Download Complex Population Dynamics: A Theoretical/Empirical Synthesis (MPB-35) (Monographs in Population Biology) fb2

Great book, read it multiple times. Great synthesis of theory and empitical work. Anyone interested in population biology should read it.

Politicians often say things like, "Let's not follow each other off a cliff like lemmings," but it's not true that lemmings really follow each other off a cliff in nature. What IS true is that some species of lemmings have huge populations some years, and become almost extinct in other years.
Why does the population of lemmings vary so much? It turns out that it has to do with the interaction of the lemmings with their food supply and their predators. When there are a lot of lemmings, then they eat all the edible vegetation and begin starving, and their predators, like weasels, thrive and kill the lemmings off, so that lemmings almost disappear. With no lemmings around, the food can grow again unmolested, and predators, having nothing to eat, die off themselves. That leaves the field clear for lemmings to thrive, and their populations grow, repeating the cycle every three to five years.
Peter Turchin formalizes all this for a wide variety of animal species -- from the Larch budmouth to the southern pine beetle, from the red grouse to the snowshoe hare. He develops mathematical models for the populations of all these species and tests the models with known data time series.
This landmark book is a must for serious professionals involved in ecology and other biological and natural sciences.

The purpose of Turchin's tome is to answer the question "Does Population Ecology Have "Laws"? (the subject of an earlier paper by him). Turchin wants to claim that population ecology is a "mature science", and uses some largely philosophical arguments to claim similarities to Newton's laws. I was not convinced.

Whilst I think he does an admirable job of explaining the current state of thinking in the field of population ecology, I believe he fails to adequately explain the first principles, leaping from the semi-firm footing of Malthusian population theory to a morass of bits of models that only partially work.

Turchin refers to the exponential law of population ecology (Malthus, 1798) as a "good candidate" for "the first principle of population dynamics". The Malthusian argument is that populations grow exponentially (at a constant rate) when "unchecked" (as Malthus would say) or as long as the "environment... remains constant" (as Turchin puts it).

Of course, this never happens, and there is no evidence presented (by any author) of any population ever having sustained indefinite exponential growth (at a constant rate). All real-world examples of exponential growth turn out to be temporary periods of exponential growth (at a constant rate).

Various other models are considered, but each with caveats and drawbacks. In short, there is - apparently - no simple law of population ecology.

==Re-interpreting Malthus==

Yet Malthus argued that (human) populations double every 25 years (when unchecked), and then explained very carefully that all human populations are checked (by war, famine, pestilence, Death, or "moral restraint")! He even explicitly states that the 25 year doubling period is the "general case" and so the doubling period would sometimes be faster, sometimes slower.

A handy way of explaining the impact of what Malthus said is to use a simple heuristic tool called the Rule of 70, whereby the annual compound interest rate is divided into 70 to get the doubling period (it becomes less accurate for larger interest rates). Thus, 1% annual growth results in doubling every 70 years, and 2% annual growth results in doubling every 35 years. Check the US Census Bureau's website, and you will see that our global population doubled from 3 billion in 1960 to 6 billion in 1999 - a doubling period of 39 years. The compound interest rates during that period were almost entirely in the 1% to 2% range. Thus, it is no surprise that the population doubled somewhere between 35 and 70 years. Try it for yourself, using any rate between 1% and 2% each year until the population doubles.

Thus, variable rate compound interest results in variable doubling periods. An almost identical argument can be made for variable negative compound interest rates of growth, resulting variable population halving times.

Imagine a balancing scale, with all the negative rates on one side, and all the positives on the other. Whenever one side "outweighs" the other by 70, you've got yourself a population doubling or a population halving (depending whether positive rates or negative rate "won", respectively).

The balancing "Scales of 70" is a simple and useful approximation, and a completely accurate model called the "Scales of e" (using Natural Logarithms) is also available.

==Simple Population Dynamics==

This "revised" Malthusian model - barely 1 page long - provides an explanation of a true law of population ecology that anyone can understand. This is a firmer "first principle" than the traditional interpretation of Malthus used by Turchin (and everyone else).

This is basically how all populations of all species grow. Variable rate compound interest. Just 4 words.