(a) Explain briefly the following terms: ecosystem; abiotic environment. (b) List three ecological factors each which are specific to: aquatic habitats; ter...

(a) Explanation of terms

Ecosystem: a natural unit made up of all the living organisms (the biotic components) in a given area together with the non-living surroundings (the abiotic components) with which they interact to form a stable, self-supporting system through which energy flows and nutrients are recycled.

Abiotic environment: the non-living physical and chemical part of a habitat, that is, factors such as light, temperature, water, dissolved oxygen, wind, pH and soil, which affect the organisms living in that habitat.

(b) Three ecological factors specific to each habitat

Aquatic habitats: salinity (of the water); turbidity (cloudiness of the water); water current (tidal/wave movement). Dissolved gases and density are also acceptable.

Terrestrial habitats: wind; humidity (of the air); topography (relief of the land). Soil (edaphic factors) and altitude are also acceptable.

(c) Relationship between green grass and a rabbit in a given habitat

The relationship is a feeding (food) relationship that also involves an exchange of gases:

• The green grass is the producer; it photosynthesises to manufacture its own food.
• The rabbit is a herbivore (primary consumer); it cannot make its own food and so feeds on the grass, thereby obtaining energy from it.
• During respiration the rabbit gives out carbon dioxide, which the grass uses for photosynthesis.
• During photosynthesis the grass releases oxygen, which the rabbit uses for respiration.
• When the rabbit defecates or dies, bacteria and fungi decompose the waste, forming humus/nutrients that the grass absorbs from the soil.

Energy therefore flows from the grass to the rabbit, while gases and nutrients are cycled between them.

(d) Pyramid of energy using four feeding levels

A pyramid of energy is drawn as horizontal bars stacked on one another, widest at the base (producers) and narrowing upwards, because usable energy is lost (mainly as heat during respiration) at each successive trophic level. The values shown (in kJ m^-2 yr^-1) are typical and confirm that each higher level contains far less energy than the one below it.

The base (producers) contains the greatest amount of energy and each higher feeding level contains progressively less, giving the upright, narrowing pyramid shape.