Even a fully constructed food web, however, can provide only a superficial and static view of the structure of biological communities. Not all the relationships between species are of equal importance in the dynamics and evolution of populations and the organization of communities. Food webs include both strong and weak interactions between species, and these differences in interaction strength influence the organization of communities. Some species, called keystone species, have a disproportionately large effect on the communities in which they occur. They help to maintain local diversity within a community either by controlling populations of species that would otherwise dominate the community or by providing critical resources for a wide range of species.

The starfish Pisaster ochraceus is a keystone species in the rocky marine intertidal communities off the northwest coast of North America. This predatory starfish feeds on the mussel Mytilus californianus and is responsible for maintaining much of the local diversity of species within certain communities. When the starfish have been removed experimentally, the mussel populations have expanded rapidly and covered the rocky intertidal shores so exclusively that other species cannot establish themselves. Consequently, the interaction between Pisaster and Mytilus supports the structure and species diversity of these communities. In other communities in which Pisaster occurs, however, the starfish has little overall effect on the structure of the community. Therefore, a species can be a keystone species in some communities but not in others.

In some forest communities in tropical America, figs and a few other plants act as keystone species but in a very different manner from the starfish Pisaster. Figs bear fruit year-round in some of these forest communities, and a large number of birds and mammals rely heavily on this small group of plant species during the times of the year when other food resources are scarce. Without figs, many species would disappear from the community.

Guilds and interaction webs

Most communities contain groups of species known as guilds, which exploit the same kinds of resources in comparable ways. The name “guild” emphasizes the fact that these groups are like associations of craftsmen who employ similar techniques in plying their trade. Guilds may consist of different insect species that collect nectar in similar ways, various bird species that employ corresponding insect-foraging techniques, or diverse plant species that have evolved comparable floral shapes with which they attract the same group of pollinators.

Guilds often are composed of groups of closely related species that all arose from a common ancestor. They exploit resources in similar ways as a result of their shared ancestry. Hence, several species within a single genus may constitute a guild within a community. A less common but not unknown occurrence is for unrelated species to make up a guild.

Because members of a guild engage in similar activities, it is not surprising that they are often competitors for the resources they share, especially when those resources are scarce. This competition among guilds emphasizes the fact that, in addition to food webs, the structure of the community is built on other types of interaction. Species not only eat one another; they compete for resources, forging a variety of interspecific interactions. Many species also interact cooperatively to search for food or avoid predators. These and other nontrophic relationships between species are as important as food chains and food webs in shaping the organization of biological communities (see below Interspecific interactions and the organization of communities).

Ecological succession

The structure of communities is constantly changing. All communities are subject to periodic disturbances, ranging from events that have only localized effects, such as the loss of a tree that creates a gap in the canopy of a forest, to those that have catastrophic consequences, which include wildfires that sweep across vast landscapes or storms that pound immense stretches of shoreline. Each new disturbance within a landscape creates an opportunity for a new species to colonize that region. New species also alter the character of the community, creating an environment that is suitable to even newer species. By this process, known as ecological succession, the structure of the community evolves over time.