COS 83-1 - Effects of ungulates on landscape heterogeneity: a mechanistic modeling approach

Background/Question/Methods

The deposition and removal of nutrients by animals can have drastic effects on nutrient cyclingand as animals move through the landscape, these zoogeochemical effects have important implications for ecosystem productivity and composition. Yet analyzing zoogeochemical effects at a landscape scale is logistical challenging. Further while the impact of grazing on nutrient cycles is well studied, and there is some work exploring the role of fecal and carcass deposition as well, research analyzing the net effects of zoogeochemical impacts remains scarce.





To address this, we present an agent-based model that examines how caribou translocate nutrients through the zoogeochemical processes of consumption, fecal deposition, and carcass deposition. Modeling the impact of caribou on nitrogen cycles over 15 years, we tease out the sole impact of consumption as well the impact of the cumulative effects of consumption and nutrient deposition (i.e. fecal waste and carcass deposition). We explored these dynamics in simulations that altered the context of environments, either initially containing heterogeneous or homogeneous nitrogen distributions, as well as three different animal densities. We then analyze changes in nutrient distributions at both the landscape and local scale. This iterative approach allows for a nuanced understanding of how animals mediate nutrient redistribution.

Results/Conclusions

In our consumption-only simu­lations, changes to caribou density created different patterns of heterogeneity at the landscape scale, with caribou increasing landscape heterogeneity in homogenous environments and decreasing landscape heterogeneity in initially homogenous environments. In the homogenous landscapes, caribou populations also crashed at high densities because the lack of animal deposition resulted in low resources across the landscape. This was not the case when considering the cumulative zoogeochemical effects, indicat­ing the return of nutrients from animals may be important for population stability. Additionally, in simulations considering the cumulative effects of caribou, increasing caribou density increased landscape heterogeneity irrespective of the initial condition (i.e. heterogeneous and homogeneous landscapes), and maintained or increased local heterogeneity in heterogeneous and homogenous landscape, respectively. Importantly, in all simulations the net impact of caribou at the individual patch level was extremely variable, suggesting that animal inputs are highly varied across the landscape. Our results indicate the movement of large ungulates such as caribou can increase the heterogeneity of available nutrients within a landscape and provide an important feed­back for population stability. Thus, the loss of large ungulates from natural ecosystems via anthropogenic activity is likely to result in less heterogeneous natural landscapes.