Ecology

May, Robert M.

doi: 10.2307/1938668pmid: N/A

Mangel, Marc; Clark, Colin W.

doi: 10.2307/1938669pmid: N/A

In the broadest sense, foraging adaptations can include problems of finding food, avoiding predation while looking for food, and reproducing. In this paper, a theory that treats these three behaviors in a consistent, unified manner, with one common currency, is presented. The theory is called unified foraging theory, although it actually pertains to a wider class of behavioral problems. The theory is based on models using Markovian decision processes and leads to quantitatively testable predictions about behavioral strategies.

Hopf, F. A.; Brown, James H.

doi: 10.2307/1938670pmid: N/A

We present a method for examining quantitative ecological data to determine whether they can be regarded as being random in character. This method allows direct comparison of two or more data sets containing different numbers of species. This permits analysis of large statistical ensembles of data, often resulting in conclusions with high statistical confidence. The method is used to demonstrate that abundances of species within communities are not distributed at random. Body mass ratios are also shown to be nonrandom in communities of desert rodents, but appear to be random in guilds of breeding birds. This method is shown to be superior to the Barton and David (1956) mathematics recently used by Simberloff and Boecklen (1981).

Meyer, Joseph S.; Ingersoll, Christopher G.; McDonald, Lyman L.; Boyce, Marks S.

doi: 10.2307/1938671pmid: N/A

Although per capita rates of increase (r) have been calculated by population biologists for decades, the inability to estimate uncertainty (variance) associated with r values has until recently precluded statistical comparisons of population growth rates. In this study, we used two computer—intensive techniques, jackknifing and Bootstrapping, to estimate bias, standard errors, and sampling distributions of r for real and hypothetical populations of cladocerans. Results generated using the two techniques, using data on laboratory cohorts of Daphnia pulex, were almost identical, as were results for a hypothetical D. pulex population whose sampling distribution was approximately normal. However, for another hypothetical population whose sampling distribution was negatively skewed due to high juvenile mortality, Bootstrap and full—sample estimates of r were negatively biased by 3.3 and 1.8%, respectively. A bias adjustment reduced the bias in the Bootstrap estimate and produced estimates of r and se(r) almost identical to those of the Jackknife technique. In general, our simulations show that the Jackknife will provide more cost—effective point and interval estimates of r for cladoceran populations, except when juvenile mortality is high (at least >25%). Coefficients of variation in the mean of r within laboratory cohorts of D. pulex were one—half to one—third the magnitude of the corresponding coefficients of variation in the mean of total reproduction and in the mean day to death (range of values of cv°[r] = 1.6 to 3.8%). This suggests that extremes in reproductive output and survival of individuals tend to be dampened at the population level, and that within—cohort variability in r is not explosive. Moreover, between—cohort variability in r can be much greater than within—cohort variability, as indicated by a statistically significant difference of 30% (P ° .01) between the high and low r values that were computed for four cohorts of D. pulex born during a 1—mo period from the same laboratory stock population. based on variability in per capita rates of increase that have been estimated for several cladoceran species, we suggest that the precision for reporting r values should in most cases be limited to two significant figures.

ter Braak, Cajo J. F.

doi: 10.2307/1938672pmid: N/A

A new multivariate analysis technique, developed to relate community composition to known variation in the environment, is described. The technique is an extension of correspondence analysis (reciprocal averaging), a popular ordination technique that extracts continuous axes of variation from species occurrence or abundance data. Such ordination axes are typically interpreted with the help of external knowledge and data on environmental variables; this two—step approach (ordination followed by environmental gradient identification) is termed indirect gradient analysis. In the new technique, called canonical correspondence analysis, ordination axes are chosen in the light of known environmental variables by imposing the extra restriction that the axes be linear combinations of environmental variables. In this way community variation can be directly related to environmental variation. The environmental variables may be quantitative or nominal. As many axes can be extracted as there are environmental variables. The method of detrending can be incorporated in the technique to remove arch effects. (Detrended) canonical correspondence analysis is an efficient ordination technique when species have bell—shaped response curves or surfaces with respect to environmental gradients, and is therefore more appropriate for analyzing data on community composition and environmental variables than canonical correlation analysis. The new technique leads to an ordination diagram in which points represent species and sites, and vectors represent environmental variables. Such a diagram shows the patterns of variation in community composition that can be explained best by the environmental variables and also visualizes approximately the "centers" of the species distributions along each of the environmental variables. Such diagrams effectively summarized relationships between community and environment for data sets on hunting spiders, dyke vegetation, and algae along a pollution gradient.

Caraco, Thomas; Gillespie, Rosemay G.

doi: 10.2307/1938673pmid: N/A

We present a stochastic model for foraging mode, where the mode is idealized as a "sit—and—wait" strategy or as a strategy of active movement between foraging sites. We direct the model's assumptions toward the behavior of certain (female) orb—web spiders, where both temporal and spatial stochasticity govern the number of prey taken by a predator. At a given web site, prey are captured randomly through time according to a Poisson process. Spatial heterogeneity is incorporated by allowing the probabilistic rate of prey capture to vary randomly among potential web sites. We assume that an efficient predator should minimize the probability of capturing fewer prey than required to survive and reproduce during a single season. The model shows that in the same environment the two strategies yield the same expected number of captured prey, but the variance in the number of prey captured is greater for the sit—and—wait predator. An approximation to the model's solution indicates that a predator should be risk—averse and employ the mobile strategy in an environment where the expected number of prey captures exceeds the requirement. But in an environment where the requirement exceeds the expected number of prey captures, a predator should be risk—prone and employ the sit—and—wait strategy.

