Carrying capacity
The carrying capacity of an
At the global scale, scientific data indicates that humans are living beyond the carrying capacity of planet Earth and that this cannot continue indefinitely. This scientific evidence comes from many sources worldwide. It was presented in detail in the
An early detailed examination of global limits was published in the 1972 book
Origins
In terms of population dynamics, the term 'carrying capacity' was not explicitly used in 1838 by the Belgian mathematician Pierre François Verhulst when he first published his equations based on research on modelling population growth.[12]
The origins of the term "carrying capacity" are uncertain, with sources variously stating that it was originally used "in the context of international
Hadwen and Palmer (1923) defined carrying capacity as the density of stock that could be grazed for a definite period without damage to the range.[16][17]
It was first used in the context of
Mathematics
The specific reason why a population stops growing is known as a
The difference between the
In the standard ecological algebra as illustrated in the simplified Verhulst model of population dynamics, carrying capacity is represented by the constant K:
where
- N is the population size,
- r is the intrinsic growth rate
- K is the carrying capacity of the local environment, and
- dN/dt, the derivative of N with respect to time t, is the rate of change in population with time.
Thus, the equation relates the growth rate of the population N to the current population size, incorporating the effect of the two constant parameters r and K. (Note that decrease is negative growth.) The choice of the letter K came from the German Kapazitätsgrenze (capacity limit).
This equation is a modification of the original Verhulst model:
In this equation, the carrying capacity K, , is
When the Verhulst model is plotted into a graph, the population change over time takes the form of a
where
- e is the natural logarithm base (also known as Euler's number),
- x0 is the x value of the sigmoid's midpoint,
- L is the curve's maximum value,
- K is the logistic growth rate or steepness of the curve [24] and
The logistic growth curve depicts how population growth rate and carrying capacity are inter-connected. As illustrated in the logistic growth curve model, when the population size is small, the population increases exponentially. However, as population size nears carrying capacity, the growth decreases and reaches zero at K.[25]
What determines a specific system's carrying capacity involves a
Software is available to help calculate the carrying capacity of a given natural environment.[26]
Population ecology
Carrying capacity is a commonly used concept for biologists when trying to better understand biological populations and the factors which affect them.[1] When addressing biological populations, carrying capacity can be seen as a stable dynamic equilibrium, taking into account extinction and colonization rates.[21] In population biology, logistic growth assumes that population size fluctuates above and below an equilibrium value.[27]
Numerous authors have questioned the usefulness of the term when applied to actual wild populations.[16][17][28] Although useful in theory and in laboratory experiments, carrying capacity as a method of measuring population limits in the environment is less useful as it sometimes oversimplifies the interactions between species.[21]
Agriculture
It is important for farmers to calculate the carrying capacity of their land so they can establish a sustainable
In other parts of the world different units are used for calculating carrying capacities. In the United Kingdom the paddock is measured in LU, livestock units, although different schemes exist for this.[30][31] New Zealand uses either LU,[32] EE (ewe equivalents) or SU (stock units).[33] In the US and Canada the traditional system uses animal units (AU).[34] A French/Swiss unit is Unité de Gros Bétail (UGB).[35][36]
In some European countries such as Switzerland the pasture (
This system can also be applied to natural areas. Grazing
Some niche market schemes mandate lower stocking rates than can maximally be grazed on a pasture. In order to market ones' meat products as 'biodynamic', a lower Großvieheinheit of 1 to 1.5 (2.0) GV/ha is mandated, with some farms having an operating structure using only 0.5 to 0.8 GV/ha.[citation needed]
The Food and Agriculture Organization has introduced three international units to measure carrying capacity: FAO Livestock Units for North America,[38][39] FAO Livestock Units for sub-Saharan Africa,[38][39] and Tropical Livestock Units.[40]
Another rougher and less precise method of determining the carrying capacity of a paddock is simply by looking objectively at the condition of the herd. In Australia, the national standardized system for rating livestock conditions is done by body condition scoring (BCS). An animal in a very poor condition is scored with a BCS of 0, and an animal which is extremely healthy is scored at 5: animals may be scored between these two numbers in increments of 0.25. At least 25 animals of the same type must be scored to provide a statistically representative number, and scoring must take place monthly -if the average falls, this may be due to a stocking rate above the paddock's carrying capacity or too little fodder. This method is less direct for determining stocking rates than looking at the pasture itself, because the changes in the condition of the stock may lag behind changes in the condition of the pasture.[29]
Fisheries
In
