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Conservation Science for a Healthy Planet

How Climate Change Canceled the Grizzly Salmon Run

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  • On an Alaskan island, one of nature’s greatest spectacles is shutting down, as brown bears abandon fish in favor of a surprising alternative- red elderberries.
  • Synchronization of salmon and elderberry availability is disrupting trophic interactions between Kodiak brown bears and salmon with potential ecosystem level impacts.
  • Most examples of these “phenological shifts” involve asynchrony between two partners. The case of the Kodiak bears represents “an under-recognized phenomenon—that of increasing synchrony of [natural] events due to climate change.

 

Ed Yong

 

…In most years, red elderberries only ripen from late August to early September, at the end of the salmon season. The two food sources don’t overlap, so the bears eat them in sequence, gorging on salmon before bingeing on berries. But, by looking at historical data, Deacy and Armstrong found that this natural timetable has changed. In Alaska, spring temperatures have increased and elderberries have been ripening earlier. In 2014, the berries ripened especially early, bringing them in sync with the spawning salmon. And it seems that whenever both items are on the menu simultaneously, the bears always choose berries.

Which makes no sense. Pound for pound, salmon contains twice as much energy as elderberries. If bears are looking to gain as much weight as possible, in anticipation of the coming winter, why would they pick the less calorific food? “All our conventional wisdom made it hard to believe that they were switching to these berries,” says Deacy.

…Robbins suggested that it’s not the total number of calories in these foods that matters, but the levels of different nutrients. In an earlier study, in which he offered captive bears a varied diet, he’d found that the animals mix and match their foods so they get around 17 percent of their energy from protein. That’s the level that allows them to gain weight most quickly. If they overload on protein, they actually lose weight.

Salmon are far too rich in protein—it accounts for about 84 percent of the energy in their flesh. But elderberries, by astonishing coincidence, comprise around 13 percent protein—far more than your typical berry, but almost exactly the optimal amount for a grizzly bear. By focusing on that single food, the bears can gain weight as fast as possible.

On average, red elderberries are ripening two and a half days earlier every decade. If that continues, they will regularly overlap with the salmon by 2070, and the unusual events that Deacy and Armstrong saw in 2014 will become the new normal.

Many scientists have shown that climate change is rescheduling nature. Warming temperatures are forcing birds to migrate sooner, insects to emerge earlier, and plants to bud and bloom before their time. These changes are disrupting many of the dances between species, forcing long-established partners to move to different rhythms. Flowers, for example, might bloom too early to catch a wave of pollinating insects.

But most examples of these “phenological shifts” involve asynchrony between two partners. The case of the Kodiak bears represents “an under-recognized phenomenon—that of increasing synchrony of [natural] events due to climate change,” says Nicole Rafferty, from the University of California, Riverside. “And the consequences of this shift in foraging behavior could be large with knock-on effects for the ecosystem as a whole.”

Species that never lived together can now interact because we’re removing the barrier of time,” says Armstrong. “We’ll see these new combinations that we never thought about, and we’ll get strong responses that no one could have ever predicted.”

William W. Deacy, Jonathan B. Armstrong, et al. Phenological synchronization disrupts trophic interactions between Kodiak brown bears and salmon. August 2017. PNAS. doi: 10.1073/pnas.1705248114

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