ECOSYSTEMS
Food webs · Succession · Biomes · Trophic Cascades — A field manual for reading the living machinery of a place.
What is an Ecosystem?
The word was coined by British botanist Arthur Tansley in 1935. An ecosystem is a community of organisms together with their physical environment, treated as a functional unit. The term replaced earlier "superorganism" thinking with something more flexible, more measurable: flows of energy and matter through linked compartments.
Eugene and Howard Odum, brothers at the University of Georgia, made ecosystem ecology a quantitative science in the 1950s — measuring radioactive tracers moving through Florida streams, calculating energy budgets of coral reefs.
"In nature, nothing exists alone." — Rachel Carson, Silent Spring (1962)
The Trophic Pyramid
Energy flows up. Mass thins out.
Each step up the food chain loses ~90% of energy as heat (the 10% rule, Lindeman 1942). A thousand kg of grass supports a hundred kg of grasshoppers, supports ten kg of mice, supports one kg of weasel.
This is why apex predators are rare. It is also why eating plants directly is more efficient than eating animals that ate plants — a single kg of beef requires ~25 kg of feed.
Food Web — North Pacific Kelp Forest
Lines = predation; arrowless because energy flows both ways at decomposition.
The Keystone Concept
In 1966, Robert Paine pried purple sea stars (Pisaster ochraceus) off the rocky tide pools of Mukkaw Bay, Washington, and watched the system collapse. Mussels colonized everything. Diversity dropped from 15 species to 8. Paine coined the term keystone species: a species whose effect on its ecosystem is disproportionate to its abundance. Wolves, sea otters, beavers, elephants, fig trees — all keystones in their domains.
Trophic Cascades
When a top predator returns, prey behavior changes — and habitat changes downstream of behavior. Wolves reintroduced to Yellowstone in 1995 didn't just kill elk; they made elk afraid of valleys. Willows along streambeds rebounded; beavers built dams; songbirds returned; even the channels of the Lamar River narrowed and stabilized. "The wolves are not running the river. But they are running the elk that are running the river." — William Ripple.
The Major Biomes
Permafrost, lichen, caribou, snowy owl.
World's largest biome. Moose, lynx, wolverine.
Oak, beech, white-tail deer, black bear.
Bison, pronghorn; Serengeti; Pampas.
Sahara, Atacama, Sonoran. Saguaro, fennec fox.
California, Mediterranean, Cape S. Africa.
Amazon, Congo, Borneo. 50% of species.
1% of ocean, 25% of marine species.
Succession
An ecosystem is not static. After disturbance — fire, eruption, glacier retreat — bare ground colonizes through predictable stages. Primary succession: starting from rock (Krakatoa 1883, Mount St. Helens 1980). Lichens → mosses → grasses → shrubs → pioneer trees → climax forest. Secondary succession: after a disturbance that left soil. Faster — abandoned New England farms returned to oak-hickory forest within 80 years.
Cycles — Carbon, Nitrogen, Water
- Carbon — atmosphere ↔ photosynthesis ↔ respiration ↔ ocean ↔ rocks. Residence time 5 yr atmospheric pool, 100M yr lithosphere.
- Nitrogen — N₂ → NH₃ via diazotroph bacteria (lightning <5%) → nitrification → assimilation → denitrification. Haber-Bosch (1913) doubled fixed N globally.
- Water — evaporation ↔ precipitation. 95,000 km³/yr cycles through atmosphere; mean residence time 8 days.
- Phosphorus — purely mineral. No gaseous phase. Ocean to rock to weathering to soil to plant to consumer to ocean. Often the limiting nutrient on land.
Tipping Points
Some systems flip rather than slide. The Sahara was green in the early Holocene; ~5,500 years ago, gradual orbital change crossed a threshold and the monsoon collapsed within centuries. The Caribbean coral reefs of the 1980s shifted from coral- to algae-dominated after disease wiped out long-spined urchins. Once flipped, hysteresis: returning to the previous state requires more than removing the original stressor.
Marten Scheffer (Wageningen) and colleagues mapped this universally — eutrophic lakes, savannas-to-desert, ice-free Arctic. The early-warning signal: variance increases as a system approaches its tipping threshold.
Indigenous Knowledge
Long before "ecosystem" was a word, indigenous land management shaped ecosystems for tens of thousands of years. Aboriginal "fire-stick farming" maintains Australian savanna mosaics. The Amazonian "terra preta" soils — anthrosols enriched with charcoal — are a deliberate Pre-Columbian technology, fertile a thousand years on. The Menominee of Wisconsin have practiced sustainable forestry on the same 235,000 acres since 1854; their stocking density is now higher than when the U.S. acquired the land.
"All things are connected like the blood that unites us. We did not weave the web of life, we are merely a strand in it." — Chief Seattle (attr.)
Watch
How Wolves Change Rivers
The four-minute viral classic narrated by George Monbiot, animated over Yellowstone footage. The clearest popular explanation of trophic cascades ever made; 45M+ views.
Coda
Aldo Leopold, A Sand County Almanac (1949): "A thing is right when it tends to preserve the integrity, stability, and beauty of the biotic community. It is wrong when it tends otherwise." The land ethic is the ecosystem concept made moral. To love a place enough to know its food web is to be embedded in it. We are not visitors to ecosystems; we are organisms within them, and our acts ripple to the apex predators we never see.