Ecological Succession: How Ecosystems Change Over Time

The parking lot that turned into a forest

Imagine a parking lot that nobody uses or cares for. In the first year, small weeds poke up through the cracks. A few years later, there are tall wildflowers and grasses, and the pavement is breaking apart. After twenty or thirty years, shrubs and young trees are growing. Leave it long enough, and a forest could stand where cars once parked.

Ecosystems are always changing. The slow, step-by-step change in which species live in a place is called ecological succession.

What is succession?

In succession, one community changes the conditions of a place, and that lets a new community move in. Each stage gets the place ready for the next.

• Early species are usually small, fast-growing and good at spreading (by wind-blown seeds or spores). They can handle full sun, wind and poor soil.
• As they live and die, they build soil, add nutrients, hold moisture and make shade.
• These new conditions suit larger, slower-growing species, which outcompete the early ones.
• Over time, the community usually gets more species, more layers (ground plants, shrubs, trees) and more stable.

There are two kinds of succession. The key difference is whether soil is there at the start.

Primary succession: starting from scratch

Primary succession happens where there is no soil at all:

• bare rock left behind when glaciers melt,
• new land made of cooled lava after a volcano erupts,
• new sand dunes along a lake shore,
• rock exposed by a landslide.

Pioneer species come first. On bare rock, these are usually lichens. A lichen is a partnership between a fungus and an alga (or certain bacteria): the alga makes food by photosynthesis, and the fungus holds water and clings to the rock. Lichens give off weak acids that slowly break down the rock surface. Bits of rock, dust and dead lichen begin to form a thin layer of soil.

Then mosses grow in the thin soil, hold more water, and add more dead matter. Next come grasses and small flowering plants, then shrubs, and finally trees. Primary succession is very slow: it can take hundreds or thousands of years.

In Ontario, glaciers covered the land until roughly 10 000 years ago. On the bare rock of the Canadian Shield, you can still see lichens and mosses growing on rock outcrops today, with trees rooted in cracks and hollows where soil has built up.

Secondary succession: starting over with soil

Secondary succession happens where a disturbance removed most of the living community but left the soil:

• after a forest fire,
• after logging,
• on a farm field that is no longer farmed,
• after a flood or a big storm,
• when a beaver dam breaks and the pond drains, leaving a wet meadow.

Because soil, seeds, roots and fungi are already there, secondary succession is much faster: often decades instead of centuries.

Example after a fire in the boreal forest: within weeks, fireweed (a tall plant with pink flowers) and grasses sprout. Jack pine cones, which often open after the heat of a fire, drop seeds onto the burned ground. Raspberries and young trembling aspen and white birch grow quickly in the sunny openings. Over many decades, shade-tolerant trees such as spruce and balsam fir can grow up underneath and eventually take over.

Example on an abandoned farm field in southern Ontario: first come annual weeds, then grasses, goldenrod and asters, then shrubs like sumac and dogwood, then sun-loving trees like white pine and aspen, and finally shade-tolerant trees like sugar maple and American beech.

Disturbances restart the clock

A mature community, like an old sugar maple forest, can stay fairly similar for a long time. But disturbances such as fire, windstorms, insect outbreaks and floods are a natural part of many ecosystems. They open up patches where succession starts again. Because of this, a natural landscape is usually a patchwork of places at different stages, which adds to its biodiversity: some species need young, sunny openings, and others need old forest.