Unit 5 Beta

Land and Water Use: the one-page sheet

5.1 The Tragedy of the Commons

A shared resource that no one controls is overused because each user gains in full from taking more while the cost is spread over everyone. Pastures, fisheries, aquifers and the atmosphere all show it. Solutions change the rewards: limits set by law, fees, assigned rights, or community rules with monitoring.

A common resource is shared, hard to fence off, and used up as it is used: open pastures, ocean fisheries, aquifers, the air.
The tragedy of the commons is not about bad people. Each user acts sensibly for themselves; together they run the resource down.
The gain from taking more goes to one user; the cost is split among all users. That gap is the cause.
Fixes change the rewards: limits set by law (permits, quotas, seasons), fees for use, private or assigned rights, or community rules with monitoring and penalties.

no single user has a reason to protect it every user takes more the resource shrinks, so each user gets less use falls back to what the resource can renew

tragedy of the commons: The overuse of a shared resource that no one owns or controls, such as an open pasture, an ocean fishery or an aquifer. Each user keeps the full benefit of taking more but shares the cost with everyone, so the resource is run down unless rules limit use.

5.2 Clearcutting

Clearcutting cuts every tree in an area: it is the cheapest way to harvest timber, but it bares the soil to erosion, fills streams with sediment, warms them so they hold less dissolved oxygen, washes out nutrients, and removes habitat and stored carbon. Deforestation is the lasting loss of forest to other land uses.

Clearcutting removes all the trees in an area at once. It is cheap and fast, and some tree species that need full sun regrow well afterward.
Costs: soil erosion, sediment in streams, warmer streams with less dissolved oxygen, nutrients washed out of the soil, more runoff and flooding, and lost habitat.
Forests are a carbon sink. Cutting and burning or decaying the wood releases carbon dioxide, and the cleared land takes up less until trees regrow.
Deforestation is the lasting loss of forest, as when land is cleared for farms, pasture, mines or towns.

roots die and leaf litter is removed, so the soil is bare more water runs off and carries soil (sediment) into the stream less light reaches algae and fish eggs are smothered the water warms and holds less dissolved oxygen, so cold-water fish decline

clearcutting: A logging method that cuts down all or nearly all of the trees in an area at once. It is the cheapest and fastest way to harvest timber, but it exposes soil to erosion, warms nearby streams and removes habitat.
deforestation: The removal of a forest, or a large part of one, so the land is no longer forest, often to make room for farms, pasture, mines or cities. It releases stored carbon as carbon dioxide and removes habitat.

5.3 The Green Revolution

The Green Revolution greatly increased grain yields from the 1940s to the 1970s with high-yield varieties, synthetic fertilizer, irrigation, pesticides and machines. It cut hunger and spared land, but it spread monocultures with low genetic diversity, fertilizer runoff, heavy water and fossil fuel use. Genetically modified crops continue the same trade-offs.

The Green Revolution (about the 1940s to 1970s) raised grain yields with high-yield varieties, synthetic fertilizer, irrigation, pesticides and machines.
Benefits: more food per hectare, fewer famines, cheaper food, and less forest and grassland cleared for farms than would otherwise be needed.
Costs: monocultures with low genetic diversity, fertilizer runoff, heavy water use, soil damage, and fossil fuels burned by machines and used to make fertilizer.
A genetically modified organism has DNA changed in a lab. GM crops can raise yields or reduce spraying, but they can lower crop diversity and pests can evolve to overcome them.

plants can carry heavy heads of grain without falling over grain yield per hectare roughly doubles the crop is a monoculture with low genetic diversity nitrate and phosphate pollute rivers and lakes

Green Revolution: The rapid rise in crop yields from about the 1940s to the 1970s, first in Mexico and then in South Asia and beyond, from high-yield grain varieties grown with synthetic fertilizer, irrigation, pesticides and machines.
monoculture: Growing a single crop species, often a single variety, over a large area. It is efficient to plant and harvest but has low genetic diversity, so one pest or disease can spread through the whole crop.
genetically modified organism: A genetically modified organism: a plant, animal or microbe whose DNA has been changed in a laboratory, for example by adding a gene from another species, to give it a new trait such as resistance to an insect pest.
synthetic fertilizer: Fertilizer made in factories, mostly nitrogen (from ammonia made with natural gas), phosphorus and potassium in forms plants can take up at once. It raises yields quickly, but nutrients the crop does not use can run off or leach into water.

