Showing papers by "Earl E. Werner published in 2004"

How dependent are species-pair interaction strengths on other species in the food web?

Abstract: In ecological theory species interaction strengths are typically described by constants or functions that depend on the densities of the two interacting species. However, if species' traits (phenotypes) are plastic, then modifications in these traits (induced by the presence of another species) could affect interaction strengths of the focal species with a number of other species in the system. The magnitudes of such higher-order effects on interaction strengths have not been reported and are not straightforward to measure. We present a methodology to quantify changes in consumer–resource interaction coefficients (a metric of interaction strength) due to effects of predators on consumer (i.e., the prey of the predator) phenotype (e.g., nonlethal or trait-mediated effects). Application of this method to studies in diverse systems indicates that predators can strongly reduce consumer– resource interaction coefficients, often in the range of 20–80%. We use analytic and simulation models to show that effects on interaction coefficients of this magnitude can lead to trait-mediated effects that contribute more strongly than density-mediated effects to the net effects of predators on consumers and their resources, and even qualitatively change model predictions. Our results strengthen previous claims that trait-mediated effects strongly influence species interactions and suggest that recent calls to quantify interaction strengths must be broadened to include examination of the variation in interaction strengths due to their dependence on densities of other species (most notably predators) in food webs.

Context dependence of nonlethal effects of a predator on prey growth

Abstract: Predators can have a large influence on their prey through induced changes in prey phenotype. Such “nonlethal” predator effects have been abundantly demonstrated empirically in both terrestrial and aquatic systems. But the extent to which changes in species traits alter short-term responses such as growth rate or probability of survival is not clear. Here we develop models to examine the nonlethal effects of predators on prey growth. Our analyses illustrate how the nonlethal effects of predators on individual prey growth depend on environmental context; e.g., factors such as focal species density, competitor density, resource dynamics, and the timescale over which the interactions occur. This context dependence arises because of complex interactions of three mechanisms; (1) the direct negative effect of induced reduction in foraging rates, which is opposed by (2) the potential positive effects of reductions in intra- and interspecific competition, and (3) resource responses to reduced foraging. We present new empirical work, and review previous work, on larval-anuran growth that is in general support of model predictions. The framework presented here can serve to facilitate the design and interpretation of experimental results and predict how the nonlethal predator effect on prey growth in natural systems will vary over time and space.