Scientists have long debated over which of two factors, resource availability (also called “bottom-up”) or consumer pressure (“top-down”), holds more sway in controlling the structure and dynamics of natural communities. This debate has been more ardent in discussions about the shallow marine systems dominated by large kelps along the California coast. Scientists generally acknowledge that biological and non-biological forces act together to influence many natural systems, including kelp forests, but differ on the relative importance of these top-down and bottom-up control mechanisms in this process (partly because it is difficult to manipulate the appropriate resources and consumers at the scales necessary to test these hypotheses).
Now, groundbreaking research at the Santa Barbara Coastal (SBC) Long Term Ecological Research (LTER) site in California is helping shed new light on this debate. Researchers at SBC examined the relative importance of nutrient availability (bottom-up), grazing pressure (top-down), and storm waves (disturbance) in controlling the standing biomass (or abundance) and productivity of giant kelp (Macrocystis pyrifera), the dominant biomass producer of kelp forests in this region. Analyzing time series data, the scientists found consistent large differences in nutrients, grazers, and waves between central and southern California. Proponents of bottom-up control would expect the cold, nutrient rich waters of central California to support more kelp biomass and production than the nutrient poor waters of southern California.
Those who favor top-down control would also expect central California to have more kelp, but would attribute this pattern to the scarcity of sea urchin grazers in this region due to the presence of sea otters, a top predator of the kelp forests.
Surprisingly, SBC researchers found that patterns of kelp biomass and production did not match either prediction. Instead, the time series data at long-term study sites in southern and central California showed that biomass and production of giant kelp were generally lower in central California, consistent with the SBC scientists’ prediction that the intense wave disturbances on the central coast could overwhelm the bottom-up and top-down forces. SBC researchers have also shown that the effects of wave disturbance on giant kelp cascade through the kelp forest food web.
These findings are important because they show that the effects of simple processes or events on a single species can have much larger impacts on the whole ecosystem. Thus, an increase in the frequency of large storms (as widely predicted by climate change models) is likely to lead not only to decreases in primary productivity of giant kelp, but also to decreases in the diversity and complexity of kelp forest food webs.