Effects of small consumers on Agropyron cristatum stands and native grasslands in the northern Great Plains

Background/Question/Methods

Small consumers have the potential to contribute to the success of Agropyron cristatum, an invasive grass in the Great Plains. Plant invasions often result in a change in small consumer species composition and abundance and these small consumers can limit the establishment of native plants. If Agropyron cristatum stands result in a change in granivores compared to native stands, granivory could be a mechanism by which Agropyron cristatum excludes native plants, particularly if granivores preferentially consume native seeds over those of Agropyron cristatum. We studied granivory in both Agropyron cristatum stands and native grasslands in southern Saskatchewan and northeastern Montana. We measured relative granivory rates in Agropyron cristatum and native grasslands and determined the effects of vegetation characteristics, granivore taxa and season on granivory rates. We also used exclosures to determine granivore seed preferences among seven common seeds of exotic and native plants.

Results/Conclusions

Granivory rates in Agropyron cristatum stands were similar to those in native grasslands. Percent cover of forbs best explained granivory rates with greater granivory as forb cover increased. No other vegetation characteristics were important. Season was also an important determinant of granivory rates with granivory increasing later in the growing season. Granivores did not prefer native seeds to exotic ones and showed little seed preference among the seven seeds, although preference was shown for a Petalostemon purpurea, a native legume. Seed removal was not related to seed mass for vertebrate granivores but invertebrates showed a weak preference for smaller seeds. Granivore exclusion had no effect on plant species abundance or composition in Agropypon cristatum stands or native grasslands after two growing seasons. The positive relationship between forb cover and predation rate is attributed to the increased small mammal abundance commonly shown with increasing forb cover. Granivore abundance commonly increases throughout the growing season resulting in greater seed removal rates later in the growing season. Unlike some plant invasions, Agropyron cristatum invasion results in stands that are of relatively similar structure and biomass to the invaded native grasslands and therefore, any change in species composition and abundance of small consumers may be minimal. We found no evidence that small consumers contribute to Agropyron cristatum expansion or stability.