Ecological Succession
Ecology organises life into levels above the individual organism. A population is a group of organisms of the same species living in one geographical area. A community is the sum of all the organisms of different species living in one area — populations can be thought of as the units of a community. An ecosystem is a group of communities interacting with their living and non-living environments.
In summary: Species → populations → communities → ecosystems
INTERACTIONS WITHIN A COMMUNITY
Species in a community interact in a number of ways:
- Competition - occurs when organisms compete for the same resources, such as food, water and space.
- Predation - occurs when one organism hunts another organism for food.
- Symbiosis - a close interaction between species, including mutualism (both species benefit), commensalism (one species benefits while the other is unaffected) and parasitism (one species benefits and harms the other in the process).
SUCCESSION
The community in an ecosystem is not fixed. Succession is the gradual change in the mix of species in an area over time, following a disturbance, moving through stages until it reaches a stable climax community. A stable ecosystem can be destabilised by external factors:
| Physical disturbances | Biotic disturbances |
|---|---|
| Flood, fire | Human activities — logging, farming, building development |
| Volcanic eruption | Disease |
| Glacier movement | Predation |
| Rise or fall of sea level; asteroids | Changes in migration patterns |
After a disturbance, the area is colonised by a series of species, each replaced by others over time. The organisms drive this change themselves: they provide food and shelter, change the composition of the soil, and decompose material, making nutrients available for the next arrivals.
PRIMARY SUCCESSION
Primary succession occurs on entirely new land with no established soil — river deltas, land exposed by retreating glaciers, sand dunes, and bare rock (for example, after a lava flow). The first colonisers are pioneer species, typically lichen or moss, which can survive on the harsh bare surface. As pioneers die and decompose, they create the first organic soil capable of sustaining plant growth.
As plants colonise, their litter and decomposing remains transform the ground itself:
- Soil depth increases and soil pH becomes altered.
- Mineral content increases, and rocks are broken down by the action of roots.
- The soil becomes aerated, and water retention increases.
These changes allow larger plants to grow, which reduce erosion through the binding action of their roots. Not all species thrive together — larger plants eventually outcompete smaller, shade-intolerant plants. Over time primary production (the biomass created by producers) increases, as does species diversity, creating more complex food webs — until a stable climax community forms.
Primary succession builds soil from bare rock: pioneers decompose into the first organic soil, which deepens and enriches until it can support a climax forest.
The community sequence: bare ground → pioneers → grasses → shrubs → climax community, over hundreds of years.
A climax community is the stable end point, with established plants and high biodiversity. 'Stable' does not mean unchanging: the community continues to change, and shifting abiotic and biotic factors continue to play a role.
SECONDARY SUCCESSION
Secondary succession is change in an established community after a major disturbance — a bushfire, flood or landslide. The disturbance destroys or damages the climax community's species but leaves behind soil rather than bare rock. Certain species are then more likely to succeed, for three reasons:
- Not all species are destroyed by the disturbance.
- Some species can regenerate faster than others.
- The new environmental conditions favour certain species over others — including species that previously could not compete.
Eventually the original species may return and outcompete the newcomers, re-establishing a climax community. Because soil already exists, secondary succession is much faster than primary succession.
| Feature | Primary succession | Secondary succession |
|---|---|---|
| Starting point | New land with no soil — river deltas, glaciers, sand dunes, exposed rock or lava | Soil left behind after a disturbance (fire, flood, landslide) |
| First arrivals | Pioneer species — lichen and moss — build the first organic soil | Species that previously could not compete |
| Speed | Very slow — soil must be created | Much faster — soil already exists |
| End point | Stable climax community | Original species may return and re-form a climax community |
REPRODUCTIVE STRATEGIES AND SUCCESSION
Within communities, different species use different reproductive strategies, and the stage of succession favours different strategies:
- r-selected species - high reproductive effort, many offspring, short life spans. They tend to flourish in the earlier, less stable stages of succession. Examples include bacteria and flies.
- K-selected species - low reproductive effort, few offspring, long life spans. They tend to flourish in the later, more stable stages of succession. Examples include large mammals and trees.
Organisms are not exclusively one or the other: r and K strategies are extremes on a continuum, and a species may use elements of both.
SUCCESSION AND NATURAL SELECTION
It is important to distinguish succession from natural selection. Succession acts at the community level, where the mix of species changes over time. Natural selection acts at the population level within one species, where organisms with favourable genetic information are 'selected' and reproduce more, changing the gene pool. Evolution is the outcome of changes in a population (for example, through mutations) combined with natural selection.