Carrying Capacity

Aug. 3, 2017

Carrying capacity is a central concept in ecological studies. Biologists define carrying capacity as the maximum population of a given species that can survive indefinitely in a given environment. It was originally applied to relatively simple population-environments such as the number of sheep or cattle that could be maintained on a grazing pasture without degrading the land for future grazing. It is a measure of sustainability because a community that is degrading or destroying the ecosystem on which it depends is using up its community capital and is living unsustainably.

Living within the limits of an ecosystem depends on three factors:

  • the amount of resources available in the ecosystem,
  • the size of the population (including birth and death rates),
  • the amount of resources each individual is consuming.

There are always limits to population growth in nature. Populations cannot grow exponentially indefinitely. Exploding populations always reach a size limit imposed by the shortage of one or more factors such as water, space, and nutrients or by adverse conditions such as disease, drought and temperature extremes. The interplay of biotic potential and density-dependent environmental resistance keeps a population in balance. Exceeding the available resources can result in population collapse.

Determining the carrying capacities for most organisms is fairly straightforward. For humans carrying capacity is much more complicated. The definition is expanded to include not degrading our cultural and social environments and not harming the physical environment in ways that would adversely affect future generations.

Earth Overshoot Day marks the date when humanity’s demand for ecological resources and services in a given year exceeds what Earth can regenerate in that year. In other words, they measure when we reach our planet’s carrying capacity. Earth Overshoot Day is hosted and calculated by Global Footprint Network, an international think tank that coordinates research, develops methodological standards and provides decision-makers with a menu of tools to help the human economy operate within Earth’s ecological limits.

To determine the date of Earth Overshoot Day for each year, Global Footprint Network calculates the number of days of that year that Earth’s biocapacity suffices to provide for humanity’s Ecological Footprint. The remainder of the year corresponds to global overshoot. Earth Overshoot Day is computed by dividing the planet’s biocapacity (the amount of ecological resources Earth is able to generate that year), by humanity’s Ecological Footprint (humanity’s demand for that year), and multiplying by 365, the number of days in 2017:

(Planet’s Biocapacity Humanity’s Ecological Footprint) x 365 = Earth Overshoot Day

Yesterday, we reached our overshoot day for the year. From now until January 1st, the 7.5 human beings on the planet are exceeding the carrying capacity of natural resources and services to sustain us. Those in developed countries will not be affected, there will still be food at the store, gas at the pumps and water in the faucets. This is not the case in other developing countries that are often exploited for their resources. This is part of the great inequities that exist between the haves and the have nots. Ecology and economy are closely correlated when trying to understand the complexity of human population carrying capacity. What is your ecological footprint?

Ecological footprint calulator: 

Earth Overshoot Video: