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Emerging signs of mass extinction?

Large-sized marine animals, more than the smaller ones, face a greater threat of extinction, says a study published in Science on 16 September.

Unheard of in the earth's history, large animals could be facing extinction as compared to the smaller ones. This picture shows a blue whale.

The authors base the analysis on comparisons with fossil records and similar situations that were encountered during the five massive extinction events that have taken place in the last 55 million years. The end-Permian event that happened 252 million years ago when reef-building animals were exterminated and the end-Cretaceous one (66 million years ago) when non-avian dinosaurs were eliminated are the two biggest extinction events.

The authors record that during previous mass extinctions, body size was either inversely associated with extinction probability or not at all. On the other hand, the present day extinction threat, they say, is biased against larger animals. This is a crucial difference because of the importance of large animals’ role in the ecosystem.

Larger animals, especially predators, are crucial in stabilising the ecosystem. Jonathan L. Payne, lead author of the study, from the geological sciences division of Stanford University, explains this in an email, thus: “[Large] animals such as whales move nutrients within the oceans by feeding in one place and defecating elsewhere. Also, large animals are often also top predators that regulate the abundances of other species.” The predatory giant sea snail, triton, is a good example of how removing an animal from the top of the ecosystem can destabilise it. As Prof. Payne puts it, “When it [triton] is removed from ecosystems, this can lead to population increase of its prey, the crown of thorns starfish. The starfish, in turn, eats corals, and so corals can suffer when triton is removed.”

The study gathers importance in its relevance to environmental change. According to Dr Payne, “In the geological record, all of the major mass extinction events are associated with evidence for large and rapid environmental change. Therefore, each mass extinction appears to have been caused by a single, large, triggering event. It is still possible that different species died out for different proximal reasons, but the overall driver appears to be singular for most, if not all, of these events.”

The dominant threat identified by the authors in this case are human fishing and hunting, rather than climate change itself.

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