SNAMP Pub #21: Relation between Occupancy and Abundance for a Territorial Species, the California Spotted Owl
Article Title: Relation between Occupancy and Abundance for a Territorial Species, the California Spotted Owl
Authors: Douglas Tempel and Rocky Gutiérrez
- Land and resource managers often use occupancy surveys to monitor species of special concern. During occupancy surveys, surveyors simply note whether the species has been detected or not.
- Occupancy surveys are less costly than mark-recapture surveys, which usually require the physical capture of individuals detected during surveys, and can cover a larger geographic area for the same amount of money.
- Metapopulation models and species-abundance distributions suggest a strong relationship between site occupancy and abundance, so occupancy may provide a useful index for population size.
- Statistical advances in occupancy analyses allow for the incorporation of factors such as detection probability, covariates, and multiple seasons.
- To test the efficacy of using occupancy studies to infer population size, we compared long-term trends estimated from both occupancy and mark-recapture data and found close agreement between the two.
- We concluded that occupancy survey data can provide reliable inferences on population trends, at least for territorial species such as the spotted owl.
Mark-recapture studies are used by land and resource managers as a means of monitoring populations and provide detailed data on species demographic rates such as survival and recruitment. However, these studies are labor intensive, relatively costly, and generally encompass a small geographic area because of these constraints. Occupancy surveys may be a more cost-efficient method for monitoring populations as they only require noting the detection or non-detection of a species (that is, whether or not the species is found during a survey).
The California spotted owl (Strix occidentalis occidentalis) is considered a ‘sensitive’ species by the U.S. Forest Service (USFS) in the Sierra Nevada because it uses late-seral forests for nesting and roosting. This relationship prompted the USFS to develop a Sierra-wide management plan for the owl. The owl is highly territorial with large home ranges, displays strong site fidelity, and breeds irregularly. Current monitoring by the USFS is done by mark-recapture studies.
We conducted our study on a contiguous 35,500-ha area on the Eldorado National Forest in the central Sierra Nevada, California. This site, the Eldorado Density Study Area (EDSA), has been part of a long term mark-recapture study of California Spotted Owls since 1986. Surveys prior to 1993 were not included in our analysis because funding constraints limited survey data collection prior to 1993. We compared trends in territory occupancy with trends in abundance, which were estimated using mark-recapture data, to assess the appropriateness of using occupancy surveys to infer population size.
We estimated that territory occupancy declined by 30.0% from 1993-2010, due to increasing territory extinction rates and decreasing colonization rates. This decline was in close agreement with the population trend that we estimated using mark-recapture data. Here, we found that the population size declined by 28.5% because of decreasing recruitment of new owls into the population (adult survival was constant).
- The realized change in population we estimated based on occupancy closely matched the realized population change estimate from mark-recapture data. This result suggests occupancy monitoring may offer an accurate, cost-effective means to monitor territorial species over large geographic areas. This relationship may be less strong for non-territorial species.
- Our study suggests that future researchers incorporate ecological covariates to explain site-occupancy dynamics because it will help inform management decisions that affect California Spotted Owls.
- However, occupancy studies do not provide information on survival and reproduction which are important metrics that help explain population change. Mark-recapture studies provide this important demographic information. The choice of study design should depend on specific research or management objectives.
Tempel, Douglas J., and R.J. Gutiérrez. 2013. “Relation between Occupancy and Abundance for a Territorial Species, the California Spotted Owl,” in Conservation Biology, 27/5: 1087-1095.
For more information about the SNAMP project and the Owl team, please see the: Owl Team Website.