Competition and temperature stress drives the evolution of species' range limits

Background/Question/Methods

Competition may play a key role in limiting species’ geographical ranges. Long-standing theory originating from Darwin suggests that competition is more important in setting range limits where abiotic environments are benign. In contrast, abiotic drivers may set range limits where environments are harsh. Empirical tests of this theory, however, have provided mixed support on the effects of competition across abiotic gradients in space. Moreover, we lack experimental tests that directly show how abiotic and biotic drivers may constrain adaptation across the range, leading to the evolution of range limits. Here, we use experimental evolution of competing duckweed species to test how competition and temperature stress interact to limit a species’ experimental range. We grew genetically diverse populations of duckweeds in “miniaturised-landscapes” with a spatial gradient in temperature ranging from benign to harsh conditions (~25C to 40C). We set landscapes either with competition (Lemna minor and Spirodela polyrhiza; Nf18) or without competition (L. minor only; Nf18) and experimentally evolved populations for 30 days (~10-15 generations). To test for adaptation to abiotic and biotic environments, we conducted full reciprocal transplants across the range and measured fitness (i.e., growth rates) of L. minor at both the range-core and range-edge.

Results/Conclusions

Our preliminary results show that temperature stress significantly reduced growth rates of L. minor at the hot, experimental range-edge (by –0.99 [95CI: –1.16 to –0.82] fronds/day compared to the benign, range-core). We find a slight but statistically significant effect of competition in reducing growth (by –0.19, –0.36 to –0.02 fronds/day compared to non-competitive landscapes). However, competition reduced growth throughout the range and was not temperature dependent. Our reciprocal transplants indicate that adaptation occurred in the benign, range-core but not at the hot, range-edge. L. minor that evolved locally at the range-core had higher growth rates than plants transplanted from the range-edge (by 0.39, 0.08 to 0.71 fronds/day). Importantly, when accounting for competition, adaptation to local, benign temperatures at the range-core was only evident when testing in non-competitive landscapes, and when plants had no history of competition. Furthermore, we did not find evidence for adaptation to hot temperatures at the range-edge regardless of competition. Overall, our data indicates that competition can affect evolutionary dynamics at the range-core by limiting adaptation to benign temperatures, whilst reducing growth rates across the experimental range. Thus, competition may be important at both benign and harsh environments, driving species’ range dynamics across abiotic gradients in space.