Climate change and the ecology of fear

He's dis­cov­ered some sur­prising things: When prey living in the inter­tidal zone sense a predator nearby, they are much less effi­cient at con­verting energy into growth. They pro­duce so much ner­vous energy in the form of ele­vated metab­o­lism, stress hor­mones, heat-​​shock pro­teins, and antiox­i­dant enzymes that there isn't a lot left over to build new tissues.

Trussell, who is chair of the Depart­ment of Marine and Envi­ron­mental Sci­ences, believes this fact can be a crit­ical driver for deter­mining how an overall ecosystem will fare. Resources and predator abun­dance will always fluc­tuate, but it's how the middle species deal with those fluc­tu­a­tions that deter­mines the way the whole system will respond. As he explained, "It's that species in the middle that have to trade off their need to feed with the risk of being eaten by a predator that's ulti­mately going to shape how the system behaves."

But there's one thing that's been missing from this cal­culus: the role of cli­mate change. Some studies have exam­ined how cli­mate change inter­acts with fear, Trussell said, but most of them haven't been very real­istic. The problem, he explained, is that they all imagine a con­stantly ele­vated tem­per­a­ture based on cli­mate change pro­jec­tions put forth by the Inter­gov­ern­mental Panel on Cli­mate Change. They don't account for nat­ural cli­mate vari­a­tion seen on a local scale.

"If you bake any­thing under a high tem­per­a­ture all the time, you're going to get an impact," Trussell said. So in new research pub­lished in the journal Global Change Biology, Trussell and his team took a dif­ferent approach. The researchers built a state-​​of-​​the-​​art system that allows them to con­stantly adjust the water and air tem­per­a­tures in their exper­i­mental habi­tats, holding them 2.5 degrees Cel­sius above actual tem­per­a­ture that is con­tin­u­ously mea­sured in Nahant.

The con­ser­v­a­tive nature of their approach makes their striking results even more sig­nif­i­cant, Trussell said. They found that when a prey species, in this case a snail called Nucella Lapillus, sensed the chem­ical cues of a predator, the inva­sive green crab, it ate fewer mus­sels and thus as expected expe­ri­enced less growth.

But when the team intro­duced the warming sce­nario, that effect was even more pro­nounced. Despite eating just as many mus­sels as in the non-​​warming sce­nario, the snails actu­ally lost weight under the warming conditions.

"You've got these ani­mals that are already burning a lot of energy because they're wor­ried about get­ting eaten," Trussell explained. "And then you throw on top of that the stress that's imposed by tem­per­a­ture increases and even­tu­ally what you end up seeing is that the ani­mals struggle to make a living."

The com­bined effects of pre­da­tion risk and warming seem to create even bigger chal­lenges for these middle species, which Trussell believes are inte­gral to the overall health of an ecosystem. The results, he said, sug­gest that the impacts of cli­mate change may be most sig­nif­i­cant in areas where the fear of being eaten is par­tic­u­larly impor­tant to com­mu­nity dynamics.

"Very rarely are species out there in iso­la­tion just dealing with tem­per­a­ture and its poten­tial increases. They're inter­acting with other species as well and all the chal­lenges that they present, Trussell said. "So looking at how cli­mate is impacting these inter­acting species is what we really need to do if we're going to under­stand the eco­log­ical con­se­quences of cli­mate change."