Water temperature and lake size explain Darwin's conundrum for fish establishment in boreal lakes.
We used a trait-based approach to disentangle Darwin's conundrum along environmental gradients and tested the contribution of earlier established invaders to biotic resistance of resident communities in boreal lakes. For this, we used a linear mixed model to test if lake surface area, water temperature, the proportion of earlier established invaders and functional distances to resident species predict the establishment of introduced species. We found that in colder and smaller lakes, introduced fishes that were ecologically distant to resident species had higher establishment success, supporting Darwin's naturalisation hypothesis. However, in warmer and larger lakes, the establishment was higher for species ecologically closer to residents, supporting the adaptation hypothesis. Therefore, the support for Darwin's hypotheses depends on how environmental gradients affect biotic interactions. Contrary to our expectation, we found that earlier established invaders seem to favour the establishment of further introduced species and did not contribute to the biotic resistance of the resident community. Our study revealed that using environmental gradients is essential to disentangle Darwin's conundrum for introduced species in boreal lakes and contribute to explain why the results of previous studies are conflicting.