Low larval densities in northern populations reinforce range expansion by a Mediterranean damselfly

Abstract

Summary Contemporary climate change triggers a poleward range shift in many species. A growing number of studies document evolutionary changes in traits accelerating range expansion (such as growth rate and dispersal‐related traits). In contrast, the direct impact of decreasing conspecific densities towards the very edge of the expansion front has been neglected. Density effects may, however, have a profound direct impact on traits involved in range expansion and influence range dynamics. In this study, we contrast the effects of high conspecific larval density typical for established populations and low larval density typical for newly founded populations at the edge of the expansion front on a set of larval traits that may affect the range dynamics in the poleward moving damselfly Coenagrion scitulum. We therefore ran an outdoor mesocosm experiment with a low‐ and high‐density treatment close to the species’ northern expansion front. Density effects on survival, growth rate and body size are scored both during the pre‐winter growth period and during the subsequent winter period. Additionally, foraging activity was scored at the end of the pre‐winter period and body condition [size‐corrected body mass, fat content and activity of phenoloxidase (PO)] was scored at the end of the winter period. The low‐density treatment had strong direct positive effects on survival, growth rate and body size of larvae before winter indicating relaxed competition. Lower foraging activity at the low‐density treatment indicated higher food availability at low conspecific densities. Interestingly, the initial density treatment had stronger effect than densities experienced at the time of quantification on survival during the pre‐freezing winter period and body condition estimates at the end of the experiment, indicating also delayed effects of the initial density treatment. Survival throughout a freezing period indicated extreme winter conditions are not likely a limiting factor in the range expansion of this Mediterranean species. The increased survival and individual growth rates (through causing shifts in voltinism) at low conspecific density will translate in increased population growth rates. Furthermore, nutritional advantages at low conspecific density may increase investment in dispersal ability. Together, these direct and delayed density‐dependent effects that gradually increase towards the expansion front are expected to accelerate range expansion.