Spatially explicit capture-mark-recapture to evaluate demographic status of the Louisiana black bear

Abstract

Louisiana black bears ( Ursus americanus luteolus ) occur in semi-isolated fragments of bottomland hardwood forest in the lower Mississippi Alluvial Valley and were listed as threatened under the United States Endangered Species Act in 1992. A population viability analysis based on radio-telemetry and capture-mark-recapture (CMR) data collected from 2002 to 2012 revealed that the probability of bears persisting in at least 1 subpopulation was >0.999, which prompted the United States Fish and Wildlife Service to remove the Louisiana black bear from the threatened species list in 2016. A post-delisting monitoring plan was developed, which included continued noninvasive CMR surveys to monitor subpopulation trends. We used genetic CMR data based on non-invasively collected hair samples for a post-delisting assessment of the demographic status of the black bear subpopulations in Louisiana, USA. The assessment included primary range in the Tensas River Basin (TRB; 2006–2020), the Upper Atchafalaya River Basin (UARB; 2007–2020), and the Three Rivers Complex (TRC; 2014 and 2021), where bears had been reintroduced beginning in 2001, and adjacent areas of possible range expansion (i.e., secondary range). We used spatially explicit closed-population capture-recapture models to estimate abundance ( N ), density ( D ), and the realized population growth rate ( λ ) for female bears at TRB, UARB, and TRC. Model-averaged estimates of N for primary range at TRB ranged from 135 (95% CI = 121–149) female bears in 2006 to 140 (95% CI = 123–157) in 2020 with mean λ of 1.003 (95% CI = 0.991–1.015), which suggested a stable primary population. An estimated 56 (95% CI = 19–92) additional female bears occurred in secondary range at TRB in 2020. Model-averaged estimates of N for primary range at UARB ranged from 35 (95% CI = 28–43) females in 2007 to 33 (95% CI = 28–43) in 2020, with mean λ = 0.996 (95% CI = 0.972–1.020), again suggesting a stable population. We estimated 3 (95% CI = 0–6) additional female bears occurred in secondary range at UARB during 2020. The estimate of N at TRC was 28 (95% CI = 18–44) females in 2014 and 42 (95% CI = 19–94) in 2021. The number of bears in the secondary ranges suggests some bears may have emigrated from the primary areas and colonized secondary areas, likely facilitated by increases in bottomland hardwood forests adjacent to the core populations. The stable primary populations, the reintroduced population at TRC, and the number of bears in secondary range adjacent to TRB indicate the overall number of Louisiana black bears has increased since monitoring began.