Native-exotic species richness relationships across spatial scales in a prairie restoration matrix

Background/Question/Methods

The diversity-invasibility paradigm suggests that increased biodiversity enables resistance to exotic invasion in native plant communities. Studies sampling plant communities at small spatial scales (< 1-m²) have found a negative correlation between native and exotic species richness, while in studies sampling at larger spatial scales (≥ 1000-m²), a positive correlation often emerges. This switch may occur because invasion resistance through competition drives species interactions at small spatial scales, while environmental heterogeneity and habitat amelioration drive native-exotic species richness ratios at larger spatial scales. It is unclear how ecological restoration treatments might affect such patterns – this is particularly important because disturbance associated with intense restoration treatments in plant communities is intended to alter the native-invasive balance, and yet the observed ratio of native to exotic species can vary across spatial scales. Our study compared slopes of native-exotic richness relationships to a null model across spatial scales in a South Puget Sound prairie undergoing active ecological restoration, using Modified-Whittaker plots in two types of fire treatment areas. Our null model was generated from our observed dataset, with random reassignments of the species origin label of “native” or “exotic” using Monte Carlo permutations at each spatial scale.

Results/Conclusions

We found that the slopes of the native-exotic richness relationships were positive over four spatial scales, but significantly lower than would be expected in a random community assembly, while the slope of the native-exotic cover relationship was significantly more negative than randomly expected at the smallest scale, where percent cover was measurable. Rather than a switch in the direction of the native-exotic species richness relationship across scales, we found support for fewer than expected exotic species across all scales and less exotic cover than expected at the smallest scale. Our results suggest a consistent directional correlation between exotic and native species richness across scales from 1-m² to 1000-m². Plots that were sampled in areas of more frequent and recent fire re-introduction (as part of ecological restoration) showed a stronger difference in the richness relationship but not the cover relationship, indicating that burning may increase native-exotic competitive dynamics in terms of total number of species but not in total cover. An analysis of native and exotic functional group cover relationships revealed that exotic graminoids may be the primary influence on the overall negative native-exotic cover relationships in this prairie system.