Organisms are made to compete for their needs from the environment. The competition as we pointed earlier could be for the food and territory, to overcome the adverse climatic conditions, to escape from predators or tp combat an infectious diskase. This is indeed the "struggle" It is this "struggb" that keeps the population numbers under manageable levels and from increasing on a logrithmic scale. We earlier referred to the reproductive potential of frogs. If the potential is fully realised, then in a short time, they would be placing a severe strain on the resources on which they depend and soon all of them would starve. However, this is not the case. Most of the population sizes do not increase enormously but they only fluctuate to asmall extent from time to time. Some other populations'however, do increase aramatically in certain seasons and decrease even more dramatically in certain other seasons. In laboratory one can grow cell cultures in which any one of the resources such as food, space etc. is limited. Under these circumstan~es the population growth rate of the cells which is exponential to begin with decreases and levels off: The resulting sigmoidcurve is characteristic of biological growth in general. (Fig. 11.1) The factors that we n. mentioned earlier are the ones which delimit the numbers or size to the carrying capacity of the environment in natural populations. For instance, the 'carrying capacity of the environment for plants is controlled by the ambunt of spact! required 'by the individuals of their own and other species. No new seeds or shoots can develop in that particular area until the older plants fall. Population sizes in animals, as mdntioned earlier can be limited by food supply. Here the number of individuals ,increases until the food shortage occurs. Such a shortage of resources would decrease,the reproduction.