Invasive ecotypes tolerate herbivory more effectively than native ecotypes of the Chinese tallow tree Sapium sebiferum.
Abstract
The evolution of increased competitive ability (EICA) hypothesis proposes that exotic plants often become invasive because they have evolved reduced allocation to defence and increased allocation to growth and reproduction in response to low herbivore loads. Previous studies with Chinese tallow tree Sapium sebiferum support EICA predictions of invasive North American ecotypes displaying rapid growth, poorly defended leaves and considerable compensation after defoliation. In contrast, native Asian ecotypes are comparatively slow growing with highly defended leaves and are negatively impacted by mechanical leaf damage. The effects of root damage on the different ecotypes are unknown. We conducted a full-factorial pot experiment designed to assess the effects of soil fertility, competition with ryegrass Lolium multiflorum and mechanical root damage on Sapium seedlings derived from seed collections obtained in the ancestral range (native Chinese ecotype) and introduced range (invasive Texas ecotype). The results, consistent with EICA predictions, revealed that Chinese ecotypes were negatively affected by severing roots, while Texas ecotypes were able to compensate for root damage. Fertilization increased growth of Chinese ecotypes, but did not reduce the negative effects of root damage enough to allow the seedlings to compensate completely. Competition increased the stem height growth of Chinese ecotypes, but did not affect shoot or root mass. Texas ecotypes were not significantly affected by any experimental treatments. Synthesis and applications. The results of this study are consistent with previous studies indicating that invasive Sapium has undergone a shift away from possessing costly herbivore defences to producing relatively inexpensive tissues that are capable of rapidly compensating for damage. Evolutionary change is increasingly being recognized as an important factor contributing to the success of exotic plant invaders. Understanding that herbivory tolerance and the compensatory capacity for damage may differ between native and introduced plant ecotypes will be essential for implementing effective control strategies for problematic invasive species.