An investigation into whether varying light intensity at a stream affects the species diversity

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An investigation into whether varying light intensity at a stream affects the species diversity


Foulden common has a variety of different habitats including grassland, young woodland, ponds, swamps, fenland, set-a-side and streams which enables a diverse range of organisms to be present in an area.

Life for plants and animals in any medium is a constant struggle. Besides the physical environment, predators, parasites and other competitors that have to be contended with; but compared with a terrestrial existence, fresh water offers a relatively stable environment.i

The small stream that runs a long the side of Foulden common is a tributary of the River Wissey, which eventually flows into the Great Ouse. The stream is spring fed so has a fairly high chalk content and so the waters are slightly alkaline. This is good for fresh water organisms, as the majority of fresh water organisms requite chalk (limestone: CaCO3) because they either have an exoskeleton or shell, both of which contain large amounts of chalk.ii

The stream is quite shallow so available light can penetrate the bottom easily however light intensity levels vary considerably along the stream due to trees and bushes which are quite dense in places, spaced well apart in other places and totally absent in other regions.

Aim: The aim of my investigation is to see whether varying light intensity at a stream affects the species diversity. Light intensities will be recorded in specific zones and samples will be taken in the stream at the zones to see if a difference in light results in a change in species diversity.

Background information: Rays of light falling on the surface of the water do not penetrate very far, and sedimentary matter, even organisms themselves, will absorb light. The most obvious effect of light is to promote the growth of the plants. The plants are mostly emergent plants so add little to the fresh water habitat and if anything maybe net removers of oxygen yet the plants do contribute food and shelter. Probably the most important species is foolcress, which is both submerged and emergent and will provide increased oxygen levels and shelter.iii

Plants will in turn affect animal life with greater number of species being present where submerged plants are predominantly found. This is mainly because they provide food and shelter yet the oxygen aspect is not so important as although plants will produce oxygen it will soon get washed downstream.

Photosynthesis is the fixation of carbon dioxide followed by its reduction to carbohydrate, using hydrogen from water. The necessary energy comes from absorbed light energy.iv There are two sets of reactions involved: the light-dependent reactions in the chloroplast lamellae and the light-independent reactions in the chloroplast stroma.

During photolysis in the light dependent stage of photosynthesis, water is broken down by light:

H2O --> 1/2O2 + 2H+ + 2e-

Oxygen is released which, in this case, is then released into the steam and may be a cause of an increase in species diversity. On a sunny day in a stream containing a number of water plants, the water can become super-saturated with oxygen in the region of weeds. At night, when photosynthesis ceases, the dissolved oxygen is at a low level due to the respiratory processes of the animals and plants themselves. Light directly controls the growth of plants and since plant life forms the basis of the animal food chains, a stretch of water rich on plant life will support a large animal community. Light also enables us to see and to be seen which is important when considering food relationships and predation.

There are two types of competition: Intra-specific which is the competition between individuals of the same species and inter-specific, which is the competition of individuals of different species

In intra-specific competition population numbers are constantly changing due to the following cycle: Abundant food is available to species so there is a high feeding rate, immigration of species and successful reproduction. The numbers of the species increase rapidly. The food supply then becomes scarce causing competition for food.

Inter-specific competition will result in the competing populations increasing in size more slowly than normal. This type of competition may result in the extinction of one of the competing populations. Examples of inter-specific reproduction are predator - prey relationships and competition for resources. In this situation as the prey population crashes, it is followed by a crash in the predator population due to a reduction in food. Consequently the prey population will then increase since the predation pressure is reduced, this is closely followed by a rise in predator numbers. This will have an effect on my results as depending on the stage of the cycle that is occurring, the number of individuals in a sample will be affected. In natural populations one species rarely out competes the other such that the population of the latter declines to zero because there are sufficient parts of the niches for two species to co exist.

Ecological succession is the changes in a community structure over time, for example in the stream the amount of plants, algae and detritus present. This may result in starvation of the species, lack of reproduction and emigration, which in turn decreases the number of individuals so the feeding rate declines and the food once again becomes abundant and the cycle, continues. This has an effect in my results as at the time that I took my sample a species may have been at any strange in the cycle which would have an obvious effect on whether the species was more or less abundant than at another time that the sample was taken.
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A Niche is defined as the precise place in a habitat where an organism lives which includes its 'role' in the community. As habitats become more complex, such as the large variety of plant species in the stream and the varied substrate as shelter, the population density increases which leads to a natural emigration of prey from the defined area. With no sediment involved in the cycle it results in both population crashing as there is nowhere for the prey to hide so the predators have an advantage, consume the prey and then have a poor numbers of ...

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