Load capacity (ecology)

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In ecology , the load-bearing capacity is the maximum number of organisms of a species (population size) that can exist in a habitat for an unlimited period of time without permanently damaging it. Often the carrying capacity is equated with the (environmental) capacity in population models that are based on the logistic equation . In English, the technical term is carrying capacity .

Definitions and conceptual history

The term was first used in the context of wildlife biology , where its use can be traced back to the late 19th century. In a more modern sense, the use goes back to the influential American wildlife biologist Aldo Leopold ; he wrote in his textbook (1933): “If the maximum density of adult individuals of a species in the wild tends to assume a maximum value that is constant over large areas, even in the habitats that are most beneficial for them, this maximum can be called the saturation point for this species . This is different from the density that can actually be maintained in a certain, less perfect habitat. While this density represents the saturation value for that particular space, it is obviously variable and habitat dependent and, to avoid confusion, it is better to refer to it as its carrying capacity. A real saturation point arises when numerous, far apart, optimal areas have the same load-bearing capacity. It should be noted that while the saturation point appears as a property of the species, the carrying capacity is a property of the particular habitat. "

In 1953, the founder of modern ecosystem research, the ecologist Eugene P. Odum, was the first to use the term in his textbook as synonymous with the capacity K in the context of the logistic equation. The exact formulation is (slightly different depending on the edition of his work): “The upper level, beyond which no further growth can take place and which is represented by the constant K, is the upper asymptote of the sigmoid curve and therefore aptly as capacity (im Original: carrying capacity). ”Due to the great influence of Odum and his work, the term has been used in this sense since then.

The load-bearing capacity, so defined, is an equilibrium density towards which a population, according to the model, would strive regardless of its initial size. If the real population density is lower, the population grows due to births or immigration from neighboring habitats; if it is higher, it shrinks due to deaths or emigration. If the real population density is very low, its growth is maximum; this maximum growth rate can be understood as a biological constant of the species, which is only limited by factors such as reproductive age / generation duration, maximum clutch / litter size, maximum life span, etc., this becomes intrinsic growth rate called. As the population increases, growth is increasingly controlled by external factors such as B. Limited food shortages. According to the model, this limitation acts proportionally, i.e. H. twice as much when doubling. Eventually the population reaches a density at which the habitat limits the number of offspring to a level at which the population size remains precisely constant, i.e. H. Mortality and birth rate keep each other in balance. If the population becomes larger, mortality prevails. In mathematical terms, the most influential model for a population with such properties is the logistic equation (one possible notation)

The population size x at time n + 1 is the model in accordance with the previous population size, the intrinsic growth rate r and the population density reached in relation to the equilibrium density of K determined. If the population is greater than K , the parenthesized term becomes negative, resulting in a negative growth rate. With growth from low population densities, the population size initially increases slowly (because there are only a few individuals who can reproduce), finally reaches a maximum value and grows more and more slowly when approaching K , so that it no longer increases when the value K is reached . When the population size is plotted against time, an S-shaped curve, a so-called sigmoid function, results .

Criticism and limitations of the concept

If one follows the equation of the load capacity with the capacity K in the sense of the logistic model, the definition becomes more precise at first. However, it then depends on the properties of the model. Since it now seems questionable whether the sigmoid growth curve of this model can represent reality with any degree of accuracy, some ecologists even advocate dropping the expression load-bearing capacity / environmental capacity entirely.

First of all, the problem is that the model is strictly deterministic, i.e. that it ignores random fluctuations in environmental conditions. This limits its applicability when, for. For example, favorable and unfavorable weather conditions make the carrying capacity appear dramatically different in successive years. Another problem is that the smooth, sigmoid growth curve only occurs at low values ​​of r in relation to K. If the factor r is too large, the development overshoots in both directions: small populations grow far beyond the value given by K, the subsequent collapse (often as a result of diseases) leads to values ​​that are far below it. Initially, there are cycles (oscillations) until the behavior finally becomes chaotic with increasing r (see under logistic equation ). If such a model is realistic, population collapses, and ultimately even the destruction of the habitat, can be almost inevitable due to demographic constraints, even at population sizes well below K. The knowledge of the carrying capacity would then no longer be associated with any practical benefit.

