摘要:

Mathematical models and empirical studies have revealed two potentially disruptive influences on ecosystems; (1) instabilities caused by nonlinear feedbacks and time—lags in the interactions of biological species, and (2) stochastic forcings by a fluctuating environment. Because both of these phenomena can severely affect system survival, ecologists are confronted with the question of why complex ecosystems do, in fact, exist. Our study analyzes the basic themes of this research and identifies five general hypotheses that, in recent years, theoretical ecologists have built into models to increase their stability against disruptive feedback and stochasticity. To counter feedback instabilities, theoreticians have considered (1) functional interactions between species that act as stabilizers, (2) disturbance patterns that interrupt adverse feedback effects, and (3) the stabilizing effect of integrating small—spatial—scale systems into large landscapes. To decrease the influence of stochasticity, modelers have hypothesized (4) compensatory mechanisms operating at low population densities, and (5) the moderating effect of spatial extent and heterogeneity. We show that modeling based on these ideas can be organized in a systematic way. We also show that the stable equilibrium state should not be viewed as a fundamental property of ecological systems, but as a property that can emerge asymptotically from extrapolation to sufficiently large spatial scales.

ISSN:0012-9615    

DOI:10.2307/1942636

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

The self—thinning rule predicts that for a crowded even—aged plant population a log—log plot of average plant mass vs. plant density will reveal a straight "self—thinning" line of slope–3/2. The rule is supported by examples from many individual populations, and by the existence of an interspecific relationship that yields a line of slope–3/2in a log—log plot displaying average mass and density data from many populations of different species. I examined and reanalyzed the evidence to evaluate the strength of support for this widely accepted rule. Some problems in fitting thinning lines and testing agreement with the rule have no truly satisfactory solution, but three improvements on commonly used methods were made: the analysis related stand biomass density to plant density because the alternative of relating average plant mass to plant density is statistically invalid; principal components analysis was used rather than regression, because regression relies on unrealistic assumptions about errors in the data; and statistical tests of hypotheses were used to interpret the results. The results of this reanalysis were that 19 of 63 individual—population data sets previously cited in support of the thinning rule actually showed no significant correlation between stand biomass density and plant density, and 20 gave thinning slopes significantly different (P<.05) from the thinning rule prediction. Four other analyses provided additional evidence against a single quantitative thinning rule for all plants: slopes of the thinning lines were more variable than currently accepted, differed significantly among plant groups, were significantly correlated with shade tolerance in forest trees, and differed among stands of the same species. The same results held for the intercepts of self—thinning lines. Despite the failure of the thinning rule for individual populations, the combined data for all populations are still consistent with an interspecific relationship of slope–3/2; therefore, the existence of the interspecific relationship does not necessarily support the within—population thinning rule. The within—population and interspecific relationships are apparently different phenomena that may arise from different constraints, so the two relationships should be tested and explained separately..

ISSN:0012-9615    

DOI:10.2307/1942637

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

Pregnant females of the viviparous lizard species Sceloporus jarrovi regulate their body temperatures at a lower level than do males or nonpregnant females. It has been suggested that such a shift in preferred body temperature during pregnancy reflects the presence of divergent optimal temperatures for the female and for development of her young; a pregnant female must compromise between these temperatures in order to maximize her fitness. We examine this hypothesis using a Leslie matrix model of life history that quantitatively predicts the mean body temperature that would optimally compromise between conflicting thermal optima for mother and embryos. The predictions of the model are in close agreement (0.4°C or less) with temperatures observed in the field. According to the model, a pregnant female maintaining the mean body temperature typical of males or nonpregnant females would have ≈13% lower fitness than a pregnant female maintaining the optimal temperature. This is mainly due to increased embryo mortality, but reduced growth and increased mortality of the female also contribute to the loss of fitness. In the model, the optimal mean body temperature depends on the precision of thermoregulation. Thermoregulation by gravid females in the field is imprecise, with a standard deviation of 1.4° during active thermoregulation. If females are (unrealistically) assumed to maintain a perfectly constant body temperature (i.e., a standard deviation of 0°), the optimal mean temperature in the model is ≈2° higher than the mean temperature of gravid females in the field. Although specifically designed to incorporate features of the physiology and life history of Sceloporus jarrovi, the model can be generalized and applied to other situations involving compromise among multiple optima.

