In a world where wildlife is threatened like never before in human history, the conservationist's mission seems clear: work to stop wildlife from going extinct, and to recover populations that have declined to the brink of extinction. In general, protecting wildlife means reducing a species' mortality by mitigating or eliminating threats. This has often been as simple as shielding or restoring a key habitat, or removing a non-native species that competes for resources.

But in today's increasingly disrupted natural environment, sometimes the biggest threat facing a species that conservationists are trying to protect and recover is … another species that conservationists are trying to protect and recover. When that happens, the solutions available to environmentalists and wildlife managers get complicated and messy very quickly. "One of the challenges that we face today in marine conservation is how to manage through success," says marine biologist Christopher Lowe, director of the Shark Lab at California State University Long Beach. "As populations recover they create new, unforeseen interactions that can often pit protected species against each other—even though many have historically interacted ecologically."

A paper published this spring in Ecology and Evolution offers one of the latest examples of this phenomenon. The recovery of once-overfished great white sharks has been touted as a conservation success story—but the recovery of a large predator means more mouths to feed. With this paper, researchers from the Monterey Bay Aquarium showed recovering great white shark populations are making it harder for another protected species to recover from past overexploitation: the sea otter. "Sea otters in California are increasingly threatened by incidental bites from white sharks that migrate to the coast to feed on seals, sea lions and occasionally scavenged whales," says lead author Jerry Moxley, a research scientist at the Monterey Bay Aquarium. "Rather than consuming otters as prey, the misdirected bite is exploratory for the shark yet consequential for the wounded otters. It's really a lose-lose, because the shark wastes energy and misses a feeding opportunity, and the otter often perishes. With no calorie-rich blubber to fuel the large predatory fish, the white sharks are left with a hairball instead of a blubber ball—and reject the meal."

There are many other examples of protected species negatively interacting with one another. In the Pacific Northwest, for example, there are renewed calls to kill seals and sea lions (protected under the Marine Mammal Protection Act) to make more salmon and other food available for endangered orcas. And in the Northwestern Hawaiian Islands National Marine Sanctuary, all fishing is illegal (except for certain traditional native practices), but government officials kill sharks to stop them from eating endangered Hawaiian monk seal pups. It is tempting for humans to intervene like this, but environmental ethicists caution against further disrupting the ecosystem in an attempt to undo damage we have already done. "If I see that you have a cancerous growth on your leg, I can't just pin you down and perform surgery to prevent your suffering, even if I am an excellent surgeon and you will benefit tremendously. I have to tread carefully, respecting your rights and wishes—partly because there's a lot about your history that I may not know, a lot about your wishes that I may not understand," says environmental ethicist Benjamin Hale of University of Colorado Boulder, author of The Wild and the Wicked: On Nature and Human Nature. "If we take action to prevent one set of animals from living their lives in their wild ways, we will be causing a cascade of effects that we cannot completely understand, unleashing futures that we cannot anticipate."

Environmentalists note that many of these complex interactions among threatened species are happening only because humans have upset the ecosystem's natural balance. "These kinds of conflicts are a product of a highly disrupted system," says Kim Delfino, the California program director of the nonprofit Defenders of Wildlife. "For sea otters, the white shark problem is really a problem of a super-small restricted sea otter population and a very degraded habitat that includes declining kelp forests. I do not think that somehow there are more great white sharks out there than there should be. For sea lions and salmon [and orcas], there are so few salmon due to how we have destroyed salmon breeding grounds, and impacts of climate change. The seals and great white sharks are only a symptom of the larger problem. Treating them will not treat the disease that is actually causing the decline in the imperiled fish and wildlife."

In the case of the great white sharks and sea otters, fixing the otter habitat may be an effective way to help otters (and other California wildlife) without hurting the sharks. "Sea otters are known to increase biodiversity and resiliency in kelp forest ecosystems," says sea otter expert Teri Nicholson of the Monterey Bay Aquarium, a coauthor on the Ecology and Evolutionpaper. "Historically these ecosystems could have served as natural refuges from larger marine predators, and may do so again. We must focus on ecosystem resilience and goals that encompass entire communities of wildlife; habitat matters. We know that risk of shark bite declines significantly with increasing kelp canopy in coastal California. If otters can establish persistent kelp forests, the refuge could buffer unwanted interactions with other predators trying to make a living in modern-day California."

Although these problematic interactions among protected species may cause plenty of headaches and heartburn in the meantime, scientists are confident that as humans learn more, we will eventually get the complexities of wildlife recovery right. "We didn't do a lot of ecological studies before driving these wildlife populations to near-extinctions," says Salvador Jorgenson of the Monterey Bay Aquarium, an expert on great white sharks and a coauthor on the shark and otter study. "Now, as they recover, we are learning about interactions between them that we hadn't anticipated. We are learning as we go. The good news for California is that, for the moment at least, we seem to be going in the right direction."