Population trend
Trend
Dean (1978) regards the White-tailed Rat as a relict species. Although conservationists have been concerned with it for over forty years, it still persists at low densities. While no empirical population estimates or trends are available, they are consistently one of the rarest species encountered and infrequent capture rates during surveys indicate that population size is low. Friedmann and Daly (2004) reported 5–10 specimens in 15–20,000 trap nights / year in the Free State Province (but only a small percentage of those trap nights took place in habitats where one expected to find the species). Similarly, with over 7,500 small mammal records collected over the current Northern Cape (concentrating in the old demarcated provincial boundaries) and current North-West (the western areas which used to be Northern Cape) provinces, only two records (from Benfontein Farm, near Kimberley) have been archived in the McGregor Museum (0.03% reporting rate, B. Wilson unpubl. data). The most recent museum record from North-West Province was from Boskop Dam Nature Reserve (de Beer 1990), but a trapping effort of 240 trap nights in 2013 did not detect the species (Power 2014). Recently, in the Blaauwberg Conservation Area, Western Cape, two studies had a trap success of 1.9% and 1.3% respectively (900 and 800 trap nights corresponding to 17 and 10 individuals respectively) (Vermeulen 2005, Specker 2006). The species may have a low sighting record as it is naturally rare and uses a burrow system.
Their patchy distribution may explain the lack of trapping success, but once a patch is found they are trapped easily if present. No home range or dispersal rate data are available, but records indicate a clumped distribution within suitable habitats. The two individuals trapped on Benfontein Farm, just east of Kimberley, were trapped in the same area, although several years apart, despite consecutive annual surveys at similar trapping effort (B. Wilson, unpub. data). Within the Free State Province, colonies do not seem to be in close proximity as, within a homogenous area-trap plot of 0.027 km2 (100 traps spaced 5 m apart with a border area of 25 m around the traps), a maximum of one colony is present (N.L. Avenant unpubl. data). Furthermore, a colony is found in approximately one in every 20 area-trap plots, which means colony density can be preliminarily estimated at one colony / 0.55 km2 or 1.8 colonies / km2 (N.L. Avenant unpubl. data). We can assume a minimum of two mature adults / colony (colonies are small, consisting of < 10 individuals). If we use maximum density as 1 colony / 0.027 km2 and minimum density as one colony / 1.05 km2 (assumed to be the positive interval between the maximum and medium density estimates), a range of population sizes can be estimated for the different AOO parameters described in Distribution (see Table 1 in the Supplementary Material PDF attached). Colony numbers and corresponding mature population size ranges over orders of magnitude, depending on the AOO estimate. The most likely density estimates (medium and minimum) range from 3,499 colonies (6,997 mature individuals) to 22,131 colonies (44,262 mature individuals). Given that colonies (breeding pairs) are suspected to be isolated and may effectively represent a single breeding unit, colony number is probably a more accurate measure of mature population size (sensu IUCN Standards and Petitions Subcommittee 2014).
Defining subpopulations for this species is difficult as we lack basic information about its ecology and distribution. However, we suspect that using vegetation types as proxies for subpopulation distribution may be an adequate approximation since the species may rely on fire to create suitable habitat patches and fire frequency will depend on the fuel load and climate of the vegetation type or bioregion. Subpopulations are suspected to be severely fragmented due to the fragmented nature of the grassland biome (Neke and du Plessis 2004), as well as the prevention of fire as an ecological process or cessation of burning as a management tool by landowners in many areas (A. Taylor unpubl. data). By treating each vegetation type within the grassland biome as a subpopulation (N = 71 subpopulations), and using the medium density estimate from Table 1 (as well as adjusting for the likely proportion of land that is occasionally burnt), average subpopulation size is 178 ± 177 colonies (357 ± 355 mature individuals), with a maximum of 760 colonies (1,521 mature individuals) in a subpopulation. However, we feel that these are overestimates given that the remaining area of the vegetation types were calculated using 1997 satellite data integrated with more recent regional data where possible, and thus mostly do not reflect the habitat lost over at least the past decade. The area currently available to each subpopulation is likely to be significantly less and thus subpopulation sizes are likely to be smaller or even locally extinct (Power 2014).