Succession, Population Cycles and Dynamic Equilibrium

Remember from Grade 7

In Grade 7 you learned the difference between primary and secondary succession, met pioneer species like lichens, and learned about limiting factors and carrying capacity. In Grade 9 you will explain why each stage leads to the next, how disturbance keeps ecosystems sustainable, and how these processes create dynamic equilibrium, a balance that is always moving.

Let's start with a burnt forest

In summer, lightning starts a fire in the boreal forest north of Lake Superior. Weeks later, the ground is black and the trees are scorched. Is the forest destroyed?

Not quite. The next spring, pink fireweed covers the ground. Aspen shoots sprout from roots that survived underground. And the heat of the fire opened thousands of jack pine cones, which had been sealed shut with resin, scattering seeds onto bare mineral soil, exactly where jack pine seedlings grow best. Jack pine is adapted to fire: in many places it needs fire to reproduce.

How succession works

Primary succession starts with no soil (bare rock left by a melting glacier, a new lava flow, or rock scraped bare by people):

1. Pioneer species such as lichens and mosses grip bare rock. Lichens release acids that slowly break down rock.
2. Dead pioneers and rock particles build a thin layer of soil.
3. Grasses and small plants move in, adding more organic matter.
4. Shrubs, then sun-loving trees, then shade-tolerant trees follow.

Secondary succession starts where soil remains (after a fire, flood, windstorm or an abandoned farm field). It is much faster, because seeds, roots and soil organisms survive.

At every stage, the organisms change the conditions: they build soil, hold moisture and create shade. Eventually the conditions favour the next group of species more than themselves. Sun-loving aspen, for example, shade out their own seedlings, so shade-tolerant spruce and fir take over beneath them.

Disturbance and sustainability

It may seem odd, but natural disturbances help many ecosystems stay sustainable:

• Fire recycles nutrients locked in dead wood back into the soil as ash.
• It opens the canopy, letting sunlight reach the forest floor.
• It creates a patchwork of young, middle-aged and old forest. Each patch supports different species, so the whole landscape has higher biodiversity.
• Some species, such as jack pine and fireweed, depend on it.

Many First Nations used controlled burning for generations to renew berry patches and grasslands and to reduce the build-up of fuel (you will look at this in Lesson 14).

But disturbance helps only within limits. If fires come too often, too hot, or over huge areas (as is happening more with climate change), forests may not have time to regrow, and the ecosystem can shift to a different state, such as grassland or shrubland. You will look at this in Lessons 11 and 12.

Populations in dynamic equilibrium

A population cannot grow forever. Limiting factors such as food, water, space, predators, disease and weather keep it near the carrying capacity of its habitat.

• If the population rises above carrying capacity, food runs short, disease spreads more easily and predators find prey more easily. Deaths rise and births fall, and the population falls.
• If it falls below carrying capacity, resources are plentiful, so the population rises.

This is negative feedback: a change triggers a response that pushes the population back toward a steady level. The population keeps changing, but around a balance point. That is dynamic equilibrium.

The lynx and the snowshoe hare

One of the most famous examples in ecology comes from Canada. For more than 100 years, the Hudson's Bay Company recorded how many lynx and snowshoe hare furs trappers brought in. The records show both populations rising and falling in a cycle of about 10 years.

1. Hares have lots of food, so their numbers rise.
2. Lynx have more prey, so more lynx kittens survive. Lynx numbers rise, a year or two later.
3. Too many hares overeat their winter food (twigs and shrubs), and more lynx catch more hares. Hares crash.
4. With few hares, lynx starve or have fewer young. Lynx crash.
5. Plants recover, and the cycle begins again.

The two species are linked by negative feedback, and the system stays in dynamic equilibrium over decades.