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

Abstract

Many populations across taxa are regulated by negative density-dependence, wherein increased population sizes lead to decreased birth rates or increased mortality. By releasing such populations from some level of these density-dependent effects, extrinsic mortality can lead to counter-intuitive results, such as no change in population size (compensation), or an increase in population size (overcompensation). These results have been documented experimentally, but there currently exists a dearth of empirical studies exploring the mechanisms behind the phenomenon. We tested the relationship between extrinsic mortality level and (over)compensation in four mosquito species -Aedes aegypti, A. albopictus, A. triseriatus, and Culex pipiens -by exposing larvae to a range of mortalities by artificial harvest or predation and analyzing the numbers of adults produced. Additionally, we examined the ability of three functionally diverse predators -Mesocyclops longisetus, Anopheles barberi, and Corethrella appendiculata -to induce (over)compensation in A. aegypti by exposing larvae to predation by either single or multiple predator species treatments. We found overcompensation across all levels of mortality in A. triseriatus and C. pipiens and at intermediate levels of mortality in A.aegypti. Low-to-intermediate levels of mortality were compensatory in A. albopictus, followed by a decrease in adult production at the highest mortality level. Predation induced compensation in the second experiment and an increase in population equilibrium, a phenomenon known as the 'hydra effect.' These results provide a better understanding of the conditions under which mortality may increase adult production or population equilibrium. Because overcompensation and the hydra effect are predicted to affect a wide range of taxa and food webs, our findings present implications for developing proper population management and pest reduction strategies.