Ecosystems

Barren, rocky habitats evolve over time to form thriving ecosystems filled with plant and animal life in a process called succession. Energy is passed to different organisms in a food chain when one animal eats another. Some energy is lost at each stage though, in processes such as respiration, which is why food chains don’t go on forever.

Succession

Succession describes the change in an ecological community over time, from a relatively sparse landscape to a stable community of several different plants and animals. There are two types of succession: primary and secondary. Primary succession is when an ecological community develops in the absence of soil (i.e. from bare rock). This may happen after a volcanic eruption which results in the formation of new rock or if the sea level lowers and exposes new land. Secondary succession is when an ecological community develops from a barren landscape in which soil is present. Secondary succession may happen after a forest fire, for example. Succession occurs in the following stages:

• The first organisms to colonise an ecosystem are pioneer species, which includes things like moss, lichen and marram grass. There is no soil to begin with, therefore nothing to absorb water. This means that the pioneer species are specially adapted live in dry, hostile conditions.

• When the pioneer organisms die and decompose, they form a basic soil called humus. This makes the environment less hostile and changes the abiotic conditions. As soil forms and more water becomes available, other plant life will be able to survive here.

• As those plants die and decompose, the soil becomes deeper and thicker. Larger plants, such as shrubs can now survive and biodiversity increases. The organisms which are best adapted to the changing ecosystem will out-compete and replace those which are less adapted.

• Eventually, a stable community of plant and animal life is formed - this is called the climax community. The ecosystem is now supporting the largest and most complex community possible and the ecosystem stops changing significantly.

Energy transfer through ecosystems

Energy enters the food chain when producers absorb light energy and convert it into chemical energy during photosynthesis. The energy is used for the plant for growth and becomes biomass (biomass describes any living matter). When primary consumers eat the plants, the energy (and biomass) is transferred to the next trophic level. The energy is eventually passed onto secondary and then tertiary consumers.

Only a small proportion (around 40%) of the energy at one trophic level is passed on to the next. The rest is lost in the following ways:

1. Not all the light energy that hits the plant will be absorbed - some will hit non-photosynthetic parts of the plant (e.g. the tree trunk). Of the light energy that does hit a photosynthetic pigment, not all of it can be absorbed because it is the wrong wavelength (e.g. chlorophyll pigments are unable to absorb green parts of the visible spectrum, which is why they appear green to us).

2. Not all parts of an organism are eaten e.g. bones, beaks and roots. The energy is transferred to decomposers.

3. Consumers may not be able to completely digest the organism - e.g. humans are unable to completely break down plant cell walls and is converted into faeces. The energy is passed onto decomposers.

Of the 40% of total energy that is passed onto the next trophic level, only a quarter (10%) is converted into biomass and will be available to the next trophic level. The remaining 30% is used in respiration, movement and maintaining body temperature. This explains why food chains rarely exceed four or five trophic levels - there isn’t enough energy remaining by the time it is transferred to the tertiary consumer.

Measuring energy transfers between trophic levels

The official method of measuring energy transfers between trophic levels is to essentially roast the organisms in the oven to dehydrate them, weigh them and calculate the difference in mass between organisms at different trophic levels. That’s all fine for plants but not such a nice experiment where animals are involved. For this reason, scientists would usually estimate the dry mass of animals instead. Here’s the method anyway:

The limitations with this approach are that it assumes that each consumer only consumes one type of organism - it wouldn’t work for consumers which feed on multiple food sources.

Net Productivity

Biomass is the amount of energy that is available to the next trophic level - it can also be referred to as net productivity. You can think of this as the amount of weight an organism has been able to pack on - the fatter the organism, the more food (and energy) will be available for the organism that is going to eat it. Net productivity is calculated by taking the gross productivity (all the energy consumed by the organism) and subtracting the amount lost in respiration.

Whenever plants are involved. net productivity is referred to as Net Primary Productivity (NPP) because they are the first organisms in the food chain. It is calculated by taking the Gross Primary Productivity (GPP) and subtracting the energy used in plant respiration.

We can also calculate how efficient energy transfer is by dividing the net productivity by the total amount of energy taken in by the organism. This number is then multiplied by 100 to convert it into a percentage.

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