A general eco-evolutionary framework for understanding bioinvasions.

Studies of bioinvasions have revealed various strategies of invasion, depending on the ecosystem invaded and the alien species concerned. Here, we consider how migration (as a demographic factor), as well as ecological and evolutionary changes, affect invasion success. We propose three main theoretical scenarios that depend on how these factors generate the match between an invader and its new environment. Our framework highlights the features that are common to, or differ among, observed invasion cases, and clarifies some general trends that have been previously highlighted in bioinvasions. We also suggest some new directions of research, such as the assessment of the time sequence of demographic, genetic and environmental changes, using detailed temporal surveys.

Enemy release but no evolutionary loss of defence in a plant invasion: an inter-continental reciprocal transplant experiment.

When invading new regions exotic species may escape from some of their natural enemies. Reduced top-down control ("enemy release") following this escape is often invoked to explain demographic expansion of invasive species and also may alter the selective regime for invasive species: reduced damage can allow resources previously allocated to defence to be reallocated to other functions like growth and reproduction. This reallocation may provide invaders with an "evolution of increased competitive ability" over natives that defend themselves against specialist enemies. We tested for enemy release and the evolution of increased competitive ability in the North American native ragweed (Ambrosia artemisiifolia: Asteraceae), which currently is invading France. We found evidence of enemy release in natural field populations from the invaded and native ranges. Further we carried out a reciprocal transplant experiment, comparing several life history traits of plants from two North American (Ontario and South Carolina) and one French population in four common gardens on both continents. French and Canadian plants had similar flowering phenologies, flowering earlier than plants from further south in the native range. This may suggest that invasive French plants originated from similar latitudes to the Canadian population sampled. As with natural populations, experimental plants suffered far less herbivore damage in France than in Ontario. This difference in herbivory translated into increased growth but not into increased size or vigour. Moreover, we found that native genotypes were as damaged as invading ones in all experimental sites, suggesting no evolutionary loss of defence against herbivores.

The role of pollination level on the reproduction of females and hermaphrodites in the gynodioecious plant Gypsophila repens (Caryophyllaceae).

In gynodioecious plant species, females are expected to have more resources available for maturing seeds because pistillate flowers are smaller, do not produce pollen, and are thus less costly that perfect flowers. The potential female advantage arising from more abundant resources is, however, likely to vary depending on whether seed production is limited by resource or pollen availability. Here we experimentally investigated the influence of pollen and resource limitation on female advantage in a gynodioecious species using two levels of pollination. Total seed production of females was always greater than that of hermaphrodites: females produced more flowers and more fruits that contained similar numbers of seeds of similar mass. Under low pollination, female and hermaphrodite plants allocated resources to increased flower production rather than to increased seed size or quality. We did not detect any influence of pollen or resource limitation on female advantage, which remained similar under low (= abundant resources) and full pollination. Outcrossed fruits performed better than selfed fruits when the same plant received both selfed and outcrossed pollen on different flowers. These differences were not greater under high pollination, possibly because resources available for each fruit did not differ between our pollen intensity treatments.