Shifting chemical defence or novel weapons? A review of defence traits in Agarophyton vermiculophyllum and other invasive seaweeds

Seaweed bioinvasions increasingly affect coastal environments around the world, which increases the need for predictive models and mitigation strategies. The biotic interactions between seaweed invaders and invaded communities are often considered a key determinant of invasion success and failure and we here revise the current evidence that the capacity of seaweed invaders to deter enemies in newly reached environments correlates with their invasion success. Particularly efficient chemical defences have been described for several of the more problematic seaweed invaders during the last decades. However, confirmed cases in which seaweed invaders confronted un-adapted enemies in newly gained environments with deterrents that were absent from these environments prior to the invasion (so-called “novel weapons”) are scarce, although an increasing number of invasive and non-invasive seaweeds are screened for defence compounds. More evidence exists that seaweeds may adapt defence intensities to changing pressure by biological enemies in newly invaded habitats. However, most of this evidence of shifting defence was gathered with only one particular model seaweed, the Asia-endemic red alga Agarophyton vermiculophyllum, which is particularly accessible for direct comparisons of native and non-native populations in common garden experiments. A. vermiculophyllum interacts with consumers, epibionts and bacterial pathogens and in most of these interactions, non-native populations have rather gained than lost defensive capacity relative to native conspecifics. The increases in the few examined cases were due to an increased production of broad-spectrum deterrents and the relative scarcity of specialized deterrents perhaps reflects the circumstance that seaweed consumers and epibionts are overwhelmingly generalists.

Ecological explanations for successful invasion of exotic plants

The intentional introduction of exotic species can increase the level of local biodiversity,enrich people’s material lives,and bring significant social and economic benefits that are also the symbols of human progress.However,along with the frequent intercourse among countries and regions,the frequency of uncontrolled crossregional migration of species is increased and there is a lack of scientific management strategy for the intentional introduction of exotic species.Exotic species invasion,which is behind habitat fragmentation,has become the second largest threatening factor to the maintenance of the global-scale level of biological diversity.Exotic species invasion can destroy the structure of an ecosystem,disturb the economic life of a society,and do harm to human health.In this paper,the authors review some of the ecological explanations for issues such as“what causes or mechanisms have led to the successful invasion of exotic species”,including the“ideal weeds characteristics”,“biodiversity resistance hypothesis”,“enemies release hypothesis”,“evolution of increased competitive ability hypothesis”,“niche opportunity hypothesis”,and“novel weapon hypothesis”.The authors also analyze and evaluate the background and theoretical basis of the hypotheses,providing explanations for some phenomena,as well as the deficiencies of these explanations.