Ecology of Florida Black Bears in the Okefenokee-Osceola Ecosystem

• Published in: Wildlife monographs no. 158; pp. 1 - 41
• Main Authors: Dobey, Steven, Masters, Darrin V., Scheick, Brian K., Clark, Joseph D., Pelton, Michael R., Sunquist, Melvin E.
• Format: Journal Article
• Language: English
• Published: The Wildlife Society 01-01-2005
• Subjects: Bears; Black bears; Diet; Hair; Litter size; Population estimates; Population size; Swamps; Wildlife ecology; Wildlife management
• ISSN: 0084-0173; 1938-5455

The population status of the Florida black bear (Ursus americanus floridanus) is problematic within many portions of its range and its potential listing as a federally threatened species has been the subject of legal debate. We studied Florida black bears in 2 areas in the Okefenokee-Osceola ecosystem in southeast Georgia (i.e., Okefenokee) and north Florida (i.e., Osceola) from 1995 to 1999 to evaluate relationships between population characteristics, habitat conditions, and human activities. Bears in Okefenokee were hunted and those in Osceola were not. We captured 205 different black bears (124M:81F) 345 times from June 1995 to September 1998. We obtained 13,573 radiolocations from 87 (16M:71F) individual bears during the study. In Okefenokee, black gum (Nyssa sylvatica) and saw palmetto (Serenoa repens) fruits were the most important foods for bears based on scat analysis. In Osceola, corn from white-tailed deer (Odocoileus virginianus) feeders was the most stable food source but saw palmetto was heavily used when available. Corn from deer feeders was not available in Okefenokee. Adult bears in Osceola were 29% heavier than those in Okefenokee ($t_{82}=3.55$, P < 0.001). The mean annual home-range size for Osceola females ($\overline{x}=30.3\ {\rm km}^{2}\pm 4.0\ [{\rm SE}]$, n = 53) varied little seasonally or annually and was almost half that of Okefenokee females ($55.9\ {\rm km}^{2}\pm 6.9$, n = 69; Z = -2.47, P = 0.014). In contrast, radiocollared females in Okefenokee expanded their home ranges during years of poor black gum production. That expansion was most apparent between autumn 1998 and 1999, when mean home-range size for Okefenokee females increased from $14.5\ {\rm km}^{2}$ to $78.4\ {\rm km}^{2}$, respectively, and included a larger proportion of upland areas open to sport hunting. As a result, 5 females were harvested in the Okefenokee study area during the 1999 bear hunting season compared with only 7 harvested from 1996 to 1998. Home ranges of adult female bears were located in areas with disproportionately high loblolly bay (Gordonia lasianthus) and gum-bay-cypress (Taxodium spp.) vegetation associations in Okefenokee and gum-bay-cypress associations in Osceola. The pine vegetation association ranked lower than most other associations within the home ranges of bears in both study areas even though much of the summer and autumn diets of bears included food items found almost exclusively in pine. Sixteen mortalities of radiocollared bears were documented in Okefenokee; hunting accounted for 11 (68.8%) of these deaths. The annual survival rate of radiocollared males in Okefenokee was 0.71 (95% CI = 0.53-0.88) whereas survival of females in Okefenokee was higher (Z = 18.87, P < 0.001) at 0.89 (95% CI = 0.83-0.95). The survival rate for females in Osceola was 0.97 (95% CI = 0.92-1.00). Overall, 67 bears (51M:16F) were killed by hunters in the Okefenokee study area from 1995 to 1999. Based on all radiocollared bears in Okefenokee, many of which resided within areas closed to hunting, we estimated an annual harvest rate of 0.22 (95% CI = 0.03-0.37) for males and 0.07 (95% CI = 0.01-0.12) for females. When we excluded those bears that were not in areas open to hunting, however, the annual harvest rate rose to 0.37 (95% CI = 0.07-0.58) for males and 0.39 (95% CI = 0.09-0.58) for females. Following a black gum shortage in autumn 1995, only 1 of 15 radiocollared females in Okefenokee produced cubs in winter 1996. That low reproductive rate was in contrast to winter 1997, which followed heavy black gum production, when 21 of 22 radiocollared females produced cubs. Reproductive output was more consistent in the Osceola study area, with 46 cubs being produced from 8, 5, and 9 litters in 1997, 1998, and 1999, respectively. To estimate population size, we maintained 88 and 94 barbed-wire hair traps during 1999 in the Okefenokee and Osceola study areas, respectively. Using DNA collected at the hair traps, mark-recapture models produced estimates of 71 bears (95% CI = 59-91) in Okefenokee and 44 bears (95% CI = 40-57) in the Osceola study area during 1999. The estimated densities in the Okefenokee and Osceola study areas were 0.12 and $0.14\ \text{bears}/{\rm km}^{2}$, respectively. Alternative density estimates based on the amount of time bears spent within study area boundaries were similar (0.11 and $0.14\ \text{bears}/{\rm km}^{2}$ on Okefenokee and Osceola, respectively). We used a population model to estimate the effect of harvest in the Okefenokee bear population. Excluding harvest, bears at Osceola experienced higher average annual population growth (λ = 1.184 ± 0.002) than those at Okefenokee (1.064 ± 0.002; $t_{18}=3.93$, P = 0.001), most