Abundance of introduced species at home predicts abundance away in herbaceous communities.

Many ecosystems worldwide are dominated by introduced plant species, leading to loss of biodiversity and ecosystem function. A common but rarely tested assumption is that these plants are more abundant in introduced vs. native communities, because ecological or evolutionary-based shifts in populations underlie invasion success. Here, data for 26 herbaceous species at 39 sites, within eight countries, revealed that species abundances were similar at native (home) and introduced (away) sites - grass species were generally abundant home and away, while forbs were low in abundance, but more abundant at home. Sites with six or more of these species had similar community abundance hierarchies, suggesting that suites of introduced species are assembling similarly on different continents. Overall, we found that substantial changes to populations are not necessarily a pre-condition for invasion success and that increases in species abundance are unusual. Instead, abundance at home predicts abundance away, a potentially useful additional criterion for biosecurity programmes.

Effects of positive interactions, size symmetry of competition and abiotic stress on self-thinning in simulated plant populations.

BACKGROUND AND AIMS: Competition drives self-thinning (density-dependent mortality) in crowded plant populations. Facilitative interactions have been shown to affect many processes in plant populations and communities, but their effects on self-thinning trajectories have not been investigated. METHODS: Using an individual-based 'zone-of-influence' model, we studied the potential effects of the size symmetry of competition, abiotic stress and facilitation on self-thinning trajectories in plant monocultures. In the model, abiotic stress reduced the growth of all individuals and facilitation ameliorated the effects of stress on interacting individuals. KEY RESULTS: Abiotic stress made the log biomass-log density relationship during self-thinning steeper, but this effect was reduced by positive interactions among individuals. Size-asymmetric competition also influenced the self-thinning slope. CONCLUSIONS: Although competition drives self-thinning, its course can be affected by abiotic stress, facilitation and competitive symmetry.

Beyond dual-lattice models: incorporating plant strategies when modeling the interplay between facilitation and competition along environmental severity gradients.

We introduce a spatially explicit model that evaluates how the trade-offs between the life strategies of two interacting plant species affect the outcome of their interaction along environmental severity gradients. In our model, we represent the landscape as a two-dimensional lattice, with environmental severity increasing from left to right. Two species with different strategies, a competitor and a stress-tolerant, interact in the lattice. We find that facilitation expands the realized niche of the competitor into harsh environments by suppressing the stress-tolerant species. Most of their coexisting range is dominated by a positive effect of one species on another, with a reciprocal negative effect from the species receiving the benefits on its benefactor ("+, -"), whereas mutualistic ("+, +") interactions are only found in the harshest part of the environmental gradient. Contrarily as assumed by models commonly used in facilitation research (e.g. dual-lattice models), our results indicate that "+, +" interactions are not dominant, and that their differences with "+, -" interactions along environmental severity gradients depend on the strategies of the interacting species. By integrating the trade-off between competitive ability and stress tolerance, our model provides a new framework to investigate the interplay of facilitative and competitive interactions along environmental gradients and their impacts on processes such as population dynamics and community organization.

Balance between facilitation and resource competition determines biomass-density relationships in plant populations.

Theories based on competition for resources predict a monotonic negative relationship between population density and individual biomass in plant populations. They do not consider the role of facilitative interactions, which are known to be important in high stress environments. Using an individual-based 'zone-of-influence' model, we investigated the hypothesis that the balance between facilitative and competitive interactions determines biomass-density relationships. We tested model predictions with a field experiment on the clonal grass Elymus nutans in an alpine meadow. In the model, the relationship between mean individual biomass and density shifted from monotonic to humped as abiotic stress increased. The model results were supported by the field experiment, in which the greatest individual and population biomass were found at intermediate densities in a high-stress alpine habitat. Our results show that facilitation can affect biomass-density relationships.

Productivity is a poor predictor of plant species richness.

For more than 30 years, the relationship between net primary productivity and species richness has generated intense debate in ecology about the processes regulating local diversity. The original view, which is still widely accepted, holds that the relationship is hump-shaped, with richness first rising and then declining with increasing productivity. Although recent meta-analyses questioned the generality of hump-shaped patterns, these syntheses have been criticized for failing to account for methodological differences among studies. We addressed such concerns by conducting standardized sampling in 48 herbaceous-dominated plant communities on five continents. We found no clear relationship between productivity and fine-scale (meters(-2)) richness within sites, within regions, or across the globe. Ecologists should focus on fresh, mechanistic approaches to understanding the multivariate links between productivity and richness.

On the balance between niche and neutral processes as drivers of community structure along a successional gradient: insights from alpine and sub-alpine meadow communities.

BACKGROUND AND AIMS: Neutral theory predicts that the diversity and relative abundance of species in ecological communities do not depend on their specific traits. This prediction remains controversial, as many studies suggest that variations in the niches of species determine the structure of communities. The aim of this study was to test empirically the relative importance of niche and neutral processes as drivers of species abundance within plant communities along a successional gradient. METHODS: Information on the abundance (density and frequency) and traits (aboveground individual biomass and seed mass) of > 90 species was collected in alpine and sub-alpine meadows of the Tibet Plateau (China). A successional gradient (1, 3, 15 and 30 years after abandonment) was established in a sub-alpine meadow. The relationships between species traits and their abundance were evaluated using regression models. KEY RESULTS: Seed mass was negatively related to both species density (r = -0.6270, P < 0.001) and frequency (r = -0.5335, P = 0.005) in the 1-year meadow. Such relationships disappeared along the successional gradient evaluated (P > 0.07 in the 3-, 15- and 30-year meadows). Data gathered in all sites showed a significant negative relationship between the average individual biomass of a given species and its density within the community (r < -0.30, P < 0.025 in all cases). CONCLUSIONS: The results show that seed mass was a key driver of species abundance in early successional communities, and that niche forces may become more important as succession progresses. They also indicate that predictions from neutral theory, in its current form, do not hold for the meadow communities studied.