Cross-scale variation in top-down and bottom-up control of algal abundance

Previous studies of top-down and bottom-up effects in Oregon rocky intertidal communities suggested that, unlike sea star–mussel dominated food chains, grazer-alga food chains deviated from simple food web predictions. However, this conclusion lacked generality due to limited replication at the site level. We explored how grazing and algal colonization and growth in different zones and multiple sites in intertidal communities varied in relation to processes varying on local to mesoscales, including oceanographic variation. Our approach employed the comparative-experimental method, in which identically-designed and replicated experiments are performed at multiple sites spanning the environmental gradient of interest. In a limpet–algae–barnacle interaction web, we tested the hypothesis that top-down and bottom-up effects were inversely related along the Oregon coast. To distinguish between the alternatives of large-scale oceanographic forcing vs. local control, our experiments spanned scales of meters (between mid and low zones), kilometers (seven sites), 10's to 100's of km (three regions), and ∼ 260 km (central and southern coasts). Identically designed and executed experiments on effects and rates of grazing were conducted in a southern coast region of persistent upwelling and two central coast regions, one with intermittent upwelling in an area of weak offshore currents, and another with intermittent upwelling in an area of strong offshore currents. Grazer effects and grazing rates varied on all scales, indicating a complex interplay of processes operating at zone, site, regional, and coastal scales. Grazing was generally highest in all mid zones and in the low zone at the middle region (Cape Perpetua) on the central coast. Growth of early successional colonists tended to be higher at the northern and southern regions (Capes Foulweather and Blanco). Grazers had no effect on barnacle cover, but their foraging was inhibited by high densities of barnacles. Differences in grazing appear to be a complex consequence of direct and indirect effects of variable oceanographic conditions, limpet recruitment, and barnacle abundance. In contrast to positive associations between predation strength and the magnitude of prey subsidies, higher grazing strength was generally associated with lower magnitude of bottom-up effects and vice versa. Hence in this system predator–prey (sea star–filter feeder) and herbivore–plant (limpet–microalgae) interactions evidently respond differently to ecological subsidies.