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Report on SAGA2/COE Symposium



A biology of ape extinction

A.H. Harcourt

Dept. Anthropology, University of California, One Shields Ave., Davis, CA 95616, USA.
ahharcourt@ucdavis.edu


     We know that species differ in their susceptibility to extinction. If that were not the case, evolution would not have occurred, and be occurring. However, while we know a lot about the path that evolution has taken, for example the decline of apes at the end of the Miocene, we know far less about why evolution took the path that it did. Thus, we do not know what differentiated the apes that went extinct from those that persisted, and why they differed. In effect, our understanding of the biology of extinction is minimal.
     To improve our understanding, I use a classic method of differentiating susceptible from persistent taxa. We know that fragmentation of habitat drives species to extinction: susceptible species cannot survive in small fragments of habitat. A comparison of the biology of taxa that exist only in large fragments (because they have gone extinct in the small fragments) with the biology of taxa that persist in the small fragments allows traits of susceptibility to extinction to be identified.
     At the end of the Pleistocene, fragmentation of habitat occurred on a massive scale, as the climate warmed. Forest retreated to mountaintops, and sea levels rose 120 m round the world, converting high areas of contiguous mainland into continental islands. The Sunda Shelf islands of south-east Asia were created by rising sea-levels between 10,000 and 6,000 years ago. They, and other such archipelagoes, have been widely used by biogeographers and conservation biologists to demonstrate and investigate extinction due to fragmentation. If species indeed went extinct as a result of fragmentation, small islands should contain fewer species than do large ones. This is the case for many Sunda Shelf taxa, including primates: the largest island contains 13 species of primate, and 3 species of ape; the smallest islands, none. Extinction is also indicated by the presence of fossils of species on islands smaller than those on which they now occur, and by the lack of any relationship between numbers of species on adjacent islands.
     The comparison of the biology of susceptible taxa (found ony on large islands) with persistent taxa (small islands) indicates that some biological traits that were expected to correlate with susceptibility to extinction did not correlate: slow reproduction; high resource needs (as judged by group mass), and specialisation (as judged by both a diet of rare, dispersed food (fruit), and a restricted altitudinal range). Other expected factors did correlate. Susceptible taxa had by comparison to persistent taxa, small populations (as judged by population density); high resource needs (as judged by large individual body mass and large home range); and were specialised (as judged by latitudinal range from the equator). While the finding that size of home range correlates with susceptibility is not new, it needs to be pointed out that in socially living species, as many primates are, year range is highly labile - it can be reduced easily by reducing group size - and it is surprising to find a labile trait associated with susceptibility. In addition, while the association of susceptibility with latitudinal range from the equator was predicted from first principles, this study is the first demonstration of such a correlation in vertebrates.
     The comparison is so far among all Sunda Shelf primates. Apes as a taxon are especially prone to extinction: they occur now on only the three largest Sunda Shelf islands. A comparison between them and monkeys shows that the apes possess a combination of the traits identified as individually risky. Thus apes occur at lower densities, are large bodied, reproduce especially slowly, and have low latitudinal range from the equator. However, they do not have a small home range compared to monkeys, perhaps because they range solitarily (orangutan) or in small groups (siamnag and gibbon).
     In conclusion, this study confirms for Asian primates as a whole, and Asian apes in particular, traits of susceptibility shown for other taxa; it questions the reasoning behind explanations for the association of one previously identified trait with susceptibility (home range); and it identifies a novel trait for vertebrates (a narrow tropical geographic range). It is suggested that the findings are applicable to understanding the path of past evolution in the primate line. And of course, the findings are relevant to predciting future extinctions, and hence to conservation management of primates in a world in which almost all primates are going to end up in highly managed, relatively small islands of remaining habitat in a surrounding sea of radically altered habitat.



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