Demographic rates and population viability of black bears in Louisiana

Abstract

The Louisiana black bear (Ursus americanus luteolus) was reduced to a few small, fragmented, and isolated subpopulations in the Lower Mississippi Alluvial Valley by the mid‐twentieth century resulting from loss and fragmentation of habitat. In 1992, the United States Fish and Wildlife Service (USFWS) granted the Louisiana black bear threatened status under the United States Endangered Species Act of 1973. Since that time, a recovery plan was developed, a reintroduced population was established, and habitat recovery has occurred. The Recovery Plan states that a minimum of 2 populations must be viable (i.e., persistence probabilities over 100 years >0.95), 1 in the Tensas River Basin and 1 in the Atchafalaya River Basin. Consequently, our objectives were to 1) estimate demographic rates of Louisiana black bear subpopulations, 2) develop data‐driven stochastic population projection models, and 3) determine how different projection model assumptions affect population trajectories and predictions about long‐term persistence. Our overall goal was to assess long‐term persistence of the bear subpopulations in Louisiana, individually and as a whole. We collected data using varying combinations of non‐invasive DNA sampling, live capture, winter den visits, and radio monitoring from 2002 to 2012 in the 4 areas currently supporting breeding subpopulations in Louisiana: Tensas River Basin (TRB), Upper Atchafalaya River Basin (UARB), Lower Atchafalaya River Basin (LARB), and a recently reintroduced population at the Three Rivers Complex (TRC). From 2002 to 2012, we radio monitored fates of 86 adult females within the TRB and 43 in the TRC. Mean estimates of annual adult survival for the TRB and TRC were 0.997 and 0.990, respectively, when unknown fates were assumed alive and 0.970 and 0.926 when unknown fates were assumed dead. From 2003 to 2013, we observed 130 cub litters from 74 females in the TRB, and 74 cub litters from 45 females in the TRC. During the same period, we observed 43 yearling litters for 33 females in the TRB and 21 yearling litters for 19 females in the TRC. The estimated number of cubs and number of yearlings produced per breeding adult female was 0.47 and 0.20, respectively, in the TRB and 0.32 and 0.18 in the TRC. On the basis of matrix projection models, asymptotic growth rates ranged from 1.053 to 1.078 for the TRB and from 1.005 to 1.062 for the TRC, depending on how we treated unresolved fates of adult females. Persistence probabilities estimated from stochastic population models based on telemetry data ranged from 0.997 to 0.998 for the TRC subpopulation depending on model assumptions and were >0.999 for the TRB regardless of model assumptions. We extracted DNA from hair collected at baited, barbed‐wire enclosures in the TRB, UARB, and LARB to determine individual identities for capture‐mark‐recapture (CMR) analysis. We used those detection histories to estimate apparent survival (φ), per‐capita recruitment (f), abundance (N), realized growth rate (λ), and long‐term viability, based on Baye...

No flood effect on recruitment in a Louisiana black bear population

“…The point estimates of recruitment probabilities during the flood years ( f 5 –f 6 ) were lower than other years, but confidence intervals extensively overlapped. Despite the flood, the Spillway portion of this population maintained an overall growth rate similar to that estimated by Laufenberg et al (; λ = 1.059; Fig. ).…”

“…However, 2‐year‐olds were available for detection (17 of 30 live‐captured bears were previously detected as 2‐year‐olds). That annual detection probability for 2‐year‐olds (0.567) translates to a weekly probability ( p ) of approximately 0.1 (assuming an 8‐week sampling period), and is roughly similar to probabilities reported by Laufenberg et al () for all age classes sampled by the hair traps. Based on those data, we concluded that cubs were likely not sampled by our hair traps.…”

“…Adult female survival is a main influence on black bear population dynamics (Freedman et al , Hebblewhite et al , Beston ) and survival probabilities in the Upper Atchafalaya River Basin population were similar to or higher than other non‐hunted bear populations in the southeastern United States (Clark and Smith , Dobey et al , Clark and Eastridge , Laufenberg et al ). High survival can often compensate for low recruitment in black bears and Laufenberg et al () reported negative density‐dependent effects between N and f in the Upper Atchafalaya. O'Connell‐Goode et al () reported that most bears did not leave the Spillway during the flood but instead remained in trees or small remnants of land that were not inundated with floodwaters.…”

“…Since the O'Connell‐Goode et al () study, Laufenberg et al () conducted a population viability analysis, which estimated that the probability of persistence of bears in ≥1 of the Louisiana populations was >0.999, and was 0.906 to 0.959 in the Upper Atchafalaya River Basin population, depending on model assumptions. Based largely on those findings, Louisiana black bears were removed from the List of Threatened and Endangered Wildlife (USFWS ) and the USFWS implemented a post‐delisting monitoring plan that prescribed annual capture‐mark‐recapture monitoring through 2022 (USFWS ).…”

“…With changes in human land uses and potential effects of climate change, however, future flooding events may become more commonplace in the areas of Louisiana occupied by bears (Milly et al 2002). Flooding could directly affect cub recruitment and possibly introduce more stochasticity than was estimated by Laufenberg et al (2016) for population viability analysis, potentially lowering probability of persistence. We speculated that, if the 2011 flood event caused high cub mortality, that age cohort would be small and such a change would be reflected in lower per capita recruitment (f).…”