Extrinsic mortality impinging on negative 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, Aedes albopictus, Aedes 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.

Aim Understanding the scales over which land use affects animal populations is critical for conservation planning, and it can provide information about the mechanisms that underlie correlations between species distributions and land use. We used a citizen science database of anuran surveys to examine the relationship between road density, land use and the distribution of frogs and toads across spatial scales and regions of the United States.

1. Population responses to extrinsic mortality can yield no change in the number of survivors (compensation) or an increase in the number of survivors (overcompensation) when the population is regulated by negative density-dependence. This intriguing response has been the subject of theoretical studies, but few experiments have explored how the source of extrinsic mortality affects the response.2. This study tests abilities of three functionally diverse predators, alone and combined, to induce (over)compensation of a prey population. Larval Aedes aegypti (Diptera: Culicidae) were exposed to predation by Mesocyclops longisetus (Crustacea: Copepoda), Anopheles barberi (Diptera: Culicidae), Corethrella appendiculata (Diptera: Corethrellidae), all three in a substitutive design, or no predation.3. Predator treatment had no significant effect on the total number of adult survivors, nor on numbers of surviving males or females. The female development rate and a composite index of performance (r ′ ) were greater with predation relative to no-predator control. No differences were detected between diverse and single-species predator treatments.4. Sensitivity analyses indicated predation effects on the number of female adults produced, despite not being affected significantly, was the largest contributing factor to significant treatment effects on the demographic index r ′ . While predation did not significantly increase the production of adults, it did release survivors from density-dependent effects sufficiently to increase population performance. This study provides an empirical test of mechanisms by which predation may yield positive mortality effects on victim populations, a phenomenon predicted to occur across many taxa and food webs.

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.

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.