Hakai Magazine

river otter eating crab
River otters feed at sea, and bring copious nutrients back to land. Photo by Jack Chapman/FLPA/Minden Pictures

The Productive Power of Poop

Referred to in polite company as a “marine subsidy,” otter feces are a key fertilizer on small islands.

Authored by

by Jess Mackie

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In the spring of 2015, John Reynolds and a team of scientists were in a Zodiac, navigating the choppy waves off British Columbia’s central coast. As they approached a small island to survey, Reynolds spotted a verdant false lily-of-the-valley bursting forth atop a rocky outcrop, its heart-shaped leaves cascading down the barren slope. “I thought, what on earth is going on up there?” recalls Reynolds, an ecologist at British Columbia’s Simon Fraser University (SFU), and at the Hakai Institute.*

Just as the rotting corpses of dead salmon help fertilize coastal forests, Reynolds and his colleagues assumed that seaweed, washed ashore, was key to subsidizing the food chain on these small, nutrient-poor islands.

But as Reynolds and the other scientists climbed to the top of the outcrop to investigate what was giving this plant a boost, he quickly realized that seaweed was “not going to be necessarily the only story here.” The unmistakable stench of ammonia drifted toward them. That, along with the trampled vegetation, meant only one thing: river otters were afoot.

Over the next three years, Reynolds and his team explored nearly 100 of the region’s 1,600 islands, collecting data on what are known as marine subsidies. River otters, it turns out, are a key source of these subsidies on nearly all of the islands they surveyed. The animals act as liaisons, traveling back and forth between the land and ocean, says Debora Obrist, a doctoral student in ecology at SFU and the study’s lead author. By feeding at sea and pooping on land, river otters fertilize the soil, fueling plant growth and indirectly providing more insects and berries for birds, she explains.

Unlike seabirds that deliver marine nutrients to land, forest-dwelling birds along the central coast—such as chickadees and sparrows—are instead nourished by the sea, says Reynolds. “To think that even these terrestrial species have a significant signature of marine world in them is pretty cool.”

River otter subsidies, says Frédéric Guichard, a biologist at McGill University in Quebec who was not involved in the research, provide an “underestimated network of connectivity” between marine and coastal ecosystems.

Yet as the scientists continued their exploration of the islands along the central coast, they began to realize that marine nutrients are having a counterintuitive impact on bird diversity. On islands where river otter poop and other subsidies are high, bird diversity is low. On islands with few subsidies, bird diversity is high.

That finding, at first, makes little sense: birds should be flocking to the abundant resources. However, Guichard says that rather than laying out a smorgasbord, these marine subsidies are creating competition. “Instead of having different patches of resources where species can survive and avoid competing with each other, you have one big patch of resources and the best species is just taking everything,” he says. So until the ecosystem reaches this tipping point, Obrist says, diversity should increase. But after that, there start to be winners and losers among bird species.

The effects of marine subsidies on island biodiversity are particularly pronounced on smaller islands, Obrist says, where inputs like river otter poop are proportionately more important. That’s because small islands have a large coastline to area ratio, lending greater significance to the marine boundary that never falls far from inland habitat. The interiors of large islands, in contrast, are relatively disconnected from the ocean.

This pattern, Obrist says, gives some support to what’s known as the subsidized island biogeography theory. Riffing off the classic ecological theory of island biogeography, in which islands that are larger and closer to the mainland have more biodiversity than smaller and more distant islands, this modified theory emphasizes the difference that size makes to subsidy effects.

Though the theory has been tested almost exclusively on arid islands, Reynolds says further research will tell whether the patterns he observed along the BC central coast prove to be a general phenomenon for rain-soaked, temperate islands.

* The Hakai Institute and Hakai Magazine are both part of the Tula Foundation. The magazine is editorially independent of the institute and foundation.