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Going to Seed: Climate Change Could Spark Small Mammal Invasion

Seeding events in forests, on the rise due partly to climate change, provide more food for invasive mammals and could be the cause of a boom in their populations
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Flickr/Anita363

Invasive animals are a scourge the world over, and on many islands they have decimated local plants and animals. New Zealand has contended with such losses for centuries as rats and stoats (short-tailed weasels) from abroad have helped to wipe out 19 bird species. These small mammals continue to threaten wildlife there today. Now climate change could ramp up that process. Scientists at Landcare Research, a government-funded research center in New Zealand, say they’ve found a troubling link between climate change and invasive mammals that could threaten biodiversity and disease containment around the world if the local pattern holds.

The link, they wrote in a study soon to be released in Ecological Applications, comes via gradual, global warming–induced disruption of periodic tree seeding in forests. This seeding, also known as masting, occurs every few years. Similar to the way trees lose their leaves in the fall, various species shed seeds such as acorns and other nuts. The seeding is a major food source for local wildlife, such that invasive mammal populations have often been seen to explode as a result. Scientists are still unsure what specifically causes masting, however data suggests that it has been happening more often, and in New Zealand invasive mammals have consistently outcompeted native species in mast years. Scientists strongly suspect climate change is a culprit, but with masting itself still so enigmatic, its interaction with climate change remains murky as well.

The study relied on masting and climate data going back to the 1970s, focusing on masting in New Zealand’s Orongorongo Valley beech trees. Masting used to happen once every six or seven years, Daniel Tompkins, research leader of Landcare Research’s Wildlife Ecology and Management team, says, but in the past decade, the average has been every three or four years. Tompkins and his team modeled how invasive mammals would respond in five different scenarios to three different masting frequencies. Their results suggest that, as masting becomes more common, the increased availability of food would turn New Zealand’s invasive mammal issue from a periodic to a chronic problem. This study is the first to investigate how invasive mammals and climate change may interact in their potential impacts on native biodiversity, Tompkins says. Invasive-borne spread of infectious disease in New Zealand is a major concern for ecologists. Opossums have been known to carry bovine tuberculosis.

Dave Kelly, an ecologist at the University of Canterbury in Christchurch, warned against overgeneralizing Tompkins’ findings. Slight changes in temperature—even a gradual increase—are unlikely to affect masting frequency, he argues. Instead, climate change could have an impact by causing more extreme  events,  like extremely hot or cold temperatures, which could lead to more frequent masting in New Zealand—and around the world. “As long as other parts of the world have plants that use the same cues, [the findings] should be generalizable,” he adds. “I would be surprised if our climate cue mechanism was not found widely outside New Zealand.”

If plant species had different cues, however, which could include different resource requirements or temperature triggers, masting frequency could be unaffected by climate change. Stephen Handler, a climate change specialist at the Northern Institute of Applied Climate Science, said climate change could actually cause masting to decrease in some areas, for example in areas suffering through severe drought.

In the U.S., invasive mammals are less common, and may not be as closely linked to masting as New Zealand’s invaders. Still, Chris Hoving, an adaptation specialist at Michigan’s Department of Natural Resources, says that masting is contributing to a growing feral swine invasion that could cost the state billions of dollars. “Pigs are a major mast consumer,” Hoving says. “More acorns, more hickory [nuts], more hard mast and soft mast would benefit them. [We’re] probably at the cusp of it becoming a major invasion.” More abundant food resources would only fuel the pigs’ expansion, and with it the diseases they’ve been known to carry, such pseudo-rabies, a viral disease prevalent in swine that can devastate livestock populations.

And in the U.S., along with feral pigs, invasive deer mice in the Southwest have been known to carry hantavirus. For this reason, among others, Tompkins would like to see other regions replicate his study. “Hopefully, this will motivate people looking at their own systems,” he says.

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