4 September 2014 Science Briefs

SNAMP Pub #30: Using integrated population models to improve conservation monitoring: California spotted owls as a case study

Article Title: Using integrated population models to improve conservation monitoring: California spotted owls as a case study

Authors: Douglas J. Tempel, M.Z. Peery, R.J. Gutiérrez

Research Highlights:


  • Researchers used an integrated population model to estimate population trends and demographic rates in a population of California spotted owls (Strix occidentalis occidentalis)

  • They observed a 50% population decline from 1990 to 2012, as evidenced by the geometric mean of the finite annual rate of population change and the resulting realized population change

  • The estimated decline was considerably greater than the approximately 30% decline from 1993 to 2010 estimated using conventional mark-recapture and occupancy approaches

Background:

The literature on integrated population modeling (IPM) to date has focused primarily on model development and evaluation with few “real-world” applications. We developed an IPM to estimate finite annual rates of population change and realized population change over a 23-year period in a demographically open population of California spotted owls in the central Sierra Nevada. Our IPM incorporated data on population counts, mark-recapture histories, and reproduction. We used a multi-state occupancy model to obtain annual “counts” of the number of adults and young produced, rather than using naïve counts that did not account for imperfect detection. These counts were then used as input data, along with the mark-recapture and reproductive data, for the IPM. The IPM allowed for the inclusion of three years of data from the beginning of the study that were omitted from previous analyses to meet the assumptions of mark-recapture models. The larger decline estimated by the IPM, relative to previous occupancy modeling, was partly due to an increase in the number of territories occupied by single owls over the study period.

Results:

We found a 50% decline in owl abundance on our study area from 1990 to 2012. Annual rates of population change were generally only slightly below 1.0, but the cumulative effect of small annual declines resulted in a large population decline over the 23-year period. The observed decline in abundance was considerably greater than declines previously estimated from separate analyses of mark-recapture and occupancy data collected from 1993 to 2010. This discrepancy was the combined result of including an additional five years of data in the IPM analysis and the increase in territories that were occupied by single owls. Apparent adult spotted owl apparent survival was high and had low temporal variation, which was consistent with prior research results. In contrast, apparent juvenile apparent survival was very low, but it was likely that a significant number of juveniles emigrated from our study area. Thus, we suspect that the true juvenile survival rate was substantially higher than apparent survival. The finite rate of population growth was most strongly correlated with the immigration rate, which was fairly high (almost 10% of individuals present in a given year were immigrants from outside of our study area).

Conclusions:


  1. The IPM provided a much more precise estimate of the total change in population size than the previous mark-recapture analysis and a somewhat more precise estimate of change than the previous occupancy analysis.

  2. The strong correlation between immigration rate and population growth rate suggested that changes in owl abundance were affected by processes occurring outside of our study area.

  3. In conjunction with recent estimates of population decline within two other regions of the Sierra Nevada, our findings have potentially important ramifications for forest management in the Sierra Nevada, where a primary goal of U.S. Forest Service management is to maintain viable spotted owl populations. Furthermore, our results suggest that rigorous monitoring of this subspecies should continue.

Full Reference:

Tempel, Douglas J., M.Z. Peery, R.J. Gutiérrez. 2014. “Using integrated population models to improve conservation monitoring: California spotted owls as a case study”, in Ecological Modelling, 289 (2014) 86-95.

The full paper is available upon request at: dtempel@wisc.edu

For more information about the SNAMP project and the California spotted owl team, please see: http://snamp.cnr.berkeley.edu/teams/owl.

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