A limiting factor or limiting resource is one that controls a process, such as organism growth or species population size or distribution. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for a species population in a specific area. The limiting factor changes the amount or organisms a population can hold. The concept is based upon Liebig's Law of the Minimum put forth by German geochemist, Justus von Liebig, in 1840. It can be easy to conceive how a "limiting" resource (say, food) controls a process (say, growth) by running low or running out. However, some biological and ecological processes are controlled by too much of a factor (such as heat) rather than too little. Or, processes may be controlled by complex interactions of factors (Shelford, 1952). Walter Taylor (1934) proposed the following broad, restatement of the law of the minimum:
The functioning of an organism is controlled, or limited, by that essential environmental factor or combination of factors present in the least favorable amount. The factors may not be continuously effective but only at some critical period during the year or perhaps only during some critical year in a climatic cycle.
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