Potential host colonization by insect herbivores in a warmer climate: a transplant experiment

We conducted a transplant experiment to investigate the potential colonization of a plant species by insect herbivores under a warmer climate. 'Acacia falcata' seeds collected from four latitudes, encompassing the current coastal range of the species (1150 km), were grown in the same soil type and climatic conditions in a glasshouse. Plants were then transplanted to two sites, 280 km north of 'A. falcata's' current coastal range; the transplant sites were 1.2 and 5.5°C warmer than the northernmost and southernmost boundaries of the species' current range, respectively. We compared the structure and composition of the herbivorous Hemiptera and Coleoptera communities on the transplants (i) to that of 'A. falcata' within its current distribution, (ii) to a closely related Acacia species ('Acacia leptostachya') that naturally occurred at the transplant sites, and (iii) among the 'A. falcata' transplants originating from seeds collected at different latitudes. Herbivory on 'A. falcata' was also compared between the transplants and the current distribution, and among transplant originating from different latitudes. Thirty species of externally feeding herbivorous Coleoptera and Hemiptera were collected from the transplanted 'A. falcata' over a period of 12 months following transplantation. Guild structure of this herbivore community (based on the proportion of species within each of seven groups based on taxonomy and feeding style) did not significantly differ between the transplants and that found on 'A. falcata' within its natural range, but did differ between the transplants and 'A. leptostachya'. Rates of herbivory did not significantly differ between the transplants and plants at sites within the natural range. There were no significant differences in herbivore species richness or overall rates of herbivory on the transplants originating from different latitudes. In conclusion, host plant identity was apparently more important than climate in influencing the structure of the colonizing herbivore community. If this result holds for other plant–herbivore systems, we might expect that under a warmer climate, broad patterns in insect community structure and rates of herbivory may remain similar to that at present, even though species composition may change substantially.