Lesson 3 of 13 · 6 min
Population density
NCERT §11.1.1
Next Saturday the group meets at a field full of Parthenium weeds with a single old banyan in the corner. 'Count them,' says Ma'am, and waits for someone to object.
The lesson in notes
In short
The size of a population reflects its status in the habitat. Competition, predation or a pesticide are all judged by the change they cause in population size.
Sizes range from fewer than 10 (Siberian cranes at the Bharatpur wetlands in any year) to millions (Chlamydomonas in a pond).
Population size is technically called population density and is written N. It need not be a head count.
Total number is usually the right measure, but it can mislead. In an area with 200 carrot grass (Parthenium hysterophorus) plants and one huge banyan, counting would make the banyan look unimportant.
In such cases per cent cover or biomass is the more meaningful measure of density.
Counting is also impractical when a population is huge, as with a dense bacterial culture in a petri dish.
Often relative density is enough: the number of fish caught per trap serves as a measure of the fish population in a lake.
Most population sizes are estimated indirectly. The tiger census in national parks and tiger reserves often relies on pug marks and fecal pellets.