| Apparent competition has been suggested as a contributing factor in the decline of mule deer (Odocoileus hemionus) populations in southern British Columbia and northern Washington. Apparent competition, whereby two prey species compete indirectly through a shared predator, can mimic direct competition by having a negative effect on one or both prey species. For cougars (Puma concolor) to be responsible for mule deer decline through apparent competition they must firstly have a numerical response which is tied to their prey either in a short term aggregative response, or long term numeric response. If present, the effects of apparent competition are magnified if the prey are not food limited, if one prey species has access to a refuge from predation that is not accessible by the second prey, if cougars do not switch between abundant prey, and if cougars do not limit their own density through a land tenure system. From 2001 to 2006 I conducted research testing the apparent competition hypothesis in a cougar, mule deer, white-tailed deer (Odocoileus virginianus) community in northeastern Washington. Under high harvest pressure the cougar population maintained itself through immigration of juveniles that were likely responding to high habitat quality/prey abundance. This suggests that cougar numerical response may ultimately be set by prey availability. I also found that, at a finer scale, cougar aggregative response is best predicted by the density of their primary prey (white-tailed deer). I found no conspecific avoidance between cougars and limited avoidance of human habitation. Finally, with the reduction of cougar populations the growth rates of both species of deer increased significantly suggesting that their populations are predator and not food limited. In conjunction, these three papers give strong support for the presence of apparent competition in this system. |
