Species exhibiting colour-polymorphism are thought to have an ecological advantage at the landscape scale, because spatial segregation of alternatively-adapted ecotypes into diverse habitats can increase the total species’ niche breadth and thus confer greater geographic range size. However, morph frequencies are also influenced by intra-populational processes such as frequency- or density-dependent social interactions. To identify how social feedback may affect clinal variation in morph frequencies, we investigated reciprocal interactions between morph-specific thermal tolerance, local climatic conditions, and social environments, in the context of a colour-morph frequency cline associated with a recent range expansion in blue-tailed damselflies (Ischnura elegans) in Sweden. Cold tolerances of gynochromes (female-like female morph) were positively correlated with local gynochrome frequencies, suggesting a positive frequency-dependent fitness benefit of being common. In contrast, androchrome (male-mimic female morph) cold tolerances were improved following recent exposure to cold weather, suggesting a beneficial environmental acclimation effect. Thus according to an environment-matching hypothesis for clinal variation, androchrome frequencies should therefore increase towards the (cooler) range limit. In contrast to this prediction, gynochrome frequencies increased at the expanding range limit, consistent with a positive frequency-dependent social feedback when invading novel climates. Our results suggest that when phenotypes or fitnesses are affected by interactions with conspecifics, beneficial social effects on environmental tolerances may i) facilitate range shifts and ii) reverse or counteract typical patterns of intraspecific interactions and environment-matching clines observed in stable populations observed over broader geographic scales.
Table S1 from 'Do group dynamics affect colour morph clines during a range shift?'
Colour morph frequencies, population densities, and sex ratios for populations of the blue-tailed damselfly (Ischnura elegans) located at the range expansion front, and ~ 500 km back from the expansion front. Please refer to the associated publication for further details.