The strong interactions among sea otters, sea urchins, and kelp are the basis of a trophic cascade, a community-level process that is influenced primarily by the actions of predators, and which has indirect effects at least two trophic levels removed from the actions of the predator. Ecologists explore community processes across scales of space and time to investigate whether trophic cascades such as these are common, and whether they are conditional on a particular set of environmental conditions. Ecological communities are structured by species composition and diversity, by the distribution and abundance of each species within the community, by the interactions among species, and by the presence of guilds and functional groups. In addition, community processes are strongly influenced by the actions of dominant and keystone species. Ecologists use food webs to construct a useful but incomplete picture of community structure. By quantifying certain attributes of food webs, such as linkage density and connectance, ecologists can compare food webs to identify unique features of community structure in a particular web. Food web structure and functioning are influenced by both consumers and producers. Ecologists are attempting to identify conditions under which bottom-up versus top-down processes are more likely to influence community structure