|Chapter 49||Table of Contents||Chapter 51|
Eco 110 - Carrying Capacity, January 15, 2057
Notes on a lecture
It was the first class of the term. I was purposefully late, because I wanted everyone to be present and impatient. I walked in, dropped my case on the wide black presentation desk and turned to face the old theatre style hall.
"Okay, here is the question: Are we collectively smarter than a vat of yeast?"
I let that sink in for a few seconds, then continued with:
"The topic for today is carrying capacity.
"Intuitively, the idea is how many animals can live on a given area of land.
"A more precise definition is: the maximum feasible load of a given species an environment can support indefinitely.
"You will note several particulars about this definition.
I used my padd to open a wall screen and put up the following points:
- Carrying capacity is species specific.
- Carrying capacity is a soft limit.
- The term 'load' is enviro-centric.
- The quantitative temptation.
I highlighted the first point and said, "How many cattle a pasture will support says nothing about the number of butterflies, although it may be related in surprising ways."
At the next highlight, I said, "Note the word 'indefinitely'. You can put more cattle in the pasture than the land will support, but when you do the herd will eat everything --- weeds, trees, roots. The cattle will not do well and the land will be degraded. A population in this situation is said to be in overshoot."
At the next item, I said, "Note the term 'load'. It refers to an environment's ability to supply the needs of a creature and to absorb or transform what it excretes or discards."
And at the last point I added, "Finally, although it is tempting to interpret carrying capacity quantitatively, and is done so for the lower orders, in the case of humans, the picture is complicated by culture and technology. For example, a million Americans present a much heavier load than a million Ethiopians.
"Now, back to the yeast.
"Consider a petri dish of agar. If I put a single bacterium on the dish with suitable light and temperature, it will begin to grow. It reproduces by budding at a known maximum rate with a pattern that looks like this."
I put up a time lapse sequence that showed a small dot, become a large dot, then a larger dot before covering half the dish and then all of it. The final picture of the seqence showed just a few cells lingering around the edges.
"The numbers of bacteria over this period of time looks like this."
I put up a graph showing cell counts over a 72 hour period, rising exponentially from one to several million and back to 10.
"In making beer a similar process occurs with yeast and emulsified grains, except on an industrial scale. The bacteria die when they have consumed all of the agar. The yeast die when they have poisoned themselves with the alcohol they produce.
"From its beginnings in Africa, the human race has spread all over the planet. From genomic comparisons and the mutation rate, we know that the human species passed through a bottleneck of some 50 or 70 individuals about 65,000 years ago.
"That gives us an approximate starting point. By the time of the Romans, there were some 170 to 300 million humans alive on the planet. Estimates vary. We reached a billion around 1800. Graphically it looks like this."
I put up a chart showing human population over the last 70 thousand years, the familiar exponential curve.
"You will notice a certain unnerving similarity to the earlier graph of bacteria counts."
I reduced both graphs to half size and put them side by side.
"That is because they are both exponential curves. That is an exam topic I am leaving for you to research. Got that? You are responsible for investigating the difference between exponential and linear curves. Okay."
"So what is it about carrying capacity? How does it work?
"In every form of life we've examined, we have learned that species will try to grow beyond the limit their environment will support. In more precise terms, the cumulative biotic potential of any species exceeds the carrying capacity of its environment. By 'biotic potential', I mean the number of offspring a creature could potentially produce."
I paused to emphasize the next statement.
"The balance is fecundity versus capacity."
I shut off all the graphics to change gears.
"So what have humans done with carrying capacity?"
"For most of our species history, we lived as hunter-gatherers in tribes of 40 or 60 individuals. Consider the range of such a tribe. Typically they would move with the seasons. maybe to follow a herd of game animals, maybe to be beside the river when the fish are running, or in the bush during berry season or in the forest during winter.
"If the tribe kills all the lions in the rangeland, they can effectively 'take over' the lions' territory. One top predator replaces another. Not much difference to the rest of the ecology.
