Adaptive Strategies Of Invasive Alligator Weed, Alternanthera Philoxeroides, In Heterogeneous Habitats

Alligator weed (Alternanthera philoxeroides (Mart.) Griseb.), a perennial herb (Amaranthaceae) native to South America, is a worldwide invasive species that has been expanding its distribution. In China, alligator weed is one of the most serious 16 invasive alien species identified by the State Environmental Protection Administration of China (SEPAC) and has resulted in enormously ecological and economic costs. Most previous studies have focused on the biological or chemical methods to control alligator weed, while few investigate its invasion mechanisms. As an amphibian plant, alligator weed can colonize both aquatic and terrestrial habitats. The natural enemies from South America can efficiently limit the growth of alligator weed in aquatic habitats, but not in terrestrial ones. A better understanding of the adaptive strategies of alligator weed in different habitats is essential for more efficient managements.Phenotypic plasticity and genetic differentiation are two alternate strategies for organisms to cope with heterogeneous habitats. Here I reported a series of comparative studies of alligator weed (native range versus introduced range; invasive alien species versus native congener), which focused on the adaptive strategies of alligator weed in different habitats. I used several techniques, e.g. common garden experiment, molecular marker analysis, and field survey, to explore the relative importance of the two alternate strategies during the invasion of alligator weed in different habitats.The main results are as follows:1) Alligator weed displayed significantly adaptive phenotypic divergences in different habitats (aquatic versus terrestrial), which resulted from phenotypic plasticity rather than genetic differentiation. These results suggest that phenotypic plasticity may be the major strategy for alligator weed to adapt to heterogeneous habitats with different water availability in China.2) The samples from Argentina (native range) and U.S.A. (another introduced range) had much higher levels of genetic diversity than those from China. However, samples from three different countries displayed similar patterns and amounts of phenotypic plasticity across the two contrasting habitats (aquatic versus terrestrial). These results suggest that phenotypic plasticity might be more important than genetic differentiation during the invasions of alligator weed into different ranges and habitats.3) Alligator weed showed significantly greater plasticity than its native congener (A. sessilis) with relation to different water availability. Alligator weed allocated about 50% of biomass to belowground roots (potential propagules) under drought condition, while the biomass allocation to flowers in A. sessilis was not sensitive to the changing water availability. These results suggest that the differences of phenotypic plasticity between the two congeners could partially explain their different ecological distribution.4) The effects of ontogenetic drift on the dynamics of biomass allocation toroots in alligator weed changed with the specific type of resource stress. Allometric analysis revealed that the water- and nutrient-stress (belowground resource) had significant effects on the ontogenetic trajectory of biomass allocation, however, light-stress (aboveground resource) only slowed down the growth of plants and had no significant effects on the ontogenetic trajectory.In conclusion, the results obtained here suggest that alligator weed could produce significant variation in different habitats through phenotypic plasticity. Phenotypic plasticity is more important than genetic differentiation during the invasion of alligator weed into different ranges and habitats. These results deepen our understanding of the invasion mechanisms of alligator weed, which may be of use to design more efficiently specific control methods. Given the importance of plastic root biomass allocation for alligator weed to colonize both aquatic and terrestrial habitats, chemical or biological agents targeting the belowground roots may be efficient in controlling the terrestrial alligator weed. I also raised some related issues that should be addressed in future studies.