02646nas a2200301   4500008004100000020001400041245009300055210006900148260001200217300001600229490000700245520185300252653001702105653001202122653002302134653002302157653001502180653001302195653001402208653001302222653002102235653001602256653001302272653000902285653000902294100001702303856002402320       2004                        eng d  a0012-965800aTrait-initiated indirect effects due to changes in consumption rates in simple food webs0 aTraitinitiated indirect effects due to changes in consumption ra  c2004///  a1029 - 10380 v853 aThis article explores the potential indirect effects due to changes in consumption-related traits of a focal prey species in each of two simple food webs. The systems considered are: one predator exploiting two independent prey, and one predator exploiting two prey that use a common limiting resource. In each system, the indirect effect of a change in a consumption-related parameter of one prey species on the population density of the other prey is determined. When prey only share a predator, the change in one prey’s density produced by a change in the other prey’s vulnerability to the predator is determined by whether or not that second prey is initially overexploited; i.e., whether its exploiter’s intake decreases with an increase in the attack rate. Increases in the vulnerability of an overexploited prey decrease the predator population and thereby indirectly increase the population size of the second prey. Increases in the vulnerability of a prey that is not overexploited have the opposite effects on both the predator and second prey populations. Both of these outcomes can be reversed by strong direct density dependence in the predator, provided the predator exhibits satiation. In the food web with a shared food and a shared resource, increases in predator vulnerability of the more vulnerable prey species always increase the population size of the more resistant prey species. However, increases in the vulnerability of the more resistant prey can either increase or decrease the population size of the more vulnerable prey species. When the more resistant prey becomes better at capturing the resource, the less resistant may either increase or decrease. Overexploitation also explains these outcomes. General principles for understanding the consequences of changes in vulnerabilities or attack rates are discussed10aCONSEQUENCES10aDENSITY10aDENSITY DEPENDENCE10aDENSITY-DEPENDENCE10aDEPENDENCE10afood web10afood webs10aFOOD-WEB10aindirect effects10aPOPULATIONS10aPREDATOR10aPREY10aSIZE1 aAbrams, P.A.  uISI:00022076660001402585nas a2200229   4500008004100000020001400041245009500055210006900150260001200219300001600231490000700247520197100254653001302225653001702238653001402255653001302269653001402282653000902296653000902305100001702314856002402331       2008                        eng d  a0012-965800aMeasuring the impact of dynamic antipredator traits on predator-prey-resource interactions0 aMeasuring the impact of dynamic antipredator traits on predatorp  c2008///  a1640 - 16490 v893 aThis article analyzes the limitations of the most widely used method for quantifying the impact of dynamic antipredator traits on food chain dynamics and discusses alternative approaches. The standard method for a predator-prey-resource chain estimates the effects of the prey’s defensive behavior by comparing population densities or fitness measures in a ’’predator cue’’ treatment to those in a no-predator treatment. This design has been interpreted as providing a measure of the ’’nonconsumptive effect’’ of the predator on the prey and the ’’trait-mediated indirect effect’’ of the predator on the resource. Other approaches involve measurements of the impact of the behavior in the presence of functional predators. The questions addressed here are: (1) How consistent are the results of different approaches? (2) How time-dependent are their results? (3) How well do they correspond to theoretical measures of effect size? (4) How useful are the measurements in understanding system dynamics? A model of a tritrophic system in which the prey species adjusts a defensive trait adaptively is used to evaluate the experimental designs. Measures of changes in prey fitness or population density in a cue treatment generally include offsetting effects of the cost of the behavior and the benefit of more resources. This means that the sign of the effect, as well as its magnitude, may change depending on when the experiment is terminated. Because predation is not present in the cue treatment, few conclusions can be drawn about the impact of the behavior on population densities or fitness of the prey in a natural setting with predators. Cue experiments often do not accurately separate trait-mediated from density-mediated effects on the resource. Most scalar measures of effects are sensitive to experimental duration and initial densities. Use of a wider range of experimental designs to measure trait-related effects is called for10aDYNAMICS10ainteractions10apredation10aPREDATOR10apredators10aPREY10aSIZE1 aAbrams, P.A.  uISI:000256540300017