5.4 Impacts of Agricultural Practices

Tilling, slash-and-burn and heavy fertilizer use each raise harvests in the short run and harm the land or water in the long run. Tilling bares the soil to erosion and loss of organic matter; slash-and-burn removes forest and releases carbon dioxide; fertilizer the crop does not use pollutes rivers, lakes and groundwater.

Tilling buries weeds and loosens the seedbed, but leaves soil bare, so wind and water erode it; it also speeds the loss of organic matter.
Slash-and-burn clears tropical forest by cutting and burning. Ash fertilizes the poor soil for a few seasons, then the plot is left and more forest is cleared.
Burning releases carbon dioxide and smoke; clearing removes habitat and carbon storage. With long fallow periods and few people, plots can regrow forest.
Fertilizer the crop does not take up runs off or leaches into rivers, lakes and groundwater, where nitrate and phosphate become pollutants.

the soil surface is bare and loose topsoil blows or washes away (erosion) soil organic matter falls and its carbon enters the air as carbon dioxide the soil holds less water and fewer nutrients, so yields fall

tilling: Turning over and breaking up the soil with a plow or disk before planting, to bury weeds and crop leftovers and loosen the seedbed. It leaves the surface bare and loose, so it increases erosion by wind and water and speeds the loss of soil organic matter.
slash-and-burn: A way of clearing land for farming by cutting the forest and burning what is left. The ash fertilizes the soil for a few seasons; when the nutrients run out the plot is left and a new one is cleared. It releases carbon dioxide and smoke and removes forest habitat.

5.5 Irrigation Methods

Irrigation methods differ in how much of the water reaches the roots: flood and furrow are cheap but lose much of it to evaporation and runoff, spray loses less, and drip loses the least but costs the most. Overirrigation causes waterlogging and salinization, and pumping groundwater faster than it recharges depletes aquifers.

About 70% of the fresh water people withdraw worldwide goes to agriculture, most of it for irrigation.
Drip is the most efficient (only about 5% of the water lost) and spray comes next (about a quarter or less lost). Flood and furrow are cheap surface methods that lose the most (furrow often about a third). Drip also costs the most to install.
Overirrigation causes waterlogging (roots without oxygen) and salinization (salt left behind when water evaporates).
Pumping groundwater faster than it recharges causes aquifer depletion: a falling water table, a cone of depression around wells, and dry shallow wells.

much of the water evaporates from the surface or soaks below the roots salt is left behind in the topsoil roots struggle to take up water, so yields fall the water table drops and shallow wells go dry

furrow irrigation: Irrigation in which water flows down shallow trenches (furrows) dug between crop rows. It is cheap and simple, but a large share of the water evaporates or runs off the end of the field.
flood irrigation: Irrigation that covers a whole field with a sheet of water, as in many rice paddies. It is the cheapest method, but much of the water can evaporate, run off or soak below the roots, and it can waterlog the soil.
spray irrigation: Irrigation that pumps water through pipes and sprays it over the crop like rain, as with a center-pivot sprinkler. It wastes less water than flood or furrow irrigation but costs more and uses energy for pumping; some spray evaporates or drifts in wind.
drip irrigation: Irrigation that delivers water slowly through tubes with small holes placed at the base of each plant. It is the most efficient method (only about 5% of the water is lost, so about 95% reaches the roots) but the most expensive to install.
waterlogging: Saturation of soil with water so that the pore spaces are full and roots cannot get the oxygen they need. Overirrigation and poor drainage cause it, and it can raise the water table toward the surface.
salinization: The buildup of salts in soil. Irrigation water carries small amounts of dissolved salt; when the water evaporates from the field, the salt is left behind, and over years it can reach levels that stunt or kill crops.
aquifer depletion: The drawing down of groundwater when it is pumped out faster than rain and snowmelt refill (recharge) the aquifer. Around a pumping well the water table drops in a funnel shape called a cone of depression; nearby shallow wells can go dry.

