Barred owls and landscape attributes influence territory occupancy of northern spotted owls.

Stan G Sovern,Brian L Biswell,Gail S Olson,Margaret Taylor,Eric D Forsman,Robert G Anthony

doi:10.1002/jwmg.793

Abstract

We used multi-season occupancy analyses to model 2 fates of northern spotted owl territories in relation to habitat amount, habitat fragmentation, and the presence of barred owls in Washington State, USA, 1989–2005. Local colonization is the probability a territory unoccupied by a spotted owl in year i would be occupied in year i + 1, and local extinction is the probability a territory that was occupied by a spotted owl in year i would be unoccupied in year i + 1. We found a negative relationship between local extinction probability and amount of late-seral forest edge. We found a negative relationship between colonization probability and the number of late-seral forest patches (higher fragmentation), and a negative relationship between colonization probability and the amount of non-habitat within 600 m of a spotted owl territory center (Akaike weight = 0.59). The presence of barred owls was positively related to extinction probability and negatively related to detection probability of spotted owls. The negative relationship between presence of barred owls and detectability of spotted owls indicated that spotted owls could be modifying their calling behavior in the presence of barred owls. The positive relationship between barred owl detections and local extinction probability suggests that because of competition with barred owls, spotted owls are being displaced. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

Highlights

We used multi-season occupancy analyses to model 2 fates of northern spotted owl territories in relation to habitat amount, habitat fragmentation, and the presence of barred owls in Washington State, USA, 1989–2005
We developed a visit-specific covariate that documented whether a barred owl was detected either vocally or visually during any visit to a territory (BAO), and we included a year-specific covariate (BAODIST) for each spotted owl territory that documented whether barred owls were detected within a 0.8-km radius of the territory center (Olson et al 2005)
When we added the barred owl covariates to our occupancy model for local extinction, we found that barred owl presence at a spotted owl territory was correlated with an increase in local extinction probability

Summary

Introduction

We used multi-season occupancy analyses to model 2 fates of northern spotted owl territories in relation to habitat amount, habitat fragmentation, and the presence of barred owls in Washington State, USA, 1989–2005. We found a negative relationship between local extinction probability and amount of late-seral forest edge. The presence of barred owls was positively related to extinction probability and negatively related to detection probability of spotted owls. The positive relationship between barred owl detections and local extinction probability suggests that because of competition with barred owls, spotted owls are being displaced. Most studies of the habitat relationships of spotted owls indicate that they are associated with late-successional forests (Forsman et al 1984, 2005; Wiens et al 2014). Occupancy modeling is advantageous because animals neither have to be detected during any given survey, nor marked to evaluate environmental relationships to spotted owl presence

Methods

Results

Conclusion