A limiting factor or limiting resource is a factor that controls a population's growth, such as organism growth or species population, size, or distribution. Limiting factors includes space, water, and food. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for an organism. An example of a limiting factor is sunlight in the rainforest, where growth is limited to all plants in the understory unless more light becomes available.
A number of potential factors could influence a biological process, but importantly only one is limiting at any one place and time. This recognition that there is always a single limiting factor is vital in ecology; and the concept has parallels in numerous other processes.
Some other limiting factors in biology are water availability, temperature, shelter, or predation. See Liebig's Law.