Strong feeding preference of an exotic generalist herbivore for an exotic forb: a case of invasional antagonism

Mechanisms of Plant Invasions of North American and European Grasslands

North American and European grasslands consist of relatively young communities that have evolved under human influences. These communities are uniformly sensitive to top-down controls and exhibit rapid changes in plant composition when the intensity and frequency of these controls are altered. These changes are intensifying due to the suite of global change factors, including the continued presence and introduction of new plant species. Establishment of nonnative plant species into grasslands requires resource opportunities generated by natural and human-induced disturbances and by niche differences of the new species. Persistence and spatial expansion require that the traits of the introduced species be compatible with a new regime of competitors, predators, pathogens, and symbionts. Plant traits of the invaders may then further facilitate the invasion process by preempting resources or by restructuring the soil microbial community and trophic food web in ways that directly or indirectly benefit the invading species.

Pinus halepensis invasion in mountain pampean grassland: effects of feral horses grazing on seedling establishment

We evaluated the mechanisms that determine the increasing abundance of Pinus halepensis in mountain pampean grasslands in Argentina that is associated with the continuous presence of feral horses. We hypothesized that direct and indirect effects of feral horse grazing on grassland may affect the establishment of P. halepensis. We conducted a field experiment to evaluate this hypothesis, studying the response of seedling emergence, survival and growth to herbaceous vegetation defoliation and soil disturbance in sites with contrasting grazing histories. We also evaluated the composition and structure of plant communities of each experimental site. Direct effects such as heavily defoliated perennial grasses and indirect effects such as reduced specific and functional richness, diversity and evenness enhanced the emergence, survival and early growth of the seedlings. High bare ground percentage and low grass biomass also positively affected seedling establishment. The experimental evidence demonstrates that the combination of invasional meltdown hypothesis and weakened biotic resistance are the mechanisms involved in P. halepensis invasion.