Modeling interactions between spotted owl and barred owl populations in fire-prone forests

Background/Question/Methods

Efforts to conserve northern spotted owls (Strix occidentalis caurina) in the eastern Cascades of Washington must deal with two challenges: 1) providing sufficient structurally complex forest habitat in a fire-prone landscape, and 2) addressing competitive interactions with a recently established population of barred owls (Strix varia).  Recent research has suggested that interference competition may result in spotted owls being excluded from barred owl breeding territories.  We developed an individual-based northern spotted owl population model (using the HexSim simulation framework) that could address the effects of displacement by territorial barred owls. Due to uncertainty regarding barred owl demographic rates, we used two approaches to model the distribution of territorial barred owls and associated displacement effects. In the first approach, we stochastically represented occurrence of territorial barred owls based on habitat characteristics in each hexagon in the study landscape. In the second approach, we explicitly modeled a population of barred owls using a dual-species implementation of HexSim. Areas occupied by territorial barred owls were attributed as being unavailable for spotted owl use in both modeling approaches.  Our models allowed us to examine potential future spotted owl distribution and population trends within the Okanogan-Wenatchee National Forest, Washington.

Results/Conclusions

We used HexSim to quantify spotted owl population size, spatial distribution of births and deaths, and other metrics, both with and without barred owl interactions.  Displacement by barred owls led to substantial differences in both size and distribution of the simulated spotted owl population. The two approaches for modeling barred owl distribution and displacement effects illustrate alternative strategies for simulating interactions between competing species to inform conservation planning.  We are also using our population models to evaluate conservation strategies for northern spotted owls in fire-prone forests using a management-focused multi-model framework that represents the interactions among forest management and fuels reduction strategies, forest growth rates, and risk of high intensity wildfire.