Large predators can mitigate nutrient losses associated with off-site removal of animals from a wildlife reserve

Animals concentrate key nutrients in their bodies. In fenced wildlife reserves where nutrient input and/or retention is low, the off-site removal of animals may constitute a significant loss of nutrients for the ecosystem. Here we add wildlife capture and removal into the phosphorus (P) and calcium (Ca) budget for a 121,700 ha fenced game reserve located in the southern Kalahari. We then use faecal P concentrations from 11 mammal herbivores >10 kg as an indicator of potential nutrient stress in this system to investigate whether the implications of nutrient loss via off-site wildlife removal may be cause for concern. Finally, we assess the role of natural predation as a mechanism to minimise the need for wildlife removal and concomitant nutrient loss. During the period 2009-2018, mean loss of P and Ca via wildlife removal was 2.9 and 6.2 kg km(-2) year(-1), respectively. This compares to 1.0 and 2.1 kg km(-2) year(-1) of P and Ca added via the provision of mineral licks. If it is assumed that natural fluxes of these elements are in steady state, then anthropogenic activities have resulted in a net deficit of 18.5 kg/km(2) of P and 40.6 kg/km(2) of Ca over the decade. We found that dry season herbivore faecal P concentrations are close to or below a widely cited minimum threshold of 2,000 mg/kg, below which most vertebrates begin suffering growth and reproductive issues. Large animals were more likely to be under this threshold. Prolonged continuation of off-site wildlife removal may result in nutrient losses that can lead to long-term ecological degradation. Natural predation levels were, however, found sufficient to mitigate the need for wildlife removal and present a management strategy whereby herbivore populations can be regulated without a loss of nutrients. Synthesis and applications. We find that the capture and permanent removal of large-bodied animals from wildlife reserves can be a significant cause of nutrient loss. Over time, in sites where nutrient input and/or retention is low, this may contribute to nutritional stress for remaining resident animals. Where possible, holistic management strategies that promote the retention of animals and carcasses within the reserve-such as the reintroduction of large carnivores-should be preferred.

Automated summary

Animals concentrate key nutrients in their bodies, and the removal of animals from fenced wildlife reserves can lead to significant loss of nutrients for the ecosystem. Faecal P concentrations can serve as an indicator of potential nutrient stress in the system. Natural predation is a sufficient mechanism to mitigate the need for wildlife removal and prevent nutrient loss.