Effects of Disease, Dispersal, and Area on Bighorn Sheep Restoration

Abstract

We simulated population dynamics of bighorn sheep (Ovis canadensis) inhabiting six discrete habitat patches in the Badlands ecosystem, South Dakota. Modeled populations were subjected to a range of potential management actions and rates of disease‐causing infection. Simulated disease varied in severity from mild (∼12% mortality) to severe (∼67% mortality), with infections imposed once, at regular intervals, or with a fixed probability each year. In the absence of disease, 200‐year extinction rates were uniformly low and insensitive to changes in colonization rate or area of suitable habitat. A single infection, accompanied by change in the area of suitable habitat or colonization rate, resulted in extinction rates of up to 40%, and large changes in average population size (up to 10‐fold with changes in area; 4‐fold with changes in colonization rate). Simulations with multiple infections, which are probably most realistic, generally resulted in extinction rates that exceeded 20% over a 200‐year period. Model results clearly showed that efforts directed toward reducing the frequency or severity of disease are of highest priority for improving the success of attempts to restore bighorn sheep populations. Increases in areas of suitable habitat or improvements to corridors between existing habitat patches were far less likely to improve persistence of simulated sheep populations than reductions in the impact of disease. Although theory predicts that enhanced movements may exacerbate effects of disease, increased colonization rates resulted in relatively small but consistent increases in persistence and average population size for all combinations of parameters we examined.

Spatio‐temporal dynamics of pneumonia in bighorn sheep

“…Our observations concur with many of the results of previous studies, but also raise questions about disease models that assume all‐age pneumonia outbreaks followed by lamb mortality at a constant or declining rate for a period of usually 1–6 years (Gross, Singer & Moses ; Clifford et al . ; Cahn et al .…”

“…Pneumonia is a significant factor limiting the distribution and abundance of bighorn sheep (Gross, Singer & Moses ; Cassirer & Sinclair ; Boyce et al . ).…”

“…The aim of this paper was to use empirical data to describe these mortality patterns in detail and to develop hypotheses about the underlying processes involved. Indeed, a lack of data has so far constrained models of pneumonia dynamics in bighorn sheep (Hobbs & Miller ; Gross, Singer & Moses ; Clifford et al . ; Cahn et al .…”

Parasite invasion following host reintroduction: a case study of Yellowstone's wolves

“…More recently, we have found evidence that Yellowstone's wolf population has suffered negative growth rates associated with CDV and mange, and that several mange-associated declines have even continued to the point of pack extinction. However, in contrast to the North American black-footed ferret (Mustela nigripes) [57,58] and bighorn sheep (Ovis canadensis) [59][60][61] reintroductions, where repeated parasite invasions via spillover have posed a substantial challenge to recovery efforts, wolf recovery has remained a success despite the invasion of infectious disease. The spatial spread of mange has been largely consistent with the hypothesis of nearest neighbour, pack-to-pack spread as opposed to a hypothesis of frequent spillover events from outside the park.…”

Predicting population survival under future climate change: density dependence, drought and extraction in an insular bighorn sheep

“…2000; Festa‐Bianchet et al . 2006) and diseases (Gross, Singer & Moses 2000), it is most likely affected by food quality and availability determined by droughts (Young 1994; Epps et al . 2004).…”