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Resource Variability (resource + variability)
Selected AbstractsResource variability, aggregation and direct density dependence in an open context: the local regulation of an African elephant populationJOURNAL OF ANIMAL ECOLOGY, Issue 1 2008Simon Chamaillé-Jammes Summary 1An emerging perspective in the study of density dependence is the importance of the spatial and temporal heterogeneity of resources. Although this is well understood in temperate ungulates, few studies have been conducted in tropical environments where both food and water are limiting resources. 2We studied the regulation of one of the world's largest elephant populations in Hwange National Park, Zimbabwe. The study period started in 1986 when the population was released from culling. Using census data we investigated changes in elephant abundance with respect to rainfall and density across the entire park and across waterholes. 3The population more than doubled since culling stopped. The population increased continuously during the first 6 years, and then fluctuated widely at about 30 000 individuals. Immigration processes must have been involved in the increase of the population size. 4Population growth rates were negatively related to previous population density by a convex relationship, and negatively related to the ratio of previous population density on annual rainfall by a linear relationship. However, only this latter model (i.e. assuming a fluctuating carrying capacity related to annual rainfall) produced realistic dynamics. Overall, population decreased during dry years when the elephant density was high. 5During dry years there were fewer waterholes retaining water during the dry season and consequently elephant numbers at waterholes increased, while their aggregation level across waterholes decreased. On the long-run elephant numbers increased only at the less crowded waterholes. 6We suggest that the interaction between population size and the available foraging range determined by the number of active waterholes during the dry season controls the park population. 7Our results emphasize the need to understand how key-resource areas cause resource-based aggregation, which ultimately influences the strength of density dependence. More specifically, this study suggests that climate variability strongly affects local elephant population dynamics through changes in surface-water availability. Finally, as dispersal is likely to be an important driver of the dynamics of this population, our results support views that a metapopulation framework should be endorsed for elephant management in open contexts. [source] Is optimal foraging a realistic expectation in orb-web spiders?ECOLOGICAL ENTOMOLOGY, Issue 4 2009WILL EDWARDS Abstract 1.,Explanations for web relocation invoking optimal foraging require reliable differentiation between individual sites and overall habitat quality. We characterised natural conditions of resource variability over 20 days in artificial webs of the orb-web spider Gasteracantha fornicata to examine this requirement. 2.,Variability in catch success was high. Day-to-day catch success in 90% (18/20) catch sites fitted negative binomial distributions, whereas 10% fitted Poisson distributions. Considered across trap sites (overall habitat), variance in catch success increased proportionally faster than the mean (i.e. Taylor's Power Law, variance = 0.54mean1.764). 3.,We compared the confidence intervals for the expected cumulative catch in randomly drawn sequential samples from a frequency distribution representing the overall habitat (based on the parameters for Taylor's power law) and the frequency distribution of expected cumulative catch within each individual catch site [via randomisation based on the mean and negative binomial exponent (k)]. 4.,In all cases and across all sample sizes, median values for the power to differentiate habitat and catch sites never exceeded 0.2, suggesting that principles involved in optimal foraging, if operating, must be accompanied by a very high degree of uncertainty. 5.,Under conditions of high resource variability, many days must be spent in a single catch site if movement decisions are based on an ability to differentiate current catch site from overall habitat. Empirical evidence suggests this is never met. This may explain why proximal mechanisms that illicit quickly resolved behavioural responses have been more successful in describing web relocation patterns than those associated with optimal foraging. [source] Competition in variable environments: experiments with planktonic rotifersFRESHWATER BIOLOGY, Issue 6 2002KEVIN L. KIRK 1.,In a constant environment, competition often tends to reduce species diversity. However, several theories predict that temporal variation in the environment can slow competitive exclusion and allow competing species to coexist. This study reports on laboratory competition experiments in which two pairs of planktonic rotifer species competed for a phytoplankton resource under different conditions of temporal variability in resource supply. 