Note that mathematically and in practical terms, MSY is problematic. If mistakes are made and even a tiny amount of fish are harvested each year above the MSY, populations dynamics imply that the total population will eventually decrease to zero. The actual carrying capacity of the environment may fluctuate in the real world, which means that practically, MSY may actually vary from year to year[45][46][47] (annual sustainable yields and maximum average yield attempt to take this into account).[citation needed] Other similar concepts are optimum sustainable yield and maximum economic yield; these are both harvest rates below MSY.[48][49]
These calculations are used to determine fishing quotas.[citation needed]
Humans
Human carrying capacity is a function of how people live and the technology at their disposal. The two great economic revolutions that marked human history up to 1900—the agricultural and industrial revolutions—greatly ramped up Earth's human carrying capacity, from 5 to 10 million people 10,000 BCE to 1.5 billion in 1900.[50] The immense technological improvements of the past 100 years—in applied chemistry, physics, computing, genetic engineering, and more—have further increased Earth's human carrying capacity, at least in the short term. Without the Haber-Bosch process for fixing nitrogen, modern agriculture could not support 8 billion people.[51] Without the Green Revolution of the 1950s and 60s, famine might have culled large numbers of people in poorer countries during the last three decades of the twentieth century.[52]
Recent technological successes, however, have come at grave environmental costs. Climate change, ocean acidification, and the huge dead zones at the mouths of many of world's great rivers, are a function of the scale of contemporary agriculture[53] and the many other demands 8 billion people make on the planet.[54] Scientists now speak of humanity exceeding or threatening to exceed 9 planetary boundaries for safe use of the biosphere.[55] Humanity's unprecedented ecological impacts threaten to degrade the ecosystem services that people and the rest of life depend on—potentially decreasing Earth's human carrying capacity.[56] The signs that we have crossed this threshold are increasing.[57][6]
The fact that degrading Earth's essential services is obviously possible, and happening in some cases, suggests that 8 billion people may be above Earth's human carrying capacity. But human carrying capacity is always a function of a certain number of people living a certain way.[58][59] This was encapsulated by Paul Ehrlich and James Holdren's (1972) IPAT equation: environmental impact (I) = population (P) x affluence (A) x the technologies used to accommodate human demands (T).[60] IPAT has found spectacular confirmation in recent decades within climate science, where the Kaya identity for explaining changes in CO2 emissions is essentially IPAT with two technology factors broken out for ease of use.[61]
This suggests to technological optimists that new technological discoveries (or the deployment of existing ones) could continue to increase Earth's human carrying capacity, as it has in the past.[62] Yet technology has unexpected side effects, as we have seen with stratospheric ozone depletion, excessive nitrogen deposition in the world's rivers and bays, and global climate change.[56][10] This suggests that 8 billion people may be sustainable for a few generations, but not over the long term, and the term ‘carrying capacity’ implies a population that is sustainable indefinitely. It is possible, too, that efforts to anticipate and manage the impacts of powerful new technologies, or to divide up the efforts needed to keep global ecological impacts within sustainable bounds among more than 200 nations all pursuing their own self-interest, may prove too complicated to achieve over the long haul.[63]
Two things can be confidently asserted regarding Earth's carrying capacity, based on the Great Acceleration of energy and materials use, waste generation, and ecological degradation post-WW II.[64] First, expansions in human carrying capacity have come at the expense of many other species occupying Earth today.[6][65] Between 1970 and today, populations of wild vertebrates have declined 60%;[66] similarly sharp declines may have occurred among insects and vascular plants,[67] although the evidence is sketchier. So our successful efforts to increase human carrying capacity have come at the expense of Earth's capacity to sustain other species.[53] As we have converted habitat and resources to our own use, other species have sharply declined—to the extent that conservation biologists speak of an incipient mass species extinction.[68]
Second, expansions in per capita wealth and the concomitant increases in per capita consumption, resource use and waste generation, tend to decrease the total number of people that can be sustained, long term.[58][69] All else being equal, a richer population, living more luxuriously, has a lower carrying capacity than a poorer, more abstemious population.[59] As affluence goes up, population must come down to remain within any theoretical carrying capacity, and vice versa.[70]
