S-Cool Revision Summary

S-Cool Revision Summary

An ecosystem includes all the living organisms that interact with one another and also with the physical and non-physical factors present.

The boundaries of the ecosystem studied are dictated by the individual carrying out the study. It may be as large as the biosphere or as small as an enclosed bacterial colony.

The following are important terms which are frequently used in ecology...

Habitat: The place where an organism lives.

Population: A group of organisms, all of the same species, and all of whom live together in a particular habitat.

Community: The total of all populations living together in a particular habitat.

Niche: The position occupied by an organism in a particular ecosystem, dependent upon the resources it uses. The more resources that are taken into account then the more carefully defined the organism's niche will be, the organism will become more specialised.

Ecosystem: The biotic community together with the abiotic environment.

In order to study an ecosystem it is essential that the way in which organisms interact with each other within the ecosystem is considered. These relationships are the biotic factors of the ecosystem.

It is also essential that the effects of the physical or non-living factors are considered. These are the abiotic factors.

Biotic factors: the biotic factors affecting an ecosystem are mainly concerned with competition either within a single population or between the members of different populations.

Abiotic factors: these are the physical factors that affect an ecosystem. They include the following:



Water Availability

Oxygen Availibility

Edaphic factors

Succession is the process by which communities colonise an ecosystem and are then replaced over time by other communities.

Pioneer species: These are the first species to occupy a new habitat, starting new communities. They have rapid reproductive strategies, enabling them to quickly occupy an uninhabited area. Many have an asexual stage to their reproduction.

Seres: These are the various stages that follow on from the pioneer species.

Climax community: This is the stable community that is reached, beyond which, no further succession occurs.

Primary Succession

This occurs when the starting point is a bare ecosystem, (e.g, following a volcanic eruption or a landslide). The pioneer species are usually lichen, moss or algae. They are able to penetrate the bare surface, trap organic material and begin to form humus.

Over several generations soil begins to form. The soil can be used by a more diverse range of plants with deeper root systems. Gradually larger and larger plants occupy the ecosystem along with a diversity of animals.

Finally a climax community is reached and the species present do not change unless the environment changes in some way.

Secondary succession

This occurs when the starting point is bare, existing soil, (e.g, following a fire, flood or human intervention). This type of succession proceeds in the same way as primary succession except that the pioneer species tend to be grasses and fast growing plants.

The factors affecting population growth, and how populations increase in numbers are important concepts in ecology as they are necessary in order to successfully study how ecosystems work.

The number of individuals per unit area of chosen habitat is known as the population density.

The population density can be affected by a number of factors:

  1. Birth: The number of new individuals born to a population
  2. Immigration: The number of new individuals joining a population.
  3. Death: The number of individuals within a population that die.
  4. Emigration: The number of individuals leaving a population

Population size can also be affected by the following:

Density dependent factors: These are any factors, dependent on the density of the population in question. Some examples of these are predation, disease and competition.

Density independent factors: These are any factors, not dependent upon the density of the population in question. Some examples of these are climate and catastrophe.

Competition is often considered to be the most important biotic factor controlling population density.

Competition between organisms may be for a number of different factors, including food, light, territory or reproductive partners.

Competition Types:

Intraspecific competition

Scramble competition

Contest competition

Interspecific competition

This describes a specific level in a food chain. The term trophic refers to nutrition.

There are four important levels in most food chains:

Producers: Organisms which convert some of the energy from the sun into stored chemical energy (usually plants).

Primary consumers: Organisms that obtain energy by consuming producers. They are herbivores.

Secondary consumers: Organisms which obtain energy by consuming primary consumers. They are carnivores.

Decomposers: These organisms form the end point of every food chain. They are bacteria or fungi that obtain their energy by breaking down dead organisms from the other trophic levels.

Each description of a trophic level will describe an organisms role in the ecosystem. Organisms may occupy more than one trophic level, (e.g, when acting as omnivores).

Transfer of energy between trophic levels is relatively inefficient. Energy is transferred from one trophic level to another as organisms are consumed.

Ecological pyramids are used as a tool to illustrate the feeding relationships of the organisms, which together make up a community.

Pyramid types:

Pyramid of Numbers

Pyramid of Biomass

Pyramid of Energy

These consider how inorganic nutrients cycle through the various trophic levels and remain constantly available.

Nutrient cycle types:

The carbon cycle

The nitrogen cycle

Nitrifying bacteria will produce nitrates from these organic nitrogen compounds.

Denitrifying bacteria are able to return nitrogen to its abiotic source by converting nitrates to nitrogen gas.

Deforestation is the rapid destruction of woodland. Although it can occur due to natural catastrophe it is most commonly caused by human intervention.