Community Regulation: Under What Conditions Are Bottom‐Up Factors Important on Rocky Shores?

Do top-down (e.g., trophic interactions) or bottomup (e.g., nutrients) effects control communities? In recent years, an increasing number of ecologists have argued that, as with most ecological controversies (Schoener 1987), this dichotomy is artificial and counterproductive. Both top-down and bottom-up factors can have important effects on community structure (e.g., Carpenter 1988, Hunter and Price 1992, Power 1992). Thus, it seems more appropriate to ask how do bottom-up and top-down effects interact and influence each other, and are the mechanisms underlying variation in each? These rather simple questions still mask enormously complex issues. However, in contrast to earlier, single-factor approaches, recent work dealing directly with the multi-factorial nature of natural communities has offered important insights. Among these are that bottom-up processes can underlie variations in community structure that can have important consequences for the influence of top-down factors (e.g., Carpenter and Kitchell 1984, 1987, Oksanen 1988, Persson et al. 1988, Power 1990). For instance, the level of primary production may determine the number of trophic levels, which in turn could have major effects on community structure (Oksanen et al. 1981, Fretwell 1987, Persson et al. 1988). It is important to define what is meant by control. As used here, control means having a major quantitative and/or qualitative effect on community structure. Quantitative effects are changes in abundance (numbers, cover, biomass), while qualitative effects are changes in community composition and/or the nature of interactions among community components. For example, increased production might increase the abundance of herbivores without changing species composition (i.e., a quantitative but not qualitative change). On the other hand, predation might reduce prey abundance, allowing both increases in abundance of competitors already present (a quantitative change)