When Does Less Equal More? Assessing The Mechanisms Driving Compensatory Mortality And The Hydra Effect

Abstract

Extrinsic mortality applied to negatively density-dependent populations can result in no change in number of survivors (compensation) or an increase in number of survivors (overcompensation) by releasing the survivors from some level of density-dependent effects. Theoretical studies have attempted to elucidate the mechanisms behind this phenomenon, but there is little empirical work testing the predictions of those efforts. The purpose of this study is to examine the relationship between the level of extrinsic mortality (i.e., percentage of mortality) and the level and likelihood of overcompensation. We test the hypotheses that 1) overcompensation is induced at low-to-intermediate levels of extrinsic mortality with additive mortality occurring above a threshold, and 2) different species exhibit varying levels and likelihood of (over)compensation due to differential responses to density. These hypotheses were tested in four container mosquitoes species (Diptera: Culicidae)-Aedes aegypti, A. albopictus, A. triseriatus, and Culex pipiens. Cohorts of 150 larvae were exposed to either artificial harvest ranging from 0-70% on day 2 or exposure to predation by 1, 2, or 3 Mesocyclops longisetus (Crustacea: Copepoda). The number of adults produced per container was recorded and analyzed by species using mixed-effects generalized linear models. Aedes triseriatus and C. pipiens demonstrated overcompensation across all mortality levels. Overcompensation was induced in A. aegypti up to the 50% mortality level, followed by compensation at 70%. Mortality was compensatory from 0-50% in A. albopictus, followed by partial compensation above 50%. Mortality from predation by M. longisetus led to greater adult production compared to artificial harvest in A. aegypti and A. albopictus and less adult 2 production in C. pipiens. Our results do not provide full support for our first hypothesis, but rather demonstrate three species-specific patterns in the relationship between extrinsic mortality level and (over)compensation. Our second hypothesis was supported. The relative levels of (over)compensation demonstrated in the four species is inversely related to their competitive abilities and responses to density. These results provide further insight on the mechanisms driving a phenomenon that is predicted to affect many taxa and food webs, and they present practical implications for developing effective strategies to control pest populations.