Competitive interactions and resource partitioning between northern spotted owls and barred owls in western Oregon

摘要: The federally threatened northern spotted owl (Strix occidentalis caurina) is the focus of intensive conservation efforts that have led to much forested land being reserved as habitat for and associated wildlife species throughout Pacific Northwest United States. Recently, however, a relatively new threat owls has emerged in form an invasive competitor: congeneric barred (S. varia). As rapidly expanded their populations into entire range owl, mounting evidence indicates they are displacing, hybridizing with, even killing owls. expansion by western North America made already complex issue more contentious, lack information on ecological relationships between 2 hampered recovery We investigated spatial relationships, use, diets, survival, reproduction sympatric Oregon, USA, during 2007–2009. Our overall objective was determine potential possible consequences competition space, habitat, food these previously allopatric species. study included 29 28 were radio-marked 36 neighboring territories monitored over 24-month period. Based repeated surveys both species, number occupied pairs 745-km2 area (82) greatly outnumbered those (15). Estimates mean size home ranges core-use areas (1,843 ha 305 ha, respectively) 2–4 times larger than (581 ha 188 ha, respectively). Individual adjacent often had overlapping ranges, but interspecific space sharing largely restricted broader foraging with minimal overlap among areas. used information-theoretic approach rank discrete-choice models representing alternative hypotheses about influence forest conditions, topography, interactions species-specific patterns nighttime resource selection. Spotted spent disproportionate amount time steep slopes ravines dominated old (>120 yr) conifer trees. Barred available types evenly owls, most strongly patches large hardwood trees flat along streams. differed relative use (greater owls) slope conditions (steeper owls), we found no young, mature, riparian-hardwood types. Mean proportional different individual 81% (range = 30–99%). best model indicated probability location substantially reduced if within or close proximity owl. pellet analysis measures food-niche dietary spatially identified 1,223 prey items from 15 4,299 24 Diets nocturnal mammals, diets many terrestrial, aquatic, diurnal rare absent Northern flying squirrels (Glaucomys sabrinus), woodrats (Neotoma fuscipes, N. cinerea), lagomorphs (Lepus americanus, Sylvilagus bachmani) primary accounting 49% total biomass respectively. moderate (42%; range = 28–70%). displayed demographic superiority owls; annual survival known-fate analyses (0.81, SE = 0.05) lower (0.92, SE = 0.04), produced average 4.4 young 3-year strong, positive relationship seasonal (6-month) probabilities proportion which suggested availability limiting factor competitive increased linearly increasing distance territory center pair all attempted nest 1.5 km failed successfully produce young. strong associations presence behavior fitness shown changes movements, reproductive output exposed levels territorial When viewed collectively, our results support hypothesis interference can constrain critical resources required successful recruitment Availability forests appeared be factors indicating further loss lead increases pressure. findings broad implications suggest heterogeneity vital rates may not arise solely because differences quality abundance also distribution newly established competitor. Experimental removal could test this whether localized control numbers ecologically practical socio-politically acceptable management tool consider strategies © 2013 Wildlife Society