Grassland invasibility and diversity: Responses to nutrients, seed input, and disturbance

The diversity and composition of a community are determined by a combination of local and regional processes. We conducted a field experiment to examine the impact of resource manipulations and seed addition on the invasibility and diversity of a low-productivity grassland. We manipulated resource levels both by a disturbance treatment that reduced adult plant cover in the spring of the first year and by addition of fertilizer every year. Seeds of 46 native species, both resident and nonresident to the community,,were added in spring of the first year to determine the effects of recruitment limitation from local (seed limitation) and regional (dispersal limitation) sources on local species richness. Our results show that the unmanipulated community was not readily invasible. Seed addition increased the species richness of unmanipulated plots, but this was primarily due to increased occurrence of resident species. Nonresident species were only able to invade following a cover-reduction disturbance. Cover reduction resulted in an increase in nitrogen availability in the first year, but had no measurable effect on light availability in any year. In contrast, fertilization created a persistent increase in nitrogen availability that increased plant cover or biomass and reduced light penetration to ground level. Initially, fertilization had an overall positive effect on species richness, but by the third year, the effect was either negative or neutral. Unlike cover reduction, fertilization had no observable effect on seedling recruitment or occurrence (number of plots) of invading resident or nonresident species. The results of our experiment demonstrate that, although resource fluctuations can increase the invasibility of this grassland, the community response depends on the nature of the resource change.