COS 143-1 - CANCELLED - Cascading effects of fitness in a territorial species

Background/Question/Methods

Trophic cascades occur when predators reduce prey abundance, thereby allowing the resources of prey to increase in abundance. An extension of this phenomenon is to individual animals. Focal individuals have immediate neighbours, who in turn also have neighbours. An immediate neighbour is as an enemy (analogous to prey), while a neighbour’s neighbour is an enemies enemy (analogous to predators) and in trophic cascades, an enemy’s enemy can be a friend. Within the context of individual animals, the potential for cascading effects is most likely realized through competition for limited resources and fitness associated with these resources. For resource limited systems, an increase in survival for one individual necessarily detracts from the probability of survival for other individuals. Using territorial North American red squirrels, we tested for cascading effects of fitness among triads of squirrels. We determined the distance between squirrel territories and developed a negative logarithmic decay function to estimate the effect of distance on competition such that competition for squirrels that live far away from each other is predicted to be minimal. Based on distance corrected competition, we modelled the potential for cascading effects of survival, and we asked whether an enemy’s enemy can be a friend.

Results/Conclusions

Depending on strength of competition, we found anywhere between 1-14% of triads display cascading fitness effects. The strength of cascading effects of fitness will depend on the synergistic effects of how close individuals live to one another and the probability of survival for enemy’s enemies and enemies. Cascading effects of fitness are predicted to be negligible in cases where focal individuals, their enemies, and their enemy’s enemies all live close to one another. By contrast, in cases where enemy’s enemies live far from focal squirrels, but relatively close to the enemies of the focal squirrel and where enemies of focal squirrels live close to focal squirrels, we observed a cascading effect of fitness. Notably, cascading effects of fitness increased by an order of magnitude when enemy’s enemies and enemies had higher survival than focal individuals. Taken together, our results are the first to identify cascading effects of fitness for any free-ranging animal system and they have potential to change how we understand first principles of evolutionary ecology.