Is isolation the major genetic concern for endangered equids?

Abstract

Although hybridization between closely related species is a natural phenomenon that can operate as an important evolutionary force, it has nevertheless contributed to the extinction of numerous species. Where the causes of hybridization are largely anthropogenic, therefore, its consequences need to be carefully assessed. Such investigations become pressing when one or both of the species concerned are endangered, since some form of management intervention may be required to limit the hybridization. Nevertheless, observations of hybridization involving species of conservation concern do not in themselves necessitate management action. Cordingley et al. (2009) report the first case of natural hybridization between two equid species; the abundant plains zebra Equus burchelli and the endangered Grevy’s zebra Equus grevyi. Since global numbers of Grevy’s zebra have declined dramatically in recent decades due to human activity, hybridization could represent a significant further risk to the conservation of the species. Fortunately, as Cordingley et al. (2009) outline, hybridization does not appear to constitute a serious threat to Grevy’s zebra because hybrids seemingly integrate into plains zebra society. Thus while the hybrids are reproductive, there appears to be little possibility of backcrossing due to behavioural isolation and gene flow is essentially unidirectional from the Grevy’s zebra to the plains zebra population. As long as these conditions persist there is little immediate threat to the Grevy’s zebra population from hybridization with plains zebra. These results are important since they suggest that conservation resources would be wasted on trying to control the hybridization and management efforts would be better directed at other, more immediate threats to Grevy’s zebra populations. The one caveat to this, however, relates to the anomalous result of two Grevy’s zebra females having plains zebra mtDNA haplotypes. While Cordingley et al. (2009) attribute this to an error in sample collection, there is an urgent need for further genetic sampling to ensure that the current findings are robust and substantiated in a broader sample. Without such confirmation it may be premature to conclude that hybridization is of little risk to the Grevy’s zebra gene pool. In essence, it would be difficult to categorically direct management efforts away from the issue of hybridization when there is any uncertainty over the scientific basis of this decision. Errors made now may not easily be undone in future years. Nevertheless if, as seems likely, the conclusions of Cordingley et al. (2009) are confirmed with further sampling, the current study does suggest that the Grevy’s zebra gene pool may still be at risk even if hybridization is not the major threat. The Grevy’s zebra within the Ol Pejeta population appear isolated from other neighbouring populations, and the main anthropogenic threats to the species suggest that the meta-population could become increasingly fragmented in isolated populations. Lessons from another endangered African equid, the Cape mountain zebra Equus zebra zebra indicate that this can have disastrous genetic consequences. Hunting and habitat destruction over the past three centuries decimated the world population of Cape mountain zebra to fewer than 100 individuals in five relict populations, and while sustained conservation efforts have elevated this number to more than 1600 animals, over 90% of the stock has derived from a single source population (Moodley & Harley, 2005). The result is low genetic variation within individual Cape mountain zebra populations, the characteristic signature of population fragmentation and drift. Moderate variation does still exist across the entire Cape mountain zebra meta-population suggesting that a management strategy focused on the mixing of the original relict populations could halt further loss of genetic diversity (Moodley & Harley, 2005). Such a strategy is far from straightforward, however, and active management is required to sustain population growth in critical populations (Watson et al., 2005; Smith et al., 2008). While the situation is far from this serious in Grevy’s zebra, evidence from the other mountain zebra subspecies,