摘要:

Lotka—Volterra (LV) equations have been used extensively to explore the possible dynamic outcomes of interspecific competition. But while there have been hundreds of papers on the mathematical properties of Lotka—Volterra models, there have been only a handful of papers that explore techniques for fitting these models to actual data, and no papers that explore the interface of experimental design and statistical inference when fitting LV equations to census data. In this paper we present a statistical analysis of Gause's experimental cultures of Paramecium aurelia and P. caudatum, using analytical methods based on maximum likelihood and Bayesian statistics. We compare the effectiveness of these two approaches in addressing several questions about competition from experimental data: Are the mutual effects of competing populations substantial? Are these competitive effects symmetrical? Are two populations expected to coexist or to eliminate each other by competition? We show that even a laboratory—derived data set with minimal variability can entail significant levels of uncertainty about the nature of the competitive interaction. We assess the errors involved in estimating the strength and symmetry of competition, and find that one's conclusions depend critically on assumptions about sources of variability in the data. We also estimate the probabilities of alternative dynamic behaviors for competing species. We use simulations to evaluate how particular experimental designs might improve our power to characterize the dynamic outcome of competition. We show that much more information is gained by running competition experiments at different starting conditions than by replicating the same experiment for a particular starting condition.

ISSN:0012-9658    

DOI:10.2307/2265613

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The relationships between biodiversity and stability were determined for both population and ecosystem traits in a long—term study of 207 grassland plots. Results demonstrate that biodiversity stabilizes community and ecosystem processes, but not population processes. Specifically, year—to—year variability in total aboveground plant community biomass was significantly lower in plots with greater plant species richness both for the entire 11—yr period and for the nine non—drought years. The change in total plant community biomass from before the drought to the peak of the drought was also highly dependent on species richness. For all three measures of total community biomass stability, multiple regressions that controlled for covariates showed similar significant relationships between plant diversity and stability. In contrast, year—to—year variability in species abundances was not stabilized by plant species richness for either all years or non—drought years. This difference between species vs. community biomass likely results from interspecific competition. When climatic variations harm some species, unharmed competitors increase. Such compensatory increases stabilize total community biomass, but cause species abundances to be more variable. These results support both the predictions of Robert May concerning the effects of diversity on population stability and the diversity—stability hypothesis as applied to community and ecosystem processes, thus helping to reconcile a long—standing dispute.

ISSN:0012-9658    

DOI:10.2307/2265614

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

We examined the spatial distribution of soil nutrients in desert ecosystems of the southwestern United States to test the hypothesis that the invasion of semiarid grasslands by desert shrubs is associated with the development of "islands of fertility" under shrubs. In grasslands of the Chihuahuan Desert of New Mexico, 35—76% of the variation in soil N was found at distances<20 cm, which may be due to local accumulations of soil N under Bouteloua eriopoda, a perennial bunchgrass. The remaining variance is found over distances extending to 7 m, which is unlikely to be related to nutrient cycling by grasses. In adjacent shrublands, in which Larrea tridentata has replaced these grasses over the last century, soil N is more concentrated under shrubs and autocorrelated over distances extending 1.0—3.0 m, similar to mean shrub size and reflecting local nutrient cycling by shrubs. A similar pattern was seen in the shrublands of the Mojave Desert of California. Soil PO_4, Cl, SO_4, and K also accumulate under desert shrubs, whereas Rb, Na, Li, Ca, Mg, and Sr are usually more concentrated in the intershrub spaces. Changes in the distribution of soil properties may be a useful index of desertification in arid and semiarid grasslands worldwide.

ISSN:0012-9658    

DOI:10.2307/2265615

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Phenomena such as disturbance play a major role in structuring ecological systems by producing a spatiotemporal mosaic of patches at different successional states. The distribution of species within the resulting mosaic depends upon an interaction between species' life history traits and the spatial and temporal structure of the ecological processes controlling species' distributions. We have used a spatially explicit simulation model (Jasper) of a serpentine grassland to examine the importance of some of these relationships, focusing primarily on the role of disturbance. The model Jasper is hierarchical in design and was developed to simulate the population dynamics of three interacting plant species: Bromus mollis, Calycadenia multiglandulosa, and Plantago erecta. Population dynamics were modeled as occurring within local sites, which were then arranged in a square array to form a landscape. Connections among sites within a landscape were made primarily through seed dispersal. Several components of disturbance architecture were varied systematically among model runs to determine their impact on population dynamics at the scale of the landscape. We considered three levels of organization in modeling disturbance: (1) overall rate of disturbance, (2) size of individual disturbances, and (3) temporal and spatial autocorrelation among individual disturbances. The results demonstrate that the impact of disturbance depends upon a complex interaction between the life history characteristics of the species making up the community and the spatial and temporal structure of the disturbance regime. For example, we found that the biggest impact on species abundance occurred in response to a shift in the temporal autocorrelation structure of the disturbance regime. Also, species diversity was found to increase at intermediate levels of disturbance (as has been shown in several other studies). However, what can be considered an intermediate level of disturbance depends as much upon the temporal autocorrelation structure of the disturbance regime as it does upon the absolute rate of disturbance. These results suggest that predicting the impact of disturbance on ecological communities will require an explicit understanding of at least some aspects of the spatial and temporal architecture of the disturbance regime.

