Novel, exotic-dominated grasslands exhibit altered patterns of beta diversity relative to native grasslands

Background/Question/Methods

Species diversity can be partitioned into alpha, beta and gamma components, and we previously documented that alpha diversity is lower in exotic than native grasslands.  Beta diversity importantly assesses species turnover and patchiness but is less well-studied.  For example, transportation of multiple species across the Earth may reduce global beta diversity.  At finer scales, however, where conservation and restoration occur, exotic species could decrease beta diversity if the same exotic species occur everywhere, or they could increase beta diversity if different exotic species establish in different places.  We sampled plant species abundances in 42 novel, exotic- and native-dominated (remnant) grasslands across a latitudinal gradient in the tallgrass prairie region, and tested whether exotic and native grasslands differed in beta diversity at three scales:  across the entire gradient, across sites within regions, and across locations within sites.  First, we compared exponential distance-decay rates across the gradient in a permutation test using Bray-Curtis similarity.  Second, we tested for differences using homogeneity of multivariate dispersion and permutation tests within southern (TX and OK), central (KS and MO), and northern (IA, MN, and SD) regions.  Finally, we calculated multivariate dispersion within each site and compared average distance to centroid in a general linear model.   

Results/Conclusions

Exotic- and native-dominated grasslands differed dramatically in beta diversity at all scales.  Contrary to expectations, exotic-dominated grasslands had higher beta diversity than native-dominated grasslands across the latitudinal gradient (difference in slope P = 0.005).  This occurred because the identity of dominant exotic species varied across the gradient, with many exotic grassland pairs exhibiting zero similarity, whereas native-dominated grasslands differed more gradually with distance.  Regional scale differences were variable, with exotic-dominated grasslands having 29% higher beta diversity than native grasslands in the south (F_1,14 = 4.9, P = 0.04), 40% higher beta diversity in the central region (marginally significant F_1,6 = 4.7, P = 0.10) and 33% lower beta diversity in the north (F_1,16 = 9.9, P = 0.004).  We hypothesize that compared to the north, in the south either more species or weaker competitors were introduced, or less stressful conditions permitted establishment of more exotic species.  Within sites, beta diversity was 26% lower in exotic-dominated grasslands (F_1,36 = 23.5, P < 0.001).  Our results provide evidence that different regional identities and abundances of exotics can alter beta diversity in unexpected ways.  Understanding how beta diversity changes with exotic introductions at multiple scales will enhance our ability to restore beta diversity.