Range expansion theory predicts that individuals at the leading edge of an expansion will evolve increased dispersal capability due to spatial sorting, higher fecundity due to selection at low densities, and will adapt to local environments. While there are strong mathematical models of range expansions and some empirical work using model systems, there are few tests of the theory in natural systems. Controlled releases of biological control agents provide natural experiments in which to study the predictions of range expansion theory. We used the rapid range expansion of Diorhabda carinulata, the tamarisk leaf beetle and biological control agent for tamarisk, to test these three predictions of range expansion theory. We collected populations of D. carinulata from four northern range core sites and four southern range edge sites. In the lab, we measured male dispersal, female fecundity, and timing of diapause induction under different photoperiods, which is linked to adaptation to different latitudes. For dispersal, we also evaluated the effects of mating status (mated or not) and rearing density on dispersal traits, as those are likely to influence propensity to fly.
Results/Conclusions
Contrary to the predictions of theory, we did not find significant differences between range core and edge in total flight distance, average flight speed, and number of flights in one hour. However, larger beetles were more dispersive than smaller beetles and there was a significant interaction between rearing density and mating status such that unmated males at low density and mated males at high density were more dispersive than beetles in the other two treatment groups. In support of theory, we found that edge females were larger and more fecund than core females. Additionally, we found that the timing of diapause matched daylengths of the collection latitudes, showing adaptation. Our results support theoretical predictions of higher fecundity at the edge and adaptation of induction of diapause to different latitudes. Our lack of support for the dispersal predictions could be due to aggregation and density-dependence in this species, which could hinder spatial sorting of dispersal ability. Alternatively, the rate of expansion could be limited by the rate of adaptation to shorter photoperiods at lower latitudes, which could also decrease the effect of spatial sorting. These results provide an important step to understanding the evolutionary dynamics of range-expanding populations in natural systems.