Synchronous timing of food resources triggers bears to switch from salmon to berries.

We are in an era of phenological (seasonal timing) change (1). One of the most consistent fingerprints of climate change is the spring advancement of life history events of diverse taxa in the northern hemisphere, from flowers to butterflies to birds. As phenologies change, so too do potential interactions among species. The speed of phenological change can differ among interacting species (2, 3), potentially disrupting crucial interactions such as pollination, foraging, or predator–prey encounters. While there is growing appreciation that climate change may cause such trophic mismatches, the effect of climate-induced synchronization on food webs has been practically unexplored. Enter Deacy et al. (4). Deacy et al. (4) study an iconic interaction: bears feeding on salmon. However, as any biologist who has walked along a salmon stream knows, bears do not feed exclusively on salmon during the salmon run; bear scat encountered along salmon streams often contains seeds from berries (Fig. 1). As mobile consumers, bears integrate across heterogeneous resource pulses that are available at different times (5, 6). For example, in the Karluk watershed of Kodiak Island, Alaska, where Deacy et al. (4) conducted their research, brown bears feed on stream-spawning sockeye salmon early in the summer and then switch to feeding on red elderberry late in the summer. The complementary timing of stream-spawning salmon and elderberry effectively prolongs the duration of high-quality foraging opportunities during the short Alaskan growing season. However, with climate change and warmer springs, elderberry phenology is changing. Elderberries ripen earlier with warmer conditions, overlapping more with stream-spawning salmon. These stream-spawning salmon—in contrast to salmon spawning in larger rivers or lakes—are the most vulnerable to bear predation (7). Deacy et al. show that bears abandon stream-spawning salmon and shift to feeding on elderberry when both resources are available concurrently (4). Indeed, aerial surveys showed reduced bear counts on spawning streams in years with higher temporal overlap of salmon and elderberry. Once elderberry became available, camera traps documented reduced bear activity in the vicinity of salmon streams; scat surveys along stream margins revealed a higher proportion of scats with berry remnants; and GPS-collared females shifted their distributions from salmon streams to adjacent hillslopes where elderberry were common. These different lines of evidence paint a coherent picture of climate-induced diet switching due to synchronized resource availability. Why bears switched from salmon to berries is an open question, but the authors postulate that elderberries might have more suitable macronutrient content, hinting that food quality and costs associated with assimilation could be playing a role. The synchronizing effect of climate is perhaps counterintuitive given that the growing season in northern temperate ecosystems is getting longer. On the one hand, wemight expect increased duration of high-quality foraging opportunities for bears with longer growing seasons. On the other hand, Deacy et al. (4) show the schedule of availability of two key foods synchronized during warm years, which reduced the ability of bears to capitalize on the two resource pulses. Which is more important to bear fitness and demography—the overall Fig. 1. Bear scat encountered along a salmon stream reveals a balanced diet. Image courtesy of Andrew Hendry (photographer).