Chapter I
Tansley's Word
By the 1930s the dominant picture of ecology, from Clements, was of plant communities as superorganisms that grow and mature. The British botanist Arthur Tansley thought this was mysticism dressed up as science. In 1935 he proposed a different unit. The organisms of a place, he wrote, cannot be separated from their physical environment, the soil, water and air, with which they form one physical system. He called it the ecosystem, a word suggested to him by a younger Oxford colleague, Roy Clapham.
The ecosystem was a system in the physicist's sense. It could be analysed by measuring what flows into it and out of it, and it did not need a life cycle or a purpose. That made it possible to study ecology with the tools of chemistry and physics.
Chapter II
Following the Energy
The first to do so was Raymond Lindeman, a young ecologist who spent five years studying a small lake in Minnesota that was slowly filling in to become a bog. He grouped the lake's organisms into feeding levels: plants and algae, the animals that eat them, the predators that eat those. Then he estimated the energy captured at each level. Energy is lost at every step, mostly as heat from respiration, so each level receives only a fraction of the energy of the one below. His paper was at first rejected as too theoretical, and published in 1942 with help from Hutchinson. Lindeman had died of liver disease a few months earlier, aged 26.
Eugene Odum and his younger brother Howard Odum made energy flow the organising idea of ecology. Eugene's Fundamentals of Ecology (1953) was the field's main textbook for decades. Howard, a student of Hutchinson's, drew ecosystems as circuit diagrams of energy flow. In 1954 the brothers measured the metabolism of a whole coral reef on Enewetak Atoll in the Pacific, a site the United States was using for nuclear tests.
Chapter III
Watersheds and a Breathing Planet
Energy flows through an ecosystem once and is lost as heat. Chemical elements cycle. In 1963 Gene Likens and Herbert Bormann began measuring those cycles at the Hubbard Brook Experimental Forest in New Hampshire. Its small valleys sit on watertight bedrock, so everything that leaves them must flow out in the stream, where it can be sampled. Comparing rain with streamwater gave a complete nutrient budget for a forest. When one valley was cut and kept bare, its streams carried away nitrogen and calcium far faster than before. The living forest had been holding its nutrients. The weekly rain samples also revealed that the rain was strongly acidic, some of the first clear evidence of acid rain in North America.
The largest ecosystem is the planet. In 1958 Charles David Keeling began measuring carbon dioxide on Mauna Loa in Hawaii. His record climbed year after year, but it also rose and fell with the seasons, lowest at the end of the northern summer, when the forests of the northern hemisphere have drawn carbon out of the air, and highest in spring, after a winter of decay. The Keeling curve shows the whole planet's vegetation breathing, on top of the steady rise from burning fossil fuels.
Chapter IV
A Closer Look: Why Food Chains Are Short
Suppose the plants of a grassland capture 10,000 kilojoules of energy per square metre each year, after their own respiration. This is their net primary production. The sunlight falling on the same square metre, at an average of 150 watts per square metre, typical of middle latitudes, delivers about 4.7 million kilojoules a year, so the plants store roughly 0.2% of it. The rest is reflected, heats the ground or drives evaporation.
At each step up the food chain, the rule of thumb is that about a tenth of the energy passes on. Most of the rest is used by the animals to live, and leaves as heat. Some is never eaten, or passes through undigested, and goes to the decomposers. So:
| Feeding level | Energy per m² per year (kJ) |
|---|---|
| Plants | 10,000 |
| Herbivores | 1,000 |
| First carnivores | 100 |
| Second carnivores | 10 |
| Third carnivores | 1 |
By the fifth level only 0.01% of the plants' energy remains. This is the main reason food chains rarely have more than four or five links. There is simply not enough energy left to support another level.
It also explains why large predators are rare and need large territories. Consider a carnivore that needs 10,000 kJ a day, or 3.65 million kJ a year. If it hunts at the level that receives 10 kJ per square metre per year, it needs the production of
and one level higher, at 1 kJ per square metre, it needs 3.65 km², ten times as much. In practice predators catch only part of what their prey produce, so real territories are larger still. The losses at each step are not bad design. The second law of thermodynamics guarantees that every transfer of energy through a living body loses much of it as heat.
The ten per cent figure is a rough average. Measured efficiencies range from a few per cent to over twenty, and are higher for cold-blooded animals, which do not spend energy keeping warm.
Chapter V
Cascades and Tipping Points
Energy flows up the food chain, but control can flow down. In 1974 James Estes and John Palmisano compared Aleutian islands with and without sea otters. Where otters lived, they kept sea urchins in check, and kelp forests flourished. Where otters had been hunted out, urchins grazed the kelp to bare rock. Such trophic cascades have since been found in lakes, streams and grasslands. The most famous claimed example, the return of wolves to Yellowstone National Park in 1995, is also the most disputed.
Ecosystems can also change suddenly. A lake enriched with fertiliser can stay clear for years and then turn green within a season, and returning it to clear water can take far larger cuts in nutrients than caused the change. Whether such tipping points can be predicted is an open question. Protecting ecosystems from being pushed past them is part of the work of conservation biology.