Carrying Capacity

“Overshoot: (v.) to increase in numbers so much that the habitat’s carrying capacity is exceeded by the ecological load, which must in time decrease accordingly; (n.) the condition of having exceeded for the time being the permanent carrying capacity of the habitat.”
Overshoot, William Catton

This is a dense post. It gathers the core ecological concepts important to my position in a single source.

Energy flow through biological systems does not behave as a blind, dumb force. Last week we looked at how this energy supplies the herbivores and layers of carnivores, how it molds food webs with a roughly 10% efficiency between layers and we ended with a ratio of respiration to biomass which stood as a thermodynamic ordering function. The conclusion provides the jumping off point for today’s examination of carrying capacity:

‘We see there are finite quantities of material and a fixed flux of radiant energy on earth. Our planet is defined by these limitations. The thermodynamic energy laws give earth its characteristic dynamics, strictly delimiting what is and what is not possible.’

An ecosystem consists of a population of species and the changing environment they find themselves in. The environment is changed in large part by the very presence of the populations dwelling there. This progression through a series of stages is called succession. Each stage is termed a sere. Imagine a bare field and watch it evolve over a century or two. The bare field becomes grassland with weeds and other pioneering species, which in turn create the conditions for shrubs of all kinds to begin growing here and there. As the years go on the shrub density increases and the first trees of a pine forest find they have the conditions they need to survive. If the pine forest in turn gives way to an oak and hickory forest and the succession ends there then that last stage, last sere, is referred to as the climax community.

1280px-Forest_succession_depicted_over_timeIf the only major influences on the ecosystem remain those that come from the living populations, natures use of energy will keep the climax community running for centuries, even millennia. It is not a wholly static metric of energy flow to production but instead it pulses, which has been found to keep the populations fit. These climax communities are characterized by the strongest adaptability to shocks compared to any other possible configuration of living things and their environment. Life alters conditions such that they become more capable of hosting life; the Gaia hypothesis. This is due to the climax community containing the highest degree of species diversity of both plants and animals that this environment can indefinitely sustain. Energy has brought forth the maximization of use before being dissipated into entropy.

The climax community is a theoretical construct for an environment that has achieved a configuration that maximizes its carrying capacity indefinitely. It is 100% sustainable.

How the process works is educationally practical; our own species is caught in a number of circumstances related to these things. The energy path can be illustrated by comparing the ecosystem’s respiration to its overall production, P/R. Think of P as the rate of primary production (plants) or total photosynthesis. Respiration, R, as before is the ‘out-gassing’ of entropy. At first a rather scrappy set of plants and animals hit the scene, what we call the invasive species. Think weeds; aggressive and competitive. They have what it takes to survive in the harsher conditions of the early stages. The primary production greatly exceeds the respiration and the ratio P/R > 1. While P exceeds R organic matter will accumulate.

Too much accumulation of organic and inorganic matter can lead to a pollution problem. If the organic pollution overwhelms the ecosystem then respiration exceeds production and the ratio P/R < 1. The theory of succession states that an ecosystem tends towards a balance between the energy fixed in the biomass and the energy cost of maintenance in respiration, that is, tends towards P/R = 1. There is not an overall accumulation of organic material in the climax ecosystem, there is very little yield. What’s there instead is a flourishing of biodiversity where cooperation and symbiosis build complex systems with sophisticated feedback loops providing a resiliency the pioneering communities cannot achieve.

Odum_9.1_CIn the climax community the amount of standing biomass is maximized but interestingly this is not always what we humans want. Agriculture is a good example of deliberately holding back the process of succession to an earlier stage so that there continues to be larger yields of crops that we can use as food. Additionally, just as one characteristic of the climax community is that it has wide biodiversity, a characteristic of the pioneering, invasive community is that it is dominated by a monoculture. There is a tendency for a single plant to dominate just as we see on our farm fields with their acres of wheat and corn. It takes an enormous amount of energy to pause the process of succession on our farms. The petrochemical dependency of agribusiness, from tractor fuel to fertilizer, is well known. It is also well known that the right pest can wipe out an entire monoculture crop. Lacking the diversity of the climax community these critical ecosystems are vulnerable to shocks.

Still, even the climax community is not permanent. The larger change of sere ceases only until something from outside the system intervenes. In the real world everything changes; sooner or later a shock will come that is larger than what the environment can adapt to and it will regress or change completely. These shocks come from changes in either the inputs, the incoming energy, food and materials or the outputs such as we see in pollution where something produced in the system overwhelms the capacity of the larger environment to absorb it and break it down.

When considering the value of this theory that nature tends to balance P/R it is worth asking how the process behaves in extremus. One of the other characteristics of the pioneer, invasive species is that because they lack the feedback loops that would limit their population growth to sustainable levels their populations are prone to a phenomenon known as overshoot and die off. The (in)famous example is yeast introduced into a vat of wine. This simple ecosystem displays many of the most critical concepts necessary for understanding the current ecological predicament mankind finds itself in. In the diagram above it’s the ‘starting algal culture’ high in production and low in respiration.