5.6 Pest Control Methods

Pesticides protect crops from insects, weeds and fungi, but they also kill nontarget organisms, run off into water, and lose their power as pests evolve resistance by natural selection. Spraying more often or switching chemicals keeps the cycle going. Genetically modified crops can cut some spraying but face the same resistance problem.

Pesticides include insecticides (insects), herbicides (weeds) and fungicides (fungi). They raise yields by protecting crops.
Most pesticides kill nontarget organisms too: pollinators such as bees, predators that ate the pests, and fish and frogs when the chemicals run off into water.
Pesticide resistance is natural selection: spraying kills the susceptible pests, the resistant ones survive and breed. Resistance is in the pests' genes, not learned.
Genetically modified crops can reduce spraying for some pests (a crop that makes its own insecticide), but pests can evolve resistance to them too.

spraying kills most pests but not those few the next generation has a larger share of resistant pests most of the population is resistant and the pesticide stops working costs rise, more nontarget organisms die, and resistance builds again

pesticide: A substance used to kill or control organisms people consider pests. Insecticides kill insects, herbicides kill weeds, and fungicides kill fungi. Most act on many species, so they also harm nontarget organisms such as pollinators, predators of pests, and fish.
pesticide resistance: The ability of a pest population to survive a pesticide that once killed most of it. A few individuals carry genes that let them survive; spraying kills the others, the survivors breed, and over generations most of the population is resistant. It is natural selection caused by people.

5.7 Meat Production Methods

Meat is produced in concentrated animal feeding operations, which are cheap and use little land on site but concentrate manure and depend on feed crops, or on pasture by free-range and rotational grazing, which spreads the impact but needs more land. Overgrazing strips plant cover and erodes soil and in dry regions leads to desertification. Eating lower on the food chain uses less land and feed.

Concentrated animal feeding operations (CAFOs, feedlots) raise many animals in a small space on grain. Cheap and land-efficient on site, but they concentrate manure, use antibiotics heavily and need large areas of cropland for feed.
Free-range and rotational grazing spread animals over pasture. Manure fertilizes the land. Rotation lets each paddock regrow, but grazing needs much more land per kilogram of meat.
Overgrazing strips plants, compacts and erodes soil, and in dry regions can lead to desertification.
Meat sits a trophic level above plants, so it takes several kilograms of feed to make one kilogram of meat. Cattle also release methane as they digest grass.

grass is eaten faster than it regrows (overgrazing) rain runs off and wind and water erode the topsoil the land becomes desert-like (desertification) ranchers crowd animals onto the land that is left, and the cycle speeds up

concentrated animal feeding operation: A concentrated animal feeding operation (feedlot): a facility that keeps large numbers of animals, such as cattle, hogs or chickens, in a small space and brings feed to them. It produces meat cheaply on little land, but concentrates manure, uses antibiotics heavily, and depends on grain grown elsewhere.
rotational grazing: Raising livestock on pasture, where the animals eat grass and other plants. In free-range grazing animals roam a large area; in rotational grazing the herd is moved from one fenced paddock to the next so grazed areas can regrow.
overgrazing: Grazing more animals on land than its plants can support, so plants are eaten faster than they regrow. Roots die, bare soil is exposed and compacted by hooves, and erosion follows.
desertification: The degradation of land in dry regions into land that behaves like a desert: lost plant cover, eroded and dried-out soil, and much lower productivity. Overgrazing, deforestation, poor farming and drought all contribute.

5.8 Impacts of Overfishing

Overfishing takes fish faster than they can reproduce, so stocks shrink and can collapse, as Atlantic cod did off Newfoundland in 1992. Bycatch kills nontarget animals, and bottom trawling destroys sea-floor habitat. Catch limits, protected areas, gear rules and closed seasons are the main fixes.