ISSN:0012-9615    

DOI:10.2307/1942638

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

The effects of fish predation and food availability on population densities and demography of zooplankton were investigated in Dynamite Lake, Illinois, USA, a lake with a high density of size—selective planktivorous fish and low food levels. Fish predators (bluegill sunfish) and food levels (phytoplankton) were manipulated in replicated, factorial—design field experiments during two summers (1980 and 1981). Overall, population densities of zooplankton were affected much more by manipulations of food availability than by manipulations of fish predation. The cladocerans Bosmina longirostris, Ceriodaphnia lacustris, and Diaphanosoma birgei were greatly increased in density by elevated phytoplankton levels in both years, in the presence and absence of fish. Demographic analysis in 1981 revealed that increased densities in response to elevated food levels resulted from both an increase in birth rates (Bosmina, Diaphanosoma) and a decrease in mortality rates (Ceriodaphnia, Diaphanosoma). The rotifers Lecane and Monostyla also increased dramatically in response to elevated phytoplankton densities. Copepods were less responsive to manipulations of food levels, but several taxa exhibited increases in density in response to increased phytoplankton abundance. Few species were reduced in density by fish predation. Ceriodaphnia density was reduced by fish more than any other species in the entire community, and the density of even this species was much more affected by food availability. In terms of percent change relative to controls, increased food availability had much more of an effect than fish predation on the density of most zooplankton species and on total zooplankton abundance. Fish predation had several effects on the size structure and life history traits of the cladocerans. All three species attained larger sizes when fish were excluded than when fish were present. Cladoceran individuals also initiated reproduction at a smaller size and produced smaller offspring in the presence of fish. The reductions in mean body size, size at maturity, and offspring size in the presence of fish were most pronounced in Diaphanosoma and Ceriodaphnia, the two largest species. Smaller body size and size at maturity apparently allow the cladocerans to reproduce before reaching a size at which they become vulnerable to size—selective fish predators. Larger size at first reproduction and larger offspring size in the absence of fish may be a response to invertebrate predators, which assume more importance in the absence of fish and prey most heavily on smaller size classes. Flexibility in these life—history traits allows the cladocerans to withstand what appears to be intense size—selective predation by planktivorous fish.

ISSN:0012-9615    

DOI:10.2307/1942639

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY

摘要:

The subtidal region at Cape Banks, New South Wales, Australia, is characterized by large areas covered solely by encrusting coralline algae in association with large densities of invertebrate grazers. Orthogonal removals of these grazers: the sea urchin Centrostephanus rodgersii; the limpets Patelloida alticostata, P. mufria, and Cellana tramoserica; and the turbinids Australium tentiforme and Turbo torquata, were done at two sites and two times to investigate the interactions among these species and their effects on the algal community. These manipulations had large effects on both the algal and animal communities, although the type and intensity of these interactions altered at different temporal and spatial scales. Initially, the removal of urchins and/or all species of limpets resulted in a rapid increase in the amount of noncrustose algae in these areas. Removal of turbinids, however, had no significant effect. Where both limpets and urchins had been removed, the cover of foliose algae continued to increase quickly and was ≈80—100% after 12 mo. In areas where only urchins were removed, the increase in the cover of algae was slower, and only approached 100% after 18—24 mo. Where only limpets had been removed, 3 mo after an initial increase in the cover of filamentous algae the cover declined to be only slightly more than in control areas. These results were similar at all sites and times. In one series of removals, the effects of some of the species of limpets were separated. In this case, the increase in the cover of algae was found to be fastest when only P. mufria was left (the urchins and other limpets removed), than when all limpets and urchins had been removed. Initially, the removal of the urchins generally resulted in a large increase in the recruitment and density of the limpets. Approximately 1 yr following the removal of the urchins,however, the density of adult and juvenile limpets of all species had declined to near zero. Removal of the larger species of limpets, P. alticostata and Cellana, also increased the rate of recruitment of the smaller species, P. mufria. Thus, these invertebrate grazers, especially the sea urchins, appeared to be necessary for the maintenance of the areas of crustose algae. Similarly, the sea urchins were found to be necessary for the continued presence of the limpets within these areas. The strong effects of the urchins on this community, and the application of the situation to theories concerning keystone species and facilitation, are discussed in relation to the important but more subtle effects of the limpets.

ISSN:0012-9615    

DOI:10.2307/1942640

出版商:Ecological Society of America    

年代:1987

数据来源: WILEY