(a) The different attributes that a population has and not an individual organism -
• Birth rate – The number of births during a given period in the population.
• Death rate – The number of deaths during a given period in the population.
• Sex ratio – It is the ratio of males to the females in a population.
• Age distribution – A population at any given time is composed of different age-groups.
(b) Population density – Population size, more technically called population density is the number of organisms of a species per unit of area is called population density.
Three different ways the population density can be measured –
• Indirect method – population density is measured without actually counting or seeing the organism. For example, pug marks and fecal pellets for the tiger census.
• Direct method – counting the number of organisms in a given area. For example, the colonies of bacteria are counted in order to determine the number of bacteria in the petri dish.
• Relative density – it is calculated when the population is too large to be counted. For example, the number of fishes caught per trap gives their total density in a given water body.
The pyramid of energy represents the total amount of energy consumed by each trophic level in a given food chain. The pyramid of energy is always upright because there is always loss of heat at each step. Therefore the amount of energy available at the top trophic levels is lesser than lower trophic levels.
The pyramid of biomass represents the total amount of living matter present at each trophic level of an ecosystem. This pyramid can be both upright and inverted. In grassland ecosystem, the pyramid of biomass is upright. The number of biomass present at tertiary level is less than the secondary level and the producer level. In pond ecosystem, the pyramid of biomass is inverted because the biomass of fishes exceeds that of zooplanktons.
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