Theory question 103
Population Ecology(a) Write short notes on each of the following:
(i) ecosystem; (ii) population; (iii) ecological succession in plants
(b) List five ways by which nitrogen can be made available to the soil for plant growth.
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(a)(i) Ecosystem: a natural unit made of a community of living organisms interacting with each other and with their non-living environment, e.g. a pond or forest; it is self-sustaining, with energy flow and nutrient cycling.
(ii) Population: all the organisms of one species living in a particular area at a particular time; it can be described by its size, density, distribution and growth.
(iii) Ecological succession: the gradual, orderly change in the plant species of an area over time; pioneer plants (e.g. lichens, mosses) colonise bare ground, change the conditions, and are replaced by grasses, shrubs and then trees until a stable climax community forms.
(b) Any five: nitrogen fixation by free-living bacteria (Azotobacter, Clostridium); fixation by Rhizobium in root nodules of legumes; fixation by blue-green algae (Nostoc, Anabaena); lightning combining nitrogen and oxygen, washed down by rain; decomposition (ammonification) of dead organisms and wastes; nitrification of ammonium compounds to nitrates; adding fertilizers/manure/compost; animal urine and droppings.
Succession: name the stages from pioneer to climax.