ISSN:0012-9658    

DOI:10.2307/2265616

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

We manipulated, in accord with global—warming predictions, the thermal regime of a permanent first—order stream near Toronto, Ontario, Canada. We examined the effects of a 2—3.5°C water—temperature increase on densities, biomass, species composition, and life histories of resident stream invertebrates. The stream was divided longitudinally at the source into two channels, one control and one experimental, and a before and after (BACI) design was employed such that one pre—manipulation year was followed by 2 yr of the temperature manipulation. Changes in the experimental channel following commencement of the manipulation included: (1) decreased total animal densities, particularly Chironomidae (Diptera); (2) earlier onset of adult insect emergence; (3) increased growth rates and precocious breeding in Hyalella azteca (Amphipoda); (4) smaller size at maturity for Nemoura trispinosa (Plecoptera) and H. azteca; and (5) altered sex ratios for Lepidostoma vernale (Trichoptera). These results partially corroborate previous laboratory and field studies. However, variation in the responses of individual target species to the manipulation was unexpected and may have been influenced by the genetic structure of local populations. We conclude that levels of gene flow among habitats may be critical to the degree of impact seen as a result of large—scale thermal perturbation (e.g., global warming).

ISSN:0012-9658    

DOI:10.2307/2265617

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Mechanisms underlying competitive effects between two species of marine snails (Tegula aureotincta and Tegula eiseni) were examined; results indicated that the interaction involved exploitation of microalgae and not interference. A behavioral experiment demonstrated that an individual of either species was less likely to feed on a recently grazed patch when the prior forager was T. aureotincta. Both snails responded only to the amount of food available on a patch, and, except at very low food levels, the likelihood that a snail would remain to feed was identical for the two species across microalgal densities. Where food was scarce, T. eiseni was more likely than T. aureotincta to feed in that patch. Density—dependent effects on microalgal standing stock differed between the snails; at low grazer densities, T. aureotincta depressed food levels to a comparatively greater degree, whereas at high grazer densities, T. eiseni had a relatively greater effect. The relative growth performance of the snails differed as a function of available food: T. aureotincta had superior body growth at high resource densities, while T. eiseni grew comparatively better at low microalgal densities. Differences in effects on and growth responses to microalgae arose from dissimilar foraging characteristics of the snails. Compared with T. eiseni, T. aureotincta moved more rapidly while feeding, harvested twice as great an area per unit time, but was unable to crop microalgae as close to rock surfaces. While T. eiseni grazed a smaller area per unit time, it removed relatively more microalgae per area grazed. These differing suites of foraging attributes may represent a trade—off to harvest food at different densities: cropping closer to the substratum (area intensive grazing) yields more food when microalgae are scarce, but precludes grazing a larger area per unit time (area extensive grazing) through fast movement, which provides a greater food intake when microalgae are abundant. If an area extensive (moves faster)—intensive (digs deeper) trade—off is a common feature of benthic grazer systems, knowledge of this aspect could provide great predictive insight into the patterns and causes of density dependence arising from exploitation.

ISSN:0012-9658    

DOI:10.2307/2265618

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The pattern of prey distribution can profoundly affect the foraging behavior and success of a predator. In pelagic marine ecosystems, where prey is often patchily distributed, predators must be able to adapt quickly to changes in the spatial patterning of prey. Antarctic fur seals feed primarily on krill, which is patchily distributed. When combined with information about swimming speed on the surface, the time taken for a fur seal to locate a new patch after leaving an old one is an indication of the distance between patches. The frequency distribution of intervals between bouts of foraging showed that fur seals foraged at two spatial distributions: (1) a fine—scale (median distance 0.18—0.27 km) represented by short (5 min) travel durations. In a study lasting 5 yr, the distributions of travel durations between bouts of feeding changed between years. These changes suggested that the structure and/or the spatial distribution of krill swarms varied between years. The behavior of fur seals suggested that there was overall clumping of prey at the fine—scale, but there was a more even spacing of prey patches at the meso—scale level. Only in 1 yr of the study (1990/1991) were there indications that fur seals had difficulty in finding enough food. Fur seal behavior suggested that there was no reduction in the number of prey patches available in that year but that prey patches were of poorer quality. The study showed how predator behavior can provide valuable information about the functional relationship between prey dispersion and predator performance.

