Natural systems develop a patchy structure, however, in most restoration plantings, seeds are mixed and species are spread uniformly. Aggregated seeding has been proposed to increase biodiversity in restorations, though it may increase invasibility. Aggregation limits interspecific interactions while increasing intraspecific interactions, and thus should reduce exclusion. Previous research supports this, but has not examined which competitive interactions, those driven by fitness differences, or those driven by niches differences, are most important in mediating spatial effects. We ask: (1) How does species aggregation affect community composition? (2) To what extent do effects of aggregation result from separating species from the same functional guild versus separating species from different functional guilds? (3) To what extent does aggregation of sown species influence invasibility? Prairie reconstruction plots were established on abandoned agricultural land. All plots were sown at constant density with the same 16 prairie species (from four functional guilds). Treatments varied only the spatial distribution of species among patches within plots, ranging between the extremes of completely uniform to monospecifically aggregated. Intermediate treatments varied functional similarity of species within patches: four species selected randomly (random), four from the same functional group (redundant), or four from different functional groups (complimentary).
Results/Conclusions
Analyses of plant composition data from the fourth year of community development revealed strong initial effects of species aggregation on a variety of community attributes. Aggregation significantly reduced the overall cover of sown species, but increased the cover of the non-sown species that naturally colonized the plots. This decline in sown species was primarily led by two of the four sown functional guilds: Grasses and Tall Forbs, which contain the most abundant and fastest growing species, and appear to have become abundant enough to spatially self-limit when aggregated. Though the increase in non-seeded species suggests that more aggregated plots were more invasible, there was no corresponding increase in non-native species. As predicted by spatial theory, aggregation increased overall species richness. This resulted from the increase in richness of non-sown native prairie species from the surrounding environment, rather than an increase in the richness of sown species. A comparison of intermediate treatments, which varied the degree of functional guild aggregation, suggests that initial effects of aggregation on community dynamics in this system are more strongly shaped by the spatial separation of functional guilds that differ greatly in fitness, rather than the separation of species with similar niches.