Armitage, Kenneth B.

doi: 10.2307/1938674pmid: N/A

Current theory suggests that population dynamics are the consequence of the reproductive strategies of individuals. Individual differences should be expressed in reproductive output, dispersal, social behavior, and recruitment. Mirror—image stimulation (MIS; i.e., exposure of the animal to a large mirror) was used as an independent measure of individuality, which could be distributed continuously or which could be grouped into two or more types. Three axes derived from a factor analysis of behavioral data obtained during MIS accounted for 85% of the variance among individual marmots. The rank order of 19 adult females on each of the three MIS axes was not correlated with the rank order of lifetime reproductive success measured as number of young weaned, number of yearlings produced, or number of young or yearlings produced per year of residency. This result suggests that individual differences are not continuous. Each female was assigned to one of three groups according to the MIS axis on which she had her highest factor score. Rankings for the number of female yearlings, number of recruits, and number of 2—yr—old resident daughters varied significantly among the MIS groups. Mean values of these measures were highest for females in the "sociability" group. Although none of eight measures of lifetime social behavior for 18 females was significantly related to the three MIS groups, several measures of lifetime amicable behavior were correlated with the production and recruitment of female yearlings. Behavior in the field is affected not only by individual behavioral phenotypes, but also by kinship and patterns of space use. Marmots may have a strategy of phenotypic plasticity. By producing young of varied phenotypes, a female increases the probability that over the long term some of her descendants will survive in varied and unpredictable social and ecological environments.

Haydock, Joseph; Ligon, J. David

doi: 10.2307/1938675pmid: N/A

Hypotheses concerning the adaptive significance of asynchronous hatching in relation to brood reduction were tested by manipulating broods of Chihuahuan Ravens (Corvus cryptoleucus) in southeastern New Mexico in 1981 and 1982. Breeding success was also monitored at the study site in 1978 and 1979. Manipulations included: (1) exchange of nestlings between nests to obtain nests with five or six nestlings of nearly equal size, (2) transfer of the largest nestling from one nest to another where it was approximately equal in mass to the smallest nestling, and (3) transfer of the smallest nestling from one nest to another nest where it was approximately equal in mass to the largest nestling. In control nests, the most productive clutch size was larger than the most common clutch size. Growth rates and masses for nestlings that fledged were similar in most cases, regardless of clutch size or hatch position. Fledging success was significantly higher in the experimental broods with equal—sized nestlings than in the control nests in which six young hatched in 1981. However, in 1982, fledging success was nearly equal in the experimental and control nests. Although growth rates were not significant lower, fledgling masses were significantly lower in the experimental broods with equal—sized nestlings. Smallest nestlings transferred to a largest nestling position in another nest fledged as well as the largest nestlings in control nests. Largest nestlings transferred to a smallest nestling position fledged as rarely as smallest nestlings in control nests. Lack's hypothesis that asynchronous hatching allows for adjustment of brood size is supported. However, our limited experimental data suggest that more young often could be fledged if hatching wee more synchronous.

Brown, Charles R.; Brown, Mary Bomberger

doi: 10.2307/1938676pmid: N/A

Colonially nesting Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwestern Nebraska, USA, are commonly parasitized by hematophagous swallow bugs (Hemiptera: Cimicidae: Oeciacus vicarius) and fleas (Siphonaptera: Ceratophyllidae: Ceratophyllus celsus). We examined to what degree these ectoparasites represent a cost of coloniality for Cliff Swallows. The number of swallow bugs per nest increased significantly with Cliff Swallow colony size. Body mass of nestling swallows at 10 d of age declined significantly as the number of bugs per nestling increased. By fumigating half of the nests in some colonies, killing the bugs, and leaving half of the nests as nonfumigated controls, we showed that swallow bugs lower nestling body mass and nestling body mass and nestling survivorship in large Cliff Swallow colonies but not in small ones. Bugs cost nestlings, on average, up to 3.4 g in body mass, and reduced survivorship by up to 50%. Parasitism by fleas showed no consistent relationship with colony size during the nestling period but increased significantly with colony size early in the season, when birds were first arriving in the study area. Flees did not affect nestling body mass or survivorship and thus, unlike swallow bugs, are probably not important costs of coloniality to Cliff Swallows. Field observations and nest fumigation experiments showed that Cliff Swallows apparently assess which nests are heavily infested with swallow bugs early each spring and select parasite—free nests, leading sometimes to alternate—year colony site usage. Cliff Swallows were more likely to construct new nests (rather than reusing old ones) in large colonies than in small colonies, probably in response to heavier infestations of ectoparasites in the existing nests of large colonies.

Richardson, Howard; Verbeek, Nicolaas A. M.

doi: 10.2307/1938677pmid: N/A

Northwestern Crows (Corvus caurinus) feed extensively on littleneck clams (Venerupis japonica) despite the high cost in time and energy incurred in digging up the clams opening them, and then extracting the edible parts. Casual observation also indicated that the crows fed preferentially on the larger clams. The purpose of this study was to quantify this apparent selectivity, to determine the energetics of the foraging process, and to compare the birds' net energy gains with the gains predicted by a random encounter model. The overall length of clams in the beach ranged from 10.0 to 43.0 mm. All sizes broke equally easily (X = 1.7 drops per clam), but the time taken to extract the meat from broken clams increased with size. Despite this, net energy gain per unit of handling time was an increasing function of clam size. Foraging crows ate clams between 24.0 and 43.0 mm long. They also dug up but rejected clams ranging from 13.0 to 35.0 mm long. The lower limit of the crows' diet, taken as the size which was eaten 50% of the times encountered, was 29.0 mm. We calculated that a diet consisting of clams @> 28.5 mm long yielded the maximum possible rate of net energy gain. These data clearly show that crows not only feed selectively on large clams but that their selectivity results in a net energy intake rate close to the maximum attainable.