1. Increases in spatial heterogeneity may provide more food, create new resources or interfere with the foraging activities of a herbivore.2. Three colonization experiments were performed in an upland stream to investigate the effects of one type of spatial heterogeneity (grooves on the substratum) on the relationship between an abundant herbivore, Agapetus monticolus (Banks, Trichoptera), and the epilithon on which it feeds.3. The results indicate that grooves do not provide any different resources or interfere with the foraging activities of A. monticolus, but may increase food abundance, although the effect of grooves on food abundance appears to vary temporally.4. Temporal variation in the effects of microspatial complexity raises the possibility that its importance to the community may be over- or underestimated unless experiments are designed to incorporate this temporal variation.