To explain the incredible diversity of life is one thing; to understand why diversity is important for life is another. What difference would it make if the world only contained a few thousand species rather than untold millions? In the Origin of Species Darwin observed:
If a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and dry herbage can be raised in the latter than the former case... The greatest amount of life can be supported by the greatest diversification of life.
Generations of research since the publication of this classic have supported Darwin's assertion. During the 1950s, Elton reviewed a number of ecological studies of many different habitat types. He concluded that the greater the diversity of organisms within an ecosystem, the more stable and productive the ecosystem is. His pronouncement echoed Darwin's. More recent experimental work points to the same conclusion.
This adds something to the idea of natural selection (competition within species), which was summarised in the previous section. It adds the idea of co-operation among species. This use of the word "co-operation" does not imply a conscious act or altruism. It means that the presence of some species helps other kinds of species to become established and flourish. As we remarked at the start of this chapter, organisms need other species because they eat one another, inhabit one another, transport one another, depend on each other's waste products and so on. The continuing survival of any species depends on the continuing survival of other species. It is a simple step to the conclusion that ecosystem diversity creates stability.
A cell is made of huge numbers of molecular components linked in a network of mutual dependence. We explored this in chapters 2-10. The network of mutual dependence confers stable order on the cell. Similarly, an ecosystem is stable and ordered because the species it comprises are linked by a network of interdependence. The more components there are, and - up to a point - the more interdependence there is among them, the more ordered and stable the network. This is a principle of complexity theory. According to one authority on complexity theory, Stuart Kauffman, both cells and ecosystems are ordered networks - but not too ordered. If a large network is too tightly interconnected, its behaviour becomes fixed and "frozen". On the other hand, if its interconnectedness is insufficient, it behaves chaotically. Kauffman argues that evolution drives biological networks towards "the edge of chaos", a state that is ordered but borders on the chaotic. In this state, behaviour is flexible and adaptable, stable and ordered but not rigid. His argument is persuasive.
So far as the interconnectedness of ecosystems is concerned, the intimacy of some relationships among species is surprising. Some flowering plants are pollinated by particular species of flying beetles. Without the beetles the plants would die out; they could not reproduce. Many of these plants, such as the tropical lily Philodendron selloum, maintain their flowers at 35oC, an ideal temperature for keeping the flight muscles of the resident beetles working. If it were not for the warm flowers, some types of beetles (which are very small animals) would have to eat their own weight of food every day in order to generate enough heat to fly - an impossible demand. So the beetles are as dependent on the flowers they pollinate as the flowers are on the beetles. If the flowers die out, so do the beetles - and vice-versa. Many comparable examples of intimate plant-insect relationships have been described, most of them species-specific.
Many species interrelationships are even more intimate. Organisms of different types are often locked in an absolute mutual dependence known as symbiosis. Lichens are familiar examples. A lichen is a symbiont comprising a fungus and a green alga; neither species can survive independently of the other. Lichens are very widespread; it is estimated that the world contains about 1014 tons of them. Herbivorous mammals such as cows and rabbits, and wood-devouring insects such as termites, eat vast quantities of cellulose, which they cannot digest; but bacteria in their guts digest the cellulose and thus provide themselves and their hosts with nutrient. Without these bacteria, most herbivores would starve; and if herbivores starved, so would the rest of us. Without termites, ant-eaters would go hungry, though the wooden structures we build in the tropics might be more durable. Without the herbivores (or termites) to provide comfortable guts to inhabit, the cellulose-digesting bacteria would not survive either. A cow is a symbiont. A termite is a symbiont. The average tree is estimated to contain some 300 fungal symbionts. The first plant is believed to have resulted from symbiosis between a fungus and an alga some 430 million years ago.
The more closely we examine the huge variety of plants, animals and fungi in the world, the more examples of symbiosis we find. Symbiosis seems to be the rule in life, not the exception. Therefore, natural selection within species takes place in a world of mutual dependence - usually intimate, often symbiotic - among species. Life progresses through competition and extinction and survives through co-operation and diversity.
(Some organisms, parasites, come to depend on others without providing anything in return except discomfort and possibly ill-health. Cooperation might be the rule in all ecosystems, but some organisms exploit rather than co-operate.)
Summarising the theory of evolution in the previous section, we described the crucial role of "environmental factors". The environment selects among variants of a species, enabling some to survive and reproduce more successfully than others. What is the environment? Clearly, it is not just the physical environment; it must include the rest of the ecosystem. And what exactly is selected? The individual? But in many cases the "individual" is a symbiont: two or more species, not one. In any case, the survival of a variant of one species has implications for the rest of the ecosystem. If one element of the network changes, the others must also change; the network adapts. The network of mutual dependence survives, but it is not the same network from generation to generation. Moreover, groups that co-operate are favoured by selection. "Cheats" that welsh on their part in co-operation and take advantage of the rest do not predominate. So although many parasites are successful organisms and survive, "cheating genes" do not generally seem to be favoured27.
Should the population be regarded - or the ecosystem as a whole - as a unit of selection? Populations change (diverge) by natural selection among individuals; species change because their populations change; ecosystems change because species co-evolve. On the other hand, some writers have argued that genes are the units of selection; the environment of a gene is other genes. We would not go along with this last point of view, but it has a respectable following. The fundamental concepts here, "selection" and "environment", are difficult to pin down. More than one meaning can be ascribed to each.
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