Theory question 103
Nutrient Cycling(a) Explain briefly how carbon (IV) oxide and oxygen are balanced in nature.
(b) List five factors that affect the distribution of plants in their habitats.
(c)(i) What is overcrowding in an ecosystem? (ii) List five effects of overcrowding in an ecosystem.
(d)(i) Distinguish between density-dependent factors and density-independent factors in an ecosystem. (ii) Give two examples of density-dependent factors. (iii) Explain briefly how density-dependent factors and density-independent factors affect the dynamic equilibrium in an ecosystem.
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(a) Green plants take in carbon (IV) oxide and give out oxygen in photosynthesis; plants, animals and decomposers take in oxygen and give out carbon (IV) oxide in respiration (and burning adds CO2). The two processes balance each other, keeping the amounts in the air fairly constant.
(b) Any five: light; temperature; rainfall/water; soil type and nutrients; pH; humidity; wind; topography; competition; animals (grazers, pollinators).
(c)(i) Overcrowding is when there are more organisms in an area than its resources can support.
(ii) Any five: competition for food, water, light and space; spread of diseases; shortage of food; build-up of wastes/pollution; stunted growth; increased death rate; emigration; lower reproduction; aggression.
(d)(i) Density-dependent factors have a greater effect as the population becomes denser; density-independent factors affect the population the same way whatever its density.
(ii) Any two: food supply; competition; disease; predation; accumulation of wastes; space.
(iii) Density-dependent factors increase deaths or lower births when the population is large and ease off when it is small, so the population is held around a balance (carrying capacity). Density-independent factors such as floods, drought, fire or sudden temperature change can kill many organisms at any density and upset the balance; the population later recovers.
In (d)(i) the difference is whether the effect depends on population size.