Logging the Onset of the Bottleneck Years

"Bottleneck" implies that some few will survive it. Everything we know about populations that massively exceed the carrying capacity of their environments suggests that extinction will be the outcome.

@darwinsdog: I'm not sure if that's strictly the case. Do you have any references to support this contention? Off the top of my head I can think of several systems which follow a boom & bust pattern (several crop pest insect species, irruptive birds, lemmings perhaps) and can also think of several situations where local carrying capacity has been exceeded leading to a crash in population, but not total extinction - in fact is this latter not regarded as a potential driver of speciation events? (It's been a while since I've looked in great detail so my evolutionary theory may be a bit rusty & out-of-date).

addendum: As I hit post it ocurred to me that it is highly likely that certain segments of the human population will survive - I'm thinking mainly of remote tribes in the world's rainforests & deserts as a first example that comes to mind.

Chris,

Population theory suggests that species tend to exist on a continuum of growth curves, which are usually referred to as either 'r adapted' or 'K adapted'. This is not to say that species are either or - rather, r and K adaptions are generally different ends of a spectrum, with most species existing somewhere along the line.

In the classic theory, r adapted species are generally small organisms which reproduce quickly (such as insects) and their populations often follow a 'boom and bust' growth curve. K adapted are generally larger animals (such as larger mammals), and their population growth curve tends to follow a classic 'S' or logistic curve, with population growth slowing as it reaches carrying capacity.

Under this theory, humans would be thought of as a K adapted species, as we fit the characteristics of that type and through most of our evolution I would suggest that was a valid idea. However, I believe that there is a key characteristic of both types which is understated in the classic theory - that of predation - and which can lead one to make false assumptions.

Population size is generally kept in check by either resource limitations (and competition), or by predation and disease. K adapted species don't have many predators, and are usually resource limited or controlled by intrinsic factors. Because they reproduce slowly, there is time for the effect of reducing excess resources to slow population growth. However, r adapted species are usually kept in check by predation, and when that predation and disease are removed (for whatever reason) the young which would not have survived to adulthood now do so, and the species experiences a population explosion which overwhelms the available resources. Inevitably, the population crashes because there are just not the resources available to sustain it.

My view is that this issue of predation and disease is critical, especially in the case of humans. For most of our evolution, human population growth was kept in check, firstly by predation and then by disease. Infant mortality in medieval times was in the order of 30 - 50%, which is similar to most wild mammals of comparable size. However, infant mortality today - even in the worst countries in the world - is less than 3%, and is less than 0.1% in most western countries. What this means is that human population is growing at an expenential rate, and I believe we are closer to exhibiting the characteristics of an r adapted species than a K adapted species.

If I am right - and I am not alone in this view - then we will reach a point in our population growth where we greatly exceed the resouces available to sustain us, and the population will inevitably collapse (we won't go extinct). What that point is I do not know, but I suspect we are pretty close one way or the other.

My apologies - I mistyped. I was out by a factor of 10 in my information on infant mortality. mea culpa!!

List of things one needs to master to survive is way shorter than many think, but if the resources deplete many of these will be in short supply. How many have f.e. smithed themselves a knife?

The difference between humans and similarly sized animals is humans are spread globally. One could argue the domesticated sheep, horses, pigs, and cows have also spread globally, but how many have actually captured one of these on a pasture? This difference may produce a global bottleneck event very few species (including humans) get through unchanged.

I predict survival teachers will be quite employed during this century, to end on a lighter tone.

mandas: Thanks for the refresher. You've confirmed for me my thoughts that darwinsdog's assertion that "Everything we know about populations that massively exceed the carrying capacity of their environments suggests that extinction will be the outcome." Is not really true.

A good way to look at your predation/disease limitations is to consider the fate of invasive species when they enter a new, naive ecosystem (a good example is the Broom (Cytisus scoparius) which, in it's native range is a small to medium size shrub but in New Zealand reaches tree-like proportions). Invasives that 'escape' their predators & diseases can cause havoc in naive ecosystems thus demonstrating the critical impact of top-down pressure on populations.

However I dispute your assertion that humans are exhibiting r-adaption characteristics. The r-adaption produces many young as so very few survive to reproduce, what humans are showing is what happens when K-strategy (less young, better survival chance) reaches its optimum.

What hasn't been covered yet in this discussion is the role of dispersal. In many species dispersal is very important, either to prevent an area reaching carrying capacity or to find new areas when it does. There is some work demonstrating how increased population density often increases the dispersal trait (a prime example is in aphids where population density is an important factor in determining when the winged migratory morphs are produced). Dispersal is an alternative to population collapse, the question is: where have humans got to disperse to?

r versus K selection: one of those oversimplifications one learns as an undergrad & has to unlearn in grad school.

During a mass extinction episode one of the few constants is that animals of large body size, both in absolute terms and in terms relative to the mean size of the clade, go extinct. Humans are animals of large body size. Anthropogenic mass extinction has been ongoing since the end of the Pleistocene and is accelerating. Never in the history of life on Earth has a large animal's population exceeded the carrying capacity of the biosphere to the extent that human population has done. As resource constraints impinge, especially constraints on liquid fuels, human population will inevitably crash. Will it crash all the way to the absorbing boundary of zero? Perhaps not. Perhaps human population will fragment into local populations between which gene flow does not occur. Perhaps these local populations will fluctuate in size over the period of several generations until novel environmental stressors, genetic & demographic stochasticity and Allee effects take them down one by one. Time will tell.

"r versus K selection: one of those oversimplifications one learns as an undergrad & has to unlearn in grad school."

Maybe it's 'cause I never went to grad school but I still find it a workable hypothesis - at least with the insects I study - and no reviewer has ever questioned it in my papers. Please enlighten me, what's the problem with it? (Oh & it's strategy, not selection).

"novel environmental stressors, genetic & demographic stochasticity and Allee effects take them down one by one"

Using that logic how does island biogeography work? Every new addition to an island ecosystem is a tiny population, if the effects you list are what is important how does anything establish?

(NB: I don't argue against the rest of your quote, though I will point out that never in the history of life on Earth has a species shown quite as much adaptability as the human species has done, both in and of itself and also developing the ability to adapt the environment to suit it, history can tell us a great deal, but not everything.)