illustration Catherina Türk
Science is made of stories, and in ecology one of the most famous starts on Tatoosh Island, off the northwestern tip of Washington State. There, in 1963, a young biologist named Robert Paine set out to uncover the rules that governed why intertidal communities are structured the way they are. Tatoosh was a perfect place to do this. Its tide pools overflowed with a varied community of mussels and barnacles, limpets and chitons and snails, and a sea star, Pisaster ochraceus, known as the ochre star.
Ochre stars are the main predator in that system, and Paine hypothesized they dictated the ways the other species behaved. But how much? And what would happen if they all disappeared? To find out, he pried off every sea star from a stretch of rock and watched to see which creatures thrived in their absence. (Mussels benefited the most, outcompeting everything else for space.) From this emerged the keystone species concept: the idea that some species’ impacts on a community are greater than would be expected given their number. “These individual populations are the keystone of the community’s structure,” Paine wrote in 1969, “and the integrity of the community and its unaltered persistence through time, that is, stability, are determined by their activities and abundances.”
Some 50 years later, though, I keep coming back to Paine’s question at its most basic, its most naïve: What happens if all the sea stars disappear?
Early in the summer of 2013, as biologists were conducting a survey at Starfish Point, in Washington’s Olympic National Park, they saw something wrong with the area’s resident ochre stars. More than one quarter had white lesions on their bodies, and most looked sickly and deflated. Some had even lost an arm or two. Sometimes the decoupled arms would be nearby, thrashing around.
This would be the first recorded sign of what scientists would come to call “sea star wasting disease. Within a couple of months, afflicted sea stars had appeared in tide pools and piers around Vancouver, Canada. (These were a different species, Pycnopodeia helianthoides, the sunflower star.) By October, sea stars were dying in Seattle and Monterey Bay. By December, in Southern California. A year later, dead sea stars had been found as far north as Alaska and as far south as Mexico.
At site after site, a numbing scenario would unfold. Signs of wasting disease would appear, and a de facto countdown would start. Sea stars stood little chance of surviving; individuals that presented symptoms were often dead in a day or so, sometimes withering away before researchers’ eyes. Entire assemblages of sea stars might be wiped out within a week. In all, more than 20 species up and down the west coast of North America would experience local die-offs.
Similar diseases had struck sea stars before, but this outbreak, biologists said, was exceptional in both its virulence and extent. In the past, the disease had affected only one or two species, in one or two spots. It had been, in the words of one biologist, “orderly.” But the current epidemic affected so many species and rampaged along an extended and exceptional length of coastline. Nor did it abate in the winter, as would be expected, when the sea temperatures cooled. Instead it only dipped somewhat, before ramping up again the following spring.
Biologists raced to figure out the cause (or causes) of wasting disease, before identifying a Densovirus as the primary culprit. But that finding was small comfort. Was the disease passed through the water, or by star-to-star contact? Did the virus itself cause the sea star to die, or did it simply weaken an individual’s immune system to the point that a sea star became more susceptible to bacterial infection? The evidence was contradictory. In any case, there was no way to stop wasting disease – no vaccine to administer to threatened sea stars, or any way to quarantine sickened ones. More ominous still was that the virus had been found in museum specimens from the early 1940s, meaning it had been present in the North Pacific for more than seven decades.
“This is clearly a sentinel event,” says Drew Harvell, a marine epidemiologist at Cornell University. “Something has changed that caused the unprecedented scale of the outbreak.” The ocean, and the life in it, is telling the scientists something, although what it is still isn’t exactly clear. But Harvell and her colleagues are sure of one thing. The decimation of the sea stars is not the last large disease outbreak they are likely to see.
