Invasibility Of Plant Communities Research Articles

Invasibility and diversity of plant communities:from patterns to processes

SummaryResearch on the function of biodiversity has stimulated a renewed interest in the mechanisms underlying the invasibility of plant communities. Conflicting results provide evidence that observational correlative studies are a dead end if isolated from manipulative experiments. Using two studies in France, Portugal and Australia we illustrate how invasion research can move from pattern observation to the investigation of processes.Artificial annual communities representing a factorial combination of one to three functional groups and one to six species per group were sown in Montpellier. Natural invasibility of the plots from the seed bank showed no strong relationship with functional or species richness. The survival of individuals of two congeneric exotic species (Conyza canadensis and C. bonariensis) introduced as seedlings did not relate to these simple diversity indices. On the other hand, their adult biomass did with a decrease in vegetative performance with increasing species richness but a decrease with increasing number of functional groups. Survival and biomass of invaders were strongly influenced by the identity of the resident functional groups.In a study of the mechanisms underlying the demographic success of the pasture weed Echium plantagineum in Australia, germinations in mixtures with varying numbers, densities, and identities of co‐germinants showed no evidence that increasing diversity increased the competitive effect. Rather, significant reduction of recruitment resulted from increased densities.These examples suggest that patterns and mechan‐ isms of community invasibility are likely to be far more subtle than people have acknowledged so far. Controlled experiments coupled with theoretical work are required to advance our understanding.

Invasibility of Experimental Habitat Islands in a California Winter Annual Grassland

In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica), at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m2 to 32 m2, had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species—rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species—rich plots were more invasible.

Effects of Soil Resources on Plant Invasion and Community Structure in Californian Serpentine Grassland

Non—native annual grasses dominate most Californian mediterranean—climate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine—derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non—native species in serpentine grassland. Factorial combinations of nitrogen, phosphorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non—native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders suppressed some native forbs. Increased macronutrient availability can increase production on serpentine—derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy—metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant communities may be directly influenced by nutrient availability, independent of physical disturbance.

Susceptibility of a tallgrass prairie to invasion by the herbicidal producing species, Festuca rubra

Disturbance, nutrients, propagule pressure, and the presence of arbuscular mycorrhizal fungi (AMF) have been shown to influence invasion in plant communities, although the relative importance of each remains unclear. A remnant of tallgrass prairie outside Cambridge, ON appears to be resilient to invasion by Festuca rubra, despite its herbicidal properties. A greenhouse experiment was conducted to determine what factors are responsible for the resilience of the native grassland, using synthesized fescue and native sod. The effects of disturbance, nutrient enrichment, propagule pressure, and the presence of AMF and phytotoxin on the establishment ability of F. rubra and the native species Sorghastrum nutans were examined. Propagule pressure was the only factor shown to influence fescue invasion and establishment of S. nutans was unaffected by all factors. These results suggest that the resilience of the native grassland may be due to decreased seed production or viability of F. rubra and not any characteristics of the grassland. Also, the restoration of native grasslands will not be limited by remnants of phytotoxin in the soil, following removal of exotic species.