Limits to exploitation: dynamic food web models predict the impact of livestock grazing on Ethiopian wolves Canis simensis and their prey.
Anticipating the consequences of, and sustainable limits to, human exploitation of ecosystem resources requires a quantitative understanding of food web dynamics. While dynamic food web models are commonly used to investigate the impact of human exploitation on marine ecosystem structure and dynamics, they are less commonly applied to human-induced disturbances in terrestrial food webs. The intensifying and expanding use of rangelands for domestic livestock production creates a potential conflict with the conservation of extensive populations of wildlife. The Bale Mountains National Park (BMNP) is an example of an area that increasingly has been used as an open-access resource by pastoralists and their herds. Livestock grazing, through its impact on herbivorous rodents, is suspected to negatively affect the critically endangered Ethiopian wolf Canis simensis, a rodent specialist. We developed a series of simple dynamic food chain models to explore the interactions between these trophic levels and how they might be affected by livestock grazing. We also explored how predictions made about these trophic dynamics are affected by the type of functional response linking the different trophic levels. If rodent and/or wolf populations exhibit extreme type 2 responses to resource availability, they can both remain at densities close to their ungrazed equilibrium as livestock density increases, but will rapidly crash once vegetation biomass collapses as a result of the increased grazing pressure. The model predicts the maximum sustainable livestock density to lie between 32 and 117 TLU.km-2, above which populations of wolves are expected to become locally extinct. Synthesis and applications. We show that dynamic trophic modelling is an informative approach when investigating the response of terrestrial ecosystems to human-induced disturbance. In particular, the models described in this paper provide a first step towards enabling managers to predict the implications of changing patterns of human impact on rangelands and support the identification of sustainable grazing grounds for livestock. The models also reveal that monitoring primary productivity alone may be a simple and effective way to detect stresses on food chains and predict the impact of increased livestock grazing on higher trophic levels. In the context of the BMNP, the models predict that some areas of high conservation value appear to be exploited unsustainably, thereby possibly affecting long-term conservation goals for the Park's flagship species.