Granivory and Granivores in Native Grasslands and Agropyron Cristatum Stands in the Northern Great Plains

Abstract

Agropyron cristatum (crested wheatgrass) is a management concern in the northern Great Plains because of the low plant biodiversity and high stability in A. cristatum stands. Small consumers have the potential to contribute to A. cristatum invasion and stability through granivory and herbivory. Little is known about ant species composition in the Northern Great Plains. I quantified seed removal rates, granivore seed preferences, and ant communities in paired native grasslands and A. cristatum stands. I examined temporal variation in vegetation factors that influence granivory rates. Finally, I determined the relationship between seed size and vertebrate granivore preferences in herbaceous and woody communities, and I sampled ants to determine species composition and how vegetation parameters influence ant communities. Granivory rates did not differ between native grasslands and A. cristatum stands. Forb cover and season were important predictors of granivory rates in 2009, with higher granivory later in the growing season and at sites with greater cover of forbs. In 2010, there were no important predictors of granivory rates. Granivory rates in A. cristatum stands were similar to those in native grasslands, possibly because the vegetative cover and litter depth of A. cristatum stands was similar to that of native grasslands. Granivores did not prefer native seeds to exotic ones and showed little seed preference. A review of vertebrate granivore seed size preferences revealed that granivores often prefer relatively small seeds in forests and relatively large seeds in herbaceous communities. However, the absolute size of preferred seeds in each community is often similar. Ant species richness was similar between native grasslands and A. cristatum stands, while ant species composition differed between the two vegetation types. However, there were no differences in total ant abundance or the abundance of functional groups between native grasslands and A. cristatum stands. Ant abundance increased with the covers of bare ground and litter. Granivory was related to variables such as forb cover and bare ground, which varied within both vegetation types. Yet, in spite of differences in plant species dominance and plant diversity between native grasslands and A. cristatum stands, there were few differences between the vegetation types in terms of granivory rates, granivore seed preferences or total ant abundance.