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Guam’s Forests Have an Unlikely Ally: Feral Pigs

Feral pigs on the island of Guam are usually regarded as destructive, alien invaders. Yet they may play a vital role in sustaining the island’s beleaguered forests.
May 23, 2018

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Shortly after World War II, brown tree snakes arrived on the Pacific island of Guam. They likely stowed away on a military plane or perhaps a cargo ship; the precise origins are shrouded in mystery, but not what happened next. The snakes, who had no natural predators on the island, proliferated at extraordinary rates. Within a few decades they’d almost entirely extirpated the island’s birds. Those birds played vital roles in spreading seeds and regenerating forests — a role now performed, unexpectedly, by feral pigs.

Typically regarded as destructive alien invaders, the pigs might be vitally important to sustaining Guam’s beleaguered forests. It’s a lesson in focusing on ecology rather than labels. “While non-native species may be neutral or detrimental in pristine ecosystems,” write ecologists led by Ann Marie Gawel of Iowa State University, “it is possible that even notorious invaders could play beneficial or mixed roles in novel ecosystems.”

Writing in the journal Royal Society Open Science, Gawel and colleagues describe how pigs and also Philippine deer were brought to Guam several centuries ago by Spanish colonists. The two non-native species are “are traditionally managed as a single entity,” they explain, and management is predicated on the notion that both species are destructive and should be eradicated.

Yet while the researchers found that high population densities of deer indeed produce over-browsed vegetation and slow-to-regenerate forests, the same doesn’t hold for Guam’s pigs. And though the deer feces they collected contained few viable seeds — when germinated, 20 scats yielded just 13 seedlings — pig feces were practically bursting with potential life.

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From 31 scats, the researchers germinated no fewer than 1,658 seedlings. The pigs “provide an ecosystem function — seed dispersal — that has been lost from Guam,” wrote Gawel’s team. “Pigs appear to be one of the last vertebrate seed-dispersers on an island that has lost its native dispersers.” Were pigs eradicated from Guam, the island’s already-damaged ecology would be further impoverished.

It’s an unusually positive assessment for a much-maligned species, and though the researchers acknowledge that feral pigs elsewhere can be destructive, they counsel open-mindedness and evidence-based assessment. That advice doesn’t just apply to pigs. “The role of non-native species must be evaluated on the basis of each habitat and ecological situation,” says Gawel’s team. As people try to restore native plant and animal communities, they might find unexpected non-native allies.

Source: Gawel et al. “Contrasting ecological roles of non-native ungulates in a novel ecosystem.” Royal Society Open Science, 2018.

Image: Craig ONeal / Flickr

About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.

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