As aforementioned, one issue with applying carrying capacity to any species is that ecosystems are not constant and change over time, therefore changing the resources available. Research has shown that sometimes the presence of human populations can increase local biodiversity, demonstrating that human habitation does not always lead to deforestation and decreased biodiversity. Another issue to consider when applying carrying capacity, especially to humans, is that measuring food resources is arbitrary. This is due to choosing what to consider (e.g., whether or not to include plants that are not available every year), how to classify what is considered (e.g., classifying edible plants that are not usually eaten as food resources or not), and determining if caloric values or nutritional values are privileged. Additional layers to this for humans are their cultural differences in taste (e.g., some consume flying termites) and individual choices on what to invest their labor into (e.g., fishing vs. farming), both of which vary over time. This leads to the need to determine whether or not to include all food resources or only those the population considered will consume. Carrying capacity measurements over large areas also assumes homogeneity in the resources available but this does not account for how resources and access to them can greatly vary within regions and populations. They also assume that the populations in the region only rely on that region’s resources even though humans exchange resources with others from other regions and there are few, if any, isolated populations. Variations in standards of living which directly impact resource consumption are also not taken into account. These issues show that while there are limits to resources, a more complex model of how humans interact with their ecosystem needs to be used to understand them.[71]
Recent warnings that humanity may have exceeded earth's carrying capacity
Between 1900 and 2020, Earths’ human population increased from 1.6 billion to 7.8 billion (a 390% increase)[72] and the size of the global economy increased from about $3.5 trillion to about $90 trillion in current dollars (an increase of over 2500%).[73] These successes greatly increased human resource demands, generating significant environmental degradation.[2] Scientific evidence from many sources indicates that humans are probably living beyond Earth's human carrying capacity, given current levels of affluence and the technology being deployed to meet people's economic demands.[citation needed]
Millennium ecosystem assessment
The Millennium Ecosystem Assessment (MEA) of 2005 was a massive, collaborative effort to assess the state of Earth's ecosystems, involving more than 1,300 experts worldwide.[2] Their first two of four main findings were the following. The first finding is:
Over the past 50 years, humans have changed ecosystems more rapidly and extensively than in any comparable period of time in human history, largely to meet rapidly growing demands for food, fresh water, timber, fiber, and fuel. This has resulted in a substantial and largely irreversible loss in the diversity of life on Earth.[74]
The second of the four main findings is:
The changes that have been made to ecosystems have contributed to substantial net gains in human well-being and economic development, but these gains have been achieved at growing costs in the form of the degradation of many ecosystem services, increased risks of nonlinear changes, and the exacerbation of poverty for some groups of people. These problems, unless addressed, will substantially diminish the benefits that future generations obtain from ecosystems.[74]
According to the MEA, these unprecedented environmental changes threaten to reduce the Earth's long-term human carrying capacity. “The degradation of ecosystem services could grow significantly worse during the first half of this [21st] century,” they write, serving as a barrier to improving the lives of poor people around the world.[74]
Critiques of Carrying Capacity with Relation to Humans
Humans and human culture itself are highly adaptable things that have overcome issues that seemed incomprehensible at the time before. It is not to say that carrying capacity is not something that should be considered and thought about, but it should be taken with some skepticism when presented as a concretely evidenced proof of something. Many biologists, ecologists, and social scientists have disposed of the term altogether due to the generalizations that are made that gloss over the complexity of interactions that take place on the micro and macro level. Carrying capacity in a human environment is subject to change at any time due to the highly adaptable nature of human society and culture. If resources, time, and energy are put into an issue, there very well may be a solution that exposes itself. This also should not be used as an excuse to overexploit or take advantage of the land or resources that are available. Nonetheless, it is possible to not be pessimistic as technological, social, and institutional adaptions could be accelerated, especially in a time of need, to solve problems, or in this case, increase carrying capacity. There are also of course resources on this Earth that are limited that most certainly will run out if overused or used without proper oversight/ checks and balances. If things are left without remaining checked then overconsumption and exploitation of land and resources is likely to occur.[75]
Ecological footprint accounting
Ecological Footprint accounting measures the demands people make on nature and compares them to available supplies, for both individual countries and the world as a whole.[76] Developed originally by Mathis Wackernagel and William Rees, it has been refined and applied in a variety of contexts over the years by the Global Footprint Network (GFN). On the demand side, the Ecological Footprint measures how fast a population uses resources and generates wastes, with a focus on five main areas: energy use, land devoted to direct settlement, timber and paper use, food and fiber use, and seafood consumption.[77] It converts these into per capita or total hectares used. On the supply side, national or global biocapacity represents the productivity of ecological assets in a particular nation or the world as a whole; this includes “cropland, grazing land, forest land, fishing grounds, and built-up land.”[77] Again the various metrics to capture biocapacity are translated into the single term of hectares of available land. As the Global Footprint Network (GFN) states:
