Conservation planning for species recovery under the Endangered Species Act: A case study with the Northern Spotted Owl

Abstract

The northern spotted owl (Strix occidentalis caurina) was listed as threatened under the U.S. Endangered Species Act (ESA) in 1990. We applied modern spatial conservation theory and models to evaluate several candidate critical habitat networks, and sought an efficient conservation solution that encompassed the highest value lands for spotted owl recovery rather than maximizing the total area of potential critical habitat. We created a map of relative habitat suitability, which served as input to the spatial conservation prioritization program Zonation. We used the spatially-explicit individual-based population model HexSim to estimate and compare simulated spotted owl population outcomes among a suite of candidate critical habitat networks that varied in size and spatial arrangement under alternative scenarios of future habitat suitability and barred owl (S. varia) effects. We evaluated simulated spotted owl population outcomes, including total population size, and extinction and quasi-extinction likelihoods for 108 combinations of candidate critical habitat networks by habitat change by barred owl scenarios, both range-wide and within 11 distinct portions of the owl's range. Barred owl encounter rates and the amount and suitability of habitat had substantial effects on simulated spotted owl populations. When barred owl encounter rates were high, changes in the amount and suitability of habitat had minimal impacts on population performance. Under lowered barred owl encounter rates, candidate critical habitat networks that included most existing high suitability habitat supported a high likelihood of long-term population persistence. Barred owls are currently the primary driving force behind poor population performance of NSOs; however, our models demonstrated that a sufficient area of high suitability habitat remains essential for recovery when effects of barred owls can be reduced. The modeling approach we employed is sufficiently flexible to incorporate new information about spotted owls as it becomes available and could likely be applied to conservation planning for other species.

Fig 1. Geographic extent of study area and modeling regions boundaries.

Fig 2. Flow chart of generalized modeling phases.

NR = nesting and roosting, F = foraging, RHS = relative habitat suitability, NWFP = Northwest Forest Plan.

Fig 3. Flowchart of development and evaluation of MaxEnt relative habitat suitability (RHS) models within each of 11 modeling regions.

Fig 4. Flowchart of steps using Zonation to identify alternative candidate critical habitat networks for northern spotted owls.

Fig 5. Flowchart of generalized steps involved in the northern spotted owl HexSim model.

Fig 6. Map of relative habitat suitability throughout the geographic range of the northern spotted owl.

Fig 7. Northern Spotted Owl strength of selection (SOS) by relative habitat suitability (RHS) bin mid-point and modeling region.

SOS is estimated by dividing the proportion of northern spotted owl site centers within a RHS bin by the proportion of the modeling region comprised of that bin (and thus represent relative densities of site centers). For values <1, we divided the SOS value into -1 to allow values <1 and >1 the potential to vary to the same extent. Without doing this, values <1 are constrained to be between 0 and 1, whereas values >1 can grow nearly infinitely. Legend acronyms are the 11 modeling regions and the mean of all regions combined.

Fig 8. Mean and 95% confidence intervals of simulated northern spotted owl population sizes among 11 composite candidate critical habitat networks and the Northwest Forest Plan (NWFP) size, based on the pessimistic habitat change scenario.

Fig 9

Maps of (a) Composite 11 (Critical Habitat designated in 2012), and (b) The Zonation scenario with 70% of habitat value, with emphasis on public lands. Although Z70PUB was more than twice the size of Composite 11, simulated spotted owl populations performed similarly.