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Q: You said only two outcomes, what are they?
MA: Well in the “R-selected” species their populations explode, filling all the ecological niches they can endure and then they suddenly collapse with the glide ratio of a rock. The “K-selected species” also explodes while food and habitat are abundant, and then they slow down as regulatory factors such as lower birth rate and reduced food availability come into play. The rate of population growth slows down to zero, and the population reaches a fairly stable level.
Q: Are we leveling off?
MA: Yes and no. No - we are still increasing the human population on earth but yes we are now slowing that rate of increase.
Q: When did it start slowing down?
MA: Somewhere in the early 60’s we reached our peak of a 2.2% increase and then we began to moderate that rate of growth. We now have about a 1.14% rate of growth which translates to doubling in 61 years.
Q: Has it been continually slowing?
MA: Yes, for the last 40 years.
Q: So if I understand correctly, we are modulating our population as we reach the limits of the environment. That sounds like good news, is it?
MA: Well, let’s take a look at a very simple equation that will help us decide the answer to that question.
r = n - m
This simple equation means that the realized intrinsic rate of growth is measured by the difference between natality (birth rate) and mortality (death rate).
And obviously zero population growth is reached when r = 0, natality equals mortality, and the population size remains constant, even though individuals are being born and are dying.
It seems simple. To follow the perfect example of a perfect “k-selected” species - we would simply expand to the limits of our environment or as we often say –“the habitat's carrying capacity” - we would then modulate our birth rate to equal our death rate and live in the land of milk and honey.
But it is, as are many things in complicated systems, not that easy. We now understand that there are both “k-selected” and “r-selected” traits in many populations. Our species is riddled with such contradictions.
Let's start with a simple concept like rate of population growth. While it is true that the overall human rate of increase is modulating - that is not true for all components of that set. Most European countries have low growth rates. In the United Kingdom, the rate is 0.2%, in Germany it is 0.0%, and in France, 0.4%. Germany's zero rate of growth includes a natural increase of -0.2%, without immigration, Germany would be shrinking, like the Czech Republic
Page #51
The Czech Republic and some other European countries’ growth rate is actually negative (on average, women in the Czech Republic give birth to 1.2 children, which is below the number to yield zero population growth, approximately 2.1 children). The Czech Republic's natural growth rate of -0.1 can not be used to determine doubling time because the population is actually shrinking in size.
So in these numbers we have a very strong trait for “k-selection”. But there is another current equally as strong - if not stronger. Many Asian and African countries have high growth rates. Afghanistan has a current growth rate of 4.8%, representing a doubling time of 14.5 years.
Q: I thought “k-selected” species also had rapid population expansion?
MA: The answer is yes; of course “k-selected” species have periods of rapid population expansion. But the kicker is the conditions under which this occurs.
Do you remember the brief phrase I said summarized the k vs. r selection?
Q: Was it quantity vs. quality?
MA: That is correct. But what it also means is that these two strategies are designed to function in different environments. Your K species uses the rapid growth to take advantage of a stable environment with ample resources while the R species responds to a disturbed environment by mass reproduction - obviously hoping that some members will survive the certain crash that always occurs.
So it is not just the growth rate but the environment that frames that growth that really defines which strategy is in play. We have in the “k-selected” species a stable environment that is density dependent. They produce few offspring with extra investment on the part of the parent, they have late maturation (again with much parental care) and they live long lives.
The “r-selected” species thrives in an unstable environment where there is density independence, they produce many offspring with limited parental investment, and they have early maturation, and live short lives.
Q: I don't understand are you saying we are not a “k-selected” species.
MA: No, we are both. Although some organisms are primarily r- or k-strategists, the majority of organisms falls between these two ecological extremes and may display traits of both. Because this concept of a continuous spectrum is such an important concept to understand - I would like to take a moment and discuss it with you.
One way to speak of a continuous spectrum is to frame it in terms of variations within a species. You are quite aware that within any species you have wide differentiation. Size, coloration, abilities etc, it is a never ending list of differences that make God's creation such a wondrous joy. These differentiations are the result of all the possible combinations of genes in an individual member of any species.
So we can say that the intermediate expression of genes results in a wide variety of variations within any species. Pretty much ABC stuff but it is incredibly important in the discussion we are having.