Abstract
When plants are introduced into new regions, the absence of their co-evolved natural enemies can result in lower levels of attack. As a consequence of this reduction in enemy pressure, plant performance may increase and selection for resistance to enemies may decrease. In the present study, we compared leaf damage, plant size and leaf trichome density, as well as the direction and magnitude of selection on resistance and plant size between non-native (Spain) and native (Mexico) populations of Datura stramonium. This species was introduced to Spain about five centuries ago and constitutes an ideal system to test four predictions of the enemy release hypothesis. Compared with native populations, we expected Spanish populations of D. stramonium to have (i) lower levels of foliar damage; (ii) larger plant size; (iii) lower leaf trichome density that is unrelated to foliar damage by herbivores; and (iv) weak or no selection on resistance to herbivores but strong selection on plant size. Our results showed that, on average, plants from non-native populations were significantly less damaged by herbivores, were less pubescent and were larger than those from native populations. We also detected different selection regimes on resistance and plant size between the non-native and native ranges. Positive selection on plant size was detected in both ranges (though it was higher in the non-native area), but consistent positive selection on relative resistance was detected only in the native range. Overall, we suggest that changes in selection pressure on resistance and plant size in D. stramonium in Spain are a consequence of 'release from natural enemies'.
Highlights
The occurrence of biological invasions has increased dramatically over the past few decades (Vitousek et al 1996; Sakai et al 2001)
We investigated the case of D. stramonium in Spain, a country where it was introduced about five centuries ago and is presently considered an invasive species (Dana-Sanchez et al 2004; Sanz-Elorza et al 2004)
In the non-native range, field observations indicated a low richness of phytophagous invertebrates on individual plants of D. stramonium (n 1⁄4 6 species, Table 2)
Summary
The occurrence of biological invasions has increased dramatically over the past few decades (Vitousek et al 1996; Sakai et al 2001) This increase has been attributed to human activities (i.e. global trade and transport) extending the range of distribution of many species to novel areas (Bossdorf et al 2005; Genton et al 2005; Hierro et al 2005). Alien plants must be able to establish and successfully compete with resident species or occupy empty niches, which may depend on, among other factors, release from their natural enemies (i.e. herbivores, pathogens and parasites) in the non-native habitat (Sax and Brown 2000; Colautti et al 2004; Wolfe et al 2004; Moles et al 2008)
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