Alligators on a beach would seem strange to anyone who learned in school that the reptiles dwell in freshwater swamps and rivers. In Florida, beach sightings are reported in local media as novelties.
But alligators in marine environments aren’t strange to Duke University marine conservation biologist Brian Silliman. His studies of alligator stomach contents in the southeast United States have turned up significant proportions of marine organisms – such as horseshoe crab shells, stingray tails and shark’s teeth – that show some alligators are quite at home in salty waters.
That research, published in 2014, was one of many data points that led Silliman and several colleagues to argue recently that large predators appear to be expanding their territories into novel habitats previously considered unconventional for their species. In a recent paper in the journal Current Biology, they documented this trend for a number of species, both marine and terrestrial, by combining data from scientific literature and government reports.
“Following long-term protection, sea otters along the northeast Pacific coast have expanded into estuarine marshes and seagrasses, and alligators on the southeast U.S. coast have expanded into saltwater ecosystems, habitats presently thought beyond their niche space,” they wrote. “There is also evidence that seals have expanded into subtropical climates, mountain lions into grasslands, orangutans into disturbed forests and wolves into coastal marine ecosystems.”
The catch, Silliman and his coauthors argue, is that these “new” habitats aren’t actually new. They believe the trend represents predators recolonizing old hunting grounds – an idea that could indicate some predators are more flexible in their habitat and more resilient than previously thought.
Silliman said these predators – “ghosts of nature’s past,” as the paper put it – were simply never studied in these novel habitats before humans chased their populations away, especially in coastal regions where people have lived the longest. Now, as predator populations recover and show up in new areas, there may be new conservation challenges as humans interact with large predators they haven’t dealt with in the past, Silliman and others said.
“We need to think about how the management of species changes in and out” of protected areas, Silliman said. “The current paradigm is to segregate humans and nature, but we can coexist in some ways.”
Seeing alligators in coastal waters along the Gulf of Mexico coast and Florida isn’t new, said Frank Mazzotti, a University of Florida ecologist who has studied alligator populations in the region as well. But he said nobody has documented the phenomenon as extensively as in Silliman’s earlier research. That alligators are being more commonly observed in coastal saltwater regions isn’t, he said, due only to their populations expanding – but is also likely because humans are expanding their own use of these same areas.
He called for better education in how people should handle alligator interactions, which happen often in Florida during mating season. “There’s nothing bad about encountering an alligator when you’re in nature,” he said. “It’s a sign of a healthy ecosystem.”
But even if people can be managed, there’s also the question of how alligators are affecting other wildlife as they show up on the coast. Most of what the alligators are eating when they go into seawater is blue crab, according to Mazzotti, which are relatively low on the food chain. This would limit their impact on the ecosystem as a whole, though their hunting could have implications for blue crab fishery managers in Florida and the Gulf states.
Silliman’s new study also cites another example of a top predator – albeit a much more cuddly-looking one – expanding its territory and eating more crab.
Sea otters eat up to 35 percent of their body weight each day to maintain the energy they need to stay warm in the chilly waters off the California coast. In recent decades, those hungry and threatened otters have expanded beyond their typical marine kelp forest habitat into marshes and other relatively protected estuaries – safe from predators such as sharks – as their populations slowly recover, according to the study.
In places like Elkhorn Slough, off Monterey Bay in California, otter populations have been increasing in the past couple of decades and eating juvenile Dungeness crabs. But Dungeness, like blue crabs, are also important foods for another top predator: humans.
Kim Delfino, director of Defenders of Wildlife’s California program, said she hasn’t heard much from crabbers about otters in Monterey Bay. Primarily, she said, that’s because West Coast crabbers have had to deal with other new and much more costly problems recently, such as harmful algal blooms. “Frankly, they’re more concerned with bigger problems,” Delfino said.
Otters in California’s estuaries could also benefit the ecosystem, Silliman said. By eating crabs, which are predators of sea slugs, the sea slugs have proliferated. Those slugs eat the algae of seagrass leaves, which has allowed seagrass beds to flourish. That, in turn, may be a boon for other fisheries, as many species use seagrass as a nursery for juveniles.
“Top predators are going to have impacts on prey, but they’ll also have cascading impacts,” he said. “The overall net effects could be positive.”
Delfino also pointed to another boon of growing otter populations in the area: wildlife viewing.
“People kayak in Elkhorn Slough specifically to see sea otters,” she said. “So there is a whole economic engine that has grown up around this return of otters to the system. It just generates huge amounts of money.”
All of those benefits need to play a bigger role in how we think about the return of predators, according to Silliman.
“If you Google ‘human-wildlife benefits’ you don’t see anything,” he said. “We always think about human-wildlife conflicts. We need a paradigm shift.”