Population: all the organisms of one species in a habitat
Community: population of all the species in a habitat
Quadrat: used to estimate the population size. the small area within the quadrat and scale up to make an estimate for larger areas. count all the organisms in a 1m2 quadrat and then multiply that by the total area of the habitat.
90 grass plants-1m2 quadrat
90 x 120=10,800