Earth's carrying capacity

The ecological model of carrying capacity becomes particularly urgent when the species under consideration is Homo sapiens and the biotope under consideration is the whole earth. In this case, the model concept is directly linked to possible social and political decisions that can affect all people. The idea that the size of mankind is limited by external ecological factors is at first abstractly plausible. However, it is difficult to agree on the size of this maximum population size. Historically of particular importance are considerations that go back to the English economist Thomas Malthus , who ultimately understood the size of mankind to be limited by a lack of food. Today it is mostly assumed that even with an expected world population of almost 10 billion people, feeding humanity as a whole would be quite realistically possible, but this does not have to apply to all regions under all conditions.

The ecological model of carrying capacity in its established form in this science is usually defined differently once it is applied to humanity. While ecologists assume the birth rate of the species they consider to be maximum until it is limited by external factors, historical experience shows that the human population does not multiply at the maximum possible rate. Here, population scientists usually use the model of demographic transition : According to this concept, societies go through four phases with increasing prosperity. In the first phase, both the birth rate and the mortality rate are very high; the population remains low due to high mortality. In the second phase, due to medical advances, better nutrition and education, mortality falls, but the birth rate initially remains high because of the social advantages of large families (e.g. child labor); this leads to strong growth. Ultimately, according to the model, the birth rate will eventually decrease with a time lag, mostly when the parents' survival and standard of living no longer depend directly on their children. In the fourth phase, the birth and death rates are equally low and the population remains constant. If this model is correct, the ecological model of carrying capacity is not applicable to human populations. The state of equilibrium K of the logistic equation results primarily from the mortality that increases with increasing population density, i.e. from hunger, epidemics and intraspecific aggression (in humans: war). In human societies, however, it is precisely those with the highest mortality that have the highest population growth.

Social sustainability and ecological footprint

Mankind is probably not primarily interested in how many people could exist on earth before their number would inevitably be decimated and limited again by hunger, epidemics and wars of distribution, but rather in how high their number may be, thus the largest Number of them would continue to have a decent existence. An attempt is therefore made to define a “social load-bearing capacity” that is below the limit of bare survival. According to a model developed by the researcher couple Paul and Anne Ehrlich together with John Holdren , the product of population size (P), consumption rate per capita / prosperity (A from affluence) and the resulting environmental damage (T) is ultimately decisive. The concept of the ecological footprint was developed based on the PAT model . The major differences in determining the earth's carrying capacity are primarily due to the different assumptions made when it comes to the question of how we should live ( living standards ( lifestyle , ecological footprint )):

Load capacity = usable surface of the earth / standard of living as ecological footprint

The biological, productive surface area of ​​the earth ( biocapacity ) was 1.68 gHa (global hectares per person) in 2014, the ecological footprint 2.84 gHa. Assuming that it cannot work in the long term if more natural resources (ecological footprint) are constantly being consumed than are growing again (biocapacity), the number of populations for which biocapacity and ecological footprint are balanced can be calculated as follows :

Population with a balanced ecobalance with current biocapacity = current world population x biocapacity / ecological footprint = 7.63 billion (as of October 2018) × 1.68 gHa / 2.84 gHa = 4.51 billion people.

If it is taken into account that the biocapacity in global hectares per person increases in the same proportion as the population decreases, this results in (4.51 + 7.57) / 2 = 6.04 billion people who under the current conditions are long-term and sustainable the earth can live.

But should and can this number be achieved? The political explosiveness becomes clear in contradicting human rights: With the "Universal Declaration of Human Rights" the United Nations stipulated in Article 25 on the one hand:

Everyone has the right to a standard of living that guarantees the health and well-being of himself and his family, ...

On the other hand, family planning is considered a human right according to the proclamation of Tehran adopted by the International Conference on Human Rights in Tehran on May 13, 1968, which was affirmed in the action programs of the World Population Conferences of 1974 (Bucharest), 1984 (Mexico City) and 1994 (Cairo) :

Every couple is granted the basic right to freely and responsibly decide on the number of their children and the time between births.

Estimates of the earth's carrying capacity

Either based on limiting factors such as food shortages or in continuation of past trends, numerous researchers have tried to determine the earth's carrying capacity for the human population. The highest estimate ever given by the British doctor JHFremlin from 1964 held 60 quadrillion (6 × 10 16 ) people as the theoretical maximum, which he derived from a physical quantity, the heat radiation of the earth into space. The density of around 120 people per square meter should be achieved by converting the planet into a multi-storey building. Occasional trips “over a few hundred meters” were therefore still possible. It is not known whether Fremlin was serious about the post.

