Abstract
Extrinsic mortality impinging on negatively density-dependent populations can result in no change in the number of survivors (compensation) or an increase (overcompensation) by releasing the population from density-dependent effects on survivorship. The relationship between the level of extrinsic mortality (i.e., percentage of mortality) and the level and likelihood of overcompensation is theoretically important, but rarely investigated. We tested the hypothesis that overcompensation occurs below a threshold value of extrinsic mortality that is related to density-dependent mortality rate, and that additive extrinsic mortality occurs above this threshold. This hypothesis predicts that survivorship vs. extrinsic mortality will: 1) be best described by a two-segmented model with a threshold; 2) have a slope >0 below the threshold; and 3) have a slope=-1 above the threshold. We also tested whether mortality imposed by real predators and random harvest have equivalent effects on adult production, and whether magnitude of overcompensation is related to species sensitivity to density-dependence. These hypotheses were tested in the container mosquitoes Aedes aegypti, A. albopictus, A. triseriatus, and Culex pipiens (Diptera: Culicidae). Cohorts of 150 larvae were exposed to random harvest of 0-70% two days after hatch or to predation by 1-3 Mesocyclops longisetus (Crustacea: Copepoda). Overcompensation occurred in A. aegypti in a pattern consistent with predictions. Aedes triseriatus showed strong overcompensation but no evidence of a threshold, whereas A. albopictus and C. pipiens had survival consistent with compensatory mortality but no evidence of a threshold. Compared to random harvest, mortality from predation yielded greater adult production in A. aegypti and A. albopictus, lesser adult production in C. pipiens, and no difference in adult production in A. triseriatus. Our results are largely consistent with our hypothesis about overcompensation, with the caveat that thresholds for additive mortality appear to occur at very high levels of extrinsic mortality. Magnitudes of overcompensation for the three Aedes were inversely related to survival in the 0% mortality treatment, consistent with our hypothesis that overcompensation is related to sensitivity to density-dependence. A broad range of extrinsic mortality levels can yield overcompensation, which may have practical implications for attempts to control pest populations.
Highlights
Population responses to extrinsic sources of mortality, such as harvesting and predation, have traditionally been assumed to result in a net reduction in population size
Previous studies have investigated the effects of extent of extrinsic mortality on population sizes (Slobodkin and Richman 1956, Fryxell et al 2005), but we have found no empirical studies testing the quantitative predictions of how extent of extrinsic mortality in one life stage affects overcompensation in the production of the stage
The mean number of adults produced with no extrinsic mortality for C. pipiens, A. triseriatus, A. aegypti, and A. albopictus were 6, 17, 38, and 47, respectively (Appendix S1 Fig. S2)
Summary
Population responses to extrinsic sources of mortality, such as harvesting and predation, have traditionally been assumed to result in a net reduction in population size. Populations regulated by negative density-dependent effects are constrained by intrinsic mortality or reductions in reproduction rates as population sizes approach and exceed the carrying capacity of their habitats (Sibly et al 2005). Extrinsic mortality affecting such populations can result in the production of the same (compensation; Fig. 1B), or a greater (overcompensation; Fig. 1A, D), number of surviving individuals in subsequent life-stages as would occur without the extrinsic mortality (De Roos et al 2007, Zipkin et al 2008, Sandercock et al 2011). Overcompensation can result in the “Hydra effect” (Abrams and Matsuda 2005, Abrams 2009), wherein extrinsic mortality results in an increase in a population’s equilibrium size
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