Ecosystem

• 1. Introduction

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  In 1935, a British ecologist named Arthur Tansley reached for a word that did not yet exist. He wanted to describe something larger than a plant, larger than an animal, larger even than a forest or a lake. He wanted a word for the whole interlocking system of living things and the physical world they sit inside. The word he settled on was ecosystem, and it had been coined for him, at his request, by a colleague named Arthur Roy Clapham. An ecosystem is a system formed by organisms interacting with their environment, its living and nonliving parts bound together through nutrient cycles and energy flows. That definition sounds tidy. The reality is anything but. How does energy actually move from sunlight into a leaf, then into an animal, then into the soil and back again? Why does a forest in a small depression behave so differently from one on a nearby steep hillside? What lets a battered landscape absorb a fire or a flood and remain recognizably itself? And what happens when it cannot, when a place loses the very features that defined it and is judged to have collapsed?

• 2. A Word For The Whole System

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  Arthur Tansley devised the concept of the ecosystem to draw attention to a single idea: the transfer of materials between organisms and their environment. He later refined his own definition, describing the ecosystem as the whole system, including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment. For Tansley, ecosystems were not simply natural units sitting out in the world. He called them mental isolates, slices of reality that a scientist chooses to study. He even offered a term, ecotope, for the spatial extent of an ecosystem.

  G. Evelyn Hutchinson, a limnologist and a contemporary of Tansley, pushed the idea toward measurement. He combined Charles Elton's work on trophic ecology with the thinking of the Russian geochemist Vladimir Vernadsky. From that fusion he argued that the availability of mineral nutrients in a lake limited the production of algae, which in turn limited the animals that fed on the algae. Raymond Lindeman carried the reasoning further, proposing that the flow of energy through a lake was the primary driver of the entire system.

  The handoff between generations made the field. Hutchinson's students, the brothers Howard T. Odum and Eugene P. Odum, built a systems approach to studying ecosystems. That framework let them trace the flow of energy and material through ecological systems, turning Tansley's mental isolate into something a scientist could quantify pool by pool.

• 3. What Holds The Reins

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  Climate is the factor that most strongly determines ecosystem processes and structure on broad geographic scales. It decides which biome an ecosystem sits within, and its rainfall patterns and seasonal temperatures govern photosynthesis, setting the ceiling on how much energy the system has to work with. These are external factors, also called state factors. They shape the ecosystem without being shaped by it in return.

  Parent material works closer to the ground. It determines the nature of the soil and the supply of mineral nutrients available to the things that grow there. Topography adds another layer, steering microclimate, soil development, and the movement of water. A small depression in the landscape and an adjacent steep hillside can host strikingly different systems for this reason alone. Time and potential biota matter too. Two ecosystems in similar environments on opposite sides of the world can behave very differently simply because they draw on different pools of species.

  Internal factors play by opposite rules. Decomposition, root competition, shading, disturbance, succession, and the mix of species present do not just respond to the ecosystem. They control it while being controlled by it. External forces decide which resources arrive. Internal forces decide whether those resources are available once they are inside, which is why the introduction of a single non-native species can cause substantial shifts in how the whole system runs.

• 4. Energy On The Move

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  Through photosynthesis, plants capture light and use it to combine carbon dioxide and water into carbohydrates and oxygen. The total photosynthesis carried out by every plant in an ecosystem is the gross primary production, or GPP. About half of that is burned back off by the plants themselves through respiration, the energy cost of growth and maintenance. What survives is the net primary production, or NPP, and it is limited by light, leaf area, the supply of carbon dioxide to the chloroplasts, water, and suitable temperatures.

  Carbon and energy ride that production into living tissue, then move outward. Some net primary production is eaten by animals while the plant still lives, feeding what is called the plant-based trophic system. The rest dies uneaten and becomes detritus, feeding a separate detritus-based trophic system. In terrestrial ecosystems, the vast majority of net primary production ends up broken down by decomposers rather than eaten by animals. In aquatic systems the balance tips, and a far higher share of plant biomass is consumed by herbivores.

  The naming of the players follows the food. Photosynthetic organisms are the primary producers. Herbivores that eat them are primary consumers, also called secondary producers. Carnivores that eat the herbivores are secondary consumers, and organisms that feed on bacteria and fungi are microbivores. Each forms a trophic level. The simple chain from plant to herbivore to carnivore is a useful fiction. Real organisms feed at more than one level, and a bird that eats both grasshoppers and earthworms ties the plant-based and detritus-based systems together, turning tidy food chains into tangled food webs.

• 5. The Work Of Rot

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  Without decomposition, dead organic matter would simply pile up, and nutrients and atmospheric carbon dioxide would run down until the system starved. Decomposition prevents that by breaking dead material apart, releasing nutrients for plants and microbes and returning carbon dioxide to the air or water. The work splits into three categories: leaching, fragmentation, and chemical alteration.

  Leaching happens as water moves through dead matter and dissolves the water-soluble parts. Freshly shed leaves and freshly dead animals are rich in these, including sugars, amino acids, and mineral nutrients, which is why leaching matters far more in wet environments than dry ones. Fragmentation does cruder work, breaking material into smaller pieces and exposing fresh surfaces for microbes. A leaf may be sealed under cuticle or bark, an animal under its exoskeleton. Animals tear through these barriers as they feed, and freeze-thaw cycles and bouts of wetting and drying do the same.