ISSN:0012-9658    

DOI:10.2307/2265619

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Much effort has focused on modeling and measuring the energy costs of free existence and the foraging strategies of animals. However, few studies have quantitatively linked these approaches to the patch structure of foods in the field. We developed an individual—based model that relates field measurements of the dispersion of benthic foods to search costs and foraging profitability of diving ducks. On Lake Mattamuskeet, North Carolina, Canvasback ducks (Aythya valisineria) eat only the belowground winter buds of the submerged plant Vallisneria americana. We measured and modeled the patch structure of winter buds at the level of potential foraging "loci," defined as contiguous circles 1 m in diameter. In the field and in the model, Canvasbacks make repeated vertical dives in such loci, foraging in the sediments by touch, before surface—swimming to another locus. We quantified first—order patchiness by fitting a negative binomial distribution to core samples taken at 50—m intervals along transects, to yield the frequencies of loci with different bud densities. Second—order patchiness was measured by taking cores at 1—m increments radiating from each sampling point, and regressing bud density at each sampling point on densities at these increments. No significant correlations were found, indicating that Canvasbacks could not predict food densities based on densities in nearby foraging loci. For the model, we generated food grids from the negative binomial distributions of core samples. Energy costs of diving were calculated by applying aerobic efficiencies (mechanical power output $\div$ aerobic power input) to biomechanical models. Unlike respirometry alone, this method accounts for effects on dive costs of varying water depth and dive duration. We used measurements of Canvasback intake rates at different bud densities to calculate profitability (energy intake minus expenditure) for each dive. Multivariate uncertainty analyses (Latin hypercube) indicated that profitability for Canvasbacks foraging on Vallisneria buds is determined mainly by food—item size and locomotor costs of descent. Bud metabolizable energy, water temperature, bud dispersion, and search and handling time coefficients of the functional response for intake rate have relatively minor influence. Individual—parameter perturbations indicated that to maintain the same foraging benefits, the total area of Vallisneria habitat would have to increase by 1.4—fold if dry mass per bud decreased from 0.10 to 0.03 g, and by 2.1—fold if water depth increased from 0.5 to 2 m. Our method allows study of interactions between patch structure and foraging energetics without detailed spatial mapping of foods, which is not feasible at appropriate scales for highly mobile benthivores. The model yields estimates of energy balance, contaminant intake, and amount and quality of foraging habitat required to sustain diving duck populations under varying environmental conditions. More accurate prediction of giving—up times and giving—up food densities will require better understanding of the time scale over which ducks balance their energy budgets.

ISSN:0012-9658    

DOI:10.2307/2265620

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Why do birds partition habitats? Possible explanations include nonoverlapping food distributions, interspecific competition, and predation. If food limits distributions, species should forage most successfully in their preferred habitats and should experience poorer food intake rates elsewhere. These same predictions hold if species distributions are shaped by exploitative competition. We tested the foraging success hypothesis and exploitative competition hypothesis as explanations of the habitat distributions of Sage Sparrows (Amphispiza belli), Black—throated Sparrows (A. bilineata), and Dark—eyed Juncos (Junco hyemalis), which spend the winter in different habitats along an elevational gradient in the Sonoran Desert of southern California. Individuals of each species were transplanted between habitats in a portable aviary and observed while foraging on naturally occurring seeds. Predictions of the foraging success hypothesis and exploitative competition hypothesis were not confirmed. Only Dark—eyed Juncos achieved their highest foraging success in their preferred habitat. Black—throated Sparrows experienced nearly identical foraging success in all three habitats, yet are confined to only one of them naturally. Sage Sparrows foraged more successfully in habitats other than that in which they naturally occur. We conclude that immediate foraging gains do not explain habitat distributions of these birds, and that interference competition, predation, or other unknown factors are responsible. We contrast this finding with that seen in Galapagos finches in which bird distributions closely matched food supply.

ISSN:0012-9658    

DOI:10.2307/2265621

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

According to life history theory, adult mortality during the breeding season may have an important influence on the evolution of several aspects of breeding ecology in birds, yet few studies have tried to quantify such mortality. We studied disappearance of Pied Flycatchers (Ficedula hypoleuca) during four breeding seasons in a woodland area in Norway provided with nest boxes. The main cause of disappearance was probably predation by the European Sparrowhawk (Accipiter nisus). Disappearance was nonsignificantly higher in females (10% per season, n = 305) than in males (7% per season, n = 269). Female disappearance peaked during egg—laying (0.53% per day), but was also high during the nest—building (0.42% per day) and nestling (0.36% per day) stages. It was low during incubation (0.05% per day), probably because less time was spent outside the nest. Low risk of predation during incubation may help to explain why female body mass remains high during this stage of breeding but drops soon after hatching. Females with selected flight feathers experimentally removed to simulate molt suffered a much higher disappearance per season (24%, n = 109) than did control females (10%, n = 305). This may help to explain why breeding and molt usually are temporally segregated activities in birds. Variation in female body mass and size (wing length, tarsus length), age, previous breeding experience, mating date, laying date, clutch size, and mating status could not account for the variation found in female disappearance. Disappearance was lower in males than in females during the nest—building period, despite the more conspicuous plumage color of males. This may be explained by the fact that only the female builds the nest. We suggest that risk of predation is an important constraint on sexual selection of male plumage color in species in which males take part in nest building.

ISSN:0012-9658    

DOI:10.2307/2265622

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY