Scale as Modifier in Vegetation Diversity Experiments: Effects on Herbivores and Predators
We conducted a meta-analysis to elucidate the role of experimental scale in field trials exploring the role of habitat diversifications on herbivore and predator abundance. Literature from a period of 18 years from 6 journals yielded a total of 25 predator and 41 herbivore abundance observations in experimental treatments with higher plant diversity compared to a control. These were divided into three size bins based on reported plot size. A clear pattern emerged for herbivore abundance whereby diversification experiments performed in small plots yielded a large negative effect on herbivores, intermediate sized plots showed an intermediate effect, and the largest plots exhibited a negligible effect. Predators were more abundant in medium sized diversified plots than in small ones, but, again, no effect could be discerned in the largest plots. These results suggest that past literature reviews and meta-analyses illustrating general trends of herbivore declines associated with increased vegetation diversity may be misleading. In particular, the effects of diversification appear to vary widely as a function of the spatial scale of vegetation plots. We speculate that effects of vegetation diversification may be enhanced when insects can move uninhibited among control and treatment plots in small scale experiments. In smaller plots herbivores may aggregate in the control plots that present a more concentrated resource, and generalist predators move to the more diverse habitat. At larger scales the ability of insects to "choose" between simple and diverse plots is diminished.
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Bommarco, Riccardo and Banks, John E., "Scale as Modifier in Vegetation Diversity Experiments: Effects on Herbivores and Predators" (2003). SIAS Faculty Publications. 95.
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