Early effects of rodent granivory on experimental forb communities

Seed predation by rodents had major direct and indirect impacts on experime ntal plantings of 16 species of herbaceous dicots of the prairie/savanna in terface in southwestern Wisconsin. We broadcast seeds in the autumn of 1997 on 18 2 X 2 m plots surrounded by rodent-proof fencing. Experimental treat ments included: (I) continuous rodent exclusion, (2) rodent access from Dec ember to April, and (3) rodent access from May to November. Highly selectiv e seed predation by meadow voles (Microtus pennsylvanicus) during the winte r directly reduced densities of the aggressive, large-seeded composite Silp hium integrifolium by 59%. Indirectly, this suppression of Silphium resulte d in 24-132% compensatory increases in the abundance of small-seeded specie s not eaten by voles, and a 33% increase in diversity of plant species as e stimated 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 successi on.