Changes in landscape composition influence the decline of a threatened woodland caribou population

Abstract

Summary1. Large-scale habitat loss is frequently identified with loss of biodiversity, but examples of the direct effect of habitat alterations on changes in vital rates remain rare. Quantifying and understanding the relationship between habitat composition and changes in vital rates, however, is essential for the development of effective conservation strategies. 2. It has been suggested that the decline of woodland caribou Rangifer tarandus caribou populations in North America is precipitated by timber harvesting that creates landscapes of early seral forests. Such habitat changes have altered the predator-prey system resulting in asymmetric predation, where predators are maintained by alternative prey (i.e. apparent competition). However, a direct link between habitat condition and caribou population declines has not been documented. 3. We estimated survival probabilities for the threatened arboreal lichen-feeding ecotype of woodland caribou in British Columbia, Canada, at two different spatial scales. At the broader scale, observed variation in adult female survival rates among 10 distinct populations (range = 0·67-0·93) was best explained by variation in the amount of early seral stands within population ranges and population density. At the finer scale, home ranges of caribou killed by predators had lower proportions of old forest and more mid-aged forest as compared with multi-annual home ranges where caribou were alive. 4. These results are consistent with predictions from the apparent competition hypothesis and quantify direct fitness consequences for caribou following habitat alterations. We conclude that apparent competition can cause rapid population declines and even extinction where changes in species composition occur following large scale habitat change.

A synthesis of scale-dependent ecology of the endangered mountain caribou in British Columbia, Canada

“…Thus, the greatest support was for increasing predation rates causing the current decline of mountain caribou. Wittmer et al (2007) found the variation in adult female survival rates among 10 subpopulations was best explained by the amount of early-seral stands within subpopulation ranges and by subpopulation density. They also found that the home ranges of caribou killed by predators had lower proportions of old forest and more mid-aged forest as compared home ranges where caribou were alive.…”

The reintroduction of boreal caribou as a conservation strategy: A long-term assessment at the southern range limit

“…As reported by pioneer naturalists, and in the harvesting records of the Charlevoix region, moose densities began to increase in the late 1890s (Jolicoeur et al, 1993). Increased prey (i.e., moose) availability for wolves (Canis lupus) translated into increased wolf density and, indirectly, increased predation risk for caribou (Bergerud & Elliot, 1986;Seip, 1992;Rettie & Messier, 1998;Wittmer et al, 2007). Anthropogenic habitat modifications also favoured the northward expansion of white-tailed deer range and an increase in beaver (Castor canadensis) densities.…”

“…This hypothesis stipulates that caribou populations suffer from increased predation risk when wolf populations are maintained at high levels by alternative prey (Wittmer et al, 2007), a precarious situation for endangered populations (DeCesare et al, 2010). In Charlevoix, the situation is slightly different as caribou demography is more constrained by black bear (Ursus americanus) predation on calves than by wolf predation on adults.…”

Dispersal from the frying pan to the fire

“…Apparent competition means that caribou living in landscapes with large amounts of early-seral-stage forest also face reduced survival rates (Wittmer et al, 2007). Thus, the rare dispersing individuals will likely find they have moved to a situation even worse than the one they left.…”

“…With the exception of one subpopulation (for which the census area expanded, yielding a higher count), all subpopulations declined between 1990 and 2006. There is a growing consensus that these declines are the result of predator-mediated apparent competition (sensu Holt, 1977) -a process also linked to declines of woodland caribou elsewhere in Canada (Seip, 1992;James et al, 2004;Wittmer et al, 2007;Latham et al, 2011). As the area of late seral-stage coniferous forests (the preferred habitat of mountain caribou) has been reduced and fragmented by timber harvest, power transmission lines and roads, populations of other ungulate species have grown.…”