YOU'VE GOTTA PAY THE RENT

by Jack Ewing

My dictionary defines "ecology" as: "The system of relationships between organisms and their environments." It defines an "ecosystem" as: "A community of organisms and their non living environment." An ecosystem can be small like a pond or large like an ocean. One ecosystem can be part of a larger one: For example a small ecosystem like a rotting tree trunk may be part of a larger ecosystem like a rainforest. Regardless of size, all ecosystems are complex. If you learn about one aspect of one ecosystem, you don't get any closer to knowing all about it; instead your increased knowledge simply opens more doors each with more questions (and answers) behind it.

You may wonder why anyone would care about an ecosystem. One reason is that ecosystems often provide services for human beings. Another is that by looking closely at the relationships between organisms within an ecosystem we can learn interesting things about human relationships with our environment. And a third is that ecosystems are just plain fascinating.

Let's try something. Close your eyes and imagine the relationships between the organisms that live in a rainforest pond. Begin with algae that grows on rocks; snails feed on the algae; crayfish lurk in crevices; fish hide underneath sunken logs and between rocks; tiny shelled creatures live in the muddy bottom; long beaked birds probe the bottom to pluck them out; caimans stalk water birds; water lilies float on the surface; frogs sit on water lilies and call to mates; mosquitos lay their eggs in shallow pools; tadpoles feed on mosquito larvae, water bugs run across the surface; turtles sun bathe on dead logs; raccoons pull crayfish out of crevices; water plants put oxygen into the water; fish snatch insects off the surface; bats catch small surface feeding fish, and it goes on and on. Just let your imagination run wild for a minute or two thinking about every living and non living thing in a pond and the infinity of relationships between them.

Much of the activity in an ecosystem is a struggle between organisms for survival. If one organism appears with some new physical characteristic that gives it an advantage and allows it to reproduce more successfully than others, the species as a whole will tend to acquire that characteristic. Stated differently, the individual with the advantageous characteristic will prosper and reproduce as will its offspring and the gene that produces the trait will increase its presence within the genetic make-up of that species. The entire inventory of genes available to a given species is sometimes called its "gene pool." Let's see how this works.

A relatively small ecosystem called a rainforest gap appears when a large tree falls, knocks over smaller trees and opens a clearing. Gaps are populated by many different plants, but the fast growing cecropia tree is usually one of the first to appear. It can grow to a height of 15 meters (50 feet) in as little as two or three years and quickly appropriates a prominent place in the gap. But nature is full of tradeoffs, and even though its extremely fast growth affords it a big advantage, the cecropia got short changed in a couple of other traits, namely strength and life span. Its watery, hollow trunk, lacks fiber and is relatively weak, leaving it vulnerable to vine growth which can cover a young tree, pull it down and finish its life early. Even when there are no vines to hamper its progress the cecropia rarely lives past 15 years. Ecologically speaking the individual tree's life will be successful only if it is able to grow to maturity, flower, seed and reproduce, thus passing on its own genes. The cecropia has developed several relationships with other species that help it accomplish these needs.

I once observed a virtual parade of birds and animals traipsing through a cecropia heavily laden with flowers and seeds. Two small flocks of two different species of toucans flew into the tree one after the other. They all hopped around on the branches snapping off flowers, tossing them into the air, catching and eating them with large boat-shaped bills. Shortly a troop of white-faced capuchin monkeys moved into the tree and the toucans left. The monkeys were less careful, grabbing a flower stem, taking a quick bite, then throwing the rest away. They never ate all of a seed bundle before discarding it and going on to another. The monkeys were followed by a lone coati that sniffed every flower before biting, then carefully ate only what it wanted, wasting nothing. Finally a large yellow-breasted bird called a "great kiskadee" landed on a branch and ate a few of the seed laden flowers. "What a generous tree," I thought. "It gives sustenance to all of those birds and mammals and probably a lot more that I didn't see today."

Later I was to learn that pure generosity is not part of nature. Everything has its price. It is true that the cecropia fed the species described above, but all of them paid for the meal by scattering cecropia seeds all over the countryside. Any place a cecropia feeder defecates, a seed has the opportunity to sprout and grow. I have found these interesting trees growing on thatched roofs, stumps of other trees, lawns and in gutters. To facilitate its mission of growing and reproducing the cecropia matures in a few short years and produces tasty flowers and seeds which attract a variety of creatures that spread them far and wide. But all the seed dispersers in the world won't be much use if vines pull down the weak trunk and end the cecropia's life before it flowers. Something else is needed to maximize success.

Remember that I said the cecropia trunk is watery and hollow? Well there are some nasty little creatures called Aztec ants that thrive in watery hollow tree trunks, especially ones like the cecropia whose sap is quite nutritious. For an ant colony the hollow trunk makes an ideal home near a dependable food supply. The sap is sometimes taken by the ants directly from nodules at the base of the leaf stems, but often it is taken indirectly. The Aztecs have learned to domesticate other insects called "mealy bugs," which suck the cecropia sap and convert it into a high energy food. Then the ants "milk" the mealy bugs and eat the calorie rich substance they secrete, just like we humans do with our dairy cows.

