Alien invasive plants are usually considered to evolve to be more competitive than native conspecifics. The evolution of increased competitive ability (EICA) hypothesis and novel weapons hypothesis (NWH) are two important theories to explain the increase of competitive ability through evolutionary perspective. EICA proposed that some exotic species reallocate resources from defense to growth, and therefore increase competitiveness. The NWH predicts that unique biochemicals of some invasive plants may provide disproportionately allelopathic, defense, or anti-microbial advantages in their non-native ranges.  

With respect to allelopathy, novel biochemicals may have stronger effects on naïve native plants from non-native ranges of the invaders, which have not adapted to the unique chemicals. Novel biochemicals are more vulnerable to naïve native plants than adapted plants from native ranges of the invaders. These two theories are based on different aspects, while they are non-mutually exclusive mechanisms, but few studies have simultaneously tested these hypotheses.   

Dr. ZHENG Yulong and his colleagues from Xishuangbanna Tropical Botanical Garden (XTBG) of Chinese Academy of Sciences investigated the importance of integrating the EICA hypothesis and the NWH in the context of invasion by Chromolaena odorata, a native of the New World but a noxious invasive in much of the rest of the tropics and subtropics. 

The researchers collected seeds of C. odorata from 15 populations in its native ranges and 16 populations in its invasive ranges. Those seeds were germinated, and the seedlings were grown in common conditions in Mexico (native range of C. odorata) and China (invasive range). They found invasive populations of Chromolaena odorata were more competitive than native populations either with or without enemy exclusion in common conditions.   

Consistent with EICA hypothesis, invasive Chromolaena plants poorly defended against aboveground insect herbivores than native populations. Chromolaena plants from the invasive range were better defended against soil-borne enemies than plants from the native range. The superior competitive ability of invasive Chromolaena plants was not based on higher biomass, and Chromolaena plants from the invasive range were not larger than those from the native range. Chromolaena plants from the invasive range produced higher concentrations of odoratin (a compound that appears to be unique to Chromolaena) and elicited stronger allelopathic effects on species native to China than natives of Mexico. 

The study suggested that invasive plant species may evolve increased competitive ability after being introduced by increasing the production of novel allelochemicals, potentially in response to naïve competitors and new enemy regimes. It also indicated that the invasion process is very complicated, and multiple factors may together affect the evolution of alien plants.  

The study entitled “Integrating novel chemical weapons and evolutionarily increased competitive ability in success of a tropical invader” has been published online in New Phytologist.