Trophic cascades and human disturbance: pumas, deer, and woody plants in the Santa Cruz Mountains

Background/Question/Methods

In a diverse landscape, predator and prey behavior is governed by tradeoffs between the costs (caloric loss or death) and benefits (forage quality or safety) of foraging in different areas.  Predator effects are traditionally thought of in terms of direct consumption, however, behavior-induced interactions driven by predation risk can have dramatic consequences for ecosystems.  Behavioral decisions of predators and prey interact with landscape features to create spatial heterogeneity in hunting grounds and prey refugia.  Differing levels of predation risk can lead prey to preferentially utilize “safe” microhabitats despite potentially suboptimal forage, and under utilize “dangerous” microhabitats despite potentially prime forage.  Differences in these use patterns can have cascading impacts on subsequent trophic levels.  In addition, human activities can alter the dynamics of these relationships.  Direct consumption and habitat fragmentation and loss change how species distribute themselves and how they use the landscape.  My research addresses how landscape attributes and human disturbance influence predation risk, and how differences in predation risk changes browse pressure.  I investigated kill site selection and the effect of housing on hunting locations by in mountain lions (Puma concolor).  I then used this information to examine how differences in hunting pressure and human disturbance influences mule deer (Odocoileus hemionus) browse activities. 

Results/Conclusions

Results suggest that pumas exhibit a threshold for human tolerance and preferentially utilize areas away from human structures when selecting kill sites.  Deer capitalize on this safety buffer created around human structures and change their activities accordingly.  As a result, deer browse on woody species is higher in “safe” areas close to human structures than “risky” areas away from human structures.  Though woody plant species composition was similar in near and far sites, browse rates were significantly different.  This suggests that deer may not be causing a shift in plant communities, however their activities may cause differences in individual plant life history or abundance.  This study suggests that humans can act as a super-predator and disrupt predator-prey and subsequent ecosystem dynamics.