Decoding death

En­vi­ron­men­tal con­tam­i­na­tion and ex­treme weather events are re­spon­si­ble for most die-offs among an­i­mals, a new study sug­gests

Down to Earth - - CONTENTS - VIBHA VARSH­NEY |

Ex­treme weather and en­vi­ron­men­tal pol­lu­tion re­spon­si­ble for mass deaths in an­i­mals, finds study

Ofirst day of the new year, N THE fish­er­folk ven­tur­ing out to sea in Chen­nai to make their daily catch were in for a sur­prise. They saw thou­sands of dead fish float­ing along the coast in Be­sant Na­gar. The rea­son for the fishes’ death re­mains un­clear. On Jan­uary 10, around 45 birds were found dead in the Sul­tan­pur Na­tional Park near Delhi. The cause of the deaths is as­sumed to be pes­ti­cide-laced crops or bird flu, but the pic­ture is not fully clear.

Such in­ci­dents are re­ported world­wide ev­ery now and then. While they do not lead to ex­tinc­tion, they of­ten kill more than 90 per cent of a pop­u­la­tion. Sci­en­tists say that while dis­eases, chang­ing cli­mate and en­vi­ron­men­tal pol­lu­tion could be re­spon­si­ble for such die-offs, there is very lit­tle re­search done on this.

Re­cently, bi­ol­o­gists in the US have stud­ied 727 die-offs, in­volv­ing 2,407 species, in the past 70 years. Most of the in­ci­dents they stud­ied were re­ported from the de­vel­oped world and were pub­lished in sci­en­tific lit­er­a­ture. The sci­en­tists an­a­lysed the re­ports and found that the mag­ni­tude of die-offs has in­creased in birds, fishes and marine in­ver­te­brates, is sta­ble among mam­mals, and has de­creased in rep­tiles and amphibians.

Dis­eases ac­counted for 26.3 per cent of th­ese die-offs, hu­man ac­tiv­ity lead­ing to en­vi­ron­men­tal con­tam­i­na­tion con­sti­tuted 19.3 per cent, biotox­i­c­ity or tox­ins pro­duced by or­gan­isms present in wa­ter caused 15.6 per cent deaths and pro­cesses di­rectly in­flu­enced by cli­mate (weather, ther­mal stress, oxy­gen stress, star­va­tion) caused 24.7 per cent deaths .The num­ber of deaths was more when mul­ti­ple causes were at play.

Dis­eases and bio-tox­i­c­ity have al­ways been linked with fish death but the re­searchers found that th­ese fac­tors are now in­creas­ingly af­fect­ing birds too. While fishes and birds have shown sus­tained mass mor­tal­ity events caused by hu­man in­ter­fer­ence, its role in mam­malian deaths have only been seen in the last three decades. The study was pub­lished in Pro­ceed­ings of the Na­tional Academy of Sciences on Jan­uary 12.

“This is the first at­tempt to quan­tify pat­terns in the fre­quency, mag­ni­tude and cause of such mass death events,” says the study’s se­nior au­thor Stephanie Carl­son, as­so­ciate pro­fes­sor at the Uni­ver­sity of Cal­i­for­nia, Berke­ley.

“Im­proved mon­i­tor­ing will help in­di­cate which an­i­mal pop­u­la­tions are most vul­ner­a­ble to en­vi­ron­men­tal per­tur­ba­tions,” says Sa­muel Fey, a post-doc­toral fel­low in the Depart­ment of Ecol­ogy and Evo­lu­tion­ary Bi­ol­ogy at Yale and the pa­per’s co-au­thor. Iden­ti­fy­ing pat­terns in the causes of mass mor­tal­ity events will help un­der­stand what the largest un­der­ly­ing causes of mass die-offs are, he adds. “Th­ese data also show that we have a lot of room to im­prove how we doc­u­ment th­ese types of rare events,” says Fey. The team says that in ad­di­tion to mon­i­tor­ing changes in tem­per­a­ture and pre­cip­i­ta­tion pat­terns, it is im­por­tant to doc­u­ment bi­o­log­i­cal re­sponse to re­gional and global en­vi­ron­men­tal change. They say there is a need to im­prove doc­u­men­ta­tion of such events in the fu­ture and cit­i­zens could play an im­por­tant role in the process.

Stephanie Carl­son, se­nior au­thor of the study, tags fish in an intermittent stream in Cal­i­for­nia. Th­ese streams cease to flow for some part of the year, killing fish in the process

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