Alexa Fredston-Hermann1, Dr Malin Pinsky2, Dr Steve Gaines1, Dr Benjamin Halpern1
1University of California Santa Barbara, Santa Barbara, United States, 2Rutgers University, New Brunswick, United States
Species around the world are shifting their ranges in response to climate change. Most empirical evidence for these climate-related range shifts comes from measuring the displacement of a population’s weighted center in space. This approach masks the complex dynamics of range edges, where climate-related colonizations and extinctions actually occur. Studies that do examine range edge displacement often use just two or three time points. We analyzed annual equatorward and poleward range edge dynamics of marine fishes in a multi-decade time-series of trawl surveys conducted on the Northeast U.S. shelf during a period of rapid warming. By examining both poleward and equatorward edges, we tested whether poleward edges show stronger evidence of climate tracking than equatorward edges, as predicted by biogeographic theory. We found that range edge dynamics were highly stochastic, and for most species, measuring the range edge in one historic year and one recent year—i.e., the resurvey method—would suggest a trend that is not consistent with the time-series as a whole. Most range edges did move north over the time-series, but many did not show a significant displacement, and some moved south. Species’ range edges were found in very different temperatures each year, and we did not find evidence for a significant shift in the realized thermal niche over time. Our results illuminate the highly variable nature of range edges and emphasize the need to understand how non-climate processes influence range dynamics.
Alexa Fredston-Hermann is completing her PhD at the Bren School of Environmental Science & Management at UC Santa Barbara. Her research explores ecological and biogeographical processes that influence Anthropocene range shifts.