In 1985, commercial fishermen started to find large numbers of dead and dying black abalone around the Channel Islands off the Southern California coast. Scientists later determined that a bacterium from the family Rickettsiaceae causes the disease. Scientists were alarmed to learn that, whenever ocean waters warmed up, the infection was more likely to spread; the initial outbreak had been detected shortly after a strong El Niño event in 1982-83. Already hard hit by overfishing, black abalone populations plummeted throughout the species’ range to the point of complete extirpation in places. In 2008, the National Marine Fisheries Service recommended that the species be listed as endangered, and in January 2009, it was. It has yet to show signs of lasting recovery.
Climate change can reach species through disease via a number of pathways, some less direct than others. A recent analysis of the resident orca population in Puget Sound, for example, showed that their respiratory systems harbor more than 50 types of potentially pathogenic bacteria and fungi – including Salmonella, Morbillivirus, and Staphylococcus. Although the effects of this stew on the orcas’ overall health are as yet unknown, what is known is that the southern residents, as the population is called, may be particularly susceptible to illnesses. Owing to the urban waters in which they swim, their blubber contains higher concentrations of polychlorinated biphenyls (PCBs) and other immune-suppressing toxins than most marine mammals. They are also subject to food-stress since populations of Chinook salmon, their preferred food are low, in part due to the effects of climate change, which creates unfavorable marine conditions and affects the salmon spawn during development.
Vibrio is a genus of bacteria, several species of which can cause severe infections in humans, usually when they are ingested in undercooked seafood. Warm saltwater is most hospitable for the growth of these pathogenic bacteria, and outbreaks typically occur between May and October. But in recent years, as sea surface temperatures have increased, both the seasonal and geographic range of Vibrio has expanded. Additionally, as the frequency of large storms is predicted to increase, so, too, are Vibrio outbreaks that often follow. In Europe, the first recorded infections of V. vulnificus in Germany followed an unusually warm summer in 1994. Since then, infections have occurred repeatedly in the Baltic Sea, where water temperatures have increased over the past two decades.
Harvell spent much of the summer of 2014 in the San Juan archipelago, in northwestern Washington, watching sea stars die and trying to find out why. Although she was alarmed by the sheer spectacle, it did not necessarily surprise her. Such a phenomenon was something she’d been predicting would happen for years. She has spent her career coordinating broader research on marine diseases, and in addition to sea stars, has studied oysters, clams, and corals, and other ailing species. For her, sea star wasting disease, while it is certainly mesmerizing as a data point, is just the latest in a larger, more worrisome trend. “That’s the frame for this,” she says. “We have to consider the role that climate change might play in the larger phenomenon of marine disease.”
In 2002, Harvell coauthored a review in the journal Science that looks precisely at the relationship between climate warming and disease risk. A warming climate, she and her colleagues argue, offers several pathways for disease risk to increase in marine environments. Bacteria are better able to propagate in warm water. Their hosts, on the other hand, might become heat stressed, which can limit immune function. Transmission thus would become easier. That might explain how, for example, a pathogen that had coexisted with sea stars for 70 years might suddenly become so severe. “Links between climate change and disease will increase the severity of threats associated with climate warming,” she wrote then. Or, as she says now, “A warmer ocean is a sicker ocean.”
Other scientists agree. “My worry,” says Peter Raimondi, a biologist at the University of California, Santa Cruz, “is that this is just one of many such events we will see.” Along the US East Coast, from Florida to New York, more than 1,800 bottlenose dolphins have been stranded since July 2013, almost all of them dead when they washed ashore, almost all of them testing positive for a Morbillivirus, similar to measles in humans. (The Unusual Mortality Event, as it’s known, is still going on.) Also on the East Coast, two separate diseases that afflict farmed oysters have crept farther and farther north, as if to follow uncurling fingers of warmer water. Within the past decade, more than two-thirds of coral reefs in the Caribbean have succumbed to a hodgepodge of diseases as higher ocean temperatures become more prevalent. More broadly, a 2015 study of the frequency of mass mortality events in Proceedings of the National Academy of Sciences found that first, they are generally increasing, and second, that disease was the most common cause.