likely due to protection from hunting and higher recruitment. Including the effects of emigration and immigration, we estimated an average annual sustainable harvest at Okefenokee of approximately 9 bears (12.6%), which was slightly less than the average 1995-1999 annual harvest of 9.4. That level of hunting in Okefenokee is sustainable but likely represents the highest exploitation rate in the region. Our mark-recapture data from Osceola suggest a high dispersal rate by subadult bears, and our population modeling data support this hypothesis; we documented bears in Okefenokee that originated from Osceola but not the reverse. We speculate that bears from the interior of the Okefenokee National Wildlife Refuge (ONWR), and to some extent northern Florida, served as a source to the population sink caused by hunting mortality in Okefenokee and in the surrounding Georgia counties. Corn from deer feeders was the most probable reason for smaller home-range sizes and greater body masses and reproductive output at Osceola. Changes in management to eliminate or reduce baiting for deer with corn would negatively affect the Osceola bear population. On Okefenokee, sporadic black gum and palmetto production influenced harvest rates and cub production and, thus, governed bear population dynamics. The U.S. Fish and Wildlife Service (USFWS) concluded in 1998 that listing the Florida black bear as threatened under the Endangered Species Act of 1973 was not warranted. That decision was largely based on the stability and protection afforded to a few subpopulations within the range of the subspecies, which includes the Okefenokee-Osceola subpopulation; our results support that conclusion. However, we suggest that metapopulation processes among the various subpopulations be given greater consideration, with the ultimate goal of managing the subspecies as a unit rather than as an assemblage of independent components. Our study illustrates the importance of travel corridors for maintaining metapopulation processes. /// El estado de las poblaciones del oso negro de Florida (Ursus americanus floridanus) es considerado problemático dentro de muchas partes de su rango de distribución y ha sido tema de debate legal su potencial clasificación como especie amenazada a nivel federal. Nosotros estudiamos el oso negro de Florida en 2 áreas dentro del ecosistema Okefenokee-Osceola en el sudeste de Georgia (Okefenokee) y en el norte de Florida (Osceola) desde 1995 hasta 1999 para evaluar relaciones entre características poblacionales, condiciones de hábitat y actividades humanas. Capturamos 205 osos negros diferentes (124M:81H), 345 veces desde junio de 1995 hasta Septiembre de 1998. Obtuvimos 13,573 radio-localidades de 87 (16M:71H) individuos durante el período de estudio. En Okefenokee, los frutos del túpelo negro (Nyssa sylvatica) y de la palma enana de florida (Serenoa repens) fueron los alimento más importantes en base a los análisis de heces. En Osceola, maíz de comederos para ciervos de cola blanca (Odocoileus virginianus) fue la fuente de alimento más importante aunque, cuando disponible, la palma de florida fue muy consumida. Los osos adultos de Osceola fueron un 29% más pesados que los de Okefenokee ($t_{82}=3.55$, P < 0.001). El tamaño medio anual del área de uso (home range) de las hembras de Osceola ($\overline{x}=30.3\ {\rm km}^{2}\pm 4.0\ [{\rm ES}]$, n = 53) varió muy poco estacional o anual mente y fue casi la mitad que el de las hembras de Okefenokee ($55.9\ {\rm km}^{2}\pm 6.9$, n = 69; Z = -2.47, P = 0.014). En cambio, las hembras con radiocollares de Okefenokee expandieron sus áreas de uso durante los años de baja producción de túpelo negro. Dicha expansión fue más notable entre el otoño de 1998 y 1999, cuando el tamaño promedio de área de uso para las hembras de Okefenokee aumentó de $14.5\ {\rm km}^{2}$ a $78.4\ {\rm km}^{2}$, respectivamente, e incluyó una mayor proporción de áreas altas abiertas a la caza deportiva. Como resultado, 5 hembras fueron cazadas en el área de estudio de Okefenokee durante la estación de caza de osos de 1999 en comparación con solo 7 cazadas de 1996 a 1998. En Osceola, ningún oso fue cazado. Las áreas de uso de las osas adultas fueron localizadas en áreas con desproporcionadamente alta presencia de laurel negro (Gordonia lasianthus) y asociaciones de vegetación de túpelo-laurel negro/ciprés (Taxodium spp.) en Okefenokee y de túpelo-laurel negro/ciprés en Osceola. La asociación de vegetación de pinares obtuvo el rango más bajo de todas las otras asociaciones dentro de las áreas de uso de los osos en ambas áreas de estudio aun cuando durante mucha parte del verano y del otoño la dieta de los osos incluyó alimentos que se encuentran casi exclusivamente en pinares. En Okefenokee se documentó la mortalidad de dieciséis osos con radiocollares; la caza fue la causa de 11 (68.8%) de esas muertes. La supervivencia anual de machos en Okefenokee fue de 0.71 (95% IC = 0.53-0.88). Las tasas de supervivencia anual de hembras con radiocollares en Okefenokee y Osceola fueron de 0.89 (95% IC = 0.83-0.95) y de 0.97 (95% IC = 0.92-1.00). Las tasas de supervivencia de machos fueron más bajas que las tasas de supervivencia de las hembras en Okefenokee (Z = 18.87,