Resource competition and apparent competition in declining mule deer (Odocoileus hemionus)

Abstract

Resource competition and apparent competition have both been suggested as the cause of mule deer (Odocoileus hemionus (Rafinesque, 1817)) decline concurrent with white-tailed deer (Odocoileus virginianus (Zimmerman, 1780)) increase. I tested for both hypotheses by conducting a “press” and “release” experiment in a mule deer, white-tailed deer, and cougar (Puma concolor (L., 1771)) community. If resource competition is causal, then predation should decrease, but other sources of mortality should increase following increased mortality of cougars and release of competing white-tailed deer. If apparent competition is causal, then predation should decrease and mule deer should increase following increased mortality of cougars and release of white-tailed deer. I accepted the apparent competition hypothesis because high mortality of female cougars and cougar population decline was associated with both white-tailed deer and mule deer population growth. Very high mortality of female cougars appeared to result in mule deer population recovery. However, high mortality of male cougars (with increased male immigration) preceding high female mortality appeared to result in sexually segregated prey-switching by females with cubs from abundant white-tailed deer to rare mule deer to avoid sexually selected infanticide. High mortality of resident male cougars may have precipitated the mule deer decline in the first place.