3.5: Limits on populations

It is a demonstrable fact that all types of organisms (as opposed to specific individuals) can produce more, sometimes many more than one copy of themselves. Consider, as an example, a breeding pair of elephants or a single asexually reproducing bacterium. Assume that there are no physical limits to their reproduction, that is, that once born, the offspring reproduce in the same way over the course of their lifespans. Allowed to continue unchecked, within a few thousand years a single pair of elephants could produce a mass of elephants larger than the mass of the Earth, an absurd, impossible outcome. We must have left something out of our calculations! As another example, let us consider a solitary, asexual photosynthetic bacterium that needs no mate to reproduce. A bacterium is much smaller than an elephant but it can produce a new bacterium at a much faster rate. Under optimal conditions our bacterium might divide once every ~20 minutes, or even faster, and would, within a few days produce a mass of bacteria greater than that of Earth as a whole. Again, we are clearly leaving something out from our thinking.

A single cell of the bacterium E. coli would, under ideal circumstances, divide every twenty minutes. That is not particularly disturbing until you think about it, but the fact is that bacteria multiply geometrically: one becomes two, two become four, four become eight, and so on. In this way it can be shown that in a single day, one cell of E. coli could produce a super-colony equal in size and weight to the entire planet Earth.… A single cell of the bacterium E. coli would, under ideal circumstances, divide every twenty minutes. That is not particularly disturbing until you think about it, but the fact is that bacteria multiply geometrically: one becomes two, two become four, four become eight, and so on. In this way it can be shown that in a single day, one cell of E. coli could produce a super-colony equal in size and weight to the entire planet Earth.

- Michael Crichton (1969) The Andromeda Strain

The ability to produce more, sometimes many more offspring than are needed to replace themselves, is a trait known as superfecundity. But unlimited growth cannot continue for very long. So what factors act to constrain superfecundity, what limits population growth? One obvious factor involves the resources needed for growth, which are limited. Thomas Malthus (1766-1834) articulated the role of limited resources as a constraint on population. His was a purely logical argument. Competition between increasing numbers of organisms for a limited supply of resources necessarily limits the number of organisms. Malthus painted a gloomy picture of organisms struggling with one another for access to these resources, with many living in a version of extreme poverty, starving to death because they could not out-compete others for the food or spaces they needed to survive and reproduce. One point that Malthus ignored, or more likely was ignorant of, is that organisms rarely behave in this way. It is common to find behaviors that limit direct struggles between organisms for resources. In some organisms, an adult has to establish, and defend, a territory before it can successfully reproduce.^[109] The end result of these types of behaviors is to stabilize the population around a steady state level, which is a function of both environmental and behavioral constraints.

An organism’s environment includes all factors that influence the organism. Environmental factors include changes in climate, as well as changes in the presence or absence of other organisms. For example, if one organism depends in important ways upon another, the extinction of the first will necessarily influence the survival of the second.^[110] Similarly, the introduction of a new type of organism or a new trait, such as O₂-generating photosynthesis, into an established environment may disrupt existing interactions and conditions. When the environment changes, existing steady state population levels may be unsustainable or some of the different types of organisms present may not be viable. If the climate gets drier or wetter, colder or hotter, if yearly temperatures reach greater extremes, or if new organisms, including for example, disease-causing pathogens, enter an area, the average population density of particular organisms may change or in some cases, if the environmental change is drastic enough, they could drop to zero, in other words populations could go extinct. Changing environmental conditions will influence the sustainable steady-state population level of an organism (something to think about in the context of global warming and the destruction or disruption of natural environments).

An obvious example of this type of behavior involves humans (→). Once constrained by disease, war, and periodic famine, the introduction of better public health and sanitation measures such as clean water, a more secure food supply, and vaccination, have led to reductions in infant mortality that resulted in explosive growth in the population. Now, in many countries, populations appear to be heading to a new steady state level, although exactly what that final population total level will be is unclear.^[111] In a number of countries, the birth rate has fallen into the low fertility domain (→), although that is no guarantee that it will stay there!^[112] In this low fertility domain (ignoring immigration), a country’s population decreases over time, since the number of children born is less than the number of people dying. This can generate social stresses.^[113] Decreases in birth rate per woman correlate with socioeconomic factors, including educational level and the reproductive self-determination, that is, the emancipation of women. Where women have the right to control their reproductive behavior, the birth rate tends to be lower. Clearly changes in the environment, and here we include the sociopolitical environment, can dramatically influence behavior and impact reproductive rates and population levels.

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