Dynamics of a black bear population within a desert metapopulation
This articles aim was to describe the dynamics of an island population of black bears in a metapopulation, within Big Bend National Park between 1988-2002. Big Bend National Park is situated in the Chihuahuan Desert of western Texas, within the park there are islands of montane forest, in a matrix of desert grasslands and shrub lands. The article defines metapopulations ‘as a set of populations within some larger area, where typically migration from one local population to at least some other patches is possible’. It was this connection between different habitat islands that the article was investigating. The types of habitat which can sustain a black bear population occur above 1500m in the mountainous regions of the park, and consist of both woodland and chaparral. Between these mountain ranges there is a matrix of inhospitable habitat, which means that each island is effectively separated. However mitochondrial and genetic data suggest that there is a link between the different island populations, through male and female migration.
Big Bend National Park encompasses 320,000ha of the Chihuahuan Desert; the area is arid with low precipitation rates of 30.5cm/annum. Precipitation rates increase as elevation increases; this is one of the main reasons why the habitat can support a population of black bears. One of the methods that was employed was to use visitor observations, these records are kept on a database and have been monitored since 1901. The information which had been was considered e.g. location, time and characteristics of the bear(s). This information was then used to try and estimate the minimum number of females and cubs which were present in any given year. The other method which was used was field data. In order to obtain this data bears were trapped, and radiotransmitter collars attached to the trapped bears which weighed more than 40kg. These collars were fitted with a mortality switch and a breakaway mechanism. This data was then used to assess the trapping success, the composition of the population and the estimated population size; they estimated the population size using consensus data from 1999-2000. The density of males and females was then calculated by dividing the estimated minimum number of bears alive, by the areas which contained the home ranges for the radiotransmitter collars. Two different survival rates for those whose mortality mode was switched on and for those whose fate was unknown, was then calculated.
The results show that the minimum number of females increased from one in 1988 to 6 in 2000, there was then a decline in 2001 to only 2. Simple correlation analysis showed that there was a strong connection between the estimated number of adult females and visitor observations. The article showed that they experienced a trapping success of only 42 captures out of a total of 1763 attempts. It is believed that 90% of the total bear population which resided within the Chisos Mountains was captured between 1988 and 2000, as in 1999 22 bears were known to be alive, this then rose to29 bears in 2000. During the autumn of 2000 a large dispersal event was recorded, 10 bears left Big Bend national Park and headed in the direction of Mexico. They travelled 47-128km from the park. Dispersal ranges averaged 76.1±33.9 for females and 92±15.4 for males. One adult female succeeded in migrating to the Sierra del Carmen and back again and was in the process of a second journey when her collar switched to mortality mode. It is thought that she either slipped off her collar, as she had done this previously, or she died of natural causes. 3 of the bears whose collars had switched to mortality mode were near Ejidos towns, poaching was believed to be the cause but was never proven. They were not able to recover all of the bears and it is believed that they had ventured far enough into Mexico, that