“Weedy” mice dominate a warming world while other small mammals suffer

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Today’s mammals are facing the twin threats of a rapidly warming planet and increasingly intrusive human activity. As usual, the big species hog the limelight. The world waits on bated breath to hear about the fates of polar bears, whales and elephants, while smaller and more unobtrusive species are ignored. But smaller mammals are still vital parts of their ecosystems and it’s important to know how they will fare in a warmer world. Now, thanks to Jessica Blois from Stanford University and a hoard of new fossils, we have an idea. As they say, all this has happened before

Around 12,000 years ago, as the Pleistocene epoch drew to a close, the mammals of North America were also dealing with multiple threats. The last Ice Age was giving way to the far warmer Holocene and at the same time, humans arrived on the scene, wiping out species after species. Some of the larger losses are familiar, such as the mammoths and ground sloths, but a new treasure trove of fossils in California’s Samwell Cave has revealed the fate of their smaller kin.

The common wisdom suggests that small mammals are relatively resistant to extinction, because they have large litters, they breed quickly and their populations grow at incredible rates (think mice and rabbits). The Samwell fossils support this idea but they also tell us that communities of small mammals were greatly affected by natural warming nonetheless. Their diversity plummeted, they became less evenly spread, and rare species became ever rarer.

Not everything suffered though – ‘weedy’ species took over this new landscape. The deer mice did particularly well, doubling in abundance between 16,000 and 13,000 years ago. These rodents aren’t fussy about their homes and they’re often the first into a new area. Opportunistic and adaptable, these generalists flourished under changing circumstances that flummoxed others. And their rise to power accounted for much of the fall in overall species evenness during this time. There are signs that deer mice are doing the same today.

To Blois, it’s clear that these changes were mainly driven by climate change. As the temperature rose, so the evenness and richness of the mammal communities fell, and the first signs of falling populations coincided neatly with the very rapid warming of the Bolling-Allerod period.  Individual species supported these general trends. The Western pocket gopher and the mountain beaver both went locally extinct and today, they’re found in much cooler parts of California. Blois thinks that these rodents tracked the cooler weather to other more hospitable areas.

Meanwhile, Blois also ruled out other possible explanations. Humans invaded North America during the end of the Pleistocene, but the shifts in small mammal populations predated them by around 1,500 years. The fall of the large beasts could have altered the local vegetation, creating new landscapes for species that scurry, but these new plant communities also appeared after the small mammal communities had already started to shift. Changing climate, it seems, is the best explanation.

Blois says that since today’s climate is changing even more quickly, our current small mammals might face a similar fate to their Pleistocene counterparts. Their communities are likely to shift towards an impoverished and uneven selection of species. In this way, they could act as a colony of furry canaries, as “harbingers of imperilled ecosystems”.

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