The impact of herbivore type and grazing pressure on benthic microalgal diversity

Benthic microalgae and the herbivores Idothea chelipes (Isopoda) and Littor ina littorea (Gastropoda) were used as an experimental model system to stud y the effect of herbivore type on primary producer diversity. Both herbivor es enhanced diversity at intermediate grazing pressures, the enhancing effe ct bring stronger with Littorina. This difference is explained bq: differen ces in the spatial heterogeneity of the grazed microalgal biofilm caused by behavioural differences in spatial grazing patterns. Idothea spreads its g razing pressure rather evenly in space ("lawn-mower" type), whereas Littori na creates pronounced spatial heterogeneity by moving slowly over the biofi lm and decimating it within the grazing tracks ("bulldozer" type). Idothea grazing did not enhance biomass patchiness beyond the level of ungrazed con trols (maximum/minimum from 1.7 to 5.5), whereas Littorina strongly increas ed patchiness (maximum/minimum from 45 to 850).