Impact of keystone species varies in different ecosystems

A groundbreaking 30-year study published in the Proceedings of the National Academy of Sciences (PNAS) has revealed that the ability of sea otters to restore kelp forests varies significantly based on local ecosystem dynamics.

The research team, led by Ryan Langendorf from the University of Colorado, analyzed three decades of data tracking kelp forest communities during sea otter reintroduction at two locations: Vancouver Island, British Columbia and San Nicolas Island, California.

"Sea otters are an iconic keystone predator that can maintain kelp forests by preying on grazing invertebrates such as sea urchins," explains the research team. "However, the effects of sea otters on kelp forests vary over their geographic range."

The study found dramatically different patterns at the two locations. Off Vancouver Island, the reintroduction of sea otters quickly led to a classic trophic cascade - otters rapidly reduced urchin populations, which allowed kelp forests to recover and flourish. However, around San Nicolas Island, the researchers observed a more muted effect, with otters, urchins, and kelp coexisting at intermediate densities for years.

By developing a new community modeling approach that treated species interactions as dynamic rates varying with community state, the team discovered why these differences occurred. While the traditional view would suggest stronger interactions in the northern region, the model revealed that the actual difference stemmed from two key factors:

1. The Vancouver Island otters had a strong but brief pulse of impact on urchins immediately following recolonization, while San Nicolas Island otters had more consistent but moderate effects.

2. In the southern California region, stronger competitive interactions among species created indirect interaction chains that buffered the impact of the otter-driven trophic cascade.

"These findings shed light on long-standing hypotheses about how interspecific interactions can alter the function of keystone species across community contexts," noted the researchers. The study provides valuable insights for marine conservation efforts, suggesting that ecosystem restoration involving keystone species may require different approaches depending on local ecological dynamics.

Reference: Langendorf, R.E., Estes, J.A., Watson, J.C., Kenner, M.C., Hatfield, B.B., Tinker, M.T., Waddle, E., DeMarche, M.L., & Doak, D.F. (2025). Dynamic and context-dependent keystone species effects in kelp forests. Proceedings of the National Academy of Sciences, 122(10), e2413360122. https://doi.org/10.1073/pnas.2413360122

Published March 3, 2025