Change in dominance determines herbivore effects on plant biodiversity

Herbivores alter plant biodiversity (species richness) in many of the world’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally

Non-native species litter reduces germination and growth of resident forbs and grasses: allelopathic, osmotic or mechanical effects?

Non-native plant species may contain allelopathic substances that might help to out-compete native vegetation. These allelochemicals may be released from live or dead plant tissues and be accumulated in the soil. We tested whether non-native species leaf litter and their leachates reduced seedling establishment and growth of native species. We subjected seeds of six native species to the effect of litter leachates of three of the most important invasive plants in Europe and to mannitol solutions with similar osmotic potential in germination chamber experiments. Additionally, we measured the effect of the same litter on emergence and growth of the native species in an outdoor pot experiment. Litter leachates delayed and reduced germination and affected initial root growth of all native species. The effects of leachates were significantly higher than those of mannitol, indicating the action of toxic, most probably allelochemical substances. Emergence of seedlings in pots was also reduced, but total biomass per pot was not affected and biomass per seedling increased. Allelochemicals may affect germination and early stages of seedling recruitment. However, these negative effects seem to cease shortly after germination, when other mechanisms such as competition may be more important. Consequently, litter-borne allelochemicals are unlikely to drive the invasion of the studied non-native species, but they may contribute to maintain mono-dominant stands reinforcing invasion success.Fil: Loydi, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; Argentina. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; AlemaniaFil: Donath, T. W.. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; AlemaniaFil: Eckstein, R. L.. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; AlemaniaFil: Otte, A.. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; Alemani

Negative and positive interactions among plants: effect of competitors and litter on seedling emergence and growth of forest and grassland species

Living plant neighbours, but also their dead aboveground remains (i.e. litter), may individually exert negative or positive effects on plant recruitment. Although living plants and litter co-occur in most ecosystems, few studies have addressed their combined effects, and conclusions are ambivalent. Therefore, we examined the response in terms of seedling emergence and growth of herbaceous grassland and forest species to different litter types and amounts and the presence of competitors. We conducted a pot experiment testing the effects of litter type (grass, oak), litter amount (low, medium, high) and interspecific competition (presence or absence of four Festuca arundinacea individuals) on seedling emergence and biomass of four congeneric pairs of hemicryptophytes from two habitat types (woodland, grassland). Interactions between litter and competition were weak. Litter presence increased competitor biomass. It also had positive effects on seedling emergence at low litter amounts and negative effects at high litter amounts, while competition had no effect on seedling emergence. Seedling biomass was negatively affected by the presence of competitors, and this effect was stronger in combination with high amounts of litter. Litter affected seedling emergence while competition determined the biomass of the emerged individuals, both affecting early stages of seedling recruitment. High litter accumulation also reduced seedling biomass, but this effect seemed to be additive to competitor effects. This suggests that live and dead plant mass can affect species recruitment in natural systems, but the mechanisms by which they operate and their timing differ.Fil: Loydi, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Centro de Recursos Naturales Renovables de la Zona Semiárida(i); Argentina. Universidad Nacional del Sur. Departamento de Biología, Bioquímica y Farmacia; Argentina. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; AlemaniaFil: Donath, T. W.. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; Alemania. Christian-Albrechts University. Institute for Natural Resource Conservation. Department of Landscape Ecology; AlemaniaFil: Otte, A.. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; AlemaniaFil: Eckstein, R. L.. Justus-Liebig University Giessen. Research Centre for BioSystems, Land Use and Nutrition. Institute of Landscape Ecology and Resource Management; Alemani