19.1 - Ecosystems

Ecology vocabulary and levels of organisation

In ecology, organisms are studied at several levels of organisation, building up from the smallest to the largest.

The levels, from smallest to largest

• Individual organism - A single living thing, such as one mouse
• Population - All the organisms of one species living together in the same place at the same time, e.g. all the mice in a field
• Habitat - The place where an organism lives, such as a field, a forest, or a rocky shore
• Community - All the populations of different species living and interacting together in one habitat or ecosystem - not just the mice, but the owls, the plants, and every other species there
• Ecosystem - The interaction of a community of living organisms with the non-living parts of their environment

The variety of all the different species of organisms on Earth, or within an ecosystem, is called biodiversity.

Biotic and abiotic factors

• Biotic factors - The living parts of the environment
• Abiotic factors - The non-living parts of the environment

An ecosystem brings together the habitat, the community that lives in it, and the biotic and abiotic factors that act on it.

An organism's niche

• The niche is the particular role an organism plays within its community.
• It covers:
  • The resources it needs, such as light and nutrients
  • The conditions it can tolerate, such as temperature
  • Its interactions with other organisms, including competition and predation

Competition as a factor on population size

• Whether a population grows, shrinks or stays steady depends on several factors acting together.
• Competition is one of these factors. When a resource organisms need is in short supply, they have to compete for it.
• This competition limits population size: if there isn't enough of a resource to go round, some individuals won't survive or reproduce, so the population stops growing.

Other factors affecting population size

• Predation - How many of the organisms are eaten by predators
• Disease - Can spread through a crowded population and reduce its numbers
• Food supply - A plentiful supply allows a population to grow, while a shortage holds it back

These factors interact, so a population settles at a size that the available resources and conditions can support.

Population growth and limiting factors

• A population growing in an environment with limited resources traces an S-shaped curve over time, called the sigmoid curve.
• The sigmoid curve has four phases: lag, exponential, stationary, and death.

The four phases of the sigmoid curve

1. Lag phase - The population grows only slowly. There are still few individuals, which are becoming established and beginning to reproduce.
2. Exponential phase (also called the log phase) - Numbers rise rapidly. There are plenty of resources, few limiting factors, and the more individuals there are, the faster new ones are added.
3. Stationary phase - The population levels off and stays roughly constant.
4. Death phase - Numbers fall.

How limiting factors shape the curve

• Limiting factors are resources and conditions that become scarce as the population grows - they set the shape of the curve.
• During the lag and exponential phases, resources such as food and space are plentiful, so growth speeds up.
• As the population becomes large:
  • These resources run short and waste products build up.
  • Competition, predation, and disease increase in the crowded population.
• The birth rate and death rate come into balance, holding the population steady at the stationary phase - around the maximum size the environment can support.
• If conditions then deteriorate, or resources are used up faster than they are replaced, the death rate exceeds the birth rate and the population enters the death phase.