ECOSYSTEMS
(a) Ecosystems, which range in size, are dynamic and are influenced by both biotic and abiotic
factors.
● To include reference to a variety of ecosystems of different sizes (e.g. a rock pool, a
playing field, a large tree) and named examples of biotic and abiotic factors.
An ecosystem is a community of animals, plants and bacteria interrelated with the physical and
chemical environment. Ecosystems can be referred to on a large scale, such as an African
grassland, a medium scale, such as a playing field, or a small scale, such as a rock pool or a
large tree.
Biotic factors—environmental factors associated with living organisms in an ecosystem that
affect each, such as predation or disease, impact each species and individual because all
species in an ecosystem are interrelated. Producers supply chemical energy to all other
organisms, whilst the numbers of primary, secondary, and tertiary consumers will affect the
populations of each other and the producers.
Abiotic factors are the nonliving characteristics of an ecosystem that affect other living
organisms, such as the pH, relative humidity, temperature, and concentration of pollutants. At
extreme values of an abiotic factor, a species may perform better or worse, or even die.
Examples include salinity, light, temperature, water and oxygen availability, and edaphic factors.
Changes in ecosystem are dynamic and may be cyclic, direction or erratic. Organisms respond
to the dynamic changes, such as by hibernating in the winter, or changing the thickness of its
coat.
(b) Biomass transfers through ecosystems.
● To include how biomass transfers between trophic levels can be measured
● the efficiency of biomass transfers between trophic levels
● how human activities can manipulate the transfer of biomass through ecosystems.
The transfer of biomass between trophic levels is inefficient and the cumulative biomass is
reduced at each stage, as living organisms need energy for physiological processes and is lost
as heat. The gross production of organic matter by autotrophic producers is only 1-3% of the
solar energy because most is reflected, some is used to sustain the photosynthetic reactions,
and other factors such as water availability limit the rate of photosynthesis. The biomass
transferred to the next trophic level, primary consumers, is limited by respiratory losses,
biomass lost in dead plant matter and parts of the plant not eaten by primary producers. Thus,
only 5-10% of the net production is passed to the next trophic level. Biomass, and hence energy,
is lost between heterotrophic levels because not all the organism is eaten, energy is transferred
to the environment as metabolic heat, as a result of movement and respiration, and biomass is
lost in excretory products. As a result, between the trophic levels the biomass is about 15-20%
and biomass is lost at each level.
Biomass transfers are measured by comparing the dry biomass at one trophic level and
comparing it, by dividing it, by the dry biomass at the lower trophic level. For this, samples of
organisms must be heated to 80 oC in order to evaporate the water. However, the wet biomass is
usually just calculated using past data. Biomass is measured in g m -2 or g m-3 in water.
Expressing it as a percentage will give an indication as to the efficiency of biomass transfer.
The primary productivity—the rate at which plants convert light energy into biomass—can be
maximised through human activities such as: irrigation which eliminates or reduces the role of
water as limiting factor, growing plants in greenhouses which raises the temperature and
