Abstract:

Aims: Exotic plants experience the selective pressure of herbivores when being transplanted from their native range to a newly introduced range. The ability for exotic plants to undergo rapid adaptive evolution of their growth and defense traits is essential for successful invasion. Insects are a common threat to non-native plants and can be divided into two categories, specialist and generalist herbivores, according to herbivores specialization. Specialist herbivores and generalist herbivores will have different selection pressures on plant growth defense traits. However, under natural conditions, the specialist-to-generalist ratio may change in space and time, and the impact of this change on the evolution of growth and defense traits of exotic plants remains unclear. To investigate how specialist-to-generalist ratio affects the growth and defense traits of invasive plant, we conducted the following experiment.

Methods: We conducted a common garden experiment in Yangpu District (121°30′ E, 31°20′ N), Shanghai City in which we grew invasive plant Alternanthera philoxeroides collected from both the native (Argentina) and the introduced range (China), and maize. After nine weeks of growth, we harvested all plants. We measured the following variables: total biomass, shoot biomass, storage root biomass, growth rate, root/shoot ratio, specific leaf area, specific stem length, branch intensity, leaf trichome density, triterpenoid saponin, flavones, tannins, and lignin.

Results: There was no significant difference between specialist-to-generalist ratio and origin on the growth and defense traits of A. philoxeroides. However, a significant difference on the storage root biomass and root/shoot ratio of growth traits of A. philoxeroides populations from different ranges was measured. The storage root biomass and root/shoot ratio of the introduced population increased with the increase of the ratio of specialist herbivores. The storage root biomass and root/shoot ratio of the native population were completely opposite to those of the invasive population. In addition, by comparing the growth defense traits between native and introduced populations, it was found that the total biomass (-21.4%, P = 0.027), shoot biomass (-22.6%, P = 0.026), growth rate (-17.5%, P < 0.001) and flavonoids content (-38.4%, P = 0.010) of introduced population were significantly lower than native population, and the branch intensity (+357.9%, P < 0.001), specific stem length (+62.2%, P < 0.001), specific leaf area (+13.6%, P < 0.001) and leaf trichome density (+221.9%, P = 0.002) of the invasive population were significantly higher than native population.

Conclusions: Our results indicate that invasive plants will respond to the continuous change of the specialist- to-generalist ratio by changing their growth traits rather than their defense traits. These results enrich the current understanding of the evolution of growth defense of exotic plants.

Key words: herbivory, specialist-to-generalist ratio, growth defense, invasive plants