Henderson's Dictionary of Biological Terms, by Eleanor Lawrence. 10th Ed.
Evolutionary Ecology, by Eric R. Pianka, 3rd Ed.
There are several factors that places limits upon the size of a population, but they can be placed into two groups, density-independent and density-dependent factors. One example of a density-independent factor is climate. Sudden climate changes, like storms, fires, floods, hurricanes and droughts can cause large changes in a population's density. Intertidal plants, and many intertidal animals attach themselves to rocks along the shoreline. Storms can pull these organisms off of the rocks, thereby killing them. This also has the effect of opening up living space in over populated areas.
Density-dependent factors vary how it affects a population based upon the population's density, number of individuals living in an area. Food and living space are examples of density-dependent factors. Each individual needs a certain amount of food in order to survive. So the total amount of available food in an area has to be divided among the total number of individuals in the population. As the density increases, the number of individuals in an area increases, the amount of food available for each individual decreases. Beyond a certain threshold, there will not be enough food for every individual in that population. This causes many individuals to die of starvation, thereby reducing the size of the population.