Biodiversity is changing on both global and local scales, motivating research to understand the consequences of these changes for how communities and ecosystems function. Here, we explore the role of life history strategies in mediating biodiversity and ecosystem functioning. In particular, we evaluate how the composition, biomass (% cover), and richness of perennial (persistence ≥ 1 year) and ephemeral (persistence < 1 year) species change along a gradient of increasing seaweed species richness on a rocky shoreline. We show that the majority of biomass is comprised of perennial species, especially where overall richness is low, whereas the majority of species are ephemeral, especially where overall richness is high. We then present and discuss the results of an 18-month field manipulation quantifying the factorial effects of tidal elevation, wave exposure, herbivore removals, thermal and desiccation stress amelioration, and nutrient additions on perennial vs. ephemeral species. In particular, the diversity of ephemeral species was strongly affected, relative to perennial species, by tidal elevation, wave exposure, and herbivory; herbivores reduced diversity of ephemeral species relative to perennials. Relative to perennial cover, ephemeral cover was greater higher on the shore, in more wave-exposed habitats, and where herbivores were removed, plots were unscreened, and/or nutrients were added. Thus, perennials and ephemerals responded differently to environmental context and experimental manipulation. We compared nitrate uptake and photosynthesis rates of ephemeral and perennial species and found that maximum nitrate uptake and photosynthesis rates of ephemerals were twice as high as those of perennials. These results highlight the disproportionate roles that ephemeral species play in mediating ecosystem-level processes. In combination with our comparisons of the diversity and cover of perennial and ephemeral species along a biodiversity gradient, these results demonstrate the utility of incorporating life history traits into our efforts to understand the functional consequences of biodiversity change.