(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 ...

(a) Short notes

• (i) Ecosystem: An ecosystem is a natural unit made up of all the living organisms (the biotic community) in a given area together with the non-living (abiotic) components of their environment, interacting as a functional whole through the flow of energy and the cycling of nutrients. Examples are a pond, a forest or a grassland.
• (ii) Population: A population is a group of organisms of the same species living together in the same habitat at a particular time and able to interbreed, for example all the tilapia fish in a pond.
• (iii) Ecological succession in plants: This is the gradual and orderly replacement of one plant community by another over a period of time until a stable, self-perpetuating community (the climax community) is reached. It begins with pioneer species (for example lichens and mosses) that colonise bare land, which change the conditions and are succeeded by grasses, then shrubs, and finally trees.

(b) Five ways nitrogen can be made available to the soil for plant growth

1. By nitrogen-fixing bacteria in the root nodules of legumes and free-living soil bacteria (for example Rhizobium, Azotobacter) fixing atmospheric nitrogen into nitrates.
2. By nitrifying bacteria converting ammonium compounds to nitrites and then to nitrates (nitrification).
3. By lightning (thunderstorms) causing nitrogen and oxygen to combine, forming oxides that dissolve in rain as nitrates.
4. By decay of dead plants and animals and their waste, releasing ammonia which is nitrified (action of decomposers/ammonifying bacteria).
5. By addition of nitrogenous fertilizers and organic manure by farmers.