Rarity and Phylogeny in Birds

Abstract

extinction risk in birds is not distributed randomly with regard to life history traits (e.g. Bennett and Owens 1997, Reed 1999) extinctions more prevalent in monotypic genera (McKinney 1997b, Hughes 1999, Russell et al 1998) losing higher taxa means that any biological innovations unique to them will also be lost, leaving an impoverished, more homogeneous fauna (Myers 1997) megasubspecies = ssp approaching sp status In birds, as in other taxa, rarity is a widespread and natural phenomenon. Our analyses on the Anseriformes have shown that, unlike the biological traits of individuals, which tend to be highly conserved between closely related species, rarity is not strongly constrained by phylogeny. This fact could arise because abundance is not partitioned equally at speciation, which is a likely outcome of certain modes of speciation. Alternatively, changes in abundance might occur subsequent to speciation. We have shown that within avian families, abundance does not appear to be a random function of taxon age. Rather, post-speciation transformation in range size may possibly follow some predictable trajectory, with young and old taxa tending to have lower abundances than species of intermediate age. These results highlight the fact that extinction risk and rarity are not synonymous. Species can be rare without being considered to be unduly threatened with extinction, and this might tend to apply to "naturally" rare young and old taxa. Human disturbance, on the other hand, has brought many previously abundant species to, or in some cases over, the brink of extinction. Such 'artificial' rarity may occur in species of any age, by affecting (a) which rare species are able to expand their ranges, (b) which common species will suffer a decline in abundance and (c) which naturally declining species are hastened on their path to extinction. This human factor tends to affect closely related species in similar ways, and so we observe phylogenetic patterns of extinction risk in birds. Given taht both population size and geographic range show no such phylogenetic pattern in birds, it seems likely that other characteristics which are phylogenetically heritable (such as body size or reproductive effort) will contribute more to the non-random allocation of extinction risk. It is interesting to note that, like extinction risk, invasiveness (as measured by introduction success) in birds also shows taxonomic selectivity. Here too, ecological specialisation (which will be influenced by factors that are heritable, such as physiology and diet) seems to play a greater role in influencing a species' chance of being successfully introduced (with a generalist more likely to be a successful invader than a specialist) than either its population size within its native geographic range, or the size of that range. We therefore arrive at a distinction between a 'natural' situation and the situation which prevails today. In the former, rarity occurs at random across the avian phylogeny. Although many of these rare species may go extinct by chance, biodiversity will be maintained; the addition of more species through speciation may even lead to an increase in diversity. In the current situation, however, human actions may have contributed to the taxonomically non-random patterns of extinction and extinction risk, with the result that not only are certain species at a higher risk of disappearing than other, but so too are the higher taxa to which they belong. To the extent that related species will tend to be exposed to similar environments, the majority of species within a clade may suffer comparable consequences from any natural disturbance, so causing taxonomic clumping of extinctions. The disproportionate loss of biodiversity resulting from human activity may therefore not be completely unprecedented; indeed, it has been documented that natural patterns of extinction tend to break down during periods of mass extinction. The frequency and intensity of human disturbance, however, may inhibit the recovery of biodiversity, which has previously relied on the origination of new species by speciation (a relatively slow process). This effect will only be exacerbated by the enormous scale of the worldwide spread of species today facilitated by humans. The primary foci of biological invasions tend to be human-altered ecosystems, and so the rapid establishment of exotic species in such areas, even when they themselves are not directly responsible for the extinction of local endemics, may remove or lessen the conditions necessary for speciation. The fact that avian introductions are biased towards certain taxonomic groups, and particularly towards ecological generalists, means that the loss of distinctive taxonomic groups across the world will tend to benefit ecologically similar species everywhere. This constitutes and extensive and sustained disruption of natural patterns of rarity and diversity and will accelerate the process of biotic homogenisation, certainly resulting in an impoverished avifauna. As Lawton states "we may not wish to destroy gorillas and bowerbirds and encourage starlings and cockroaches. But we seem powerless to do anything about it". The worry with the present wave of extinctions is that continuing human-imposed selection for such "pest and weed" ecology may mean that distinctive ways of life, among teh birds and among other taxa, are lost forever.