Ecosystems and Energy Transfer — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

Species are adapted to life in a particular habitat.

The abundance and distribution of a species in a habitat are determined by a combination of biotic and abiotic factors. Biotic factors are living factors that result from the activity of other organisms: intraspecific competition (individuals of the same species competing for resources), interspecific competition (between different species), predation, disease, parasitism and food availability. Abiotic factors are non-living factors, such as temperature, light intensity, pH, soil moisture, salinity, oxygen or carbon dioxide concentration and wind. For example, low light reduces plant growth in a woodland floor, and a low oxygen concentration in a polluted river favours organisms that tolerate it.

Species occupy a specific niche in a habitat.

The abundance and distribution of species can be found by counting all the organisms in a habitat if the area is small, but usually sampling is used (see the next lesson).

Intraspecific competition occurs when members of one species compete for the same limited resources such as food, mates and space, and it tends to control population size.

Random sampling selects sample sites at random, for example by using a random number table to generate co-ordinates on a grid laid over the habitat and placing quadrats at those points. This avoids bias and is suitable for areas that are uniform. Systematic sampling places sample sites at regular intervals, and is used to investigate a change along a gradient.

A frame quadrat is a square frame (commonly 0.25 m² or 1 m²).

Transects are lines laid across a site to measure abundance and distribution across a habitat, useful for determining how species abundance and distribution change along a gradient, for example at increasing distance from a field margin or perpendicular to the water's edge on a rocky shore.

Results can be shown on a kite diagram.

Planning tipwhen asked to describe or design an investigation, include what you will vary, how you will measure the dependent variable (e.g. percentage cover), how you will ensure validity (control other variables, random placement, enough repeats) and the choice of technique and why.

Use a frame quadrat for plants or slow-moving animals in a small area, and random sampling in a uniform habitat.

Ecosystems are dynamic

• they change constantly.
• The process of ecosystem change over time is succession, during which biotic and abiotic conditions change. Primary succession starts on bare ground such as rock or sand dunes, with no soil;
  • secondary succession occurs where soil already exists, for example after a fire or abandoned farmland.

Often the new colonising species change the environment so that it becomes less suitable for the previous species.

In photosynthesis, producers convert light energy into chemical energy stored in biological molecules.

Worked example

• grass in a meadow converts light energy to carbohydrate at a rate of 17 500 kJ m^-2 yr^-1 (GPP) and releases 14 000 kJ m^-2 yr^-1 by respiration (R).
• NPP = 17 500 - 14 000 = 3 500 kJ m^-2 yr^-1.

On a sand dune system, the youngest dunes near the sea are colonised by pioneer species such as marram grass, which tolerates salt, wind and low water, and whose roots bind the sand.

In photosynthesis, plants convert light energy into chemical energy in biological molecules.

Losses between trophic levels include

• not all of an organism is eaten (bones, roots, shells);
  • not all that is eaten is digested and absorbed (lost in faeces);
  • energy is lost as heat through respiration;
  • energy is lost in excretion (urine) and movement.

Energy efficiency = (net productivity of the trophic level / energy received from the previous level) x 100.

Worked examplea toad receives 10 000 kJ m^-2 yr^-1 and has a net productivity of 1 000 kJ m^-2 yr^-1.

Efficiency of biomass transfer = (biomass transferred / biomass intake) x 100. Check that both quantities are in the same units; if both are in kg no conversion is needed.

Worked example

• a consumer ingests 35 kg of biomass and 4.1 kg is incorporated into its body.
• Efficiency = (4.1 / 35) x 100 = 11.7%, which is 12% to two significant figures.

Farmers increase the efficiency of food production by reducing energy losses: keeping animals indoors (less movement and heat loss), controlling parasites and pests, and feeding crops directly to humans rather than to livestock, since eating at a lower trophic level wastes less energy.

Because only about 10% of energy is transferred at each step, a given area of land can support far more people if they eat plants directly than if the plants are fed to animals first.