THE PROGRAMME► Research ► Population Ecology

Intrinsic mechanisms of the recovery of Ethiopian wolves after a rabies epidemic
Prepared by Jorgelina Marino, June 2005

A blossoming body of theoretical studies explores the ways in which life history traits and behavioural decisions can influence population dynamics of social carnivores. Theory and few empirical studies alerts us that populations of comunal breeders with reproductive suppression, such as the Ethiopian wolf, may suffer delayed or negative growth at low densities, because of their need for helpers for survival and/or reproduction. In general, such Allee effects in social mammalas might be mediated by territoriality, dispersal or reproductive suppression. The fall and rise of Ethiopian wolves after a disease epizootic in Bale provided a fascinating natural experiment to test some of these predictions with a model species: in spite of being solitary foragers, wolves are highly social, living in family untis that communally defend a territory and help to raise pups of the dominant pair, without the selective pressures imposed by natural competitors or the need for cooperative hunting.

Wolf numbers in Bale dropped to less than half during rabies epizootics in the early 90s, and by 1997 densities were still very low. By 2000, however, wolves and packs in two well-studied local populations had recovered to pre-epidemic levels. Fourteen years of population monitoring allowed us to unveil the process underlying recovery, by comparing the demography and spatial distribution of wolf packs in two time periods: one of high population density in a saturated environment - before epizootics - and a subsequent period of low density and population recovery -after epizootics. The wolves that survived the epizootic faced a new situation in which the subordinates' chances for dispersal and reproduction, previously highly constrained, were enhanced by the possibility of exploiting vacant territories and breeding opportunities. But did they do so?

Pack enlargement predominated among the packs that survived, as opposed to the options of pack subdivision into smaller groups or colonization by breeding pairs. The surviving packs rapidly enlarged their territory, occupying areas vacated by the extinction of neighbouring packs. Limited recovery thus may have resulted from reproduction being initially constrained to fewer breeding units, as no new packs formed until 1997 -increasing to the previous maximum of six by 1999. The shape of the relationship between population growth rate and population size (Figure A) showed declining growth rates above a maximum rate at around ~1 wolk/km2 -so far a case of typical density dependence as if populations were regulated by competition for a critical resource, most likely the rodent prey. However, low population growth at low density appeared to unveil the action of some Alle effect. This however, was not explained by decreasing litters sizes, or reduced survival in small packs. Indeed, it was the pattern of increasing reproduction with increasing density (measured as the total number of pups annually born in the population) which closely matched the pattern of population growth (Figure B).



Figure: Low rates of population growth at low density (a) coincide with lower fecundity (b), as constraints to pack formation limited the number of breeding females in the recovering population.

Limited recovery thus resulted from delays in the formation of new breeding units, with effects recognizable at the population level. A lower male to female sex ratio evidenced a tendency for females to remain in the recovering populations, in contrast with the pre-epizootic period of saturation. Subordinate females that would have otherwise faced long-distance dispersal, remained in the expanding, surviving packs, or dispersed short distances to form new pack on the edge of the natal territory. In one case, a large pack split into two.

The behavioural strategy adopted by wolves after a severe reduction is in agreement with the hypothesis that securing and inheriting a familiar high-quality area are the greatest advantages of group-living in Ethiopian wolves. Previous studies proved that group size determines the outcome of territorial boundary clashes; this study showed that expansionism also prevails at reduced densities. All along the evolutionary history of these wolves, remaining at home may have been favoured by an intense competition for spatially restricted rodent-rich habitats. Arguable this strategy form the bases of wolves' today resilience to small population sizes in always shrinking Afroalpine `islands'.

Relevant publications

Courchamp, F., Grenfell, B. & Clutton-brock, T. (1999). Population dynamics of obligate cooperators. Proceedings of the Royal Society of London, 266: 557-563.

Kruuk, H. & Macdonald, D.W. (1985). Group territories of carnivores: empires and enclaves. In: Behavioural ecology: the ecological consequences of adaptive behaviour, pp. 521-536. Eds. R.M. Sibly & R.H. Smith. Blackwell Scientific Publications: Oxford.

Sillero-Zubiri, C., King, A.A. & Macdonald, D.W. (1996). Rabies and mortality in Ethiopian wolves (Canis simensis). Journal of Wildlife Diseases, 32(1): 80-86.