Energy flow
Most ecosystems take their energy from the sun. Photosynthesis in trees and plants converts the solar energy into organic molecules, known as chemical energy. Photosynthetic organisms, such as plants, are therefore known as producers and are the start of food chains and food webs.
A food chain is one simple way of demonstrating the flow of energy through an ecosystem. Food chains begin with a producer, such as Scots Pine bark or Oak leaves. This is the first stage, or trophic level (TL), of the chain. Animals which feed on the producers are the first (primary) consumers. These animals, such as beetles or slugs, are the next (second) TL of the food chain. Carnivorous animals at the third TL are secondary consumers. These might be spiders or blackbirds, who feed on beetles or slugs respectively. The next animals in the chain are tertiary consumers, then quaternary, quinary and so on. Trophic levels continue in the same fashion: TL3, TL4 and so on. A proportion of energy is lost at each trophic level.
“Most organisms feed on more that one other kind of organism, and more details of their feeding relationships can be shown in a food web.” (Jones & Jones, 1997). One example is the Crested Tit who feeds on beetles, but also eats spiders. A food web is therefore a collection of interrelated food chains.
Nutrient Cycling
“Ecosystems are self-sustaining. One of the reasons is because they recycle nutrients.” (Dawson, 2003) The Carbon and Nitrogen Cycles depict how carbon and nitrogen flow within ecosystems.
Carbon in the atmosphere is mostly Carbon Dioxide (CO2), which plays an important role in supporting life. As far as woodland ecosystems are concerned, carbon is taken from the atmosphere by plants, which use photosynthesis to convert carbon dioxide into carbohydrates. These carbohydrates are passed on to animals during feeding. CO2 is released back into the atmosphere through respiration performed by plants and animals. CO2 is also put back into the atmosphere when fungi and bacteria break down the carbon compounds in dead animals and plants, converting the carbon to CO2. Woodland areas store large quantities of carbon as peat and other fossil fuels, such as coal, which accumulate when “dead plant material cannot rapidly be broken down by decomposers, such as where soils are waterlogged and oxygen is in short supply” (Jones & Jones, 1997).
Earth’s atmosphere is about 78% Nitrogen (N), which is essential for many biological processes and crucial for life on Earth. Key phases of the N cycle associated with woodland ecosystems are:
N fixation is the conversion of atmospheric Nitrogen (N2) into a form readily available to plants and, therefore, animals. N Fixation happens in various ways, but in a woodland ecosystem it is mainly performed by free-living bacteria such as Rhizobium, which live in legume root nodules.
Ammonification occurs when bacteria or fungi convert the organic N within the remains or waste of a plant or animal to ammonia.
Nitrification is the conversion of ammonia to nitrates, performed by soil-living bacteria. “Other types of nitrifying bacteria, for example Nitrobacter, further oxidise nitrite ions to nitrate ions.” (Jones & Jones, 1997).
Denitrification is the reduction of nitrites back into N gas (N2), completing the N cycle. The process is performed by bacteria such as Pseudomonas.
Primary data collected at fieldwork site
Secondary data: Mean readings taken in Nagshead Nature Reserve
From “Ecosystems and Human Activity” (1994) RSPB
Conclusion
The primary and secondary data show a clear difference between the woodlands. The depth of humus and litter layers is greater in coniferous areas, while deciduous woodland receives more light below the canopy. Soil holds more moisture, and is less acidic, in deciduous areas.
Acidic conditions in coniferous woodland effect the speed of nutrient processing. There are fewer decomposers, hence deeper layers of humus and litter.
The food chains and food webs support the findings that there are more nutrients available to animals and plants. Food chains can be longer in deciduous woodland ecosystems because there is more energy available, while food webs can be larger and more varied for the same reason.
References
Class notes, 2007
Dawson, B., 2003: AS Success Biology; Letts Educational
Jones, G. & Jones, M., 1997: Advanced Biology; Cambridge University Press
RSPB, 1994: Ecosystems and Human Activity; Collins Educational
