Life in the balance: our failure to recognize our connection with the global ecosystem lies behind the biodiversity crisis facing our planet today

Natural History, June, 1998 by Niles Eldredge

Our failure to recognize our connection with the global ecosystem lies behind the biodiversity crisis facing our planet today.

Life is abundant, colorful, and richly diverse. Biodiversity is that spectrum of life from the smallest bacteria to the giant redwoods, from algae to wild dogs. It is all the world's millions of living species and all the places in which they live--the vast array of ecological systems ranging from the polar ice caps, with their relatively few species, to the forests and grasslands of the Tropics, which teem with species. The interconnections of all these species and ecosystems form a global ecosystem now roughly 4 billion years old.

Energy flows through the individual systems as animals consume plants that have trapped solar energy through photosynthesis; these animals are eaten by other animals, and all are destined to die eventually and be recycled by microbes and fungi. Energy also flows between ecosystems. What happens in the Amazonian rainforest ultimately affects not only the conspicuous mammals and birds of that forest but also its river fauna. Runoff from the rivers affects the ocean and all its life. Our failure to recognize our part in this complete system lies behind the biodiversity crisis facing the planet today.

Take the air we breathe, for example. The atmosphere close to Earth's surface is mostly inert nitrogen (79 percent), which in itself is a good thing--an atmosphere richer in oxygen than it already is (20.9 percent) would literally fan the flames of out-of-control wildfires. But it is oxygen that most of us mean when we talk about "air." Oxygen is absolutely essential to all living things except for a very few forms of microbial life that utilize other chemical pathways to break down the nutrients on which they live. All the rest--most microbes, plants, fungi, and animals--require a constant supply of oxygen just to exist. Although no one seriously thinks that our supply of oxygen is in imminent danger of collapse, it is important to realize just where the daily replenishment of this most precious resource comes from: most of the world's fresh supplies of oxygen are produced by single-celled, microscopic plantlike organisms floating near the surface of the oceans--supplemented, of course, by the photosynthesizing activities of terrestrial plants. Consider also that green plants filter out noxious gases, utilizing carbon dioxide in the very act of photosynthesis, and absorb other noxious effluents, including particulate matter from dirty air.

Of course, plants do more for us than enrich and cleanse the atmosphere. The Amazonian rainforest, for instance, controls the water cycle over a vast region because its trees transpire a tremendous amount of water every day. Clearing forests leads to the massive erosion of soil no longer held in place by tree roots. Earth is losing 25 billion tons of topsoil each year through erosion--a direct reflection of our conversion of land with natural vegetation into farmland and a portent of the difficulties that he ahead for us because of our continued reliance on agriculture. Consider for a moment the net effect of increased erosion from denuded lands on the global system. Coral reefs typically fringe the shorelines in tropical oceans, and coral reefs worldwide are beginning to show distress. Reefs, for all their massive structure, are actually made up of delicate colonial animals that are extremely sensitive to the silt content in water. They quickly die as the clear tropical waters become clouded with particles of clay and quartz, as has happened recently in Belize. Coral reefs also provide a protective barrier to delicate ecosystems, such as the mangrove wetlands fringing the southern tip of Florida, breeding grounds for countless species of fish and "shellfish." Without such habitats, our fisheries--already on the verge of collapse in many places, over-taxed as they are by often ruinous fishing practices--would soon be in even worse shape.

Oxygen is only one of the elements provided to animals courtesy of other living things. Nitrogen, an essential component of all proteins, is another. Despite its plentiful supply, virtually no organisms--not even plants--can extract nitrogen from the atmosphere and incorporate it directly into their bodies. Only a few forms of bacteria (aptly called nitrogen-fixing bacteria) can accomplish the task. Some plants, such as the legumes (peas and their relatives), harbor nitrogen-fixing bacteria amid their roots. All the nitrogen in our bodies comes from eating plants, or eating animals that have eaten plants. The cycle is complete when organisms that decompose dead plants and animals produce ammonia, which is then converted by other bacteria into nitrates that plants can pick up again directly from the soil and convert into free atmospheric nitrogen.

And what about the carbon cycle? Carbon is crucial to the construction of our very bodies, but it must return to the system so more bodies can be built. In the arid Tropics, the carbon cycle depends entirely on termites. No termites, no plant decay--and without these agents of decomposition, life would soon simply cease as vast mounds of dead plants accumulated and sat there. Termites cannot break down the strong carbon bonds of cellulose, the prime constituent of plants, any better than we can. They need help, just as plants need bacteria to extract nitrogen from the atmosphere. Bacteria (and some protozoa) fill the bill. Spirochetes (related to those that cause syphilis and Lyme disease) housed in the guts of termites actually break down the cellulose into organic compounds that the termites can use. Without those lowly microbes--and their plant-eating termite hosts--there would be no carbon cycle, and we would all soon be in trouble.