrepancy between these measures resulted from sex—specific differences in adult age distribution: males matured at earlier ages and/or smaller sizes than females, and large males suffered higher mortality than large females. Such life history differences between the sexes can in turn be related to patterns of mating effort and parental effort. Apparently, selection for mating effort in males to defend territories and parental effort in females to produce large clutches has resulted in sexual maturation at different ages but in similar advantages of large body size. The influence of mating effort and parental effort on sexual dimorphism is discussed for other anuran species as well as birds and mammals. It is suggested that an observed body size dimorphism in any species in which individuals grow appreciably as adults could result from a difference in the age distributions of males and females as well as from differential growth rates.

ISSN:0012-9658    

DOI:10.2307/1936704

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

The theory and procedures for using electrically heated taxidermic mounts to measure the standard operative temperature, Tes, of natural habitats have been examined using the American Goldfinch, Carduelis tristis, Tesis a thermal index directly related to the metabolic requirements of a specific endothermic animal. Tesmay be calculated from a thermal energy budget analysis. However, direct measurement of Tesusing heated mounts as “environmental temperature thermometers” is simpler and allows measurements in confined spaces, such as nocturnal roots, which are inaccessible to conventional instrumentation. For the goldfinch, heated mounts predict metabolic heating less evaporative cooling with an uncertainty of ±5% for temperatures below the thermal neutral zone and winds of 0−1.5 m/s.

ISSN:0012-9658    

DOI:10.2307/1936705

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Differences in fire frequency and the rate of secondary succession following fire have had a major effect on the present composition of forest vegetation in a 4500—ha undisturbed watershed in the subalpine zone of the Medicine Bow Mountains, southeastern Wyoming. USA. Periodic fire coupled with slow secondary succession has perpetuated lodgepole pine forest on the upland, while mature Engelmann spruce—subalpine fire forests have developed in sheltered ravines and valley bottoms where fire is less frequent and succession following fire is more rapid and/or more direct. A graphic model is presented showing the relationship between topographic position, fire—free interval, and the occurrence of mature forests dominated by spruce and fire.

ISSN:0012-9658    

DOI:10.2307/1936706

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

A graphical theory of habitat selection is built in steps. The theory treats two species in an environment with two usable patch types in a matrix of unusable space. The first step assumes habitat selection is density independent and free of search costs. The second assumes density independence, and the third assumes neither. The first two steps produce results already known from earlier theories. The third, however, requires a new analytical device, the isoleg, which is a line in a two—dimensional—state space of the two species' densities. An isoleg is a set of points in such a density space, such that on one side of the set, individuals of a species optimize their foraging by being strict habitat selectors, whereas on the other side, they do so by using at least a bit of a poorer patch. The population dynamics of the competitors is analyzed using their isolegs. The isolegs allow us to deduce that the zero isoclines of the species are warped into nonlinear forms capable of producing competitive coexistence. It is shown that at the equilibrium point of this coexistence, there may be no overt competition remaining. The difficulties this presents to the field investigator are mentioned, and a modified definition of interspecific competition is suggested.

ISSN:0012-9658    

DOI:10.2307/1936707

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Web orientation (direction that spiders faced) in a forest—dwelling orb web—building spider, Micrathena gracilis (Araneidae), was related to the microhabitat where webs are constructed. Individuals in shaded (closed) microhabitats faced predominantly north/south, while those in well—lighted microhabitats (open and patchy) faced east/west. Several variables were examined in an effort to explain the patterns that emerged. Wind speeds were found to be so low within the interior of the forest that available instrumentation could not measure them. Nethertheless, webs were not aligned with prevailing winds. There were no significant differences in web site parameters (height, bridge thread distance) between microhabitats. Artificial web studies indicate that prey abundance is greater in open microhabitats and at lower heights, though prey movement through all microhabitats is nondirectional. Spider body temperature, however, was positively correlated with the amount of solar radiation striking the animal, which varied with both microhabitat and direction of orientation. The compass orientation of a spider's web influenced the amount of radiation to which the spider was exposed, and thus affected its body temperature. A model is proposed that relates the time an individual spider can spend on its web to the amount of heat accrued by the spider. Both too little or excessive body heat might affect activity. Since time on the web is positively correlated with total prey captured, more time spent on the web effectively increases the availability of prey. Web orientation is suggested as a behavioral thermoregulatory mechanism which allows Micrathena gracilis enough flexibility to exploit a variety of microhabitats and to maximize its time on the web, thus optimizing its prey take in each habitat.

