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Scale-dependent responses of species richness to experimental manipulation of productivity and disturbance in Californian coastal grasslands

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Abstract

Question

Relationships between species richness and environmental drivers such as productivity and disturbance are sensitive to the scale over which they are measured, but the extent to which this scale-dependence is important for experimental studies conducted over small scale ranges is not well known. We ask whether the response of species richness to experimental manipulation of productivity and disturbance varies across small spatial scales (0.016–4 m2). We show that species–area relationships are well suited to summarize cross-scale responses of species richness, and ask whether the responses of species–area relationships to experimental manipulations are more consistent than richness at any single scale.

Location

Northern Californian coastal grasslands.

Methods

We applied disturbance and productivity reduction treatments over 4 yr at two sites. We assessed changes in species richness over five grain sizes, encompassing a 256-fold range of plot size. This allowed us to construct a species–area relationship for each experimental plot in each sampling year. We used the slope of the species–area relationship to summarize changes in species richness across multiple spatial scales.

Results

Richness responses were scale-dependent and complex, causing changes at any one scale to be difficult to interpret. Disturbance either increased or had no effect on richness, while reducing productivity had idiosyncratic effects among sites, scales and years. Both treatments, however, had consistent and interpretable effects on the species–area relationship. Reducing productivity increased the slope of the species–area relationship, while disturbance decreased it.

Conclusions

Our results suggest that the productivity–richness and disturbance–richness relationships are scale-dependent, and that improved generality could be achieved by focusing attention on the response of the species–area relationship to these factors.

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