The roles of competition and habitat in the dynamics of populations and species distributions

Abstract

The role of competition in structuring biotic communities at fine spatial scales is well known from detailed process-based studies. Our understanding of competition's importance at broader scales is less resolved and mainly based on static species distribution maps. Here, we bridge this gap by examining the joint occupancy dynamics of an invading species (Barred Owl, Strix varia) and a resident species (Northern Spotted Owl, Strix occidentalis caurina) in a 1000-km study area over a 22-year period. Past studies of these competitors have focused on the dynamics of one species at a time, hindering efforts to parse out the roles of habitat and competition and to forecast the future of the resident species. In addition, while these studies accounted for the imperfect detection of the focal species, no multi-season analysis of these species has accounted for the imperfect detection of the secondary species, potentially biasing inference. We analyzed survey data using models that combine the general multistate-multi-season occupancy modeling framework with autologistic modeling, allowing us to account for important aspects of our study system. We found that local extinction probability increases for each species when the other is present; however, the effect of the invader on the resident is greater. Although the species prefer different habitats, these habitats are highly correlated at the patch scale, and the impacts of invader on the resident are greatest in patches that would otherwise be optimal. As a consequence, competition leads to a weaker relationship between habitat and Northern Spotted Owl occupancy. Colonization and extinction rates of the invader are closely related to neighborhood occupancy, and over the first half of the study the availability of colonists limited the rate of population growth. Competition is likely to exclude the resident species, both through its immediate effects on local extinction and by indirectly lowering colonization rates as Northern Spotted Owl occupancy declines. Our analysis suggests that dispersal limitation affects both the invasion dynamics and the scale at which the effects of competition are observed. We also provide predictions regarding the potential costs and benefits of managing Barred Owl populations at different target levels.

Tamm Review: Management of mixed-severity fire regime forests in Oregon, Washington, and Northern California

“…BDOs appear to be displacing NSOs from suitable habitat, particularly in moist valley bottom settings that are preferred by BDOs (Singleton et al, 2010;Singleton, 2013;Yackulic et al, 2014;Wiens et al, 2014). BDOs are also increasing in number and distribution in the Sierra Nevada, and are an increasing risk factor for CASPOs (Keane, 2014).…”

“…(4) Today, competitive interactions with recently established BDO populations are impacting NSO populations. BDOs are apparently displacing NSO from valley bottom habitats they prefer (Singleton, 2013;Yackulic et al, 2014). (5) Other prey species like the bushy-tailed woodrat occupy early-or mid-seral patches within MSForest landscapes because preferred mast species are more common there.…”

Potential for camera-traps and spatial mark-resight models to improve monitoring of the critically endangered West African lion (Panthera leo)

“…Population estimation from camera surveys could be expanded to other species, although caution should be exercised when using cameras spaced and sited for large carnivores to detect other non-target or secondary target species (Meek et al 2014;Mann et al 2014). Increased camera surveys in the region could allow for baseline population estimates of many mammalian species from a common survey effort, and thus a better understanding of community interactions such as competition between predator populations and predator-prey dynamics (Yackulic et al 2014).…”

Forest ecosystem restoration: Initial response of spotted owls to partial harvesting

“…Woodrats and northern flying squirrels most likely co-dominated owl diets in the Mixed Conifer areas, woodrats predominated in Coast Redwood study areas (Ward et al, 1998), while northern flying squirrels likely were most prominent in owl diets in the Douglas-fir zone (Forsman et al, 2004). During our study, northern barred owls (Strix varia), a major competitor for prey and nest sites (USFWS, 2011;Wiens et al, 2014;Yackulic et al, 2014), were abundant only in the Douglas-fir study area near Springfield, Oregon at an approximate ratio of 3 barred owl pairs per spotted owl pair. During our studies, we were aware of two locations with barred owls in the Klamath Falls study area, one location with a pair in the Ft. Bragg study area and none in the Chico study area.…”