Each city, state or nation’s Ecological Footprint can be compared to its biocapacity, or that of the world. If a population’s Ecological Footprint exceeds the region’s biocapacity, that region runs a biocapacity deficit. Its demand for the goods and services that its land and seas can provide—fruits and vegetables, meat, fish, wood, cotton for clothing, and carbon dioxide absorption—exceeds what the region’s ecosystems can regenerate. In more popular communications, we also call this “an ecological deficit.” A region in ecological deficit meets demand by importing, liquidating its own ecological assets (such as overfishing), and/or emitting carbon dioxide into the atmosphere. If a region’s biocapacity exceeds its Ecological Footprint, it has a biocapacity reserve.[77]
According to the GFN's calculations, humanity has been using resources and generating wastes in excess of sustainability since approximately 1970: currently we use Earth's resources at approximately 160% of capacity.[78] This implies that humanity is well over Earth's human carrying capacity for our current levels of affluence and technology use. According to the GFN:
In 2022, Earth Overshoot Day fell on July 28. Earth Overshoot Day marks the date when humanity has exhausted nature’s budget for the year. For the rest of the year, we are maintaining our ecological deficit by drawing down local resource stocks and accumulating carbon dioxide in the atmosphere. We are operating in overshoot.[79]
The concept of ‘overshoot’ can be seen as equivalent to exceeding human carrying capacity.[80][76] According to the most recent calculations from GFN, most of the world's residents live in countries in ecological overshoot (see the map on the right).
This includes countries with dense populations (such as China, India, and the Philippines), countries with high per capita consumption and resource use (France, Germany, and Saudi Arabia), and countries with both high per capita consumption and large numbers of people (Japan, the United Kingdom, and the United States).[77]
Planetary Boundaries Framework
According to its developers, the planetary boundaries framework defines “a safe operating space for humanity based on the intrinsic biophysical processes that regulate the stability of the Earth system.”[55] Human civilization has evolved in the relative stability of the Holocene epoch; crossing planetary boundaries for safe levels of atmospheric carbon, ocean acidity, or one of the other stated boundaries could send the global ecosystem spiraling into novel conditions that are less hospitable to life—possibly reducing global human carrying capacity. This framework, developed in an article published in 2009 in Nature[81] and then updated in two articles published in 2015 in Science[55] and in 2018 in PNAS,[82] identifies nine stressors of planetary support systems that need to stay within critical limits to preserve stable and safe biospheric conditions (see figure below). Climate change and biodiversity loss are seen as especially crucial, since on their own, they could push the Earth system out of the Holocene state: “transitions between time periods in Earth history have often been delineated by substantial shifts in climate, the biosphere, or both.” [55]
The scientific consensus is that humanity has exceeded three to five of the nine planetary boundaries for safe use of the biosphere and is pressing hard on several more.[82] By itself, crossing one of the planetary boundaries does not prove humanity has exceeded Earth's human carrying capacity; perhaps technological improvements or clever management might reduce this stressor and bring us back within the biosphere's safe operating space. But when several boundaries are crossed, it becomes harder to argue that carrying capacity has not been breached.[83] Because fewer people helps reduce all nine planetary stressors, the more boundaries are crossed, the clearer it appears that reducing human numbers is part of what is needed to get back within a safe operating space.[84][85] Population growth regularly tops the list of causes of humanity's increasing impact on the natural environment in Earth system science literature.[86] Recently, planetary boundaries developer Will Steffen and co-authors ranked global population change as the leading indicator of the influence of socio-economic trends on the functioning of the Earth system in the modern era, post-1750.[87]
See also
- Biocapacity – Estimate of an ecosystem's production of certain biological materials
- Ecological footprint – Individual's or a group's human demand on nature
- Ecological overshoot – Demands on ecosystem exceeding regeneration
- Inflection point – Point where the curvature of a curve changes sign
- Overpopulation – When a population of a species exceeds the carrying capacity of its environment
- Overshoot (population) – Phenomenon in which populations temporarily exceed carrying capacity of environment
- Population ecology – Study of the dynamics of species populations and how these populations interact with the environment
- Population growth – Increase in the number of individuals in a population
- r/K selection theory – Ecological theory concerning the selection of life history traits
- Tourism carrying capacity
- Toxic capacity
Further reading
- Kin, Cheng Sok, et al. "Predicting Earth's Carrying Capacity of Human Population as the Predator and the Natural Resources as the Prey in the Modified Lotka-Volterra Equations with Time-dependent Parameters." arXiv preprint arXiv:1904.05002 (2019).
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