Increased reproductive capacity and physical defense but decreased tannin content in an invasive plant

Plant invasions create novel plant-insect interactions. The EICA （evolution of increased competitive ability） hypothesis proposes that invasive plants will reallocate resources from defense to growth and/or reproduction because they have escaped from their co-evolved insect natural enemies. Testing multiple herbivory by monophagous and oligophagous herbivores and simultaneous measurement of various plant traits will provide new insights into the evolutionary change ofinvasive plants. In this context, we conducted a common garden experiment to compare plant growth and reproduction, chemical and physical defense, and plant responses to herbivory by different types of herbivores between invasive North American populations and native East Asian populations of mile-a-minute weed, Persicariaperfoliata. We found that invasive mile-a-minnte exhibited lower biomass, flowered earlier and had greater reproductive output than plants from the native range. Compared with native populations, plants from invasive populations had lower tannin content, but exhibited higher prickle density on nodes and leaves. Thus our results partially support the EICA hypothesis. When exposed to the monophagous insect, Rhinoncomimus latipes and the oligophagous insects, Gallerucida grisescens and Smaragdina nigrifrons, more damage by herbivory was found on invasive plants than on natives. R. latipes, G. grisescens and S. nigrifrons had strong, moderate and weak impacts on the growth and reproduction of mile-a-minute, respectively. The results indicate that mile-a-minute may have evolved a higher reproductive capacity in the introduced range, and this along with a lack of oligophagous and monophagous herbivores in the new range may have contributed to its invasiveness in North America.

Enemy release at range edges:do invasive species escape their herbivores as they expand into new areas?

Aims We test the hypothesis that invasive plant species at their range edges experience lower herbivory and allocate less to defense at the edge of an expanding range edge than from more central parts of their distribution,during secondary invasion in a new range.Invasive plants are often able to spread rapidly through new areas.The success of invasive species in new ranges is frequently attributed to enemy release in these new areas and associated evolutionary changes minimizing allocation to defense in favor of growth and reproduction.Enemy release could also explain rapid advances of invasive species upon arriving in new habitats.If invasive species accumulate enemies over time in a new location,then these species may experience a release from their enemies at expanding range fronts.Enemy release at these range fronts may accelerate range expansion.Methods We used populations of four woody invasive species within the invaded range,and four native control species.We quantified leaf herbivory and leaf physical defense traits at both range central and range edge locations,over two 1-month sampling periods,sampled 7 months apart.Important Findings Herbivory at the range edge did not differ to the range center but patterns were not consistent across species.There was a trend for lower herbivory at the range edge for Lantana camara,which was reflected in lower leaf toughness.Overall,leaf toughness was greater at the range edge location across invasive and control species.Physical defenses were different among range locations in a few species,though most species show the same trend,suggesting higher herbivory pressures at the range edge location or differences may be due to climatic factors.Leaves of L.camara were significantly less tough at range edges,suggesting that some species can potentially escape their enemies at range edges.However,our results overall do not support the hypothesis that plants at the edge of their ranges experience reduced impact from their enemies.