476 Ecology, 86(2), 2005, pp. 476–486  2005 by the Ecological Society of America GRASSLAND INVASIBILITY AND DIVERSITY: RESPONSES TO NUTRIENTS, SEED INPUT, AND DISTURBANCE KATHERINE L. GROSS, 1,2,5 GARY G. MITTELBACH, 1,3 AND HEATHER L. REYNOLDS 1,4 1 W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 49060-9516 USA 2 Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 USA 3 Department of Zoology, Michigan State University, East Lansing, Michigan 48824 USA 4 Department of Biology, Indiana University, Bloomington, Indiana 47405 USA Abstract. The diversity and composition of a community are determined by a com- bination 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 grass- land, the community response depends on the nature of the resource change. Key words: competition; disturbance; diversity; fertilization; grasslands; invasibility; resource flux; resource manipulations; seed addition; species richness. INTRODUCTION What determines species richness and community composition at the local scale? Ecologists have long focused on species interactions within the community as a primary factor determining local community struc- ture. The outcome of these interactions depends on the magnitude and distribution of abiotic factors (e.g., re- sources, pH, temperature) and on the distribution of traits in the co-occurring species (Tilman 1988, Lons- dale 1999). However, communities do not function in isolation. Just as the dispersal of individuals may link the dynamics of populations in a metapopulation, the dispersal of species may link the dynamics of com- munities across a landscape (Tilman 1997). This ‘‘meta-community’’ view (Mouquet and Loreau 2002, 2003, Cottenie et al. 2003, Leibold et al. 2004) holds that the composition of any local community results from the interplay of within-community interactions and the dispersal of species between communities. In Manuscript received 13 January 2004; revised 31 May 2004; accepted 22 June 2004; final version received 26 July 2004. Cor- responding Editor: S. Lavorel. 5 E-mail: kgross@kbs.msu.edu addition, the response of communities and ecosystems to environmental change (e.g., nitrogen deposition, in- creased temperature) may vary depending on the spe- cies present and their traits. Norberg et al. (2001) and Norberg (2004) suggest that the ability of communities to function as complex adaptive systems will depend on the number and types of species that may colonize from the regional species pool. Thus, dispersal rates, recruitment limitation, and community invasibility may play vital roles in determining local community diver- sity and composition, as well as the response of com- munities to environmental change. Species invasions are of critical importance in their own right and ecologists are interested in the factors that make communities more or less invasible (Levine 2000, Mack et al. 2000). Increases in nutrient avail- ability have been hypothesized to increase invasion success. For example, species invasions to California serpentine grasslands are enhanced by nitrogen addi- tion (Huenneke et al. 1990). The spatial or temporal heterogeneity of nutrients could also affect invasion success through effects on niche availability. Distur- bance (i.e., biomass removal or reduction; Grime 1979,