by Roger Bourke White Jr., copyright March 2004
The mix of species on Earth is constantly changing: Some species are thriving, others are languishing. New species are appearing, and existing species are disappearing. Is there rhyme or reason to all this churning?
In this article I will talk about two ecological dynamics involving positive feedback loops and how one of these can apply to our species, Homo Sapiens. The two dynamics are the boom-bust dynamic and the eco flip-flop dynamic. Here are some quick summaries of each.
Species populations are also affected by the fortunes of other species in their environment. A simple example is that predators thrive when prey is abundant. But the relation can be more subtle as well. Two species can interact in a way where if one becomes dominant it suppresses the other, and vice versa. When this condition exists, we have an "eco flip-flop" -- named after the electronic flip-flop circuit that has the same characteristic: One state will remain dominant until an outside disturbance upsets the equilibrium, then the other state will become dominant and remain so until there is another disturbance.
A species thrives when the environment the species lives in becomes more favorable, and a species withers when the environment becomes less favorable. At the same time, through natural selection, the species is also adapting to the changes occurring in its environment.
This adaptation is taking place by changes in the species gene pool. The gene pool is the sum of all the genes in the species in a given breeding area. Each member has a slightly different mix of genes, so the gene pool is not the same as what is contained in each body. Changes to the gene pool over time are called genetic drift. Genetic drift is the changes in the species genotype (gene pool) caused by random mutations plus natural selection. Natural selection weeds out those mutations in the genetic drift that are unfavorable. Natural selection success is determined by all the things that affect the species’ environment… climate, food, predators, disease… everything. And these are constantly changing. So mutations that used to be successful may become unsuccessful because the environment changes. When change in the environment is changing what is successful in the gene pool, I call that “pushing the gene pool”. It’s pushing it to a new more successful mix. Breeding, artificial selection done by mankind, is another example of pushing the gene pool.
One of the first examples of genes adapting to environment was discovered by Charles Darwin as he studied the Galapagos finches. He found that the few colonizing finches that originally made it to the islands had radiated in their development into several subspecies, each adapted to a different part of the varied microclimates of the Galapagos Islands.
Subsequent researchers have discovered that the finches' adaptation is even more subtle than that: Each species adapts from one generation to the next with the varying cycles of plentiful rain and drought that make up the islands' climate. One change is in beak size. In times of rain, the beak size of a species will grow larger to accommodate the larger, softer nuts that are characteristic of lush vegetation times. In times of drought, beak size will shrink to accommodate the harder, smaller nuts of the drought vegetation cycle. It has been discovered in recent years that both beak sizes are held within the finch genotype -- no new mutations are required. It's just a question of which genotype is being expressed within a particular generation of birds. Mutations create other forms of genetic drift in the finches, but changing beak size requires no new mutations.
Normally, a species' adaptation to its environment is subject to diminishing returns: the species expands quickly, until it consumes all the "easy" resources, and then reaches equilibrium when it must continue growing only by consuming "tougher" resources. Usually a single resource becomes the limiting factor that caps further growth. The single resource could be lack of a critical food, lack of satisfactory shelter, or the onset of a deadly disease that is only sustained by a high population density.
As a species thrives, it changes its environment -- more of the local biomaterial becomes part of the species as it eats, and then part of its excreta, and the nests or homes it builds become a larger part of the physical landscape. Normally these changes contribute to the diminishing returns of thriving in the environment, but sometimes they do not. Sometimes the changes a species makes improves the environment for the species. When this happens, there is positive feedback, and the species undergoes a massive boom in population for a while. For example, coral cannot grow on sand or mud, but coral likes to grow on coral. So starting a coral reef is difficult, but once it gets established, it will thrive and spread with new coral growing on the foundations laid by previous generations. Another example is that pine trees prefer an acid soil, and pine needles dropped by the pine trees tend to acidify soil. When a stand of pine trees gets established, the bed of needles acidifying the soil will discourage deciduous competitors from establishing nearby.
If a species makes its environment only modestly more favorable, and if the effect is short-lived, it will have little effect on the evolution of the species. But if the species can dramatically improve its own environment, and the effect is long-lived, it will effect how the species evolves. Natural selection will adapt the species to thrive even more in the feedback-affected environment, which can make the feedback loop even stronger. Using the pine trees as an example, if many generations of pines grow in the acid soil, then the descendants will become even more acid-tolerant than their predecessors.
The long-term result is the species becomes better and better adapted to living when the species is thriving, and less and less well adapted to living when the species population is scarce and not changing the environment. If this adaptation becomes well established in the species, the species is vulnerable to a calamity which changes the environment in a way that breaks the species’ dominance. Again, using the pine trees as an example, if the pines become so accustomed to acid-rich soil that they lose their ability to grow well in non-acid soil, then they will have a hard time reestablishing after a fire, if the fire burns down the pines and the soil returns to neutral PH.
This evolving to fit a thriving environment and then having a calamity occur may have been what killed off the Passenger Pigeon in the eastern US about the time Europeans were settling the region, and it may explain the sardine decline off the California coast in the 1930's. Before its demise, the Passenger Pigeon was the dominant migrating bird of the North American east coast. When it migrated, its flocks would darken the skies. Then, all of a sudden, it was gone, as in ... extinct. No one knows why this happened, but one cause may have been that the Passenger Pigeon had somehow massively improved its own ecological niche for a long time, adapted to the improvement, then hit a calamity. When the environment reverted back to "normal" (not affected by Passenger Pigeons), the Passenger Pigeon calamity survivors found themselves in an environment that they were poorly adapted to, and the survivors perished as well. This is a boom-then-bust-to-extinction cycle, and it is probably a fairly common one in the history of our planet.
