Context dependency of top‐down, bottom‐up and density‐dependent influences on cheetah demography

Abstract

Research on drivers of demographic rates has mostly focused on top predators and their prey, and comparatively less research has considered the drivers of mesopredator demography. Of those limited studies, most focused on top‐down effects of apex predators on mesopredator population dynamics, whereas studies investigating alternative mechanisms are less common. In this study, we tested hypotheses related to top‐down, bottom‐up and density‐dependent regulation of demographic rates in an imperilled mesopredator, the cheetah (Acinonyx jubatus). We used a 25‐year dataset of lion density, cheetah density and prey density from the Mun‐Ya‐Wana Conservancy in South Africa and assessed the effects of top‐down, bottom‐up and density‐dependent drivers on cheetah survival and reproduction. In contrast to the top‐down and bottom‐up predictions, both adult and juvenile cheetahs experienced the lowest survival during months with high prey densities and low lion densities. We observed support only for a density‐dependent response in juvenile cheetahs, where they had a higher probability of reaching independence during times with low cheetah density and low prey density. We did not identify any strong drivers of litter size. Collectively, our results indicate that high apex predator abundance might not always have negative effects on mesopredator populations, and suggest that context dependency in top‐down, bottom‐up and density‐dependent factors may regulate demographic rates of cheetahs and other mesopredators. Our results highlight the complexities of population‐level drivers of cheetah demographic rates and the importance of considering multiple hypotheses of mesopredator population regulation.

Habitat complexity and lifetime predation risk influence mesopredator survival in a multi-predator system

“…We could not explicitly investigate whether long-term exposure to predation risk reduced cub survival through direct or indirect effects. The majority of cub mortality in our study system is a result of predation 48 , but cubs also experience non-predation mortality such as starvation or injury 22,48 , and indirect effects of predation risk could have reduced cub body condition, which might increase susceptibility to predation. In the absence of direct predation, the long-term risk of predation has been found to cause changes in the morphology 49 , behavior 50,51 , physiology 52,53 , and demography 54,55 of a variety of species.…”

“…We limited our prey analyses to these species because they comprised 82% of cheetah kills in the study area 65 . We estimated prey abundance using hierarchical distance sampling models with spatial covariates on both the abundance and detection processes, and used our top model to extrapolate prey abundance over our entire study period 48 . Our resulting prey density rasters depicted the average number of prey within 400 m 2 cells for each season of each year.…”

“…If a cheetah was removed from the reserve for management purposes, we censored that individual animal from analyses 48 . Because lion and cheetah density can vary greatly within a season, and because cubs can be born and become independent at any time during the year, we conducted our analysis on a monthly timescale to best reflect the conditions that might be driving survival 48 . Because survival of cubs from the same litter might not independent, we first ran a Chi-square test of independent survival 72 to test this assumption, with the null hypothesis being that survival of cubs is independent.…”

“…To run this analysis, we randomly selected half (n = 19) of the monitored litters and ran a survival model (see below for information on model structure) without any individual covariates, to estimate monthly cub survival. We used the results of this model to estimate the expected number of living cubs at independence, which we defined as 16 months post-birth 48 . When then repeated this procedure 50 times and ran a Chi-square test on the observed vs. expected number of survived cubs.…”

“…Additionally, because juvenile cheetahs stay with their mothers for variable amounts of time 75 , we could not incorporate a regular age structure into our models. Thus, we used a multi-state approach to estimate survival for both cubs and adults simultaneously 48 . We specified the two model states as cub (juvenile cheetahs dependent on their mother) and adult (cheetahs that were independent from their mother) and did not incorporate immigration or emigration because our population was a closed population.…”