Native Plants, Habitat Restoration, and Other Science Snippets from Athens, Georgia

Wednesday: 29 November 2006

Population Growth  -  @ 07:09:57
We've reached the last two weeks of lectures in the semester. In the general biology courses, they're talking about populations and evolution (the latter, unfortunately, is receiving increasingly shorter attention these days, almost a "filler" in case there's a day or two available). As usual, maybe 3/4 of UGA students aren't able to tell you how many people there are in the world (6.7 billion), how many expected in 2050 (10-12 billion), or even how many in the US (300 million, and remarkable because it's been in the news lately). About the same number don't particularly care, except as factoids to be memorized for the final, or perhaps as brief soundbites to be considered in-between cell-phone calls.

But population growth, and the sophistocated mathematical models to describe it, are interesting. While these students don't see the first equation, the graphs do tweak their interest, however briefly.

Here's exponential growth, unhindered by environmental resistance factors, or limiting factors.

The curve starts out slow and then picks up speed in an accelerating fashion. It's the sort of curve that you might see for a short period of time for bacteria on a petri dish, or what we're now seeing for human population growth, currently.

A word about human population growth. In the late 1700s, Thomas Malthus wrote a treatise on this subject, pointing out that populations grew geometrically (as you see above) and food supplies grew arithmetically, that is, linearly. There would come a time, he hypothesized, when the numbers of people would be greater than their food supply and then would come chaos, famine, and massive death from starvation. Malthus' treatise was one of the foundations for Charles Darwin's chain of reasoning leading to natural selection as a force behind evolution of populations.

The usual cohort has gleefully seized upon Malthus and pointed out that over the two centuries since, none of his predictions has come to pass, and fallaciously conclude that therefore they will never come to pass. Malthus, of course, could not predict scientific innovations that could abruptly increase food sources - such things as the Industrial Revolution that supported the chemical synthesis of nitrogen fertilizers, the production of pesticides in the 1940s, and the Green Revolution that saw the emergence of more productive hybrid plants in the 1960s. Gene technology is predicted to be the next Revolution that will up the food supply.

It probably goes without saying that many environmental problems can be attributed to all of these innovations: nitrogen fertilizer runoff into rivers and the ocean, pesticide organic chemicals as pollutants that cause deformities and mutations in animals at the top of the food chain, and the periodic massive failures of crops (corn in the 1970s, for instance) when hybrid plants are found to harbor defects that permit massive disease to demolish crops. There's considerable suspicion that there will be similar environmental repercussion from gene technology - and for the first time the consequences are at least under discussion.

Well, back to population growth. That exponential growth is limited by a lot of things - an increasing lack of food, water, space, air; an increase in disease and predators; these are limiting factors. In a very simplistic way, species respond differently to these limiting factors:

Species with individuals that are large, have few kids, take care of their kids, and have long life spans generally show K-selection, the upper left panel. They eventually level off to a carrying capacity, the number of individuals the environment can support. The population remains, stably, at a constant level.

The upper right panel shows R-selection. These are species, such as insects and many kinds of plants, that have individuals that are rather small, short-lived, produce tons of kids, and don't take care of their kids. They overshoot the carrying capacity and because of any number of things, perhaps their rapid reproductive growth, strip their food supplies. The population crashes. Gradually it recovers and then the cycle is repeated.

From a human point of view the left panel seems rather benign (although out of keeping with what we expect of a constantly growing economy) and the right panel looks dangerous (but is the panel that our constantly growing economic expectations is encouraging).

Which panel suits humans best? Currently we're still in an exponential phase, and we would simplistically expect that because of the formulaic descriptions of K- and R- selected species that humans would naturally be K-selected.

Not necessarily. Remember that humans have the ability to (intelligently or foolishly) alter their environment to maintain population growth. This enables them to enter into the regime of R-selected species, with a suddenly burgeoning population that strips the environment.

If you've dabbled in anthropology, geography, sociology - you're probably familiar with this kind of graph - age structure. That link, by the way, is an interesting one in which you can select any country and discover its age structure. Check out India, and despair. China, for all its huge population, is interesting. Ethiopia, despite its famines, maintains a depressing amount of population growth. Contrast with, say, Germany.

An age structure graph shows the number of individuals in each age bracket. The top half of the figure (A) shows a large number of immature kids (blue) compared to middle age (green) and older people (red). Fast forward 25 years, when those blue kids become green fertile people, and you can see that this population is growing rapidly, and over the next 25 years there will suddenly be a *lot* more people, no matter what.

The bottom two panels show other forms the age structure graph can take - a stable population is more or less up and down (B), and a declining population is more like an upside-down pyramid (C). There are countries that represent all of these alternatives, but human growth globally currently resembles the top one - continued growth.

So now, are humans K-selected or R-selected?

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