(For more details on the nature of the Passenger Pigeons’ extinction and how their inability to adapt to a new environment contributed to it, see http://en.wikipedia.org/wiki/Passenger_Pigeon#Attempts_at_preservation or the Smithsonian Encyclopedia at http://www.Si.edu/encyclopedia_Si/nmnh/passpig.htm)
Sometimes a species seems to boom dramatically as if they are experiencing a positive feedback loop of the sort mentioned above, and then these species appear to experience a dramatic decline. But unlike the Passenger Pigeon, they don't become extinct. Instead, at some future point, they repeat their dramatic expansion. They will dramatically thrive and wither, repeatedly, but without much rhyme or reason; the thriving is not cyclic. If the thriving and withering is dramatic and repeated, the species may be half of an "eco flip-flop" system.
In an eco flip-flop system, two species are competing for setting up a thriving environment. If species A is dominant, it is suppressing species B and forcing it to thrive only in marginal niches around the main environment, niches that A can't thrive in. Species A may, for instance, eat Species B babies as "dessert" (the "main course" is some other food), but not Species B adults. If Species A suffers a calamity and declines in population, many Species B babies thrive, and B becomes the dominant species. Species B may eat Species A as children and adults, and Species A is driven into the niches where Species B can't thrive, and it waits its turn. Species A will revive and become dominant when Species B suffers a calamity, and it can no longer protect its babies from being eaten by Species A.
Baby eating is just one way a flip-flop could be sustained. A disease that is deadly to B but benign in A could be another flip-flop mechanism, and there are dozens of other possibilities.
What distinguishes a flip-flop are the following characteristics:
I don't have a clear cut example of an eco flip-flop, just a hint of one. In the 1930's the population of sardines off the Monterey Coast of California declined dramatically and put a big crimp in the fishing industry there. (Environmentally, the Monterey Coast resembles the famous fishing grounds off the Peruvian coast, and those are dominated by anchovies, a similar fish.) People at the time blamed the decline on overfishing. But subsequent research was done in the area, and core sampling off the California coast shows that sardine population has come and gone in the area many times in the past. It may be that the sardines are part of an eco flip-flop with some other as yet unidentified species. If so, the overfishing may have been a calamity that set a flip flop in motion, and "the other species", whatever it is, is now dominant. (Overfishing may have been responsible, but other natural causes, such as disease, are just as likely to have been the culprit.) If the sardines are part of a flip-flop system, at some point in the future, that other species will suffer a calamity, and the sardines will come back.
The sardines coming and going may be an example of an eco flip-flop. Eco flip-flops will not be numerous in the environment compared to the total number of species, but the system is simple enough that there should be plenty operating, and they should be easy to spot, should scientists choose to look for them.
Homo sapiens, mankind, is a boom species: Humans modify their environment to make it more suitable for humans. In fact, thanks to its fantastic language and tool using abilities, mankind is probably the most intense environment modifier on the planet. Its only competition for the title of most intense would be from some forms of primitive algae or bacteria, that environment modify a lot because they have huge biomasses; no other existing multi-cellular species comes close.
This means that mankind is in a positive feedback loop with the environment: Mankind is modifying the environment, and mankind's genes are drifting to optimize for the modified environment.
An example of where there is tight tension in the genetic makeup of mankind is childbirth. In prehistoric and historic times, childbirth has been a hazardous process for both mother and child. Childbirth has historically been a leading cause of death for women and unborn children. This risk is caused by carrying the baby for such a long term (nine months), which allows the baby to get very large inside the mother. Yet, the natural selection process of evolution has decided that this huge risk to mother and child is lower than the risk of carrying the baby for a shorter term and delivering a smaller, less mature baby to the outside world. Apparently the risks to mankind's survival of delivering shorter term babies are even more huge.
But now that civilized mankind has modern medicine, much of the risk of long term babies has been eliminated. Now in the US, about 20% of all babies are delivered Caesarian (C-section). How will this affect the gene pool? It means that the formerly tight constraint that limited how long a baby could be in a mother and how big it could get -- the constraint of death on delivery -- is gone. This means that the gene pool will now push for even longer term babies, even bigger babies, and be successful.
When the gene pool pushes successfully for longer term, bigger babies, it means that mankind is becoming adapted to having modern medicine always available. Suppose that in ten generations (about 200 years), the term for the average baby lengthens to nine months and a week, and all babies are routinely delivered by C-section. What happens if mankind has a catastrophe, and modern medicine is no longer available, and because of that, C-sections have to be done in primitive, unsterile conditions? Those women that carry a baby to full term will face the risk of a C-section in unsterile conditions. Only that small fraction of women who give birth prematurely -- nine months or earlier -- will be able to do vaginal deliveries, and their babies will be premature by the standards of that time. Either way -- vaginal or C-section -- mankind's reproductive success is going to plunge until mankind can either restore modern medical conditions or until the gene pool can readapt to a nine month standard term. Either way, it will be a spooky time to be a Homo Sapiens.
This is just one example. There are thousands of pressures and tradeoffs being applied to the human gene pool. As we become more and more civilized, the environmental pressures on the gene pool are changing, which means that the direction the gene pool is drifting in is changing. Like it or not, mankind is building a gene pool that is well suited to Modern Man's environment, not Stone Age Man's environment.
Should mankind find itself suddenly forced to live and prosper in a Stone Age environment, the species will be in for some tough sledding, and it’s very likely that mankind will go suddenly extinct, much as the Passenger Pigeon did.
What can we do about this? Read on in the “Save Mankind with Neolithic Park” section.
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