Ord’s kangaroo rat (Dipodomys ordii) COSEWIC assessment and status report: chapter 9
Population Sizes and Trends
Since 1995, there has been considerable research on the life history, physiology, and landscape ecology of Ord’s kangaroo rats in Canada. However, population monitoring has not been a priority. The high turnover rates of subpopulations confounds making opportunistic population estimates alongside other focused research (Kenny 1989, Gummer and Robertson 2003b). Therefore, estimation of general population sizes and trends is difficult. Thus general trends in population size are probably best inferred from trends in the amount and quality of habitat.
These shortcomings aside, Kenny (1989) estimated the population size of kangaroo rats for the Great Sand Hills, Burstall Sand Hills, and Cramersburg Sand Hills in 1985 by combining live trapping results (densities) with interpretation of total potential habitat from aerial photographs. Kenny’s estimates likely represent 50% of the total Canadian habitat at the time. Since it is unlikely that all patches were simultaneously occupied, Kenny’s (1989) estimates should be viewed as liberal. Gummer (1997b) estimated the Middle Sand Hills population based on mark-recapture data in 1995. These estimates represent approximately 40% of the habitat at the time, but had broad confidence intervals owing to relatively low recapture success.
An important characteristic of the Canadian population of Ord’s kangaroo rats is large fluctuations. These have been inferred from survival rates of animals in relatively small study areas (Kenny 1989, Gummer 1997a, Gummer and Robertson 2003b, Gummer and Robertson 2003c, Gummer unpubl. data) and assumes that population fluctuations documented in the Middle Sand Hills (e.g., Gummer 1997a) are representative of the Canadian range.
Kenny (1989) estimated the maximum population size for the Great Sand Hills, Cramersburg Sand Hills, and Burstall Sand Hills to be 1370, with 95% confidence limits of 1120 and 1690. If this represented 50% of the Canadian population then the total population estimate would have been 2740 (2240 to 3380). Using Kenny’s (1989) over-winter survival estimate (25%), the total population during the subsequent spring, the seasonal low-point, would have been approximately 685 (560 to 845). Kenny (1989) suggested that kangaroo rat abundance during his study was low due to ongoing drought.
Gummer (1997b) estimated the population in the Middle Sand Hills in 1995 to be 3000 (2180 to 4160) at its seasonal peak. The corresponding total population estimate would be 7 500 (5 450 to 10 400). Based on Gummer’s (1997a) over-winter survival estimate (10%), the total population during the subsequent spring was approximately 750 (545 to 1040), similar to Kenny’s (1989) estimate. Thus it is likely that at least in some years the Canadian population of Ord’s kangaroo rat numbers less than 1000 individuals during early spring.
The Canadian population of Ord’s kangaroo rats experiences seasonal population fluctuations due to high summer reproductive output and low over-winter survival (Kenny 1989, Gummer 1997a, Gummer and Robertson 2003b). The population can decline by an order of magnitude (≤ 10% survival) during winter (Gummer 1997a). There is also a high frequency of local extinctions (i.e., sand dunes, road segments; Kenny 1989, Gummer and Robertson 2003b), further evidence of the severity of population fluctuations. In addition to within-year, seasonal fluctuations, strong inter-annual fluctuations in population size are likely, albeit difficult to quantify. The current data are not sufficient to evaluate inter-annual variation, nor long-term trends in population size. The loss of the population near Hilda (see Distribution) may be indicative of an overall decline in the Canadian population.
There is strong evidence that the natural habitats of kangaroo rats have changed drastically in recent decades. It is reasonable to assume that there have been long-term population declines, concomitant with historical losses of natural sandy areas. Because kangaroo rats colonize anthropogenic habitats, rates of decline of natural habitats do not necessarily equate to declines in total population size, per se, but are strongly suggestive of declines in habitat quality.
The likelihood of a rescue effect is negligible. The nearest conspecifics occur over 270 km to the south, beyond the dispersal capabilities of kangaroo rats. It is unlikely that translocations of animals from more southern localities would be effective or appropriate because the Canadian population may be endemic.
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