Translocation of an imperilled woodrat population: integrating spatial and habitat patterns

Abstract

Many species have strong habitat preferences that directly influence population viability. For successful reintroduction of threatened populations that rely on habitat structures, the correct placement of artificial structures is also important to population persistence. In this paper, we present a hierarchical approach to the problem of translocating animals that rely on permanent habitat structures, in which we first use population dynamics data to identify areas of suitable habitat, and then identify optimal configuration for habitat structures. We use data collected from a non-endangered, conspecific population of the endangered riparian woodrat (Neotoma fuscipes riparia) to examine the degree to which the distribution of dens in translocation sites might influence the likelihood that animals persist in their new environment. We characterize the habitats in which dens occur, analyze their spatial clustering, and compare them to temporal changes in population status for sex and age classes. We compare the potential efficacy of translocation efforts using spatial analysis versus solely habitat-based approaches and identify the optimal spatial configuration of dens that should be considered in this translocation effort. We found that patterns of habitat use were positively correlated with overstorey cover, and animal weight was positively correlated with understorey cover. Woodrats appear to select den locations on the basis of understorey cover, but also benefit from dense overstorey cover and distance to nearest tree. Our results suggest that in translocation efforts, artificial dens should be placed in clusters within a radius of 15 m, as values above and below this value showed negative correlations with body mass. Translocations should occur after reproductive events, which occur in April and August for woodrats in southern California. Our analyses provide practical guidelines in determining appropriate timing and spacing for translocation events in the context of animal condition, minimizing disease transmission, and reproduction.

Habitat Use of the Key Largo Woodrat (Neotoma floridana smalli)

“…Occupancy also showed a trend toward higher occupancy along abandoned roads (p=0.12). Contrary to some of the previous research (Olsen 1973;Balcom & Yahner 1996;Gerber et al 2003), no forest metrics were found to be relevant to either the presence or occupancy of a woodrat nest.…”

“…All artificial and natural nests were photographed, described, and mapped using a GPS unit (eTrex Venture® HC, Garmin™). Since woodrats are known to use multiple nests clustered within their territories (Linsdale & Tevis 1956;Gerber et al 2003), I conducted adaptive surveys to increase the probability of finding additional nests (Smith et al 2004). This involved walking in a spiral out from the nest I originally found up to a distance of 30 m away ( Figure 4).…”

“…as nest substrate, since it offers protection against most predators (Brown et al 1972;Smith 1995). Similarly, dusky-footed woodrats (Neotoma fuscipes) will often choose areas thick with poison oak (Toxicodendron diversilobum), which may act both as a deterrent and a persistent ground cover to conceal their ground movements (Gerber et al 2003). The Allegheny woodrat is more or less limited to the rock formations and cliff faces of the Appalachian Mountains, which are thought to be more easily defended from predators (Newcombe 1930;Poole 1940;Balcom & Yahner 1996).…”

“…Woodrats can exhibit strong habitat preferences (Cameron & Rainey 1972;Wright 1973), and have been observed to choose nest sites on the basis of the defensibility of core substrate against predators (e.g., cacti; Brown et al 1972;Smith 1995), surrounding vegetation (e.g., Poison Oak; Gerber et al 2003), over-and understory cover (Olsen 1973;Gerber et al 2003), and variation in tree species assemblages (Balcom & Yahner 1996).…”

“…On the basis of previous studies that report woodrats nesting in trash piles and other anthropogenic substrates (Goodyear 1985;Shaw 1994;McCleery et al 2006;Winchester et al 2009) and the suggestion that woodrats may be limited by nest substrate (Winchester 2007;Winchester et al 2009), I hypothesized that woodrat nests would be more numerous and have higher occupancy rates in areas associated with human disturbance (old roads, paved roads, young forest). I also hypothesized that woodrats would choose sites on the basis of finer-scale forest structure traits that may not be captured in a forest age metric such as canopy cover and foliage density (Olsen 1973;Balcom & Yahner 1996;Gerber et al 2003).…”