Contrasting effects of herbivory on responses of a native and an alien plant to soil nutrient heterogeneity

Soil nutrient heterogeneity can influence the performance of both native and alien plants. However, such effects may be modified by native generalist herbivores and this modification may differ between native and alien plants due to differences in co-evolutionary histories with the herbivores. In a greenhouse experiment, we grew two native and two alien plants either alone or in native-alien pairs in homogeneous or heterogeneous soils, with or without the native generalist herbivore Spodoptera litura . Total biomass of the native plant Alternanthera sessilis and the alien plant Celosia argentea was lower in the heterogeneous than in the homogeneous soils, indicating an overall negative effect of soil nutrient heterogeneity. Although the heterogeneity × herbivory interaction was not significant, post-hoc comparisons revealed species-specific patterns. For A. sessilis , the lowest total biomass occurred in heterogeneous soils under herbivory, whereas the other treatment combinations did not differ significantly. In contrast, for C. argentea , total biomass differences between homogeneous and heterogeneous soils were evident only in the absence of herbivores. Belowground biomass analyses suggest that the contrasting responses were likely to result from species-specific, herbivore-induced trade-offs between the benefits and costs of root foraging precision in heterogeneous soils. These effects, however, were not observed for the native plant Achyranthes bidentata or the alien plant Amaranthus retroflexus . Competition with alien plants reduced aboveground biomass of both native plants, whereas the alien plants were unaffected by competition with natives. Competition did not alter the effects of soil heterogeneity or herbivory. These results indicate that soil nutrient heterogeneity and native generalist herbivores may jointly shape the performance of native and alien plants in a species-specific manner, regardless of competition between them.