​Sustainable Ecology

Carrying Capacity

Carrying capacity is a well-known ecological term that has an obvious and fairly intuitive meaning : "the maximum population size of a species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment".  Unfortunately that definition becomes more nebulous the closer you look at it – especially when we start talking about the planetary carrying capacity for humans.  Ecologists claim that our numbers have already surpassed the carrying capacity of the planet, while others (notably economists and politicians...) claim we are nowhere near it yet!

 

Referring to the above Graph, where the Red Overshoot Line intersect at the second point on the Horizontal Graph Line a degraded Carrying Capacity is already in Motion before Humans realise the severity of the Ecological Resource Overshoot. The Arrows that defines the degraded Carrying Capacity of the Ecology on the Graph is where is there less Resources available for Earth Biosphere's Natural Carrying Capacity. Earth's Species at this point in time are having their Comfort Zones Compromised by Over Resource Consumption and are on the Endangered list of Extinction.
 
I think the confusion arises because we intuitively conflate two very different understandings of the phrase. I call them the “outside” view and the “inside” view.
 
The “outside” view of carrying capacity (I call it CCo) is the view of an observer who adopts a position outside the species in question. It’s the typical analytic/synthetic view of an ecologist looking at the reindeer on St. Matthew’s Island, or at the impact of humanity on other species and its own resource base. CCo is the view that is usually assumed by ecologists when they use the naked phrase “carrying capacity”, and it is an assessment that can only be arrived at through deductive reasoning. From this point of view humanity passed CCoa while ago. It probably happened between 1850 and 1950, depending on what factors you draw into your assessment, but we certainly passeed it before 1975.  As you will see below, my estimate for when we passed it is surprisingly early - well before we were aware that it might be a problem.

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When we say that humans have “expanded our carrying capacity” through technological innovation, we are using our "inside" voice. From the experiential, subjective, species point of view, we have indeed made it possible for the environment to support ever more people. This is the only view that matters at the biological, evolutionary level. In humans it is this perspective that encourages constant innovation in the face of scarcity.

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The combination of our immense intellectual capacity for innovation and our biological inability to step outside our chauvinistic, anthropocentric perspective has made it impossible for us to avoid landing ourselves in our current insoluble global ecological predicament.

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Overshoot

When a population surpasses its carrying capacity it enters a condition known as overshoot.  Because carrying capacity is defined as the maximum population that an environment can maintain indefinitely, overshoot must by definition be temporary.  Populations alwaysdecline to (or below) the carrying capacity.  How long they stay in overshoot depends on how many stored resources there are to support their inflated numbers.  Resources may be food, but they may also be any resource that helps maintain their numbers.  For humans one of the primary resources is energy, whether it is tapped as flows (sunlight, wind, biomass) or stocks (coal, oil, gas, uranium etc.).  A species usually enters overshoot when it taps a particularly rich but exhaustible stock of a resource.  Like oil, for instance...
 
Population growth in the animal kingdom tends to follow a logistic curve: an s-shaped curve that starts off low when the species is first introduced to an ecosystem, then at some later point rises very fast as the population becomes established, then levels out as the population saturates its niche. 

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Humans have been on the front end of our logistic curve for our entire history. Our population has risen very slowly over the last couple of hundred thousand years, as we (very) gradually developed the adaptive skills we needed in order to deal with our varied and changeable environment (language, writing and arithmetic). As we developed and disseminated those skills our ability to modify our environment grew, and so did our growth rate. 
 
If we had not discovered the stored energy resource of fossil fuels, we would probably be following the green curve in the chart below, and would be well on our way to achieving balance with the energy flows in the world around us, with our numbers settling down somewhere aroud two billion. This is the road not taken. 
 
The road we have taken is the one in red, on which our numbers and consumption have been driven well past the world's long-term carrying capacity, deep into overshoot territory. As we partied hearty since 1900, we have degraded the flow-based carrying capacity of the biosphere by messing up the earth, air and water. As a result, when the party ends, the resulting correction will take us back well below a carrying capacity of two billion, since that no longer exists - we have already "eaten" a big chunk of it while we were in overshoot.