Effects of a flooding event on a threatened black bear population in Louisiana

The Louisiana black bear, Ursus americanus luteolus, is listed as threatened under the Endangered Species Act as a result of habitat loss and human-related mortality. Information on population-level responses of large mammals to flooding events is scarce, and we had a unique opportunity to evaluate the viability of the Upper Atchafalaya River Basin (UARB) black bear population before and after a significant flooding event. We began collecting black bear hair samples in 2007 for a DNA mark-recapture study to estimate abundance (N) and apparent survival (phi). In 2011, the Morganza Spillway was opened to divert floodwaters from the Mississippi River through the UARB, inundating >50% of our study area, potentially impacting recovery of this important bear population. To evaluate the effects of this flooding event on bear population dynamics, we used a robust design multistate model to estimate changes in transition rates from the flooded area to non-flooded area (psi(F -> NF)) before (2007-2010), during (2010-2011) and after (2011-2012) the flood. Average N across all years of study was 63.2 (SE = 5.2), excluding the year of the flooding event. Estimates of psi(F -> NF) increased from 0.014 (SE = 0.010; meaning that 1.4% of the bears moved from the flooded area to non-flooded areas) before flooding to 0.113 (SE = 0.045) during the flood year, and then decreased to 0.028 (SE = 0.035) after the flood. Although we demonstrated a flood effect on transition rates as hypothesized, the effect was small (88.7% of the bears remained in the flooded area during flooding) and phi was unchanged, suggesting that the 2011 flooding event had minimal impact on survival and site fidelity.