There is an interesting side note about the Aztec ants which are known for their extremely aggressive behavior. In nature all organisms must live in such a way as to conserve energy. If the energy required for a given activity doesn't bring substantial benefit to the individual, or in this case the colony, the activity is usually not part of that specie's behavior. Aggressiveness requires large energy outputs, yet the Aztec ants are so aggressive that they war against any other colony of ants occupying the same tree and fight until one of the two is totally annihilated and a single colony remains in possession of the entire tree. The interesting thing is that the victorious colony doesn't even bother to rob the brood of the conquered. Every individual is killed. It has been suggested that the Aztec's practice of domesticating mealy bugs, which provide them with a surplus of calories, allows them the luxury of being able to commit genocide against others of their own species.

As interesting as Aztec aggression may be, it doesn't help us to understand the cecropia and its vine problem? Like I said earlier, in nature everything has its price. The cecropia may provide the Aztec ants and their mealy bugs with room and board, but not without charging rent, which is paid by cutting vines. Any vine that tries to grow on a cecropia tree gets cut by the ants before it even gets started. You can check this out for yourself. Go look for cecropia trees and see if you can find one with a vine. I can assure you that you won't find any. While you're at it whack the trunk a couple of times, and then lean against it for a while. You may have the opportunity to experience first hand the nastiness of the Aztecs.

Within the small ecosystem of a rainforest gap, an area of less than half a hectare (about an acre) we see the intricacy of the relationships between a few organisms on a single tree. But, as I said earlier, in nature, if you dig a little deeper, there is always more to be learned. Let's say, for example, that a colony of Aztec ants has somehow acquired a gene that deters it from cutting vines. Rainforest gaps are replete with vine growth. Without the protection of the ants a young cecropia tree would soon be covered and toppled. In this hypothetical case the tree would not flower and reproduce, the ants would lose their home and sustenance, and the colony would quickly perish. The odd gene that prevented the well established behavior of vine cutting would not be passed on, but instead would promptly be eliminated from the gene pool.

This is the consequence of not paying the rent. By the other token, if the cecropia doesn't live up to its end of the bargain and doesn't provide the ants with a hollow trunk for a home or nutritious sap for food, or for some other reason creates an unfavorable environment for Aztec ants, it will not be colonized by the ants, will be pulled down by vines prior to flowering and reproducing, and the gene that created the predicament will be eliminated from the cecropia gene pool. The incidence of any gene that does not enhance the species' chances of survival will be reduced and genes that do allow the species to flourish will become more abundant in the gene pool. Human beings have evolved an extremely high level of intelligence that has allowed our species to dominate the planet. Our intelligence has given us technology capable of overriding some of the ecological limitations that keep other organisms in check. For example, I have had diabetes since I was 29 years old. Had I been born 100 years earlier, I probably would not have lived to 30 years. Prior to the 1930s and the development of the technique of injecting insulin, many diabetics died before reproducing and the incidence of the diabetes gene in the gene pool was kept fairly low. It was not entirely eliminated because many diabetics, like myself were able to reproduce before the onset of their disease. Even so, the incidence of the gene and the prevalence of diabetes was much lower then than it is today.

Prior to the advent of surgery, a gene for a pelvis too narrow to permit natural birth would result in the mother's and child's death during childbirth. Neither the mother nor her offspring would survive to pass the gene into future generations. After decades of modern surgery and the ease of cesarean sections, the gene for a narrow pelvis is much more abundant in the human gene pool than before. This is not a problem as long as modern medical techniques are available. If however, some major planetary disaster were to take place and the level of available medical services to diminish drastically, the genes for diabetes and narrow pelvis would soon be severely reduced in the gene pool.

The genetic trait that gave humanity a superior intellect has enabled us to invent agriculture, the domestication of animals, and medical technology, all of which have allowed our population to increase to the point that the impact on our own environment is alarming. Our activities are gobbling up arable land, depleting available fresh water supplies, contaminating our air, water and food, and even changing our climate. Our population is now at 6.4 billion and still climbing. When I was born in 1943 world population was around 2.6 billion, about 40% of today's. Human impact on the planet at that time however, was probably less than one-tenth that of today. Our technology has increased many fold in the last half century and each advance in technology brings a greater environmental impact. Increased knowledge has not given us the ability to live sustainably, but rather the ability to deplete our planet's riches more effectively. It is ironic that the intelligence that allows us to dominate the Earth has also allowed us to create a global reality dangerous to the future of our own species. In other words the genes that gave us our intelligence may have given us too much of a good thing.

Humanity's big challenge in the next few decades is to use our intellect to turn the present situation around by reducing our impact on the planet and living in harmony with other species. We must learn to think of Earth's natural resources as an endowment, learn to live off of the interest and stop depleting the principle. This is the ecological rent that our species must pay for our continued existence on this planet. Should we fail, the presence of the genes that produced our superior intellect will diminish in the gene pool, and we may very well end up with the dinosaurs.

For anyone who is interested in further reading, I highly recommend Collapse - How Societies Choose to Fail or Succeed, by Jared Diamond.

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