Carrying Capacity

Carrying capacity is best known today as a means of expressing the finitude of Earth's resources relative to the human population. However, the concept has been used in a remarkably wide range of fields beyond demography and geography, including anthropology, ecology, population biology, range and wildlife management, engineering, medicine, and law. Considered across all these uses, carrying capacity may be defined as the maximum or optimal amount of some X that can be conveyed or supported by some encompassing entity or place Y. The cogency and utility of this definition depend on the X and Y in question, however, and an understanding of its complex history is necessary to evaluate carrying capacity as a concept.

In the 1840s, when the term originated, carrying capacity referred to the mechanical or engineered attributes of manufactured objects or systems, specifically ships. It arose in disputes over the tariffs and duties applied to steamships, as compared with the sailing vessels for which earlier laws and practices had been designed. Later in the 19th century, the concept was extended to things such as commuter rail systems, electrical lines, lightning rods, irrigation ditches, and pipelines.

The term carrying capacity was applied to living organisms and natural systems beginning in the 1870s. Examples include how much weight pack animals could haul, how much pollen certain bees could carry, and how much floodwater a bayou could transport. In the late 1880s, the term began to be used to describe the number of livestock a given quantity and quality of land could support. This marked an important reversal, as an earlier “Y” became an “X” being “carried,” in a more figurative sense, by the environment in which it lived. In the 1920s and 1930s, early game managers applied this concept to quail and deer, and in the 1940s, British colonial administrators applied it to human populations in Africa as part of resettlement and agricultural development schemes. Also in this period, biologists and chemists conducted laboratory experiments in which invertebrates and microorganisms were found to display sigmoid population growth curves when provided a stream of nutrients under controlled environmental conditions. This observation led to the rediscovery of Pierre-François Verhulst's logistic equation (showing population growth and decline as an S-shaped curve), which would provide subsequent ecologists with a means of quantifying and modeling carrying capacity.

After World War II, two additional types of carrying capacity concepts emerged concurrently. One retained fauna as its object but transformed the epistemological basis of carrying capacity from inductive and applied to deductive and theoretical. This ecological concept has been poorly supported by empirical research, however, and is now seen to have only limited, heuristic value. The other shifted the object of the concept to humans and expanded its scale to regions, continents, and the entire globe, giving rise to the neo-Malthusian sense of carrying capacity that pervades the general use of the term today. When applied to living populations, however—and especially humans—static conceptions of carrying capacity are empirically unsupportable, while variable ones are theoretically incoherent.