Ecosystems
What is an Ecosystem?
It is quite complicated getting an idea of exactly what an ecosystem is, it can be thought of as all the biological organisms within an area of study as well as the abiotic factors (sunlight, pH etc), criucially, ecologists are looking at the interactions between these and so there are many layers.
The different components of an ecosystem can be thought of as existing at different scales. A Population is a group of individuals from the same species, and populations from various species make a community. The area that these communities live in is called a habitat.
Within an ecosystem, a species occupies a niche, this is best described as the particular role it has for instance a penguin has a niche of living on land in the antarctic and feeding on fish. According to the competitive exclusion principle no two species can occupy exactly the same niche so to return to our penguin example, if another animal came to be living on land and feeding on fish it would be in competition with the penguins, over time this new species would cause penguins to go extinct or each would have to have slightly different niches, perhaps living in different parts of the antarctic or by eating different fish.
Overall, an ecosystem is a community of fairly uniform habitat (rainforest, pond) that consitsts of interactions between organisms and their physical environment (abiotic factors).
Population Stability
The size of a population in an ecosystem tends to stay fairly constant, albeit with fluctuations. However, there are certain fators that will affect the sizes of populations outlined in the table below.
| Factor | Effect |
|---|---|
| Abiotic | Things like temperature, pH and mineral nutrients are vital for growth. If these were to change, it would make a certain niche less favourable. |
| Predation | The populations of predators and prey are very closely linked. Other factors (other than prey) may reduce the population of the predator, and this in turn influences the prey. |
| Competition | As previously discussed, species have a certain niche, whilst they cannot occupy exactly the same, they very often have overlapping niches which results in competition. Where two species occupy the same niche, only one or the other can survive. For example: the red and grey squirrel in Britain. The grey (Sciurus carolinensis) was introduced in the 19th centuary and has replaced the native red (Sciurus vulgaris). In some areas (near Norwich for example) they do coexist, but together their numbers are lower than if one or the other wholly populated the area, this is inter-specific competition. |
Succession
Many millions of years ago, the earth was devoid of any visible life but over time organisms began to colonise. The sequence of events is called succession and ends with what is known as the climax community and is the most stable community possible in that particular area. So succession could be thought of as the process of getting to the climax community.
There is generally a certain pattern to this process - where small plants initially grow for example, which changes the environment and makes it suitable for other species. Take the example of a sand dune where a sequence of succession can be seen moving away from the sea, so it is on a geographic gradient.
Next to the shore are pioneer plants such as grasses, these make the dune more stable for more complex organisms. And this process continues and the dune is made more stable until the most stable community is reached: the climax.
Other example of where succession can occur are on recently formed volcanic islands or wetland formation. In each instance the climax community is again woodland.
Secondary succession happens in a habitat that has been previously colonized but has been distrubed; for example, an area of forest that has been cleared. Secondary sucession usually occurs more quickly than primary succession because it is made more favourable as a result of previous colonization by existing factors (seeds, roots, soil fertility).
Updated: 11 April 2012