Obvious and Cryptic Vole Suppression of a Prairie Legume in Experimental Restorations

Ecological conditions may amplify or diminish effects of herbivory. We use replicated prairie plantings established in 1997 to test for the effects of two components of ecological conditions, planting density (high or low density at a 10-fold difference) and planting season (spring or fall), on meadow vole (Microtus pennsylvannicus) herbivory of the prairie perennial Desmanthus illinoensis Michx. (Fabaceae). Over 3 yr, Desmanthus counts declined in plots planted at high density and in December compared with those planted at low density and in June. In 2000, obvious suppression of Desmanthus by voles was reduction of numbers: overall, Desmanthus was 58% less common in plots with vole access than in those without and was virtually eliminated where voles had access in December plantings. In June plantings, direct but cryptic suppression by voles reduced plant biomass ( 51%), fecundity ( 44%), and seed mass ( 50%). In June plantings, Desmanthus adults produced nearly 3000 fewer seeds, each with half the mass, where voles had access compared with where they did not. Desmanthus abundance was much reduced in December plantings, where remaining individuals were smaller and had lower reproductive output than in June plantings, with vole herbivory accentuating seasonal effects. Our results offer insights into the reasons why populations of this legume are sparse in much of central North America and why restoration plantings of this plant sometimes fail.