Benthic microalgal diversity enhanced by spatial heterogeneity of grazing

This study presents model experiments on the effect of the spatial pattern of herbivory on primary producer diversity. Microalgal biofilms (periphyton ) were exposed to different mixtures of two benthic herbivores, the isopod Idothea chelipes and the gastropod Littorina littorea. The herbivores are s imilar in their feeding selectivity but differ strongly in the spatial patt ern of grazing. Idothea did not increase the spatial heterogeneity of algal cell densities beyond the level of ungrazed controls (<1 order of magnitud e between local minima and maxima at the 1 mm(2) scale). Littorina grazing, in contrast, created a pronounced spatial heterogeneity with maximum:minim um ratios of almost 3 orders of magnitude. When algae were exposed to mixtu res of both gazers, the spatial heterogeneity of microalgal cell densities increased with an increasing proportion of Littorina in the herbivore mixtu re. Algal species richness, diversity and evenness also increased with incr easing proportions of Littorina, and was highly significantly correlated wi th the spatial heterogeneity of cell densities.