Ecology is 'the study of the interrelationships between organisms and their natural environment, both living and non-living'
A popular aspect of ecology is the relationship of the relative proportions of predator and prey. When more predators are introduced to an environment, they eat more prey animals. This tends to increase the population of predators, since they get more food and can thus live longer to reproduce more. But it also reduces the food supply of the predators, which contrariwise causes the predators to live shorter lives and to reproduce less. However, since predators tend to eat herbivores, increased predation of herbivores will allow primary producers to increase in popularion, creating a phenomenon called a Trophic Cascade  In some ecosystems, the populations of predator and prey cycle up and down wildly, but usually they reach an equilibrium.
The first significant contribution to the theory of population ecology was that of Thomas Malthus, an English clergyman, who in 1798 published his Essay on the Principle of Population. Malthus introduced the concept that at some point in time an expanding population must exceed supply of prerequisite natural resources, i.e., population increases exponentially resulting in increasing competition for means of subsistence, food, shelter, etc. This concept has been termed the "Struggle for Existence". 
- Martin, E. and Hine, R. (2008) 'A Dictionary of Biology' Oxford University Press, Oxford, U.K.
- Estes, James A. and Terborgh, John, Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature. Island Press, 2010. ISBN 10597264873.
- Rockwood, Larry L. Introduction to Population Ecology. Wiley Blackwell, 2006. ISBN 9781405132633. Chapter 10.