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Anna L. Hargreaves, Rachel M. Germain, Megan Bontrager, Joshua Persi, and Amy L. Angert (March 2020)
Global synthesis finds species interactions strongly affect fitness, but only broadly drive local adaptation in tropics
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Adaption to local conditions drives biological diversification, but what drives local adaptation? Famous experiments have shown populations adapt to their local climates, soils, even pollution, but how often they adapt to their local communities has remained a mystery. To solve it, researchers at McGill University and the University of British Columbia synthesized more than 125 studies testing local adaptation in over 100 species.
“As field biologists we’d seen firsthand how strongly interactions can affect fitness,” says Anna Hargreaves, who lead the study. “I thought for sure local adaptation to interactions was widespread and we just needed a meta-analysis to show it. But—surprise!” The authors found frequent local adaptation across studies and that negative interactions like being eaten or outcompeted strongly reduced performance. Nevertheless, local adaptation was not stronger or more common when experiments left interactions intact.
So why don’t interactions drive local adaptation even though they affect fitness? Maybe, the authors suggest, interactions vary too much to exert consistent divergent selection, as individuals move around and populations grow and crash. Or maybe we’re looking in the wrong places. Interactions are often thought to be strongest in the tropics, but most data are from temperate zones. When the authors analyzed tropical data separately a stronger signal of local adaptation to interactions emerged.
“It doesn’t explain why interactions don’t drive local adaptation more often in temperate ecosystems, but it is intriguing evidence that interactions might be more evolutionarily important in the tropics,” says Hargreaves. The authors say more direct tests of what drives local adaptation, especially studies of positive interactions and tropical species, are needed before we know the real scope of local adaptation to interactions.
Adaptation to local conditions can increase species’ geographic distributions and rates of diversification, but which components of the environment commonly drive local adaptation—particularly the importance of biotic interactions—is unclear. Biotic interactions should drive local adaptation when they impose consistent divergent selection; if this is common we expect transplant experiments to detect more frequent and stronger local adaptation when biotic interactions are left intact. We tested this hypothesis using a meta-analysis of transplant experiments from >125 studies (mostly on plants). Overall, local adaptation was common and biotic interactions affected fitness. Nevertheless, local adaptation was neither more common nor stronger when biotic interactions were left intact, either between experimental treatments within studies (control vs. biotic interactions experimentally manipulated) or between studies that used natural vs. biotically-altered transplant environments. However, the effect of ameliorating negative interactions varied with latitude, suggesting that interactions may promote local adaptation more often in tropical vs. temperate ecosystems, though few tropical studies were available to test this. Our results suggest that biotic interactions often fail to drive local adaptation even though they strongly affect fitness, perhaps because temperate biotic environments are unpredictable at the spatiotemporal scales required for local adaptation.