Does functional form of non-native invaders influence plant species composition along a climatic gradient?

Background/Question/Methods

Invasion by non-natives is a major cause of degradation of native ecosystems. This is especially true of North American grasslands, which now occupy a mere one percent of their historical range. Often, the extent of habitat loss caused by invasive plants depends on the environmental conditions and the functional form of the invader.  We evaluated plant community composition of grasslands invaded by non-native C₃ and C₄ grasses across a climatic gradient (Central and Southern Great Plains).  Plant species composition was quantified using the point-intercept method in invaded and native sites at three military bases: Chanute Air Force Base (AFB) in Illinois, Ft. Riley Military Base (MB) in Kansas, Tinker AFB in Oklahoma.  Sites were invaded by Bothriochloa sp. (Tinker AFB), a C₄ grass, Festuca arundinacea (Chanute AFB), a C₃ grass, or both Bothriochloa sp. and Bromus inermus (Ft. Riley MB). We compared plant species diversity, richness, and native and non-native plant species abundances among sites.

Results/Conclusions

Preliminary results suggest that plant diversity and richness in all invaded sites were lower than the native sites and that sites invaded by Bothriochloa spp. showed the lowest diversity and richness overall. Sites invaded by the C₃ grasses B. inermus and F. arundinacea showed much higher diversity and richness than those invaded by the C₄ invasive Bothriochloa spp. grass. Invasion by the C₃ grass, F. arundinacea in the C₃ dominated grassland (Illinois) exhibited a higher proportion of additional non-native species than the C₄ dominated site invaded by the C₃ grass, B. inermus. We found higher non-native species richness in C₃ dominated grasslands (Illinois) following invasion by a C₃  non-native grass and reduced species diversity and richness in the C₄ dominated grasslands (Kansas, Oklahoma) invaded by a C₄ non-native grass.  These results suggest invasion of grasslands by non-natives of the same functional form, in contrast to those invaded by different functional forms, may be a larger challenge for restoration efforts to restore native grasslands.