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Unprece­dented wave of large-​mammal extinc­tions linked to (ancient) humans

pub­lished 11 May 2018 | mod­i­fied 11 May 2018

Body size pat­terns of mam­mals affected as hominins expand across the globe

Megafauna extinctionsMegafauna extinc­tions.
Image: The Uni­ver­sity of New Mexico.

Researchers have demon­strated that mam­mal bio­di­ver­sity loss, a major con­ser­va­tion con­cern today, is part of a long-​term trend last­ing at least 125,000 years. As archaic humans, Nean­derthals and other hominin species migrated out of Africa, what fol­lowed was a wave of size-​biased extinc­tion in mam­mals on all con­ti­nents that inten­si­fied over time.

A new study titled Body size down­grad­ing of mam­mals over the late Qua­ter­nary, pub­lished on 20 April in the jour­nal Sci­ence, is the first to quan­ti­ta­tively show that human effects on mam­mal body size pre­dates their migra­tion out of Africa and that size selec­tive extinc­tion is a hall­mark of human activ­i­ties and not the norm in mam­mal evolution.

The research, funded by a National Sci­ence Foun­da­tion (NSF) grant, was led by Dr. Felisa Smith at The Uni­ver­sity of New Mex­ico, along with col­leagues from Uni­ver­sity of Cal­i­for­nia San Diego, Uni­ver­sity of Nebraska-​Lincoln and Stan­ford Uni­ver­sity. The researchers showed that body-​size down­grad­ing — the loss of the largest species on each con­ti­nent over time — is a hall­mark of human activ­ity, both in the past and present.

Our study sug­gests that all of these mam­mal extinc­tions are part of a long-​term trend.

Rose­mary Elliott Smith, co-​author, Depart­ment of Math­e­mat­ics, Uni­ver­sity of Cal­i­for­nia, San Diego, USA

If this trend con­tin­ues into the future researchers warn, the largest ter­res­trial mam­mal in 200 years will be the domes­tic cow.

One of the most sur­pris­ing finds was that 125,000 years ago, the aver­age body size of mam­mals on Africa was already 50 per­cent smaller than on other con­ti­nents,” said Smith, a pro­fes­sor in the UNM Depart­ment of Biol­ogy who has stud­ied megafauna extinc­tion for more than 15 years. “We sus­pect this means that archaic humans and other hominins had already influ­enced mam­mal diver­sity and body size in the late-​Pleistocene.”

This find was par­tic­u­larly sur­pris­ing because Africa is a larger con­ti­nent and typ­i­cally, larger land masses house and sup­port larger mam­mals. But, it appears that by the late Pleis­tocene, hominins had already reduced the diver­sity of mam­mals there. Over time, as humans migrated around the globe, extinc­tions of the largest mam­mals fol­lowed. These giant mam­mals included the woolly rhi­noc­eros, mam­moths, lla­mas, camels and giant ground sloths as well as fero­cious preda­tors such as the short-​faced bear, and the scim­i­tar and saber-​toothed cats.

In their research, Smith and her col­leagues found that this decline fol­lows the global expan­sion of hominins over the late-​Quaternary, includ­ing the Pleis­tocene and Holocene Peri­ods. A num­ber of the­o­ries have been devel­oped over the years to explain more recent extinc­tions such as those at the end of the last ice age, includ­ing human hunt­ing, cli­mate change, dis­ease, and even a cos­mic impact such as an aster­oid or comet. How­ever, ear­lier work­ers had not focused on extinc­tions this far back in time.

Our study sug­gests that all of these mam­mal extinc­tions are part of a long-​term trend. This was fas­ci­nat­ing because it only occurred after the arrival of early humans,” said co-​author Rose­mary Elliott Smith.

By quan­ti­fy­ing mam­malian extinc­tion selec­tiv­ity, the researchers doc­u­mented what hap­pened to mam­mals as early humans left Africa through the com­pi­la­tion of exten­sive data includ­ing mam­mal body size, cli­mate, extinc­tion sta­tus and geo­graphic loca­tion over the last 125,000 years. They also used the con­ser­va­tion sta­tus of mod­ern mam­mals to model diver­sity and body size dis­tri­b­u­tions for 200 years in the future. The researchers inves­ti­gated and demon­strated the role of body size and diet on the like­li­hood of extinc­tion. These data were eval­u­ated in light of cli­mate change and human migra­tion pat­terns over the same time frame.

They demon­strated size-​selective extinc­tion was already under­way in the old­est inter­val, occurred on all con­ti­nents, within all trophic modes, and across all time inter­vals. More­over, the degree of selec­tiv­ity was unprece­dented in 65 mil­lion years of mam­malian evo­lu­tion. The dis­tinc­tive selec­tiv­ity sig­na­ture impli­cates hominin activ­ity as a pri­mary dri­ver of tax­o­nomic losses and ecosys­tem homogenization.

The researchers also exam­ined the poten­tial influ­ence of cli­mate on extinc­tion risk and selec­tiv­ity over time. They found that for 65 mil­lion years, changes in cli­mate did not result in more extinc­tions, nor was there a greater ten­dency for large-​bodied mam­mals to go extinct. “You just don’t see extreme size selec­tiv­ity for mam­mals until the late-​Pleistocene,” said Kate Lyons, a co-​author on the study. “Past cli­mate changes don’t result in size-​selective extinction.”

We sus­pect that in the past, shifts in cli­mate led to adap­ta­tion and move­ment of ani­mals, not extinc­tion” said co-​author Payne, “Of course, today ongo­ing cli­mate change may result in extinc­tion since most megafauna are lim­ited in how far they can move.” If the loss of large-​bodied mam­mals con­tin­ues into the future and all the cur­rently threat­ened ani­mals are lost, the largest mam­mal on earth in 200 years may be a domes­tic cow.

Because megafauna have a dis­pro­por­tion­ate influ­ence on ecosys­tem struc­ture and func­tion, past and present body size down­grad­ing is reshap­ing Earth’s bios­phere. By com­par­ing extinc­tion events with the entire record of mam­mal turnover over the past 65 mil­lion years, the researchers demon­strated that body size and diet did not influ­ence extinc­tion risk for mam­mals for most of their evo­lu­tion­ary his­tory. These results high­light a star­tling point. The role of ancient and mod­ern humans on large mam­mals has been vastly under­ap­pre­ci­ated researchers say.

Megafauna play a really impor­tant role in ecosys­tems,” said Smith. “which we are just begin­ning to appre­ci­ate. For exam­ple, as they walk their mas­sive size com­pacts the soil, which can lead to changes in gas exchange or water tables. They change the struc­ture of veg­e­ta­tion through their brows­ing and help main­tain open grass­lands. They burp methane, a green­house gas and even influ­ence the dis­tri­b­u­tion of nitro­gen and phos­ph­er­ous on the land­scape. We are not entirely sure what the poten­tial loss of these ‘ecosys­tem engi­neers’ could lead to. I hope we never find out.”

(Source: Uni­ver­sity of New Mex­ico news release, 19.04.2018)

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