The yeast consumes the nutrients from the sugary grapes producing a population explosion. The more yeast, the faster the sugars are consumed. It is party time in the wine vat, what ecologists refer to as the stage or age of exuberance. As we have learned, all ecosystems are defined by limits and this one is no different. There is a limited supply of the sugar nutrients and the expanding population is drawing down that resource; it is consuming it faster than it can be renewed. This ballooning population exceeds the carrying capacity of the vat with no way to replace what is being lost. We say the population of the yeast is in overshoot. Because this stage is unsustainable it will not be sustained; there will be a population crash until the resources can recover to a level adequate to sustain it. In the vat the pollution the yeast produce, the alcohol and carbon dioxide (our fermentation), fill the environment until the yeast is no longer able to survive. In that vat there is a die off to the point of extinction. There was a balance point in the vat between the number of yeast, their waste products and the ability of the mash to recycle them but this balance point was over shot. By exceeding the carrying capacity the yeast harmed the environment’s future carrying capacity, creating a downward spiral leading it, in this case, to zero. These words form the key take away: drawdown, overshoot, crash and die-off.

This ecological analysis highlights the difference between ecosystems that are sustainable and those that are not. To sustain the existing carrying capacity indefinitely the community populations can only be using renewable resources and those only at a rate that allows them to be renewed. This is an important point. Oil, for instance, is technically a renewable resource since new oil is being formed today, but the process takes millions of years. For all practical purposes oil is a non-renewable resource so with every barrel that we burn we draw down the total that remains in a usable form. Using any non-renewable resource is going to have the same characteristics. These are not methods of enlarging human carrying capacity, only exceeding it. Are there ways then that truly enlarge carrying capacity?

Another method is available. Humans are masters at this as well, the art of taking over other primary and secondary production for our own use. Every acre taken over for human settlement and raising our plant and animal foods increases the earth’s human carrying capacity at the expense of other species. This is simply the way it is, a necessary result of life unfolding on a finite planet. History is a tale full of examples of one kingdom usurping the lands and resources of another. These are other examples of leveraging the takeover method in human affairs. In these cases, all other things being equal, the addition to the carrying capacity can be a permanent one. For most of human history this has remained the case because the contextual environment in which human settlement occurred retained its integrity. The larger global environment was able to provide the inputs we required and process our outputs successfully. We were fairly successful at diverting a large percentage of the world’s life support capacity from supporting other life forms towards supporting ourselves.

Around 500 years ago something new came to be. It had taken all of pre-history and all of history up to the year 1500 for our species to multiply to 500 million; half a billion individuals scattered throughout the globe. At this time the European lands were full and population pressures kept the balance between the number of people and the carrying capacity of their environments more or less in balance. A famine or plague could eliminate large percentages of the population but in time the population would recover.

WorldPop_1970Then the Europeans discovered the new world, a whole hemisphere rich in natural resources. The largest takeover in human history commenced and an Age of Exuberance was begun which we are still living in today. An Age of Exuberance that is now coming to a close. Some nations seem to get away with supporting a population much greater than what could be provided for from their own lands. In these cases in order to form a complete analysis we need the idea of ghost acreage developed by George Borgstrom. Such nations were drawing upon invisible carrying capacity, which is capacity located elsewhere on the planet. “When Columbus set sail, there were roughly 24 acres of Europe per European. Life was a struggle to make the most of insufficient and unreliable resources. After Columbus… a total of 120 acres of land per person was available in the expanded European habitat – five times the pre-Columbian figure!” (Catton, 1980, pg. 24)

It took another three centuries for the human population to double. The globe sustained one billion of us around the year 1800. The operations of takeover had proven to be powerful, enough so that many ecologists who study these things think that perhaps about one billion people is the balance point of global, indefinite, carrying capacity for the human species in pre-industrial ecosystems.

The Age of Exuberance became the new normal; from now on, year after year, growth was expected. The renaissance and scientific revolutions, the war for independence and the drafting of the American Constitution all took place against this background. Fetters like the dead weight of superstitions which had haunted our ancestors were thrown off right and left. The new abundance supported widespread literacy for the first time as leisure hours spread among the peoples. Technological improvements became the definition of progress and all began to feel entitled to a “perpetually expansive life” as Catton had it. A belief in limitlessness began to become common currency in our cultures. With the western enlightenment came a humanism that dared to assert it would be possible by man’s own ingenuity to improve his fortunes, improve the ‘state of nature’ and improve his social relations. An Age of Exuberance indeed. It is not difficult to understand how, as again Catton had it, we have inherited the conviction that mankind is “largely exempt from nature’s constraints.”

The Age of Exuberance would have run its course as the benefits of takeover were again maximized but for another fortuitous discovery. James Young, a Scottish chemist, noticed petroleum seeping out of a coal mine in Alfreton, Derbyshire. In 1847 he managed to distil light thin oil that could be used in lamps along with thicker oil good for lubricating machinery. In 1848 he started a small business refining crude. Mankind began a draw down not from elsewhere but elsewhen. Now we were dealing with another kind of ghost acreage, call it phantom acreage. It was the combination of the New World and the fossil fuel based industrial age together that created the 500+ year Age of Exuberance and Abundance we have now learned to take as the normal state of affairs.

Next week will continue our look at how carrying capacity plays out in the real world where we will meet the strange denizen, Homo Colossus.