As human activities play an increasingly important role in affecting ecosystem processes, the ability to predict the direct and indirect effects of these impacts on these processes becomes a priority. For example, a better understanding of how competition between native species and domestic species affect trophic interactions and other key ecosystem processes is crucial to assessing the sustainability of current human land-use patterns and informing the development of relevant management strategies. Food webs provide insight into the trophic structure and energy flows of ecosystems (Vander Zanden, Casselman & Rasmussen 1999; Cohen, Jonsson & Carpenter 2003), the factors affecting trophic dynamics, as well as the trophic effects of disturbances (Jenkins, Kitching & Pimm 1992; Townsend et al. 1998). Dynamic trophic modelling can be informative when exploring the ecological issue of biomass partitioning between competing species, for example (Fox 2005), as well as the more general impact of human-induced changes in trophic relationships on an ecosystem’s structure and dynamics (Walters, Christensen & Pauly 1997).
There are significant associations between human population density and biodiversity hotspots in Africa (Balmford et al. 2001; Sachs et al. 2009). At these sites, conflicts over natural resources are frequent (Stewart 2002) and often centred on contested access to land (Peluso 1993). An example of this human–wildlife conflict is being played out in and around Ethiopia’s Bale Mountains National Park (BMNP) (06°41′N, 39°03′E and 07°18′N, 40°00′E). The Bale mountains represent the largest area of afroalpine habitat in Africa (Yalden 1983), and harbour a diverse array of endemic and range-restricted species, including the largest remaining populations of Ethiopian wolves Canis simensis Rüppell, 1840. The Ethiopian wolf, with a total population of <500 individuals, is the rarest canid in the world. They are at the top of one of the most simple, yet critical, food chains in afroalpine pastures (Sillero-Zubiri, Tattersall & Macdonald 1995), preying upon the rodent fauna, especially the endemic giant mole rat Tachyoryctes macrocephalus and two species of murine rodents, Arvicanthis blicki and Lophuromys melanonyx. These species combined represent an estimated 90% of the wolves’ diet (by volume) (Sillero-Zubiri & Gottelli 1995), and feed primarily on the above-ground parts of grasses and flowering plants (Alchemilla spp. in particular) on the afroalpine pastures (Yalden 1988).
Core wolf ranges in the BMNP are located in the Web valley, Morebawa and on the Sanetti plateau. Ethiopian wolf populations have been affected by diseases transmitted from domestic dog populations (Randall et al. 2006) but livestock grazing, through its impact on the wolves’ rodent prey, is considered to represent a more profound long-term threat to their persistence (Stephens et al. 2001; Vial 2010). Livestock density estimates for 2007/2008 based on distance-sampling along 500 km of transects (Vial 2010) showed densities of cattle and caprines (sheep and goats) to be c. 195 tropical livestock units (TLU) km−2 (95% CI: 125–325 TLU·km−2) in the Web valley, c. 149 TLU·km−2 (95% CI: 69–342 TLU·km−2) for Morebawa and 49 TLU·km−2 (95% CI: 13–194 TLU km−2) for the Sanetti plateau in 2007–2008 [1 TLU = 1·5 cattle, 11 caprines or 1·5 equid (Boudet & Riviere 1968)]. It has been shown that rodent biomass declines as livestock density increases along a grazing gradient and that rodent density increases in response to the experimental removal of livestock grazing using exclosures (Vial 2010). Such experiments have also revealed some evidence that the impact of livestock on rodent biomass is concurrent with changes in the vegetation. In particular, the removal of livestock has a positive impact on the biomass of Alchemilla spp. and grasses, and results in smaller home ranges and higher reproductive success for L. melanonyx, an indication that resources may be more limited for some rodent species on grazed sites (Vial 2010).
We apply classical and modified Lotka-Volterra (LV) predator–prey models to explore the potential effects of exploitative competition between livestock and rodents for primary production in BMNP and its possible repercussions on trophic levels higher in the food chain. Sustainability can then be evaluated in terms of the food chain’s ability to withstand disturbances (Holling 1973), that is the capacity of a system to absorb change while preserving its structure and dynamics. There are two common ways of measuring this property: stability and resilience. Here, we will use stability as ‘the propensity of a system to attain an equilibrium condition’ (Holling 1986) (quantified as the coefficient of variation around mean equilibrium biomasses), while we will define resilience as the ‘domain (quantified here as a livestock stocking rate) over which disturbance can be experienced’ while still retaining an equilibrium condition (sensuHolling 1973). These models provide a framework for the objective examination of biomass flows and standing crops, an approach rarely adopted when considering the following ecological questions: What is the impact of increasing competition (in our case livestock competing with rodents) on the food chain’s stability and resilience? Does this impact depend on the type of functional response that links the different trophic levels?
More specifically for our case study, we also ask whether livestock densities in the Web valley, Morebawa and the Sanetti plateau exceed our models’ estimated resilience to disturbance thereby threatening Ethiopian wolves’ persistence in part of their range in BMNP?