Grassland Invasion by Non-native Grass Species: Ecological Issues of Multiple Species at Multiple Trophic Levels
Greer, Mitchell John
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Invasion of native grasslands by non-native grasses is of great economic and ecologic concern. Non-native grasses have potential to alter ecosystem functions, habitat quality, disturbance regimes, and feedback loops. These alterations can have bottom-up or top-down effects that may negatively influence grasslands at multiple trophic levels. I conducted three integrated studies to assess non-native grass invasion: 1) on the competitive interactions between native vs. non-native grass species with community-level implications; 2) possible utilization of allelopathic biochemicals as an invasion mechanism; and 3) possible alterations in the small mammal communities with implications for ecosystem-level function. My first study indicated that both invasive and native species varied in mycorrhizal dependency along a continuum from obligately to facultatively dependent. Native species biomass production was consistently reduced when planted into `away' soil, as compared to `home' soil. Increased biomass production of native grasses was consistently observed following additions of native prairie soil to steam-pasteurized soil from the invaded sites, indicating invasive feedbacks may occur through alterations in biotic communities. My second study indicated that Bothriochloa spp. may gain a competitive advantage through the use of allelopathic biochemicals. However, it is unclear if these allelopathic effects directly hinder competitors, or indirectly hinder competitors through alterations of soil microbial communities. Determination of allelopathic biochemicals was not definitive. Data from my third study indicate that invasion of B. ischaemum into the native grasslands lowered all abundance metrics for deer mice (Peromyscus maniculatus), while increasing all abundance metrics for hispid cotton rats (Sigmodon hispidus), as compared to native grassland controls. Species-specific models show cotton rats select vegetation that supplies aerial predator avoidance and deer mice select habitat that increases foraging efficency. Alterations in these small mammal communities may have profound effects on ecosystem functioning. Our research indicates non-native grass invasion alters native communities on multiple trophic levels. I propose management practices for restoration may be most successful if determined on a species-specific and site-specific basis, as different species appear to use different mechanisms for successful invasion into native prairies.
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