Search

Exotic plants do not always escape enemies in new areas; instead, they may be damaged by co-invading non-native enemies or by enemies recruited from native species. However, even when such enemies exist in an invader's new range, escape may still be possible if particular habitats offer low-damage refuges. In this study, herbivory on the non-native common burdock ( Arctium minus (Hill) Bernh.) were compared between open and understory habitats, with high and low sunlight exposure respectively, on five sites in Ontario, Canada. Open-habitat populations showed relatively reduced herbivore damage; this may be due to herbivores occurring less abundantly in open habitats or burdock expressing greater defences in open sites. These results emphasize that enemy release is not an absolute effect, but can depend on an individual's habitat. As well, they suggest herbivory potentially may help to confine burdock primarily to open habitats. [ABSTRACT FROM AUTHOR]

Native floras of oceanic islands are among the most threatened on Earth. For example, only 1% of intact dry-forest ecosystems remain on Réunion, harbouring numerous endangered native species. Alien species invasion is one of the more important threats facing these ecosystems, as it has been hypothesized that, over time, the abundance of alien species will increase more than that of native species. To explore this, we studied floristic changes over a 16 year period (1995-2011) in six dry-forest remnants. Species richness and abundance increased for both alien and native species over this period, but at a significantly higher rate for the abundance of invasive alien plants. Despite this, Jaccard's coefficient indicates a high level of similarity between the communities of 1995 and 2011. Also, the single site that benefited from invasive species management experienced the largest increase in native species, both in terms of specific richness and abundance. This study underlines the importance of permanent plots in studying the dynamics of invaded communities and in understanding plant succession. It also suggests a potential temporal persistence of island dry-forest communities, despite the expansion of alien species, and suggests the development of a new pathway in secondary succession where native and alien species coexist. [ABSTRACT FROM AUTHOR]

We examined the type and magnitude of invasive plant ( Centaurea stoebe subsp. micranthos (Gugler) Hayek) effects on the demographic parameters of a rare plant ( Cirsium pitcheri Torr. & A.Gray), using two seed addition experiments with three treatments in two dune habitats - lakeshore linear dunes and high perched dunes on Lake Michigan, USA. Treatments were near Centaurea stoebe subsp. micranthos, near a native plant, or in the open. Seedling emergence, establishment, and juvenile survivorship were significantly higher on perched dunes, with higher surface soil moisture, than on lakeshore dunes. Treatment effects varied significantly with habitat and life stage. Seedling establishment decreased near Centaurea stoebe subsp. micranthos on lakeshore dunes, but increased near any neighbor on perched dunes. However, juvenile survival was significantly lower near Centaurea stoebe. subsp. micranthos in both habitats. The probability of reaching the adult flowering stage was much lower, and occurred later, for plants near Centaurea stoebe subsp. micranthos, especially on lakeshore dunes. Thus, Centaurea stoebe subsp. micranthos had a strong negative effect on establishment, survival, and flowering of Cirsium pitcheri, but interaction intensity varied with habitat and life stage. The results suggest that coupling seed addition with watering and invasive plant removal could bolster populations of this rare endemic plant. [ABSTRACT FROM AUTHOR]

The Enemy Release Hypothesis predicts that exotic plants gain an advantage over native competitors by losing their natural enemies while invading new regions. However, this assumes that those enemies reduced the performance of these invaders in their native ranges, and this may not be true if an invader is highly tolerant of herbivory. We used a field experiment to test the herbivore tolerance of a North American annual, common ragweed (Ambrosia artemisiifolia L.), which is known to have lost insect herbivores while invading Europe. We clipped leaves to simulate damage by folivores and removed meristems to simulate apical mortality caused by stem borers, and measured the consequences for growth and reproduction. Stem biomass was only reduced by defoliation far in excess of native-range natural damage, while seed production was unaffected by our treatments. Severely damaged plants maintained seed production by allocating relatively more aboveground biomass to reproduction. These results suggest that for the most part, damage by natural enemies may have little impact on this highly tolerant plant; consequently, enemy release may not have provided a significant advantage to this species in Europe. As well, biological control by insect folivores is unlikely to succeed unless it results in very high levels of damage. [ABSTRACT FROM AUTHOR]