The predators of specie serve as a natural control of the species' population growth. Population is the major functional unit of the ecosystem. It plays a particular role in the energy flow and in the cycling of materials or nutrients. To a certain extent, predation keeps low population density of species in the community. To a probable greater extent, the size of the said population is affected by that of the species on which it preys.
Thus, in the food chain grass-mouse-snake-hawk, where the mouse eats grass and is then ate by the snake, which, ever, is taken by the hawk, an increase in the number of snakes and a decrease in grass supply will significantly decrease the number of mice. Likewise, an increase in snake population will increase the number of preys for the hawks, allowing quite a number of the latter to survive. As the number of hawk increases, the population of snakes will be lessened as they become preys.
The rates of productivity or turnover at each level in a food chain reveal the dynamics of an ecosystem, especially the size of every organism's population. By measuring changes at the bottom of a food chain, it is possible to predict changes at the top. Populations vary in numbers of individuals from time to time. The changing population is measured by counting and recording individuals from an area at different times. This is what you call as population density. This is the number of individuals of specie in a definite area at a specific time.
If organizations are placed in a new habitat that is favorable for their development, the population will increase at a rate. Certain factors control population size in proportion to density. These factors may include birth rate, reproductive rate, death rate, parasitism, disease and predation depend to an extent on the population density at any given time.