  Chemical alteration is mostly the province of bacteria and fungi. Fungal hyphae produce enzymes that pierce the tough structures around dead plant material and break down lignin, opening access to cell contents and the nitrogen locked in the lignin. Fungi can ferry carbon and nitrogen across their hyphal networks, so unlike bacteria they are not dependent solely on what lies immediately around them.

  The pace of all this varies. Decomposition rates run lowest under very wet or very dry conditions and highest in moist soils with adequate oxygen. Waterlogged soils, especially in wetlands, go short on oxygen and slow microbial growth. Temperature drives microbial respiration, so warmth speeds the breakdown. Freezing temperatures kill soil microorganisms, which hands a larger role to leaching. As soil thaws in spring, that shift can release a sudden pulse of nutrients.

• 6. Shocks And Recovery

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  F. Stuart Chapin and coauthors define disturbance as a relatively discrete event in time that removes plant biomass. The list runs from herbivore outbreaks and treefalls to fires, hurricanes, floods, glacial advances, and volcanic eruptions. When a perturbation strikes, an ecosystem moves away from its starting state. Its tendency to stay close to that equilibrium despite the blow is its resistance. Its capacity to absorb the shock and reorganize while keeping essentially the same function, structure, identity, and feedbacks is its ecological resilience.

  The severity of a disturbance sets the path of recovery. A volcanic eruption or a glacial advance and retreat strips the ground down to soils without plants, animals, or organic matter, and the system must begin again through primary succession. Lighter blows, like forest fires, hurricanes, or cultivation, leave more behind and lead to secondary succession with a faster recovery. The more severe and more frequent the disturbance, the longer the return. Resilience thinking folds humanity into the picture, treating people as an integral part of the biosphere who depend on ecosystem services and must maintain their capacity to withstand shocks.

  The past lingers in the present. The forests of eastern North America still carry legacies of cultivation that ceased in 1850, when large areas reverted to forest. In the lakes of eastern Siberia, methane production is governed by organic matter that accumulated during the Pleistocene, a reminder that an ecosystem is always recovering from something.

• 7. Following The Nitrogen

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  Most terrestrial ecosystems are nitrogen-limited in the short term, which makes the nitrogen cycle a central control on how much they can produce. Over the long term, phosphorus availability can become critical instead. Nitrogen is one of the primary macronutrients, alongside phosphorus and potassium, the elements plants need in the largest amounts. Calcium, magnesium, and sulfur follow as secondary major nutrients, with a roster of micronutrients including boron, copper, iron, manganese, molybdenum, and zinc needed in small quantities.

  Until modern times, nitrogen fixation was the major source of nitrogen for ecosystems. Nitrogen-fixing bacteria live either symbiotically with plants or freely in the soil, and many members of the legume family host them. The arrangement is expensive. Supporting nitrogen-fixing symbionts can cost a plant as much as 25 percent of its gross primary production under controlled conditions. Some cyanobacteria fix nitrogen too, and as phototrophs they run photosynthesis as well.

  The modern picture is dominated by people. Anthropogenic nitrogen inputs account for about 80 percent of all nitrogen fluxes in ecosystems, arriving through acid deposition from burning fossil fuels, ammonia evaporating off fertilized fields, and dust. Once nitrogen is in the soil, a chain of microbial processes moves it along. Mineralization releases ammonium, nitrification converts ammonium to nitrite and nitrate, and under nitrogen-rich, oxygen-poor conditions denitrification turns those back into nitrogen gas. Mycorrhizal fungi, symbiotic with plant roots, trade phosphorus and nitrogen compounds for plant carbohydrates, a pathway that may account for more than 70 teragrams of plant nitrogen assimilated each year.

• 8. Reading And Repairing

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  The Hubbard Brook Ecosystem Study began in 1963 in the White Mountains of New Hampshire. It was the first successful attempt to study an entire watershed as an ecosystem, using stream chemistry to monitor the system and building a detailed biogeochemical model. The long-term work paid off in 1972 with the discovery of acid rain in North America, after which researchers documented the depletion of soil cations, especially calcium, over the following decades. The American ecologist Stephen R. Carpenter warned that small-scale microcosm experiments can be irrelevant and diversionary if they are run without matching field studies at the ecosystem scale.

  The stakes of that science are spelled out by the Millennium Ecosystem Assessment, a synthesis by over 1000 of the world's leading biological scientists. It sorted ecosystem services into four categories, provisioning, regulating, cultural, and supporting, and called natural systems humanity's life-support system. Of the 24 ecosystem services it measured, only four had improved over the previous 50 years, while 15 were in serious decline and five were in a precarious condition. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, or IPBES, was created to sharpen the link between science and policy, modeled on the role of the Intergovernmental Panel on Climate Change.

  When management is applied to whole ecosystems rather than single species, it becomes ecosystem management, built on the principle that intergenerational sustainability is a precondition for management, not an afterthought. Where degradation has gone too far and a system has lost its defining features, it is considered collapsed, though unlike species extinction that collapse can be reversible. Integrated conservation and development projects try to treat protection and human livelihood together rather than separately, which is why ecosystem restoration is counted among the ways to advance the Sustainable Development Goals.