The progress of succession greatly depends on the activity of successive colonists, each of which modifies the environment to either facilitate or inhibit the colonization by other species. Facilitation is seen, for example, in the fixation of nitrogen by Dryas and alders that allows spruce trees to become established in Glacier Bay (see Figure 56.15B). At later stages of succession, though, competition by spruce trees for nitrogen, light, and water inhibits other tree species from establishing themselves. Likewise, dwarf lupines helped facilitate the establishment of later successional plants but, in turn, were inhibited by multiple insect herbivores, which were able to control the pace of primary succession.

Why do we see this interplay between facilitation and inhibition as succession progresses? In most post-disturbance successional sequences, the early pioneer species are exposed to physically challenging conditions that can be tolerated only with the help of other species. For example, in secondary succession, many of the first organisms to arrive on bare soil after a disturbance are detritivores, which process dead organic matter and release nutrients, thus facilitating the establishment of early colonizing plants. Early successional species that can disperse to these habitats and tolerate and modify the abiotic environment will likely facilitate other species without those capabilities. As succession progresses, species that are less stress-tolerant but are larger, longer-lived, and competitively superior will likely come to dominate as the conditions improve, inhibiting earlier pioneer species. While there is typically a transition from early facilitation to later inhibition in most successional sequences, as species diversity increases, a variety of both positive and negative interactions naturally arise, further shaping the pattern of succession.