AboutZoos, Since 2008


Early stages of Earth’s 6th mass extinc­tion event are here, sci­en­tist warns

pub­lished 25 July 2014 | mod­i­fied 25 July 2014

The planet’s cur­rent bio­di­ver­sity, the prod­uct of 3.5 bil­lion years of evo­lu­tion­ary trial and error, is the high­est in the his­tory of life. But it may be reach­ing a tip­ping point.

African elephant atlantaIn a new review of sci­en­tific lit­er­a­ture and analy­sis of data pub­lished on 25 July in Sci­ence, an inter­na­tional team of sci­en­tists cau­tions that the ongo­ing loss and decline of ani­mals is con­tribut­ing to what appears to be the early days of the planet’s sixth mass bio­log­i­cal extinc­tion event.

Since 1500, more than 320 ter­res­trial ver­te­brates have become extinct. Pop­u­la­tions of the remain­ing species show a 25 per­cent aver­age decline in abun­dance. The sit­u­a­tion is sim­i­larly dire for inver­te­brate ani­mal life. And while pre­vi­ous extinc­tions have been dri­ven by nat­ural plan­e­tary trans­for­ma­tions or cat­a­strophic aster­oid strikes, the cur­rent die-​off can be asso­ci­ated to human activ­ity, a sit­u­a­tion that the lead author Rodolfo Dirzo, a pro­fes­sor of biol­ogy at Stan­ford, des­ig­nates an era of “Anthro­pocene defaunation.”

Across ver­te­brates, 16 to 33 per­cent of all species are esti­mated to be glob­ally threat­ened or endan­gered. Large ani­mals — described as megafauna and includ­ing ele­phants, rhi­noc­er­oses, polar bears and count­less other species world­wide — face the high­est rate of decline, a trend that matches pre­vi­ous extinc­tion events.

Larger ani­mals tend to have lower pop­u­la­tion growth rates and pro­duce fewer off­spring. They need larger habi­tat areas to main­tain viable pop­u­la­tions. Their size and meat mass make them eas­ier and more attrac­tive hunt­ing tar­gets for humans. Although these species rep­re­sent a rel­a­tively low per­cent­age of the ani­mals at risk, their loss would have trickle-​down effects that could shake the sta­bil­ity of other species and, in some cases, even human health.

For instance, pre­vi­ous exper­i­ments con­ducted in Kenya have iso­lated patches of land from megafauna such as zebras, giraffes and ele­phants, and observed how an ecosys­tem reacts to the removal of its largest species. Rather quickly, these areas become over­whelmed with rodents. Grass and shrubs increase and the rate of soil com­paction decreases. Seeds and shel­ter become more eas­ily avail­able, and the risk of pre­da­tion drops. Con­se­quently, the num­ber of rodents dou­bles — and so does the abun­dance of the disease-​carrying ectopar­a­sites that they harbour.

Where human den­sity is high, you get high rates of defau­na­tion, high inci­dence of rodents, and thus high lev­els of pathogens, which increases the risks of dis­ease transmission
Rodolfo Dirzo, lead author, pro­fes­sor of biol­ogy at Stan­ford Uni­ver­sity and senior fel­low at the Stan­ford Woods Insti­tute for the Environment »

“Who would have thought that just defau­na­tion would have all these dra­matic con­se­quences? But it can be a vicious cir­cle,” said Dirzo.

The sci­en­tists also detailed a trou­bling trend in inver­te­brate defau­na­tion. Human pop­u­la­tion has dou­bled in the past 35 years. In the same period, the num­ber of inver­te­brate ani­mals — such as bee­tles, but­ter­flies, spi­ders and worms — has decreased by 45 per­cent. As with larger ani­mals, the loss is dri­ven pri­mar­ily by loss of habi­tat and global cli­mate dis­rup­tion, and could have trickle-​up effects in our every­day lives.

For instance, insects pol­li­nate roughly 75 per­cent of the world’s food crops, an esti­mated 10 per­cent of the eco­nomic value of the world’s food sup­ply. Insects also play a crit­i­cal role in nutri­ent cycling and decom­pos­ing organic mate­ri­als, which helps ensure ecosys­tem pro­duc­tiv­ity. In the United States alone, the value of pest con­trol by native preda­tors is esti­mated at $4.5 bil­lion annually.

Dirzo said that the solu­tions are com­pli­cated. Imme­di­ately reduc­ing rates of habi­tat change and over­ex­ploita­tion would help, but these approaches need to be tai­lored to indi­vid­ual regions and sit­u­a­tions. He said he hopes that rais­ing aware­ness of the ongo­ing mass extinc­tion — and not just of large, charis­matic species — and its asso­ci­ated con­se­quences will help spur change.

“We tend to think about extinc­tion as loss of a species from the face of Earth, and that’s very impor­tant, but there’s a loss of crit­i­cal ecosys­tem func­tion­ing in which ani­mals play a cen­tral role that we need to pay atten­tion to as well,” Dirzo said. “Iron­i­cally, we have long con­sid­ered that defau­na­tion is a cryp­tic phe­nom­e­non, but I think we will end up with a sit­u­a­tion that is non-​cryptic because of the increas­ingly obvi­ous con­se­quences to the planet and to human wellbeing.”

(Source: Stan­ford Uni­ver­sity news release, 24.07.2014)

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