Overfishing is catching fish faster than they reproduce. It is the tragedy of the commons at sea.
Bycatch is everything caught that was not the target: dolphins, sea turtles, sharks, seabirds, young fish. Much of it dies.
Bottom trawling drags heavy nets across the sea floor, destroying corals, sponges and the places where young fish shelter.
A fishery collapse can take decades to recover, and removing a top predator or a forage fish changes the whole food web.

the stock shrinks year after year fewer eggs are laid, so fewer young join the stock catch holds steady for a while, hiding the decline the fishery collapses and the food web around it changes

overfishing: Catching fish faster than the population can replace them by reproduction, so the stock shrinks year after year. It is a tragedy of the commons in the open ocean.
bycatch: Animals caught by fishing gear that were not the target of the catch, such as dolphins, sea turtles, sharks, seabirds and young fish. Much bycatch dies and is thrown back.
fishery collapse: The drop of a fish stock to a small fraction of its former size, so that catches fall sharply and fishing for it is no longer worthwhile. A collapsed stock may take decades to recover, or may not recover.

5.9 Impacts of Mining

Mining extracts ore and coal by surface methods (strip, open-pit, mountaintop removal), which remove overburden over large areas, or by subsurface methods, which are more dangerous for miners. Waste rock and tailings release acid mine drainage and toxic metals; mining also causes erosion, sediment and dust, and clears habitat. Land reclamation restores mined land, and lower-grade ores mean more waste per kilogram of metal.

Ore is rock with enough valuable mineral to mine at a profit. As rich ores run out, lower-grade ore means more rock moved and more waste per kilogram of metal.
Surface mining (strip, open-pit, mountaintop removal) removes the overburden to reach shallow ore: cheaper and safer for miners, but it destroys habitat and leaves huge waste piles.
Subsurface mining uses shafts and tunnels for deep ore: less land disturbed, but more dangerous for miners and the ground above can sink.
Mine waste causes acid mine drainage, metal pollution, sediment and dust. Land reclamation regrades the land, replaces topsoil and replants it.

the minerals react to form sulfuric acid the pH of nearby streams falls toxic metals enter streams and groundwater fish and insects die and the stream can stay polluted for decades

ore: Rock that contains enough of a valuable mineral, such as copper, iron, gold or lithium, to be mined at a profit. As the richest ores are used up, lower-grade ores must be mined, which moves more rock for each kilogram of metal.
surface mining: Mining that removes the soil and rock above a deposit to reach ore or coal near the surface. It includes strip mining, open-pit mining and mountaintop removal. It is cheaper and safer for miners than subsurface mining but destroys habitat over large areas.
subsurface mining: Mining deep deposits through vertical shafts and horizontal tunnels, leaving the surface mostly in place. It disturbs less land but is more expensive and more dangerous for miners (cave-ins, explosions, poor air) and can cause the ground above to sink.
overburden: The soil and rock that lie on top of an ore or coal deposit and are removed in surface mining. Tailings are the finely ground waste left after the valuable mineral is extracted from ore; both are piled near the mine and can release metals and acid.
acid mine drainage: Acidic water that flows from mines and mine waste. Sulfide minerals exposed to air and water form sulfuric acid, which lowers the pH of streams and dissolves toxic metals such as lead, mercury and arsenic from the rock.
land reclamation: Restoring land after mining: filling and regrading pits, replacing topsoil and planting native vegetation, so the land can support plants and wildlife again. In the United States a federal law requires coal mines to post money for it in advance.

5.10 Impacts of Urbanization

Urbanization replaces soil and plants with impervious surfaces. Rain runs off instead of soaking in, so floods rise higher and faster, groundwater recharge falls, and runoff carries pollutants and heat to streams. Coastal pumping can cause saltwater intrusion, dark surfaces create urban heat islands, and urban sprawl spreads all of this outward.

