Science: A wing and a prayer
Extinction beckons for over 1,000 bird species, and it seems nature wants it that way, Peter Bennett investigates
Monday 11 May 1998
My "cosmic mega-tick", the one to be seen above all others, is the hyacinth macaw. It's the largest of the parrots, with a native habitat in the Pantannal region of central Brazil.
Unfortunately, it is prized by collectors, and its numbers have declined dramatically as a result of trapping and habitat loss, such that only a few thousand birds remain in the wild. Its relatives, the Glaucous and Spix macaws, are already believed to be extinct in the wild.
I've been lucky. In 1988 I did see this magnificent bird in its native habitat. But I keep wondering: what will determine the survival of this species? Why is it that birds such as the Californian condor, bald ibis, night parrot and hyacinth macaw teeter on the brink of oblivion, while many other species appear secure? Do threatened species all share some unfortunate quality that renders them more at risk? Or are they just unlucky?
Human disturbance, the principal cause of current extinctions, does not appear to affect all species equally. In fact, bird species vary greatly in the extent to which they are extinction-prone.
Certainly, human actions threaten the survival of an increasing number of birds, Revent events, such as the destruction of the rainforest habitats in south-east Asia, have highlighted this problem: in 1994, Dr Nigel Collar and his colleagues at BirdLife International estimated that of the 9,672 living bird species , 1,111 (or 11.5 per cent) are threatened by extinction. But it's not evenly distributed among species.
Since 1993 I have been attempting to understand why. Working with Dr Ian Owens, of the University of Queensland, I compiled a database of detailed information on the biology of more than 3,000 threatened and secure species.
Since we didn't know what might be the key aspect, we measured more than 50 variables about each bird, such as body size, clutch size, lifespan, mating habits, diet and habitat. For each species we also constructed a phylogeny - a type of family tree identifying related species based on molecular data. Then we added data about the conservation status of each species.
At this point, we could begin our statistical analysis, to see how evolution sorts the survivors from the strugglers. Our main goal was to test whether threatened species simply arise at random, or whether something about their biology predisposes them to extinction.
To do this we asked three specific questions. First, is the risk of extinction randomly distributed among families? Second, which families (if any) contain more or fewer threatened species than would be expected by chance? Third, is species' variation in the risk of extinction associated with biological characteristics, such as variation in body size or fecundity?
The first answer: extinction is not a bolt of lightning that strikes among bird families at random. Certain families contain a surprisingly large proportion of threatened species, with eight - the parrots, pheasants, albatrosses, rails, cranes, cracids, megapodes and pigeons - containing more than would be expected by chance.
Only one of the 143 bird families contained significantly fewer threatened species than expected by chance: woodpeckers. Thus, human disturbance has affected bird families in different ways, some are especially vulnerable to extinction while others are relatively secure.
But why? Does a bird's evolutionary past influence its ability to meet contemporary challenges to its survival? Yes, we found, it does. Species from the highly extinction-prone families tended to be large-bodied and have small egg clutches.
Diversity in these traits evolved in the early evolutionary history of birds, many tens of millions of years ago. Since then, it seems that in many bird families these critical aspects have changed very little. Low reproductive rates which may have evolved millions of years ago have now predisposed certain bird families to extinction.
Birds with small clutch sizes take longer to recover their numbers if they are reduced to small sizes; therefore they are more likely to become extinct if an external force severely reduces their numbers.
But a biological characteristic like clutch size is not easy for birds to change - unlike, say, alterations in feeding behaviour which may make some species more flexible and able to adapt to environmental changes and human disturbance. Particularly worrying here is that a number of bird families contain a small number of species in total, but a high proportion of threatened species.
For example, the only species of kagu is threatened, all three species of kiwi are threatened and two of the four cassowaries are threatened.
In another study we have devised an index that quantifies the importance of each bird species in terms of representing overall global biological diversity in birds. This index shows that some of these small families represent a large fraction of avian biological diversity. Thus, the loss of species from these families would result in a disproportionately large loss of avian diversity. Every effort should be made to identify these species and ensure that they are in protected areas.
What does this mean for the hyacinth macaw, and for those who would wish to see it? The problem is that they have very low productivity: studies by Dr Charles Munn, of the New York Zoological Society, have shown that only 10 to 20 per cent of wild macaws in pristine rainforest in Peru's Manu Biosphere Reserve breed in a given year. Furthermore, pairs usually fledged only one young, and one-third of the nests he studied failed.
Low reproductive rates, together with high survivorship, is a successful evolutionary pattern. But when macaws suffer a population crash due to trapping or environmental catastrophe, their ability to recover by increasing productivity is severely constrained by their evolutionary history. They can do little about it because their reproductive strategy evolved millions of years ago amongst their ancestors.
Strange though it may seem, the bird-watching desires of the future may be limited by avian patterns of behaviour that became fixed even before humans were walking on two feet.
Dr Peter Bennett is a research fellow at the Institute of Zoology, Zoological Society of London. He will give a talk on this topic, "How to become rare", tomorrow at 5.30pm at the Zoological Society of London's (Outer Circle, Regent's Park) Scientific Meeting. Attendance is free.
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