Specialist Insect Herbivore and Light Availability Do Not Interact in the Evolution of an Invasive Plant

The significance of biomass allocation to population growth of the invasive species Ambrosia artemisiifolia and Ambrosia trifida with different densities

Background

Ambrosia artemisiifolia and Ambrosia trifida are globally distributed harmful and invasive weeds. High density clusters play an important role in their invasion. For these two species, the early settled populations are distributed at low densities, but they can rapidly achieve high population densities in a short period of time. However, their response to intraspecific competition to improve the fitness for rapid growth and maintenance of high population densities remains unclear. Therefore, to determine how these species form and maintain high population densities, individual biomass allocations patterns between different population densities (low and high), and plasticity during seedling, vegetative, breeding and mature stages were compared. In 2019, we harvested seeds at different population densities and compared them, and in 2020, we compared the number of regenerated plants across the two population densities.

Results

Most biomass was invested in the stems of both species. Ambrosia trifida had the highest stem biomass distribution, of up to 78%, and the phenotypic plasticity of the stem was the highest. Path analysis demonstrated that at low-density, total biomass was the biggest contributor to seed production, but stem and leaf biomass was the biggest contributors to high-density populations. The number of seeds produced per plant was high in low-density populations, while the seed number per unit area was huge in high-density populations. In the second year, the number of low-density populations increased significantly. A. artemisiifolia and A. trifida accounted for 75.6% and 68.4% of the mature populations, respectively.

Conclusions

High input to the stem is an important means to regulate the growth of the two species to cope with different densities. These two species can ensure reproductive success and produce appropriate seed numbers. Therefore, they can maintain a stable population over time and quickly form cluster advantages. In the management, early detection of both species and prevention of successful reproduction by chemical and mechanical means are necessary to stop cluster formation and spread.

Contrasting effects of specialist and generalist herbivores on resistance evolution in invasive plants

Invasive alien plants are likely to be released from specialist herbivores and at the same time encounter biotic resistance from resident generalist herbivores in their new ranges. The Shifting Defense hypothesis predicts that this will result in evolution of decreased defense against specialist herbivores and increased defense against generalist herbivores. To test this, we performed a comprehensive meta-analysis of 61 common garden studies that provide data on resistance and/or tolerance for both introduced and native populations of 32 invasive plant species. We demonstrate that introduced populations, relative to native populations, decreased their resistance against specialists, and increased their resistance against generalists. These differences were significant when resistance was measured in terms of damage caused by the herbivore, but not in terms of performance of the herbivore. Furthermore, we found the first evidence that the magnitude of resistance differences between introduced and native populations depended significantly on herbivore origin (i.e., whether the test herbivore was collected from the native or non-native range of the invasive plant). Finally, tolerance to generalists was found to be higher in introduced populations, while neither tolerance to specialists nor that to simulated herbivory differed between introduced and native plant populations. We conclude that enemy release from specialist herbivores and biotic resistance from generalist herbivores have contrasting effects on resistance evolution in invasive plants. Our results thus provide strong support for the Shifting Defense hypothesis.