What happens to defenses when plants leave their herbivores and natural enemies behind, but gain new interactions?
One approach to understanding the relationship between the biotic environment, herbivore defense, and plant success is to compare populations differing in evolutionary history among environments that differ in the type or strength of biotic interactions. For example, it is hypothesized that the differences in biotic pressure between the native and introduced ranges of plants contribute to the evolution of increased competitive ability in invasive plants. That is, in the absence of herbivory, plants can reallocate energy used for defense towards growth and reproduction and, over long enough evolutionary time, populations can diverge in traits related to these key biotic interactions. We have been examining these differences in biotic interactions and plant traits in a range of invasive plants.
We have an ongoing collaboration with researchers in the US and China examining differences in biotic interactions among native (China) and invasive (US) populations of Chinese tallow tree (Triadica sebifera) and associated trait change related to invasive success. In collaboration with Evan Siemann (Rice University), Jianqing Ding, Wei Huang, & Qiang Yang (Wuhan Botanical Garden), Bo Li (Fudan University), Jianwen Zou & Ling Zhang (Nanjing Agricultural University), Yi Wang (LSU), Chris Gabler (UTRGV), and Gregory Wheeler (USDA), we have explored cross-continental differences in plant success through a series of greenhouse, field, and reciprocal transplant common garden experiments (see publication page for more info).
Chinese tallow tree is especially interesting because it produces extrafloral nectar, which functions in attracting the natural enemies of herbivores, such as predaceous ants. Extrafloral nectar can be consititutively produced by both invasive and native populations, although damage increases the production and sugar content of the nectar.