Urbanization covers land with impervious surfaces. Less rain soaks in and more runs off, so floods rise higher and faster and groundwater is recharged less.
A coastal city that pumps groundwater faster than it is recharged can pull seawater into its aquifer: saltwater intrusion.
Cities are warmer than the countryside, an urban heat island: dark surfaces with low albedo absorb sunlight, and fewer plants cool the air.
Urban sprawl spreads low-density development onto farms and natural land, which adds roads, driving and paved area per person.

rain cannot soak in (less infiltration) streams rise higher and faster, so floods are more frequent groundwater is recharged less, and streams run lower between storms it carries oil, metals, salt, litter and heat into streams

urbanization: The growth of cities and the shift of a population from rural areas into urban areas. Over half of the world's people now live in cities.
urban sprawl: The spread of low-density development, such as suburbs, shopping centers and roads, outward from a city onto farmland and natural land. It increases driving, paved area and cleared natural land per person compared with compact growth.
impervious surface: A surface that water cannot soak through, such as pavement, roofs, parking lots and packed ground. Rain that falls on it runs off instead of infiltrating, so runoff and flooding increase and groundwater recharge falls.
saltwater intrusion: The movement of seawater into a coastal freshwater aquifer when fresh groundwater is pumped out faster than it is recharged. Wells near the coast begin to pump salty water.
urban heat island: A city that is warmer than the countryside around it, especially at night. Dark pavement and roofs absorb sunlight (low albedo), buildings and traffic release heat, and there are fewer plants to cool the air by transpiration.

5.11 Ecological Footprints

An ecological footprint is the land and water area needed to supply a person's or population's consumption and absorb its carbon dioxide; a carbon footprint counts emissions only. Footprints rise with diet, energy use and goods, and they are far larger in wealthy countries. Footprint per person times population gives a country's total, and comparing it with biocapacity shows whether resources are used faster than they renew.

An ecological footprint is the land and water area needed to supply what a person or group uses and to absorb its carbon dioxide, often in global hectares (gha) per person.
A carbon footprint counts only greenhouse emissions, in metric tons of carbon dioxide; it is one piece of the ecological footprint.
Footprints grow with consumption: diet (especially meat), energy use, travel and goods. Wealthy countries have footprints several times those of poor countries.
When the world's total footprint is larger than its biocapacity, resources are used faster than they renew (overshoot).

cropland, pasture, forest, fishing grounds and built land are used to supply them forest area is counted to absorb it, the largest part of most footprints in wealthy countries people are using resources faster than nature renews them forests, fisheries and soils are run down and carbon dioxide builds up in the air

ecological footprint: The area of biologically productive land and water needed to supply the resources a person or population uses (food, wood, fibers, space for buildings) and to absorb its carbon dioxide waste. It is often given in global hectares per person and compared with the biocapacity available.
carbon footprint: The total amount of carbon dioxide (and other heat-trapping gases, counted as carbon dioxide) released by a person, product or activity, usually in metric tons of carbon dioxide per year. It counts emissions only, so it is one part of an ecological footprint.

5.12 Introduction to Sustainability

Sustainability means meeting present needs without reducing future generations' ability to meet theirs. For a living resource such as fish or timber, the maximum sustainable yield is the largest harvest the population can replace each year, at about half its carrying capacity for logistic growth. Environmental indicators such as biodiversity, food production, carbon dioxide and temperature, population and resource depletion show whether a system is becoming more or less sustainable.

Sustainability: meeting today's needs without reducing the ability of future generations to meet theirs. Living resources used no faster than they regrow; wastes released no faster than they are absorbed.
Maximum sustainable yield (MSY) is the largest harvest a population can replace each year. For logistic growth it is at about half of the carrying capacity (K/2).
Harvesting right at MSY is risky: estimates are uncertain and bad years happen, so managers usually harvest less.
Environmental indicators track sustainability: biodiversity, food production, atmospheric carbon dioxide and temperature, human population, and resource depletion.

resources are plentiful, so each fish leaves many young it adds the most new fish per year the population stays the same year after year: the maximum sustainable yield the population shrinks and adds fewer fish each year, so a steady harvest drives it down further

sustainability: Using resources in a way that meets present needs without reducing the ability of future generations to meet theirs: resources that regrow, such as fish and forests, are used no faster than they regrow, and wastes are released no faster than the environment can absorb them.
maximum sustainable yield: The largest amount of a resource that regrows, such as fish or timber, that can be harvested each year without reducing the population over time. For a population with logistic growth it is taken when the population is about half its carrying capacity, where it grows fastest.
environmental indicator: A measurement that shows the condition of an environment and how it is changing, such as biodiversity, food production, atmospheric carbon dioxide and temperature, human population, and the remaining supply of resources. Indicators show whether a system is moving toward or away from sustainability.

