Does the driver of native richness impact invasion–diversity relationships? In high stress systems, do negative invasion–diversity relationships emerge from biotic resistance (as predicted by the diversity–resistance hypothesis) or from stress acting as a selective driver of native richness? Do invasion–diversity relationships change when abiotic and biotic legacies, rather than stress, act as non-selective drivers of richness?Location:
Calcareous fens, Wisconsin, USA.Methods:
We compared plot-level relationships between native and invasive species richness in 220 plots in 11 calcareous fens, six of which had stress alleviation and community impoverishment due to historic ploughing. We measured nutrient availability (resource stress) and saturation stress (non-resource stress), native richness and invasive richness in each plot.Results:
Residual maximum likelihood (REML) regression found a negative correlation between native and invasive richness in never ploughed plots, but no relationship was found in ploughed plots. REML multiple regression found that after accounting for saturation and nutrients, there was no relationship between native and invasive richness in ploughed or never ploughed plots. Saturation stress predicted low invasive richness and high native richness in never ploughed plots. Time since abandonment did not predict invasive richness. Non-metric multidimensional scaling (NMDS) found most invasive species present only in areas with the least saturation stress.Conclusions:
Negative relationships between native and invasive richness can result when the main driver of native richness favour native species (such as stress), and therefore may not support the diversity–resistance hypothesis. When the main driver of native richness does not select between native and invasive richness, there may be no relationship between native and invasive richness, even at very small spatial scales.