ISSN:0012-9658    

DOI:10.2307/1936708

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Experiments utilizing epilimnetic water were conducted to determine the rate of ammonium regeneration, due to zooplankton excretion and microbial mineralization processes, in relation to the rate of inorganic nitrogen assimilation by phytoplankton. The euphotic zone of dimictic, meso—oligotrophic Castle Lake is characterized by a rapid depletion of both nitrate and ammonium soon after spring thaw. External inputs of nitrogen are minimal during this period and levels of inorganic nitrogen remain low until fall overturn. The rate of ammonium assimilation was high relative to nitrate assimilation, and was significantly correlated with the rate of ammonium regeneration. The importance of this rapid turnover of nitrogen to phytoplankton growth is consistent with the results of previous Castle Lake studies, and with the current conceptual model of primary production in the mixed layer of nitrogen—deficient marine ecosystems.

ISSN:0012-9658    

DOI:10.2307/1936709

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

Some statistical “distributions” which, when plotted on an “arithmetical” basis, do not in the least resemble a normal or Gaussian distribution, become very similar to one when plotted on a logarithmic or “geometrical” basis. In this paper I examine in particular the difference of two declining exponential curves, and, as a special case, a simple single declining exponential. On a logarithmic base these curves are somewhat skewed, but none the less are very close to a lognormal, and nearly all the variance can be accounted for (something like 99%), but there seems to be a limited range of “standard deviations” that can be accommodated. In human ecology, for instance, women's age at first marriage, which among western nations is known to be close to a lognormal, can be slightly better fitted by the difference of two declining exponentials, and a physical interpretation can be given to the latter, though not so easily to the former. The same thing is true of the failure of industrial products under repeated impact, or under prolonged stress. The apparently lognormal distribution of commonness and rarity differs from these in having a much greater value of the standard deviation, and it is not so easily explained. This may be because it is, in fact, a totally different phenomenon, but there is also a possibility that some modification or extension of the present paper may point the way to an explanation.

ISSN:0012-9658    

DOI:10.2307/1936710

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY

摘要:

We examined the seasonal importance of vertebrate predators in potentially regulating the abundance and diversity of the benthic macroinvertebrates in the littoral zone of a soft—bottom reservoir that receives thermal effluent from a nuclear production reactor. Thirty—six predator (fish and turtle) exclusion cages (4 m2) were placed in shallow water at six locations along a thermal gradient in Par Pond, a 1100—ha cooling reservoir on the Savannah River Plant near Aiken, South Carolina, USA. An additional 36 control plots (4 m2) were also set up. Cages were in place during three, 3—mo test periods beginning in September 1977. Estimates of benthic density, taxon richness, and distribution within functional groups (defined by feeding mechanism) were calculated for each test period. Effects of temperature on predator—prey relationships were also determined. Experimental results of this study suggest that vertebrate predation was not the fundamental parameter organizing the benthic macrionvertebrate community in the littoral zone of this reservoir. Neither taxon richness nor density of total macroinvertebrates was conclusively related to predator treatment. Relationships between predator treatment and community response (changes in density and taxon richness) were generally unaffected by either plot locality, temperature fluctuations from thermal effluent, or seasonal changes. When data from caged and control plots were pooled, however, both location and water temperature individually had direct impacts on the benthic community. From our results and other field studies we hypothesize that individual species of "keystone" benthic predators (Paine 1969b) do not occur in the littoral zone of freshwater lentic environments with soft bottoms. This hypothesis is based on the following four arguments. First, environmental heterogeneity should reduce predator efficiency relative to that found in rocky intertidal systems where keystone predation may be present. Second, relative food web complexity should reduce the community importance of an individual predator species. Third, the apparent relatively greater resource partitioning of space and food in the freshwater littoral zone hinders monopolization of rate—limiting environmental resources by a single dominant prey species. Fourth, the effects of predation may not be extensive enough in time or area to provide sufficient resource space (and thus food) for exploitation by fugitive species. Possible regulation by a guild of predators and cases where the general hypothesis might be falsified are discussed.

ISSN:0012-9658    

DOI:10.2307/1936711

出版商:Ecological Society of America    

年代:1981

数据来源: WILEY