Читать книгу Ecology - Michael Begon - Страница 106

Opinions naturally differ regarding both the ethics and the practical benefits of keeping wild animals in captivity in zoos, but the current reality is that zoos play an integral role in the conservation of many species, especially those, like many mammals, that are large and inherently attractive to the general public. Hence, in managing these animals, we need to understand their patterns of survivorship, and to know in particular if there are general rules organising these patterns that would not only describe the species for which we have good data, but also allow us to predict patterns for similar or related species when currently available data are sparse. Lynch et al. (2010) therefore reviewed what was known about the survivorship of captive mammals – 37 species, including primates, artiodactyls (cattle, sheep, deer, etc.), carnivores, bats, seals and the giant panda – and some of their results are summarised in Figure 4.12. They were more interested in the shapes of the survivorship curves (and for example whether they were type 1, 2 or 3) than in absolute values, and all data sets were therefore scaled to the maximum longevity of the species concerned. They then fitted all datasets to a general survivorship function with two shape parameters, α and β, which allowed the different curves to be classified and either grouped together or distinguished (Figure 4.12). Broadly speaking, with increasing values of α/β, mortality shifted towards being more evenly distributed throughout life, rather than being concentrated at the start; and with decreasing values of αβ, mortality shifted towards including senescence – a period of increased mortality at the end of life – rather than decreasing steadily with age.

Figure 4.12 Distribution of the shapes of survivorship curves for 37 species of animals kept in zoos. (For a full list of species names, see the original text.) A generalised survivorship function with two parameters, α and β was fitted to all datasets, allowing each to be located in logα–logβ space. The shapes themselves are illustrated in the insets, referring to the four starred locations, as survivorship on linear and semilogarithmic scales (solid and dashed lines, respectively; see Figure 4.11) and as distributions of mortality (histograms) between birth and maximum longevity (L). Among those species, the locations of artiodactyls (), carnivores () and primates () are indicated, plus five further species ().

Source: After Lynch et al. (2010).

Despite wide variations in size, longevity and taxonomic affiliation, most of the curves were, in essence, type 2, with some element of type 1 (senescence) or type 3 (early mortality). The variation that did exist was significantly associated with the species’ taxonomic order: the artiodactyls showed the least evidence of senescence, the carnivores the most, with the primates somewhere in between (Figure 4.12). This taxonomic variation was in turn associated with variations in age to weaning (relative to lifespan) and litter size, suggesting ‘syndromes’ of associated life history traits. We return to the whole topic of the patterns in life histories and their possible causes in the next chapter. For now, though, the results do provide us with grounds for believing that, based on this analysis, even for species where we have little or no prior knowledge, managers in zoos can make educated predictions with some confidence about likely patterns of mortality, and act accordingly.