Transient dynamics of invasive competition: Barred Owls, Spotted Owls, habitat, and the demons of competition present

Abstract

Abstract. The recent range expansion of Barred Owls (Strix varia) into the Pacific Northwest, where the species now co-occurs with the endemic Northern Spotted Owl (Strix occidentalis caurina), resulted in a unique opportunity to investigate potential competition between two congeneric, previously allopatric species. The primary criticism of early competition research was the use of current species' distribution patterns to infer past processes; however, the recent expansion of the Barred Owl and the ability to model the processes that result in site occupancy (i.e., colonization and extinction) allowed us to address the competitive process directly rather than inferring past processes through current patterns. The purpose of our study was to determine whether Barred Owls had any negative effects on occupancy dynamics of nesting territories by Northern Spotted Owls and how these effects were influenced by habitat characteristics of Spotted Owl territories. We used single-species, multi-season occupancy models and covariates quantifying Barred Owl detections and habitat characteristics to model extinction and colonization rates of Spotted Owl pairs in southern Oregon, USA. We observed a strong, negative association between Barred Owl detections and colonization rates and a strong positive effect of Barred Owl detections on extinction rates of Spotted Owls. We observed increased extinction rates in response to decreased amounts of old forest at the territory core and higher colonization rates when old-forest habitat was less fragmented. Annual site occupancy for pairs reflected the strong effects of Barred Owls on occupancy dynamics with much lower occupancy rates predicted for territories where Barred Owls were detected. The strong Barred Owl and habitat effects on occupancy dynamics of Spotted Owls provided evidence of interference competition between the species. These effects increase the importance of conserving large amounts of contiguous, old-forest habitat to maintain Northern Spotted Owls in the landscape.

Government-sponsored Falconry Practices, Rodenticides, and Land Development Jeopardize Golden Eagles (Aquila chrysaetos) in Western China

“…Persistence and colonization rates did not differ between study areas, suggesting habitat suitability was comparable across the study area. Colonization probabilities that increase from an initial low point can be expected in studies like ours that begin with a sample of recently occupied territories because high initial occupancy leaves few territories available for colonization (Dugger et al 2011). Apparent declines in occupancy and persistence over the study period may also be artifacts of our sampling design: because we started with a nearly saturated sample, the initial decline and subsequent increase in these rates may actually reflect the return of the sample to an unknown average level of occupancy.…”

“…The studies described above focused on relatively smaller areas chosen for high densities of nesting Golden Eagles, whereas we may have observed lower occupancy rates, in part, because our larger study area incorporated more marginally suitable habitat in which some territories were only sporadically occupied. This is the first study, to our knowledge, to estimate the occupancy dynamics of persistence and colonization for Golden Eagle territories, although dynamic occupancy models have been used to analyze longterm datasets on other raptors, including Peregrine Falcons (Falco peregrinus; Bruggeman et al 2016) and Spotted Owls (Strix occidentalis; Olson et al 2005, Dugger et al 2011. Persistence and colonization rates did not differ between study areas, suggesting habitat suitability was comparable across the study area.…”

“…To avoid fitting large numbers of models for all possible combinations of covariates, we developed models using a stepwise approach (e.g., Olson et al 2005, Dugger et al 2011: (1) we considered a suite of models for p, while maintaining a general structure on other parameters (i.e., group þ linear time trend). We considered models for p in which detection among years was constant, year-specific, and a function of additive and quadratic trends; and detection within seasons was constant and a function of additive trends.…”

Effects of Fire and Commercial Thinning on Future Habitat of the Northern Spotted Owl

“…Unoccupied California Spotted Owl territories had a lower probability of re-occupancy after timber harvest, even when habitat alterations comprised <5% of a territory (Seamans and Gutiérrez 2007). In addition, Barred Owls (S. varia), which out-compete spotted owls (Dugger et al 2011), use younger and more open forests compared to Northern Spotted Owls (Wiens 2012).…”

“…Furthermore, owls often move outside core areas (USFWS 2011). Lastly, available habitat outside existing cores may become important to owl recovery, particularly if spotted owls are displaced from higher quality habitat by Barred Owls (Dugger et al 2011).…”