Changing demographics in the ex situ giant panda population
Demography refers to the characteristics of a population's age structure, growth rate and vital statistics, such as reproductive and survival rates. Perhaps the most telling feature of the changing demography of the global ex situ giant panda population is its growth, specifically an increase in total number of individuals living in zoos and breeding centres (Fig. 21.1). However, how much of this 6% annual growth rate is due to adding pandas from nature (i.e. catching more pandas for zoos) versus improved ability to produce captive-born young? Until 1989, virtually all growth was due to importing wild-caught (i.e. born in the wild or wild-born) individuals. In the late 1980s, however, the trend began to change, with a significant increase in captive-born offspring. Since 1990, the latter component of the population has expanded about 12% per year while there has been a 5% annual decline in the number of wild-born or -caught pandas (see Fig. 21.1). Thus, the population is in transition from a 'sink' (one whose survival relies on wild-caught animals) to a 'source' (one that produces captive-born animals without the need for capturing animals from nature).
An increasing proportion of captive-born animals is indicative of improved overall reproduction and/or survival. We next examined how reproductive and survival success has changed over time. We first focused on fecundity, defined as the number of births in a year divided by the number of reproductive-age giant pandas. This was calculated separately for males (six to 29 years of age) and females (five to 20 years) and then averaged. Results showed that fecundity had increased exponentially over the last ten years at about 10% per year (p — 0.004;
r2 = 0.59, using a weighted exponential regression; Fig. 21.2). While fecundity was only about 10% in 1990, it had risen to 40% by 2001.
This increased fecundity was not due to increased litter size. Since 1995, average litter size has remained at 1.52 with 50% of births producing litters of one cub, 48% producing two and 2% producing three cubs. Rather, the fecundity surge has been due to a 6% annual increase in the proportion of animals breeding (r2 = 0.49; p — 0.01, using a weighted exponential regression), a rise that has been almost identical for males and females. Based on these trends, 23% of the females can be predicted to breed on an average basis (with a standard deviation of ± 6%). Thus, there is good news - an impressive rise in fecundity - but also not-so-good news in that still only one in four adult giant pandas reproduces. This dependence upon so few individuals to do all the breeding has significant implications on population genetic viability (see below). In particular, the number of contributing captive-born males is exceptionally low, mostly because of sexual incompatibilities with females (see Chapter 5).
It is important to distinguish between reproductive success in wild-caught versus captive-born giant pandas. Historically, most of the reproduction has occurred in the former (Fig. 21.3). With gradually fewer wild-caught pandas in the population, reproduction will probably decrease unless captive-born pandas can increase their reproductive contributions. The proportion of births from the latter group is increasing.
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