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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.Understanding how the strengths of species interactions are distributed among species is critical for developing predictive models of natural food webs as well as for developing management and conservation strategies. Recently a number of ecologists have attempted to clarify the concepts of "strong-" and "weak-interactors" in a community, and to derive techniques for quantifying interaction strengths in the field, using metrics that are consistent, comparable, and of relevance to theoreticians. In this paper, we examine potential biases in different empirical approaches to quantifying variation in interaction strengths within and among natural communities.Using both simulated and published data, we explore the behavior of four commonly used or recently proposed empirical measures of the strength of consumer-prey interactions. The type of index used, the experimental protocol, and the underlying model of predatorprey interaction all strongly influence one's perception of both (1) the distribution of interaction strengths among species (e.g., presence of "keystone" species), and (2) the specific identity of the interactions that appear to be most important. Raw treatment differences tend to emphasize effects on very abundant prey, while the three proportional indices tend to emphasize effects on extremely rare prey. Two of the proportional indices are inherently asymmetric about zero, and they inflate positive or negative effects, respectively. When predators exhibit a saturating functional response, the three proportional measures of per capita effect are biased toward a skewed distribution of interaction strengths dominated by effects on the rarest prey. Predator interference causes the per capita measures to emphasize the effects of rare predators. Estimates of per capita effects are also problematic when (1) the per capita effects are back-calculated from experiments designed to measure collective effects (e.g., predator exclusions), and (2) the collective effect of a predator is constant across a wide range of predator densities, as may be common for keystone predators. Finally, since all of the indices show time-dependent behavior, they are differentially suited for different experimental protocols (e.g., short-term vs. long-term results, or community initially near vs. far from equilibrium). All the indices explored here have the potential to provide useful, complementary information about ecological impacts of species in natural communities. In this analysis, we attempt to clarify what each index actually measures and the conditions under which each is most...

Chytridiomycosis is an emerging infectious disease that has recently been reported in amphibian populations throughout the world. It has been associated with many cases of population declines and extinctions. In some areas of the Sierra Nevada of California the disease appears to be the causal factor in the rapid extinction of local populations of the mountain yellow‐legged frog, Rana muscosa, within a few years of the first detection of the disease. In other areas, however, R. muscosa populations appear to persist for many years, despite high levels of infection in tadpoles. Here we present simple models of the dynamics of the disease within an individual lake and ask whether our current understanding of the disease is consistent with the field survey observations of: (a) extinction due to the disease over a wide range of host population sizes, and (b) persistence of frog populations with the disease at some sites. Despite our laboratory observation of chytridiomycosis being invariably lethal to postmetamorphic frogs, the observed long‐term persistence of infected frog populations can only be explained if at least some infected adult frogs survive and reproduce.

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It has been argued that ecological theory has not been useful to the practice of biological control. The purpose of this article is to show how recent theoretical advances may reverse this situation. We first discuss four issues that have arisen in the development of theory for biological control. (1) Recent work has clarified and resolved earlier disagreements concerning the effect of aggregation of the parasitoid to local host density on the stability of parasitoid—host models; this work emphasizes that such aggregation increases the ability of the parasitoid to reduce pest density. (2) There has been disagreement over the conditions under which stability, in the mathematical sense and on a local spatial scale, is an appropriate goal for classical biological control of insect pests, and whether a metapopulation may sometimes provide a more appropriate framework. We also comment on (3) relative size of refuges, which has been proposed as a unifying concept, and (4) density dependence and ratio dependence. We then discuss recent models using a stage—structured approach, particularly those that compare potential biological control agents or agents that have been differentially successful in practice. We argue that this approach has already produced valuable insights into the factors operating in several field situations. It demonstrates the importance of identifying which pest stages are most injurious to the crop, since the natural enemy that wins in competition may not be the most effective at suppressing the crucial pest stage(s): in general, the winning parasitoid reduces the density of the host stage attacked by its competitor below the level at which the latter can maintain a positive growth rate when at low density. A useful criterion is that pest equilibrium density is suppressed most by the parasitoid species that needs the fewest host individuals to allow a female parasitoid to replace herself in the next generation. Caveats are that this may apply only under equilibrium conditions and that the pest stage most suppressed is the one we wish to control. We discuss the connection between successful biological control and evolutionary considerations.

Lyme disease (LD), the most frequently reported vector-borne disease in the United States, requires that humans, infected vector ticks, and infected hosts all occur in close spatial proximity. Understanding the spatial dynamics of LD requires an understanding of the spatial determinants of each of these organisms. We review the literature on spatial patterns and environmental correlates of human cases of LD and the vector ticks, Ixodes scapularis in the northeastern and midwestern United States and Ixodes pacificus in the western United States. The results of this review highlight a need for a more standardized and comprehensive approach to studying the spatial dynamics of the LD system. Specifically, we found that the only environmental variable consistently associated with increased LD risk and incidence was the presence of forests. However, the reasons why some forests are associated with higher risk and incidence than others are still poorly understood. We suspect that the discordance among studies is due, in part, to the rapid developments in both conceptual and technological aspects of spatial ecology hastening the obsolescence of earlier approaches. Significant progress in identifying the determinants of spatial variation in LD risk and incidence requires that: (1) existing knowledge of the biology of the individual components of each LD system is utilized in the development of spatial models;(2) spatial data are collected over longer periods of time; (3) data collection and analysis among regions are more standardized; and (4) the effect of the same environmental variables is tested at multiple spatial scales.