In the third chapter of On the Origin of Species, Darwin explored evolution’s ecological context from a range of angles that cover many of the issues that have come to be addressed by the science now called ecology.
First, recall from the first post in this series that, in the third chapter, Darwin introduced the concept of Natural Selection (the topic of the fourth chapter): Given the struggle for existence, “any variation, however slight and from whatever cause proceeding, if it is in any degree profitable to an individual of any species in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring” (Darwin 1859, p.61).
As Darwin explored these “infinitely complex relations to other organic beings and to external nature” in chapter 3, here are some of the ecological ideas he addressed (with page numbers referring to the original first edition).
Struggle for existence leads to Character-displacement among close relatives.
Climatic gradients, mirrored in altitudinal gradients, p.69 (not all original ideas to Darwin, but he was attuned to all such considerations)
Disturbance equilibrium, i.e., more species coexist if the situation is disturbed than if it isn’t, p. 67 “If turf which has long been mown …be let to grow, the more vigorous plants gradually kill the less vigorous.”
Self-thinning, p. 67-8
Admittedly, Darwin was not always correct, e.g., p. 76: “the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera.” However, it now seems that exotic taxa less related to native species are more invasive (Strauss, S. et al. 2006 PNAS 103(15): 5841-5845).
The angle on ecology that is most significant in relation to the challenge of integrating ecological dynamics into evolutionary theory is indirect effects, i.e., interactions between species that result from chains of interactions, not direct interactions between the two species, p. 71
Many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. I will give only a single instance, which, though a simple one, has interested me. In Staffordshire, on the estate of a relation where I had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man; but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with Scotch fir. The change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another: not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath. The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath; and the heath was frequented by two or three distinct insectivorous birds. Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception that the land had been enclosed, so that cattle could not enter. But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hill-tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live. When I ascertained that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted clumps. But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle. In one square yard, at a point some hundreds yards distant from one of the old clumps, I counted thirty-two little trees; and one of them, judging from the rings of growth, had during twenty-six years tried to raise its head above the stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food.
The next installment in the series—the last—says something more abut chapter 3 that ties together the strands of these posts. But, let me end with a shout-out to a blog on research addressing the challenge of integrating ecological dynamics into evolutionary theory: http://evol-eco.blogspot.com, “The EEB and flow: All things ecology and evolutionary biology”:
there is conspicuously little blogging of recent advances in ecology and evolutionary ecology. Thus an ecology blog was born. Contributors include a diverse group of junior researchers who post about recent exciting papers and ideas…