Exotic- and native-dominated grasslands exhibit ecosystem service tradeoffs across a latitudinal gradient

Background/Question/Methods

The replacement of native-dominated ecosystems by novel, exotic-dominated systems may alter multiple ecosystem functions and services, but this idea has become recently controversial. In particular, it has been suggested that exotic species do not differ from natives. However, multiple ecosystem services and functions have not been compared directly between existing native- and exotic-dominated systems. If the native-exotic status of ecological systems does not affect functioning, multiple ecosystem services should be similar among native and exotic systems. Alternatively, there could be tradeoffs in ecosystem services if exotics provide increased levels of some services but declines in others relative to native systems. We tested whether there were ecosystem functioning and service tradeoffs among 21 native- and 21 exotic-dominated grasslands across a latitudinal gradient spanning the tallgrass prairie region, USA. We sampled plant species diversity, aboveground primary productivity, forage quality, proportion of C₃ biomass, and bee pollinator abundances during peak biomass seasons of 2010 and 2011. Relative abundances of exotics ranged from 0% to 30% in native sites and from 70% to 98% in exotic sites. We used ANCOVA to determine whether variables differed between exotic and native grasslands across the latitudinal gradient.

Results/Conclusions

Native and exotic grasslands differed in multiple ecosystem functions and services, and differences sometimes depended on latitude. Plant species diversity and richness were 66% and 58% lower, respectively, and forage digestibility (ADF) was 5% higher in exotic sites. Aboveground biomass was higher in exotic sites at low latitude, but lower in exotic sites at high latitude. The proportion of C₃ biomass increased linearly from 29% to 55% with latitude in native sites as expected. However, exotic sites exhibited two alternate states in proportion of C₃ biomass compared to native sites, very strong C₄ dominance at southern latitudes, and very strong C₃ dominance at northern latitudes. Bee pollinator numbers did not differ between grassland types. Our results suggest that as native systems were replaced by exotics due to human activities, exotics altered important latitudinal gradients, and tradeoffs in multiple ecosystem functions and services occurred. Transitions to higher exotic abundances could promote additional tradeoffs in ecosystem services and functioning.