An integrated population model reveals source-sink dynamics for competitively subordinate African wild dogs linked to anthropogenic prey depletion

Many African large carnivore populations are declining due to decline of
the herbivore populations on which they depend. The densities of apex
carnivores like the lion and spotted hyena correlate strongly with prey
density, but competitive subordinates like the African wild dog benefit
from competitive release when the density of apex carnivores is low, so
the expected effect of a simultaneous decrease in resources and dominant
competitors is not obvious.  Wild dogs in Zambia’s Luangwa Valley
Ecosystem occupy four ecologically similar areas with well-described
differences in the densities of prey and dominant competitors, due to
spatial variation in illegal offtake. We used long-term data to fit a
Bayesian integrated population model (IPM) of the demography and dynamics
of wild dogs in these four regions. The IPM used Leslie projection to link
a Cormack-Jolly-Seber model of area-specific survival (allowing for
individual heterogeneity in detection), a zero-inflated Poisson model of
area-specific fecundity, and a state-space model of population size that
used estimates from a closed mark-capture model as the counts from which
(latent) population size was estimated. The IPM showed that both survival
and reproduction were lowest in the region with the lowest density of
preferred prey (puku, Kobus vardonii, and impala, Aepyceros melampus),
despite little use of this area by lions. Survival and reproduction were
highest in the region with the highest prey density, and intermediate in
the two regions with intermediate prey density. The population growth rate
(λ) was positive for the population as a whole, strongly positive in the
region with the highest prey density, and strongly negative in the region
with the lowest prey density. It has long been thought that the benefits
of competitive release protect African wild dogs from the costs of low
prey density. Our results show that the costs of prey depletion overwhelm
the benefits of competitive release and cause local population decline
where anthropogenic prey depletion is strong. Because competition is
important in many guilds and humans are affecting resources of many types,
it is likely that similarly fundamental shifts in population limitation
are arising in many systems.