Abstract
Nonlethal disturbance of animals can cause behavioral and physiological changes that affect individual health status and vital rates, with potential consequences at the population level. Predicting these population effects remains a major challenge in ecology and conservation. Monitoring fitness-related traits may improve detection of upcoming population changes, but the extent to which individual traits are reliable indicators of disturbance exposure is not well understood, especially for populations regulated by density dependence. Here we study how density dependence affects a population’s response to disturbance and modifies the disturbance effects on individual health and vital rates. We extend an energy budget model for a medium-sized cetacean (the long-finned pilot whale Globicephala melas) to an individual-based population model in which whales feed on a self-replenishing prey base and disturbance leads to cessation of feeding. In this coupled predator-prey system, the whale population is regulated through prey depletion and the onset of yearly repeating disturbances on the whale population at carrying capacity decreased population density and increased prey availability due to reduced top-down control. In populations faced with multiple days of continuous disturbance each year, female whales that were lactating their first calf experienced increased mortality due to depletion of energy stores. However, increased prey availability led to compensatory effects and resulted in a subsequent improvement of mean female body condition, mean age at first reproduction and higher age-specific reproductive output. These results indicate that prey-mediated density dependence can mask negative effects of disturbance on fitness-related traits and vital rates, a result with implications for the monitoring and management of marine mammal populations.
Highlights
A major challenge in ecology and conservation is to understand and predict how populations respond to natural and human-induced changes in their environment
We use the energy budget model of Hin et al [21] to explore how nonlethal disturbance in the form of lost foraging days affect individual body condition, vital rates and population dynamics for a marine mammal population that is regulated by density dependence acting through depletion of its prey
Density-dependent population consequences of disturbance affected individual-level energetics, which led to changes in vital rates: calf survival and the fraction of pregnant females decreased, and age at first reproduction increased (Fig 1)
Summary
A major challenge in ecology and conservation is to understand and predict how populations respond to natural and human-induced changes in their environment. We use the energy budget model of Hin et al [21] to explore how nonlethal disturbance in the form of lost foraging days affect individual body condition, vital rates and population dynamics for a marine mammal population that is regulated by density dependence acting through depletion of its prey. Tailored for pilot whales, the model of Hin et al [21] provides a realistic representation of how the life history of a medium-sized cetacean arises from the dynamics of energy acquisition (through prey foraging) and energy expenditure (on metabolism, growth, lactation and gestation). The rate of milk consumption by calves (in MJ assimilated energy day-1) was given by: ILða; S; F;
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