After several years of studying brown bear ecology on Alaska’s Kodiak Island, I grew used to walking up streams into scenes of carnage. Where bears had killed and eaten spawning sockeye salmon, streambeds were littered with fish heads, jaws and whole carcasses, and plants on the stream banks were flattened. But at the peak of the stream spawning run in 2014, I was puzzled to find no bears or salmon parts. Salmon were dying naturally after spawning and piling up in streams, intact.
I’ve spent the last three years trying to solve this ecological puzzle. After extensive field and lab work along with researchers from Kodiak National Wildlife Refuge, Flathead Lake Biological Station and Oregon State University, we arrived at a fascinating conclusion.
In warm years, another favorite bear food — red elderberries — ripened early enough to overlap with the salmon season. This forced bears to choose between the foods. Surprisingly, almost all bears opted for berries over salmon. This choice has likely altered food webs, and will become increasingly common with expected climate warming.
Our team was struck by the bears’ seemingly counterintuitive switch. Why would bears stop eating a high-protein food loaded with energy? Quickly, though, we realized that our work was an example of a more global concern: What happens when climate change alters nature’s schedule?
Timing is everything
Among the most apparent consequences of a warming climate are shifts in phenology — the timing of key biological events like hatching, blooming or migration. Researchers have found that timing is changing in all types of organisms, but some species are more sensitive to temperature changes than others.
As a result, nature’s timetable is slowly becoming scrambled. Some species that have evolved together, such as songbirds and caterpillars, are drifting apart in time. Others, such as elderberries and salmon, are drifting together. Species which once were temporally separated are now able to interact, with unpredictable results.
In a typical year on Kodiak Island, the bears we study eat spawning salmon in small streams during midsummer, shift to berries in late summer and finally switch back to catching salmon in rivers and lakes in fall. This pattern provides bears with a continuous supply of high-quality foods. The bears can be in only one place at a time and can eat only so much each day, so they benefit when their resources are spread through time. When their key foods overlap in time, they must choose which to eat and which to skip.
Tracking bear diets
Each year, a team including myself, Kodiak Refuge biologist Bill Leacock, field technician Caroline Deacy and several volunteer crew members contended with swarming insects, rain and thick brush to collect data on salmon runs, berry crop timing and bear behavior. We worked out of a remote field camp accessible only by float plane, without phone reception or internet access.
We developed multiple data sources on bear feeding habits, each of which filled in part of the ecological puzzle. First we placed 12 time-lapse cameras along streams to see how bears responded to salmon runs before and after berry ripening. Next we used GPS collars to track female bears before, during and after the red elderberry season.
To make sure that we were not just witnessing a local phenomenon, we analyzed data collected during aerial surveys of bears fishing at streams and rivers across southwestern Kodiak Island. Finally, we conducted a scat survey to make sure that bears were eating elderberries instead of some mystery food. Together, our data showed that bears switched to eating red elderberries even when streams were packed with spawning salmon!
Why swap fish for fruit?
Why this happened is still an open question, but evidence suggests the bears were responding to protein content in their food choices. In captivity, bears offered a buffet of foods will not simply choose the most energy-rich option – that is, food that is 100 percent fat. Instead, they select a balanced diet that includes a moderate amount of protein, or around 17 percent of their total caloric intake. We don’t know exactly why 17 percent is a magic number, but it maximizes the rate at which bears gain weight.
Spawning salmon have burned through their fat stores, and their bodies are about 80 percent protein. Most common berries, such as blueberries, contain very little protein, but red elderberries are about 13 percent protein, so they help bears fatten quickly.
The main worry with respect to bears’ health is that increasing overlap between foods will force bears to choose between them. This would be like having to choose between eating breakfast and lunch, both served at 8:00 a.m., and then going hungry until dinner. Luckily Kodiak is a bear paradise with many suitable foods, including genetically diverse salmon populations that spawn at different times in different habitats. Bears that skip early runs of stream-spawning salmon can still catch salmon that spawn later on rivers and beaches. Diverse salmon runs ensure that bears will always have something to eat.
However, in the northwest United States, once-robust salmon populations are now dominated by homogeneous hatchery populations. Here, increasing overlap between foods would likely have a larger impact on predators such as bears. The key lesson for conservation is that disruptions caused by climate change will be less harmful to the species we care about if we keep nature complex and intact.
Impacts beyond streams
What about the rest of Kodiak’s ecosystem? Salmon accumulate nutrients in their bodies as they grow in the ocean and then deliver these nutrients into fresh water when they head upstream to spawn. When they die after spawning, their bodies provide fertilizer for plants and tasty snacks for scavengers.
Bears spread the bounty onto land by carrying fish from streams and leaving partially consumed carcasses far from water. This makes salmon available to smaller animals that cannot capture fish themselves, and fertilizes plants far from spawning streams. When bears ditch salmon, this carcass distribution stops, potentially harming species that depend on bear-caught salmon.
When people think about how wildlife is impacted by a warmer world, they often think of overheating animals or polar bears standing on melting icebergs. We discovered a more subtle effect of warmer temperatures: By rescheduling bears’ feeding options, climate change dramatically altered bear behavior, halting an iconic predator-prey interaction. Scientists, naturalists and even gardeners are seeing changes in biological timing throughout nature, so we should expect to witness more surprising species interactions in the future.