How Plant Communities Assemble: Succession and Competition

On May 18, 1980, Mount St. Helens erupted and buried roughly 230 square miles of Washington forest under ash, mud, and rock. Ecologists expected a lifeless wasteland for decades. Yet within two years, a scrappy purple wildflower called the prairie lupine was blooming in the pumice. Because lupines host bacteria that pull nitrogen from the air into the soil, they fertilized the ground for the grasses and shrubs that followed. That is plant community ecology in fast-forward: a predictable, staged rebuilding of life driven by which species can survive first and who they make room for next.

By the end of this lesson you will be able to: - Explain primary vs. secondary succession and give a real example of each. - Describe how competition for light, water, and nutrients sorts species into niches. - Identify foundation and keystone species and predict what happens when they are removed. - Read a simple field survey to infer a community's successional stage.

Succession is the change in a plant community over time. Primary succession starts on bare rock or ash with no soil at all, such as a new lava flow on Hawaii or the Mount St. Helens blast zone. Pioneer species like lichens and lupines arrive first because they tolerate harsh, low-nutrient conditions and actually build soil. Secondary succession happens where a disturbance (a wildfire, a plowed field left fallow, a hurricane) clears the plants but leaves the soil and seed bank intact. It moves much faster. An abandoned Midwestern farm field goes from annual weeds, to perennial grasses, to sun-loving shrubs and pine, and finally to a shade-tolerant hardwood forest of oak and maple, often within 100 to 150 years.

Two plants cannot occupy the exact same niche indefinitely; one will eventually outcompete the other. In a forest this plays out as a race for light. Fast-growing but short-lived species like aspen shoot up quickly in full sun after a fire, but their own shade lets slower, shade-tolerant seedlings like sugar maple grow up beneath them. Decades later the maples tower over the dying aspens. Belowground the competition is for water and nutrients. This is why you rarely see two species with identical rooting depths dominate the same patch: differences in root depth, bloom time, or nutrient needs let species partition resources and coexist.

A foundation species is abundant and physically structures the community, like the kelp in a kelp forest or the oaks in an oak woodland; remove it and the habitat itself collapses. A keystone species has an impact far larger than its numbers. Beavers are a classic keystone: by damming a stream one beaver family converts a fast, shaded creek into a sunlit wetland, creating habitat for dozens of new plant species that could never establish in flowing water.