Bottom-Up and Top-Down Forces in Tide Pools: Test of a Food Chain Model in an Intertidal Community
A simple food chain model of community structure was used to evaluate the roles of bottom-up and top-down factors in a rocky intertidal community. Predictions of the model were modified to incorporate known variation in the strength of species interactions and nutrient delivery rates along a wave-exposure gradient. To test the predictions of the model, I manipulated nutrients and consumers in tide pools chiseled into mudstone benches at two sites that varied in degree of wave exposure. The pools were located in the mid-intertidal zone between ;1 and 1.5 m above mean lower low water (MLLW), at Boiler Bay, Oregon, USA. The focal organisms were the benthic macroalgae and mobile invertebrate herbivores that dominate naturally occurring tide pools at this site. I manipulated nutrient levels and the abundance of herbivores in these tide pools in a fully factorial randomized block design replicated six times at a wave-exposed and a wave protected site. The experiment was maintained for two years (1994–1996). The abundances of herbivores and macroalgae were monitored in the spring, summer, and fall of each year. I measured primary productivity in the tide pools during the summer. Herbivores had a negative impact on algal abundance. The total effect of herbivory, and the efficiency of herbivores per se, on algal abundance was lower at the wave-exposed site. Nutrient additions had a positive effect on algal abundance, but this effect was reduced at the wave-exposed site. Nutrients also appeared to increase algal productivity, but only where herbivore abundance was low. Algal abundance patterns were generally consistent with model predictions for bottom-up, top-down, and hydrodynamic effects. In contrast to model predictions, herbivore abundance did not respond to the nutrient treatment. The decoupling of consumers from resource dynamics is interpreted to be the result of an herbivore preference for noncalcified seaweeds with higher potential growth rates. In wave-protected pools, where nutrients were most limiting and consumers were most efficient, seaweeds with the potential to translate elevated nutrient levels into growth had no effective refuge from consumers. The difference in scale between resource patches (tide pools) and the foraging range of the dominant herbivore, Tegula funebralis, may have augmented the ability of this herbivore to virtually exclude fleshy seaweeds from wave protected pools. Expanding the domain of applicability of food chain models requires the incorporation of consumer preferences, variation in plant growth rates, environmental gradients, and differences in the relative scales of resource patches and foraging ranges of consumers.