The native-exotic success relationship depends on vegetation indices in a California serpentine grassland

Background/Question/Methods Diverse communities may be more resistant to invasion due to a lack of niche space for invaders. Negative native-exotic richness relationships (NERRs) could support this hypothesis, but results have been inconsistent at small spatial scales, and positive NERRs are observed at large scales. Also, absolute or relative cover may be better indices of exotic success than richness. NERRs may also be the product of environmental conditions acting on natives and exotics in similar or opposite ways instead of reflecting causal relationships. We hypothesized that relationships between native and exotic success depend on both vegetation indices and spatial scale, and that exotic success would show stronger relationships to environmental variables than native richness. We surveyed vegetation and soil across topographic gradients varying in resource availability in a California serpentine grassland. Vegetation was assessed at 3 spatial scales: 0.25m², 2.5m², and 250m². We took soil samples adjacent to vegetation plots and analyzed them for nutrients, water availability, and toxic elements. We analyzed the relationships between three measures of native and exotic success (richness, absolute abundance, and relative abundance) using Spearman’s rank correlations. We assessed models incorporating different environmental data and native richness as predictors of exotic success using Akaiki’s Information Criterion.

Results/Conclusions We found mostly negative relationships between the various indices of native and exotic success. Of 17 correlations, 15 were significantly negative and only two (absolute native cover vs. absolute exotic cover at the intermediate scale, and native and exotic species richness at the large scale) - were non-significant. At both small and intermediate scales, native species richness had stronger negative correlations with absolute and relative exotic cover than with exotic richness.  While richness data may be the easiest to produce, absolute or relative cover data likely give a better indication of the extent of biological invasions. Environmental data suggest that grazing regime and resource availability were strong predictors of both native richness and exotic cover.  In this system, environmental conditions may play a stronger role than native diversity in repelling invaders.