Dispersal and fecundity evolution during the natural range expansion of a biocontrol agent

Eliza Clark, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, Amanda Stahlke, Bioinformatics and Computational Biology, University of Idaho, Moscow, ID, Ellyn V. Bitume, US Forest Service, Hilo, HI, Dan Bean, Palisade Insectary, Colorado Department of Agriculture, Pallisade, CO, Paul Hohenlohe, Biological Sciences, University of Idaho, Moscow, ID and Ruth A. Hufbauer, Agricultural Biology, Colorado State University, Fort Collins, CO

Background/Question/Methods
Evolutionary theory predicts that individuals on the edge of an expanding range may become differentiated in fitness and dispersal from individuals at the core due to the expansion process. Expansion load and genetic drift in small populations may decrease fecundity at the edge. Alternatively, abundant resources and low competition at the edge could select for increased fecundity. Serial founder events and spatial sorting by dispersal ability during range expansion can lead to an evolved increase in dispersal ability at the edge. Dispersal can also depend on the external environment (i.e. population density) and internal state (i.e. mate status) of individuals. Here, we evaluate evolution of fecundity and dispersal ability between core and edge populations of a biological control agent, the tamarisk leaf beetle (Diorhabda carinulata), which has been expanding its range since 2010. We evaluated egg production on the first day of reproduction as a measure of early fecundity and female body mass upon adult emergence as an estimate of potential long-term fecundity, since insect body size is generally correlated with fecundity. We used flight mills in the laboratory to evaluate the emigration propensity and flight ability of mated and unmated males reared at high and low densities.
Results/Conclusions
We found that individuals from the edge of the range laid on average 1 more egg in the first 24 hours and were 7% larger than individuals from the core. This indicates that selection at low densities for increased fecundity is likely dominant over expansion load and genetic drift at the edge of this range expansion. For dispersal, we found that emigration propensity (total number of flights and probability of at least one flight during a 1-hr trial) increased in edge populations under certain treatment conditions, particularly for unmated individuals reared at low density. Flight ability (total distance flown during a 1-hr trial) increased in edge individuals relative to core individuals across the density and mating treatments, but the difference was largest in unmated individuals. These results support theoretical predictions of dispersal evolution during range expansion, but in a natural system where heterogeneous environments impose many other selection pressures. We also show that external and internal conditions present across the range are important factors in the evolution of dispersal. This work can help us better predict the rate of dispersal and population growth of other biocontrol agents and invasive species that are undergoing range expansions.