Species and biomass dependence of an indirect effect of vegetation on seedling recruitment

Vegetation can indirectly affect seedling recruitment by providing habitat for seed predators. We tested whether this indirect effect was generally important or was restricted to rare situations. We measured the effects of seed predators and vegetation on seedling recruitment of three herbaceous species (representing a range of seed masses) in each of five habitats (representing the local range of vegetation biomass in herb-dominated communities). Species' response to experimental removal of ground cover and/or the addition of a cage was recorded. Cages were designed to exclude both vertebrate and invertebrate predators. If vegetation controlled seedling recruitment indirectly by providing a habitat for predators, then removing vegetation would increase seedling recruitment to a greater extent in the absence of cages than when cages were present. We detected indirect effects in three of the fifteen possible species x vegetation biomass combinations. An indirect effect was found only for species with relatively large seeds and in habitats with an intermediate amount of vegetation biomass. Our results show that this indirect effect is not always important, and suggest that it is dependent on both species and vegetation biomass.