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Migratory Herds of Wildebeests and Zebras Indirectly Affect Calf Survival of Giraffes

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Migratory Herds of Wildebeests and Zebras Indirectly Affect Calf Survival of Giraffes

Derek E Lee et al. Ecol Evol.

Abstract

In long-distance migratory systems, local fluctuations in the predator-prey ratio can exhibit extreme variability within a single year depending upon the seasonal location of migratory species. Such systems offer an opportunity to empirically investigate cyclic population density effects on short-term food web interactions by taking advantage of the large seasonal shifts in migratory prey biomass.We utilized a large-mammal predator-prey savanna food web to evaluate support for hypotheses relating to the indirect effects of "apparent competition" and "apparent mutualism" from migratory ungulate herds on survival of resident megaherbivore calves, mediated by their shared predator. African lions (Panthera leo) are generalist predators whose primary, preferred prey are wildebeests (Connochaetes taurinus) and zebras (Equus quagga), while lion predation on secondary prey such as giraffes (Giraffa camelopardalis) may change according to the relative abundance of the primary prey species.We used demographic data from five subpopulations of giraffes in the Tarangire Ecosystem of Tanzania, East Africa, to test hypotheses relating to direct predation and indirect effects of large migratory herds on calf survival of a resident megaherbivore. We examined neonatal survival via apparent reproduction of 860 adult females, and calf survival of 449 giraffe calves, during three precipitation seasons over 3 years, seeking evidence of some effect on neonate and calf survival as a consequence of the movements of large herds of migratory ungulates.We found that local lion predation pressure (lion density divided by primary prey density) was significantly negatively correlated with giraffe neonatal and calf survival probabilities. This supports the apparent mutualism hypothesis that the presence of migratory ungulates reduces lion predation on giraffe calves.Natural predation had a significant effect on giraffe calf and neonate survival, and could significantly affect giraffe population dynamics. If wildebeest and zebra populations in this ecosystem continue to decline as a result of increasingly disrupted migrations and poaching, then giraffe calves will face increased predation pressure as the predator-prey ratio increases. Our results suggest that the widespread population declines observed in many migratory systems are likely to trigger demographic impacts in other species due to indirect effects like those shown here.

Keywords: asynchronous reproduction; juvenile survival; life history; mammal; match–mismatch; natural enemy; population dynamics; predator–prey ratio; prey switching.

Figures

Figure 1
Figure 1
Study area in the Tarangire Ecosystem of northern Tanzania. Thick gray lines delineate the five sites sampled, thin gray lines are roads and tracks, black lines are rivers and watercourses, and light gray areas are lakes
Figure 2
Figure 2
Giraffe apparent reproduction (black line; calves per adult female per year, an index of neonatal survival) and apparent calf survival (gray line; probability of survival for four months following first detection) decline with increasing lion predation pressure (lion density/primary prey density) in the Tarangire Ecosystem, Tanzania 2012–2014

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Cited by 3 PubMed Central articles

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