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Expiry dates

The dodo is most certainly dead. But when did the species finally disappear? A statistical approach allows estimation of the date, and could be applied to other extinctions, both past and present.

In 1923, the USS Tanager landed a scientific expedition on Laysan, in the northwest Hawaiian Islands. Rabbits, introduced in the late 1800s, had reduced the native vegetation to a wasteland and the island's endemic honeycreeper, the Laysan `apapane, to a few individuals. During the 1923 visit, a strong storm blew through Laysan, whipping up the dust. When it settled, the honeycreepers were gone, with the time of the species' demise known to the day along with details of the causes of its decline and death. Partly recorded by an early film, the story of the birds' end is unique. Typically, however, we have only a vague idea of when a species goes extinct and the reasons why it did so. On page 245 of this issue, Roberts and Solow1 provide a method for estimating dates of extinction. Their application is for the most famous recent extinction of all — that of the dodo, discovered on Mauritius then driven to extinction in the seventeenth century.

Roberts and Solow have used a statistical method, involving a formulation known as the Weibull distribution, and have applied it to the record of dodo sightings by sailors and others. Their key result is an estimate of the probable range of dates for extinction itself, rather than a last sighting, without knowing the underlying statistical distribution of the times observers recorded the species. This result has applications for both past and future extinctions.

Take, for instance, the controversy over whether dinosaurs became extinct on the last and worst day of the Cretaceous, because of the effects of an asteroid impact, or whether the group was in terminal decline millions of years before that. Similar arguments apply to the demise, some 60,000 and 10,000 years ago, of the huge animals that inhabited Australia and the Americas, and of Pacific island bird faunas over the past two millennia. Was the cause a brutal first contact with humans or a consequence of changing climate? For an incomplete fossil record, the last records of species will inevitably precede the date of their extinction, so there will always be a decline in the number of species known to be alive before some posited cataclysm. Extraordinary data have always supplied the key evidence for cataclysms — in the case of the asteroid and the dinosaurs, the iridium layer, seen in rocks, that is thought to have been produced by the impact. The indictment of humans for recent extinctions gains credibility from narrow windows of human coexistence with their victims. For example, the bones of many bird species barely coincide with charcoal and other features of human presence on Pacific islands.

Roberts and Solow's analysis1 is applied to ordinary data, however — the dates when now-extinct species were recorded. For each species, one could generate the ranges of the predicted date of extinction. Across all the species involved, do these ranges support the hypothesis of a sudden, contingent death, or slow, independent decline, typical of the known background rate of normal extinctions2? The answer could be a mix, of course. Application of Roberts and Solow's method might suggest which dinosaurs (if any) had predicted dates of extinction well before the rest of their kind. In this case, the records are obviously not sightings, as in the case of the dodo, but fossils dated to various times.

The major extinctions now unfolding suggest another application. For well-known species groups, 10% or more are “threatened”. That is, expert opinion expects their demise within a few decades. Birds are the best documented; some 11% are “threatened” and 2% “critically endangered”, usually meaning that there is considerable uncertainty about whether the species is still alive. The cover of a catalogue of threatened birds3 depicts two more species from Hawaii — the nukupu`u, seen only sporadically over the past few decades, and the akialoa, not seen in 50 years. The catalogue's title, Birds to Watch, implies that there is some hope for these species. Indeed, species sometimes reappear after long absences; the Cebu flowerpecker is one4.

There is a problem with retaining hope for species missing in action, however. If we chose the date of extinction to be, say, 50 years after the last record, then we record no extinctions for the past 50 years. Naive interpretations of counts of the number of extinctions per decade might conclude that extinction rates have declined in the past 50 years. Roberts and Solow's result provides a way of converting records of critically endangered species into predictions of what species might still be found and those for which the expiry date has passed. Other methods, using different statistical approaches individually or in combination, might be applied to the same end.

And what about the dodo — a universal symbol of stupidity and its fatal consequences? The familiar portrait is particularly unflattering, but we forget that so, too, were other bird drawings of that vintage. According to The New Shorter Oxford English Dictionary, the dodo “became extinct”, perhaps implying that it deserved its fate. Put more accurately, however, the entry would read “humans wrecked its habitat, introduced species that ate it and perhaps directly bludgeoned the flightless birds into oblivion” (Fig. 1) — oblivion most likely coming, at least in the statistical terms of Roberts and Solow's analysis, in 1690, 28 years after the dodo's last confirmed sighting.

Figure 1: Humans set about the fauna of Mauritius.


In the background, dodos are being bludgeoned to death; in the foreground, a species of parrot (Lophopsittacus sp.) is being killed. The latter is thought to have outlived the dodo. But not long afterwards, probably in the early part of the eighteenth century, this parrot followed its flightless fellow inhabitant of Mauritius down the grim path to extinction. The illustration is by Julian Pender Hume, based on a woodcut from a Dutch journal by Willem van Westzanen.

A more important question is on what date did we seal its fate? Or that of the Laysan `apapane? Or that of the Cebu flowerpecker — for its re-discoverers found only three individuals? Exactly when did human actions put these and other species into irrevocable decline? Roberts and Solow do not tackle this much more difficult matter. We must do so, of course, if we are to prevent the extinctions of the large fraction of species now threatened.


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Pimm, S. Expiry dates. Nature 426, 235–236 (2003).

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