2.,For both species pairs, Keratella cochlearis dominated under all conditions of temporal variability, and the other species (Brachionus calyciflorus or Synchaeta sp.) almost always went extinct. Increasing temporal variation in resource supply slowed competitive exclusion but did not change competitive outcome or allow coexistence. 3.,Rotifers show a gleaner,opportunist trade-off, because gleaner species have low threshold resource levels (R*) and low maximum population growth rates, while opportunist species have the opposite characteristics. In the competition experiments, the gleaner always won and the opportunists always lost. Thus, a gleaner,opportunist trade-off was not sufficient to facilitate coexistence under conditions of resource variability. Instead, the winning species had both the lowest R* and the greatest ability to store resources and ration their use during times of extreme resource scarcity. [source] Asymmetric Abstraction and Allocation: The Israeli-Palestinian Water Pumping RecordGROUND WATER, Issue 1 2009Mark Zeitoun The increased attention given to international transboundary aquifers may be nowhere more pressing than on the western bank of the Jordan River. Hydropolitical analysis of six decades of Israeli and Palestinian pumping records reveals how ground water abstraction rates are as asymmetrical as are water allocations. The particular hydrogeology of the region, notably the variability in depth to ground water, variations in ground water quality, and the vulnerability of the aquifer, also affect the outcome. The records confirm previously drawn conclusions of the influence of the agricultural lobby in maintaining a supply-side water management paradigm. Comparison of water consumption rates divulges that water consumed by all sectors of the farming-based Palestinian economy is less than half of Israeli domestic consumption. The overwhelming majority of "reserve" flows from wet years are sold at subsidized rates to the Israeli agricultural sector, while very minor amounts are sold at normal rates to the Palestinian side for drinking water. An apparent coevolution of water resource variability and politics serves to explain increased Israeli pumping prior to negotiations in the early 1990s. The abstraction record from the Western Aquifer Basin discloses that the effective limit set by the terms of the 1995 Oslo II Agreement is regularly violated by the Israeli side, thereby putting the aquifer at risk. The picture that emerges is one of a transboundary water regime that is much more exploitative than cooperative and that risks spoiling the resource as it poisons international relations. [source] Diversity,stability relationships in multitrophic systems: an empirical explorationJOURNAL OF ANIMAL ECOLOGY, Issue 5 2003Priyanga Amarasekare Summary 1The relationship between diversity and stability is crucial in understanding the dynamics of multitrophic interactions. There are two basic hypotheses about the causal link between diversity and stability. The first is that fluctuations in resource abundance allow consumer coexistence, thus increasing diversity at the consumer trophic level (resource variability hypothesis). The second is that interactions between coexisting consumer species reduce consumer efficiency and dampen population fluctuations, thus increasing consumer,resource stability (consumer efficiency hypothesis). 2The two hypotheses lead to three comparative predictions: (i) fluctuations should be greater (resource variability) or smaller (consumer efficiency) in resource populations with coexisting consumer species, compared to those invaded only by the consumer species superior at resource exploitation; (ii) average resource abundance should be greater (resource variability) or smaller (consumer efficiency) in resource populations with greater fluctuations; and (iii) removal of the consumer species inferior at resource exploitation should increase or not affect resource population fluctuations (resource variability), or always increase them (consumer efficiency). 3I tested these predictions with data from a host,multiparasitoid community: the harlequin bug (Murgantia histrionica) and two specialist parasitoids (Trissolcus murgantiae and Ooencyrtus johnsonii) that attack the bug's eggs. 4Local host populations with coexisting parasitoids exhibited smaller fluctuations and greater average abundance compared to those with just Trissolcus, the species superior at host exploitation. Local populations that lost Ooencyrtus, the species inferior at host exploitation, exhibited an increase in host population fluctuations compared to those that did not. 5The results contradict the expectations of the resource variability hypothesis, suggesting that host population fluctuations are unlikely to be driving parasitoid coexistence. They are consistent with the consumer efficiency hypothesis, that interactions between coexisting parasitoid species dampens host population fluctuations. I discuss the implications of these results as well as possible caveats. [source] | ||||||||||