Scale, disturbance and productivity control the native-exotic richness relationship

The relationship between native and exotic species richness may be highly context-dependent. Spatial scale, including both plot size (grain) and study area (extent), is likely to influence this relationship, as are environmental conditions such as resource availability and disturbance intensity. We used experimental manipulations of soil fertility and disturbance in a California coastal grassland to directly examine the sensitivity of the native–exotic richness relationship (NERR) to these factors across five grain sizes and two spatial extents.

The slope of the NERR was a function of grain size, extent and treatment. Over small spatial extents, native and exotic richness were usually uncorrelated. Across a larger extent, NERRs were negative in control plots, neutral in disturbance plots, and positive in plots with experimentally reduced soil fertility. These patterns were strongest for small grain sizes. We verify the importance of spatial grain in determining the NERR, and emphasize the role of spatial extent.