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A brown bear grabs a salmon in Alaska.
In many places, warm spring temperatures now arrive earlier in the year—due to climate change—prompting some species to forward shift the timing of important lifecycle events. The effect of such shifts in the Karluk watershed of Kodiak Island, Alaska has been found to alter the feeding behavior of Kodiak brown bears, which could have potentially serious consequences for the island’s ecosystem.
Omnivorous brown bears take nourishment from foods as they become seasonally available. On Kodiak Island, they typically consume salmon during spawning season, then turn to elderberries that ripen as the spawning season ends. A team of ecologists, led by William Deacy and Jonathan Armstrong of Oregon State University, investigated whether the early ripening of elderberries, caused by anomalously high spring temperatures, changed what Kodiak bears eat. Aerial surveys, time-lapse cameras, and GPS collars provided location information for the bears over time, while fecal analyses provided information on what bears were eating. Salmon abundance and spawning timing were estimated using time-lapse photos and historic salmon data, and elderberry ripening dates were estimated using time-lapse photos and a model drawing upon decades of historic air temperature data.
In the seasons when elderberries ripened early, there was a greater overlap in berry and salmon availability. The researchers found that, during these overlaps, the Kodiak bears switched from eating salmon to eating berries on adjacent hillsides. Though Deacy says that the bears will not likely be affected by this food switching, the behavior disrupts an important ecological link. Kodiak bears can kill 25 to 75 percent of spawning salmon. They move salmon nutrients from water to land, making carcasses available to animals that don’t go in the water. As abandoned carcasses rot, and as salmon passes through the bears’ digestive systems, salmon-derived nutrients are taken up in the soil, which increases plant productivity.
If this salmon/elderberry overlap continues, species that depend on bear-killed salmon may become less productive. Deacy notes, “Shifts in timing caused by warming may have more potential to change ecosystems than the warming itself.” (Proceedings of the National Academy of Sciences)