The first commandment of the Georgia Guidestones (1980) is: "Keep the human race below 500 million in perpetual balance with nature."

Gorbachev , founder of the International Green Cross himself, explains the ecological crisis as a population crisis and calls for a reduction of the world population by 90 percent.

The National Strategy for a Sustainable America, a panel of experts that advised US President Bill Clinton between 1993 and 1999, came to the conclusion in 1996 in response to the 1992 Earth Summit in Rio de Janeiro that the world population should not exceed 500 million people.

The Biodiversity Convention, as the world's most comprehensive convention in the field of nature conservation and development policy encompassing species diversity , genetic diversity and the diversity of ecosystems, does not contain any numerical data on the earth's carrying capacity.

Most of the published estimates of carrying capacity lie in a value range between approximately 1 and 12 billion people and thus on a scale that humanity has exceeded or will reach in the near future. However, the bases of all these estimates are not very reliable.

See also

Web links

Individual evidence

  1. ↑ Lexicon of Biology. Volume 14, Spektrum Akademischer Verlag, Heidelberg 2004, ISBN 3-8274-0339-1 .
  2. ^ Aldo Leopold: Game Management. Charles Sccribener's Sons, New York 1933, p. 51; quoted based on: Andre A. Dhont: Carrying capacity - a confusing concept. In: Acta Oecologica. Vol. 9 No.4 (1988): pp. 337-346.
  3. ↑ EP Odum: Fundamentals of ecology. Saunders, Philadelphia, USA 1953, p. 122.
  4. ↑ Eugene P. Odum: Fundamentals of Ecology. Volume 1: Basics. Translated and edited by Jürgen and Ena Overbeck. Thieme Verlag, 1980 (corresponds to the 3rd edition, 1973), ISBN 3-13-382302-7 , p. 289.
  5. ↑ cf. z. E.g. M. Begon, JL Harper, CR Townsend: Ecology: individuals, populations, communities. Birkhäuser, Basel / Boston / Berlin 1991, ISBN 3-7643-1979-8 or M. Schaefer: Ökologie. Biology dictionaries. 3. Edition. Fischer, Jena, ISBN 3-334-60362-8 .
  6. ↑ cf. z. BMA Hixon: Carrying capacity. In: SE Jørgensen, BD Fath (Ed.): Encyclopedia of Ecology. vol. 1. Elsevier Press, Oxford, UK 2008, pp. 528-530.
  7. ↑ cf. Andre A. Dhont: Carrying capacity - a confusing concept. In: Acta Oecologica. Vol. 9 No. 4 (1988), pp. 337-346.
  8. ↑ vg. Herwig Birg: population development. (Information on political education, 282). Revised new edition 2013. Published by the Federal Agency for Civic Education. download as pdf
  9. ↑ Country database of the World Population Foundation
  10. ↑ Nikos Alexandratos, Jelle Bruinsma: World agriculture towards 2030/2050: the 2012 revision. FAO Food and Agriculture Organization of the United Nations, Agricultural Development Economics Division. ESA Working Paper No. 12-03.
  11. ↑ for central Africa cf. z. B. Maurice King: demographic disentrapment. ( Memento of the original from December 3, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.leeds.ac.uk
  12. ↑ H. Ronald Pulliom, Nick M. Haddad: Human Population Growth and the Carrying Capacity Concept. In: Bulletin of the Ecological Society of America. 75: 141-157 (1994).
  13. ↑ Basia Zaba, lan Scoones: Is carrying capacity a useful concept to apply to human populations? In: Basia Zaba, John Clarke (Eds.): Environment and Population Change. Derouaux-Ordina, Liege 1994.
  14. ↑ Open Data Platform. Retrieved March 23, 2019 .
  15. ↑ Benjamin Seiler: One billion is enough !. In: ZeitenSchrift Nr. 65 (2010)
  16. ^ A b Joel E. Cohen: Population Growth and Earth's Human Carrying Capacity. In: Science. 269 ​​(1995), pp. 341-346.
  17. ↑ JH Fremlin: How many people can the world support? In: New Scientist. 415 (1964), pp. 285-287.