Individual study: Effect of family support on the success of translocated black-tailed prairie dogs Cynomys ludovicianus on the Vermejo Park Ranch, New Mexico, USA
Shier D.M. (2006) Effect of family support on the success of translocated black-tailed prairie dogs. Conservation Biology, 20, 780-790
When undertaking translocations of highly social species, as well as predation, founder group composition may play a critical role in post-release survival. In this study the fitness of black-tailed prairie dogs Cynomys ludovicianus translocated with or without their family groups was compared. Black-tailed prairie dogs are considered keystone species in North American grassland ecosystems. They have declined by around 98% from the number that occupied the Great Plains before European settlement. Most remaining colonies are small (<40 ha) and isolated, hence translocation may be considered appropriate to establish new colonies in former areas given that the habitat is suitable. Keeping family groups together during translocation is however not easy as it requires exhaustive trapping of individual families. Therefore, the main goal of this study was to determine whether maintaining family groups could be more effective and economical than translocating individuals without regard for family membership.
Study site: Black-tailed prairie dogs were studied on the Vermejo Park Ranch in Colfax County, New Mexico (36°N, 104°W, elevation 1,850 m). During the springs of 2001 and 2002, 10 uninhabited sites within the historical range of the prairie dogs were selected. Pairs of family translocated (FT) and non-family translocated (NFT) sites were matched for soil, vegetation cover, slope, proximity on the ranch, and sex and age-class ratios to control for these factors. To limit dispersal and allow them to acclimatize to the release site, sites were prepared for "soft release" by installing 20 acclimation cages at each site.
Translocation: The translocation comprised 973 wild-caught black-tailed prairie dogs (232 juvenile males, 212 adult males, 269 juvenile females, 260 adult females) divided into two treatment groups: FT (moved in intact families; n = 484 adults [≥1 year old] and juveniles into five sites, 87–100/site) and NFT (moved without consideration of family membership; n = 489 into five sites, 88–103/site). Areas where animals were released are referred to as 'sites' but then 'colonies' once prairie dogs established burrow systems at these sites.
Animals in the FT groups were individually marked and observed until all members of a family had been determined. NFT animals were trapped without regard to family membership. Fitness parameters (e.g. survival, reproductive success) were measured by retrapping all marked animals remaining at release sites and observations undertaken in the summer following release.
Cost-effectiveness: To determine how cost-effective FT was in the present study compared with NFT, the cost of each method in terms of hours invested (including mark, recapture, and observation for the FT treatment) was calculated. This was used to project the total cost of establishing a given number of prairie dogs (defined as number of prairie dogs present on the site) 2 years after translocation. Calculations were based on data from the three colonies established during the 2001 release.
Family translocated prairie dogs were 5-times more likely to survive and had significantly higher reproductive success than those translocated without families. Predation was an important factor in translocation success, but family translocation significantly reduced the success of predators on newly established prairie dog colonies.
Post-release survival was also affected by the timing of release. Late season (August) translocations had the highest survival regardless of predation pressure and translocation method; this was more evident for juveniles than adults
The results of this translocation study demonstrate the importance of considering translocations of whole family units. The results also suggest that conservation efforts involving translocations of similar highly social mammals may also benefit through the maintenance of social groups through family translocation which may enhance initial survival and reproduction.
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