Exotic- and native-dominated grasslands exhibit differences in ecosystem services in the Northern Plains

Background/Question/Methods

Exotic species are homogenizing biota and lowering diversity across the globe, but we do not know how they differ from natives in affecting ecosystem services.  If native-exotic status of ecological systems does not affect functioning, ecosystem services should be similar among native and exotic systems.  Many grassland types exist, including those that are dominated by native species (e.g., never plowed remnant grasslands) and those that are dominated by exotic species (e.g., previously plowed, over-seeded, or heavily invaded grasslands).  However, we do not know how these grassland types differ with respect to plant species diversity, pollinator abundances, aboveground primary productivity, forage quality, and species turnover throughout a growing season.  We sampled plant variables and the abundance of potential pollinators (bees and flies) in three exotic-dominated grasslands and three native-dominated grasslands (unplowed prairie remnants) in the Northern Plains tallgrass prairie region, central Iowa, USA.  Exotic- and native-dominated grasslands averaged 94% and 14% exotic plant abundance, respectively, in late summer.  Samples were collected four times during the 2010 growing season (May – September) and repeated measures ANOVA was used to determine if these variables differed between the two types of grasslands.

Results/Conclusions

Native and exotic grasslands differed in their levels of species diversity and their within season temporal dynamics.  Diversity was lower in exotic than native grasslands.  Exotic grasslands were two times less diverse than native grasslands in early May and six times less diverse by mid-September, which resulted from increasing levels of diversity in native grasslands over the growing season that were missing in exotic grasslands.  Aboveground biomass was 21% higher in early May and peaked earlier in exotic grasslands than it did in native grasslands.  However, exotic grasslands had 26% lower aboveground primary productivity than native grasslands during the growing season.  Surprisingly, total number of pollinators was not affected by grassland type.  Temporal synchrony (Bray-Curtis similarity) was 40% higher in exotic than native grasslands across the growing season, indicating exotic grasslands had higher temporal niche overlap than natives.  Our results suggest that, contrary to the perception that both grassland types function similarly, exotic- and native-dominated grasslands function differently.  As exotic species replaced natives due to human activities, some important ecosystem services provided by native systems were traded off for services provided by exotic systems.  Continued replacement of native systems by exotic systems may promote this shift in ecosystem services.