“There’s a classic narrative when it comes to these things,” says Sam Fey, a researcher at Yale University who was lead author of the paper. “You see a report on a single event, and scientists speculate on the causes so far as anyone can tell.” You might see some anecdotal hand-waving about climate change or point-source pollution, but the events themselves are usually treated as one-offs, and are rarely put in larger context; Fey’s study was the first to try to identify patterns in such mass mortality events. He thinks we are perhaps too fascinated by the general carnage to take the necessary step back. “It’s like the biological version of a cop show, or a murder mystery,” he says. “It’s hard to look away.”
Even with the increasing number of outbreaks linked to a warming sea, scientists caution that, with disease – and especially with marine diseases – it can be hard to disentangle their effects from other causes of mortality. (Fey’s study, for instance, found that the most severe mass mortality events often had several causes.) One problem when trying to assess the severity of an outbreak is that there is often little in the way of baseline data with which to compare it. Also, the ocean is so big that it is difficult to know what is happening beyond the slim wedge of it that people can monitor. Unless the disease strikes something economically significant, it isn’t likely to seem like that big of a deal to the public at large.
“I would say, at least at this point, we’re much more engaged in our nearshore systems,” says Colleen Burge, a biologist at the University of Maryland. “These are places we’re able to see and study, they’re easier for us to understand, just easier to watch.” She points out as well that marine disease does not always cause mass mortality events like the sea stars’. “Disease is a pretty normal part of any population,” she says. “Just as it is with us.” (That said, she is “blown away” by the media response to sea star wasting disease.)
For her part, Harvell is trying to drum up support for a legislative fix that she hopes will help scientists and managers react more robustly to future outbreaks. The bill, called the Marine Disease Emergency Act, was introduced in February 2015 by Representative Dennis Heck of Washington. It would provide emergency funding for scientists trying to get their arms around an outbreak – something Harvell says is sorely lacking, given the slow pace necessary to get most research grants. Right now, there is little to help identify, monitor, or forecast marine diseases, to say nothing of mitigating their effects once they spread. Unfortunately, the bill is still languishing in the House Subcommittee on Water, Power, and Oceans.
“We’re not very well equipped once something is in the ocean to do anything about it,” Harvell says. “On land, you could quarantine, cull, or vaccinate, but none of these options is available at sea.”
It has now been three years since the initial detection of sea star wasting disease, and most of the decimated sea star populations have yet to recover to anything approaching their former numbers. Although the disease is no longer as prevalent and some juvenile sea stars have shown up in California, in the Pacific Northwest most rocks remain bare. No one is sure when, or if, the sea stars will come back, or what the communities that come back to will look like.
illustration Catherina Türk
In their absence, there is already evidence that the places they left are restructuring themselves. In Monterey and surrounding areas, sea urchins now blanket the seafloor. When sea stars were around, the sea urchins wedged themselves into rocky cracks and crevices to avoid their voracious relatives. But so great are their numbers now that biologists worry for swaths of California’s great kelp forests. Sea urchins are dedicated consumers of kelp holdfasts, chewing through them and sending the blades adrift, clear-cutting the seafloor, in effect. As the kelp forest goes, so goes the shelter of many species of fish, crabs, and other species of economic importance.
A couple of hundred miles to the south, however, the scene is quite different. There, small populations of sea urchins have started to succumb to a peculiar condition. They progressively lose their spines, like they are going bald. To some scientists, the scenario has an unsettling familiarity. They suspect the balding urchins may be suffering from a wasting disease similar to the one that ravages sea stars. (Both sea stars and urchins belong to the same phylum, Echinodermata.) But they aren’t entirely sure yet, having not had the chance to study it. Will disease – if it is even a disease – devastate urchins the same way it is ravaging sea stars? They will know soon enough.
Eric Wagner writes about science and nature from his home in Seattle, where he lives with his wife and daughter.
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