Dual-track habitat management needed to prevent extinction

Posted on July 1, 2013

Habitat fragmentation has led to dramatic reductions in the range and population size of many freshwater species.  One such species is the inland cutthroat trout (Oncorhynchus clarkii), which is now absent from over 90% of its historic range and is listed as threatened.   Populations of inland cutthroat trout are limited to small sections of headwater streams where their isolated nature puts them at risk of local extinction.  Biologists at the University of Nevada sampled trout from 13 rivers and creeks to examine the impact of stream size and stream habitat  on population viability and extinction risk.  The effective population size (the number of individuals contributing offspring to the next generation) was related to the density of stream pools, but, counterintuitively perhaps, this relationship was an inverse one.  The likely reason for the relationship was the fact that pool habitats are commandeered by older (breeding) fish.  As a result, the younger age classes are excluded and the genetic diversity of the population is reduced.  Another unexpected result was the lack of a relationship between habitat variables and the risk of extinction.  These findings show that factors that affect population viability don’t necessarily affect extinction risk, which suggests that management actions to increase short-term population persistence might be ineffective in safeguarding genetic diversity and population survival in the long term.  It follows that the best management strategy is to simultaneously increase instream habitat diversity (to maximize the survival of all age classes) and the overall amount of occupied stream habitat. 

Reference:  Peacock, M.M. &  Dochtermann, N.A.   2012.  Evolutionary potential but not extinction risk of Lahontan cutthroat trout (Oncorhynchus clarkia henshawi) is associated with stream characteristics.  Canadian Journal of Fisheries and Aquatic Sciences 69,615–626.