Early Effects of Rodent Granivory on Experimental Forb Communities

Seed predation by rodents had major direct and indirect impacts on experimental plantings of 16 species of herbaceous dicots of the prairie/savanna interface in southwestern Wisconsin. We broadcast seeds in the autumn of 1997 on 18 2 × 2 m plots surrounded by rodent-proof fencing. Experimental treatments included: (1) continuous rodent exclusion, (2) rodent access from December to April, and (3) rodent access from May to November. Highly selective seed predation by meadow voles (Microtus pennsylvanicus) during the winter directly reduced densities of the aggressive, large-seeded composite Silphium integrifolium by 59%. Indirectly, this suppression of Silphium resulted in 24–132% compensatory increases in the abundance of small-seeded species not eaten by voles, and a 33% increase in diversity of plant species as estimated by Simpson's D. Vole herbivory and granivory during the spring and summer had no influence on overall plant abundance or species richness. As these synthetic communities mature, initial winter suppression of Silphium integrifolium and the compensatory response of small-seeded composites and legumes not eaten by voles may foretell a divergence of community succession.