"Once all the other predators have been killed off, the domestication of herd animals would simplify hunting and possibly increase the load on their rangeland, but not the carrying capacity.
"Then came the biggie: the advent of agriculture. The land that supported 40 or 60 hunter-gatherers would then support hundreds of farmers. The carrying capacity of the rangeland was significantly increased by the change in human culture. Note however that there would be a loss of biodiversity in both plant and animal realms.
"In time, the ingenuity of humans gradually increased the productivity of the land. With science, humans began to learn how things grow. In the 19th century, the German Justus Liebig, while investigating the chemicals of life, discoverd his 'Law of the Minimum' as you learned last term. For those of you who need reminding, Liebig's Law states that the least available necessary element is what limits the growth of a plant or an ecosystem. With this knowledge, fertilizers could be employed optimally.
This sets up a situation where 'trade' can be a relevant factor in considering carrying capacity. If I have a rich potassium source and you have a rich nitrogen source, we can trade and both increase our productivity. Thus enter the complexities of international relations.
With the knowledge of Mendelian genetics, humans began cross breeding plant varieties and selecting preferable strains. New varieties of plants that resisted disease or that had higher yields were produced. The discovery of DNA by Watson and Crick in the twentieth century eventually allowed this process to proceed by direct genetic manipulation.
"One key factor here is external energy. In the industrial era, humans began to draw down the reserves of 'buried sunlight', the fossil fuel resources sequestered over the previous millions of years. The energy was used to make fertilizers and pesticides, to pump water and to power farm equipment. At its height, industrial agriculture required 10 calories of external energy to produce one calorie of food. Think about that for a second.
"In energy systems theory, there is the concept of EROEI -- Energy Returned Over Energy Invested. This is similar to the financial concept of ROI -- Return On Investment. In order to break even, a businessman needs an ROI of at least 1, preferably much higher. He definitely would not touch a project with an ROI of 0.1. And yet, that is precisely what humans did all over the world with the energy of the food system. Why?
"The collapse of the petroleum economy may have made the Great Hunger inevitable even if the oceans had never risen."
I put up a new graphic with the following points:
- [Capacity increase?]
"You may have noticed these terms in the preceding description. These are all manipulations which increase the human share of carrying capacity.
"Now about this time, someone usually asks, 'Why not increase the carrying capacity itself?
"It seems to make sense, but there is a trap embedded in it. Do you see it?
"Doubling or quadrupling the carrying capacity itself will not change our fundamental situation. Sooner or later the population would increase until we were once again hitting the new limit. Humanity needs to develop a kind of homeostatic mechanism to manage its population level --- or Mother Nature, call her Gaia or Medea as you will --- will employ her harsh methods.
"So, on what does carrying capacity depend?
"If we look at the food chains, they all end up at plant life.
"How much do plants on land and ocean grow in a year?
"The term for this vast measure is 'Gross Primary Production.' And due to satellite measurements, we are able to measure it fairly accurately. It is of the order of 125 billion tonnes per year.
"If we subtract the amount the plants need to maintain themselves, the useable portion remaining is called the 'Net Primary Production.' Both measures are in units of mass of carbon per unit area per year (g C/m^2/yr).
"Here's your assignment.
"Research the percentage of NPP which humans appropriate. Look at the history of previous estimates. Start with Rees, Wackernagel and the Ecological Footprint Network in the twentieth century and work through more modern estimates. I'd like at least 1500 words in two weeks.
"So where are we now as a global species?
"Human population peaked at approximately 8.5 billion before the Great Hunger.
"The population now is below 6 billion and still falling.
"It remains an open question how seriously we will have degraded the environment before an equilibrium is established."
I put up the graphic of human population change again.
"Before we see just where this plot ends."
"Okay I think that is enough for today."
Excerpted from _The Bottleneck Years_ by H.E. Taylor
For further information, see
A Gentle Introduction.
If you want a copy, see
Last modified July 23, 2013