Going against the flow: non-diffusive organismal movement influences local and meta-ecosystem functioning

Matteo Rizzuto, Shawn J. Leroux, Eric Vander Wal, Yolanda F. Wiersma and Travis R. Heckford, Department of Biology, Memorial University of Newfoundland, St. John's, NF, Canada, Oswald J. Schmitz, School of the Environment, Yale University, New Haven, CT

Background/Question/Methods
Many organisms move over landscapes, routinely crossing ecosystem borders and thus acting as vectors to connect ecosystems over time and space. Meta-ecosystem ecology has emerged as a theoretical foundation to study the effects of such movement on local and landscape level ecosystem properties and processes. The majority of models investigating the effects of organismal movement on local and meta-ecosystem functions model the movement of organisms as diffusion. This reduces model complexity and provides a reasonable approximation for certain types of organismal movement. However, diffusive movement further assumes that organisms exclusively move along concentration gradients of energy or nutrients availability. Yet, empirical evidence abounds with cases of animals moving against natural resource gradients. In this talk, we derive a meta-ecosystem model for non-diffusive herbivore movement across ecosystem borders and investigate its influence on local and meta-ecosystem functions. We apply separation of time scales to partition the herbivore movement coupling two local ecosystems from local ecosystem dynamics. We then use a mixed analytical and numerical analysis to study how herbivore movement across a soil fertility gradient influences local and meta-ecosystem biomass and primary and secondary production.
Results/Conclusions
Separating the time scale of local ecosystem dynamics and herbivore movement reveals that herbivore movement, which connects local ecosystems, can lead to strong trophic cascades across ecosystem boundaries. Herbivores leaving the donor ecosystem release primary producers from foraging pressure, which translates in local declines in limiting nutrients. Conversely, as herbivores enter the recipient ecosystem, increasing foraging pressure leads to the collapse of primary producers and to an increase in the recipient ecosystem’s nutrients stock. This spatial cascade compounds with landscape characteristics to influence local and meta-ecosystem functions. In homogeneous landscapes, the trophic cascades arising from non-diffusive herbivore movement establishes primary and secondary productivity differences among local ecosystems that are independent from local soil fertility conditions. Conversely, in heterogeneous landscapes, this spatial cascade exacerbates differences in local ecosystem fertility, leading to lower productivity in less fertile ecosystems. Our model results show that non-diffusive herbivore movement can influence local and meta-ecosystem processes and functions. By translocating biomass and nutrients as they move across ecosystem borders, herbivores can establish spatial patterns of resource availability. As well, moving against gradients, they may reinforce them—effectively engineering their environment. Our work supports and strengthens recent calls to more explicitly consider the role consumers play in shaping and maintaining biogeochemical cycles.