5.13 Methods to Reduce Urban Runoff

Methods to reduce urban runoff let rain soak in or be held near where it falls: permeable pavement, rain gardens, green roofs, rain barrels, trees and parks, and building upward and with transit so less land is paved. They lower flood peaks, cut sewer overflows, trap pollutants and recharge groundwater, at the cost of building and upkeep.

The goal is to let rain soak in, or be stored and released slowly, near where it falls, instead of piping it straight to streams.
Permeable pavement lets water through gaps or pores into gravel and soil. It must be swept or vacuumed so it does not clog.
Rain gardens and planted swales collect runoff in low spots where plants and soil soak it up and trap sediment. Green roofs hold rain in soil and plants and cool buildings.
Other methods: rain barrels and cisterns, more trees and parks, building vertically to cover less ground, and public transit so fewer roads and parking lots are needed.

it is held in soil and plants or passes into the ground less runoff reaches storm drains during the storm flood peaks downstream are lower and storm sewers overflow less sediment and some pollutants are trapped, and streams keep flowing between storms

permeable pavement: Pavement that lets water pass through it into a gravel layer and the soil below, through gaps between pavers or pores in the concrete or asphalt. It cuts runoff and recharges groundwater, but costs more and must be cleaned so its pores do not clog.
rain garden: A shallow, planted low spot that collects runoff from a roof, driveway or street and lets it soak into the ground. Deep-rooted plants and soil slow the water and trap sediment and some pollutants.
green roof: A roof covered with soil and plants. It soaks up rain and releases it slowly, cools the building and the air around it, and gives habitat to insects and birds; it costs more to build because the roof must carry the extra weight.

5.14 Integrated Pest Management

Integrated pest management keeps pests below damaging levels by monitoring them, preventing outbreaks, and using biological, physical and cultural controls first and targeted pesticides only when counts pass an action threshold. It cuts pesticide use, protects natural enemies, pollinators and water, and slows resistance, but it takes more knowledge, labor and time.

Integrated pest management (IPM) combines methods to keep pests below the level that causes real damage, using as little pesticide as possible.
Steps: monitor pests, set an action threshold, prevent problems (changing which crop grows in a field each year, resistant varieties, timing of planting), and control pests with the narrowest tool first.
Biological control uses natural enemies: ladybugs and lacewings eat aphids, tiny wasps lay eggs in caterpillars. Physical methods include traps, barriers and hand weeding.
Benefits: less pesticide, lower costs, less harm to nontarget species and water, slower resistance. Drawbacks: it takes knowledge, labor and time, and can be slower to act.

they know when pests are near a damaging level and when they are not far less pesticide is sprayed than on a fixed schedule predators and pollinators survive and keep pests in check the pesticides that are used keep working longer

integrated pest management: Integrated pest management (IPM): a way of controlling pests that combines monitoring with biological, physical, cultural and, only when needed, targeted chemical methods, to keep pests below the level that causes real economic damage while using as little pesticide as possible.
biological control: Biological control: using a pest's natural enemies, such as predators (ladybugs, lacewings), parasitic wasps or disease organisms, to keep the pest's population down.

5.15 Sustainable Agriculture

Sustainable agriculture keeps farmland productive for the long run. Contour plowing, strip cropping and terracing slow water on slopes; no-till farming and windbreaks protect the soil surface; crop rotation, legumes and perennial and cover crops protect fertility and break pest cycles; organic fertilizer adds nutrients and organic matter; and liming corrects acidic soil. Each works by slowing water or wind, or by building soil.

Erosion control on slopes: contour plowing (rows across the slope), strip cropping (alternating strips of row crops and dense crops) and terracing (flat steps).
No-till farming plants into the last crop's residue without plowing, and windbreaks of trees slow the wind. Both protect the surface.
Crop rotation breaks pest cycles, and rotating in legumes adds nitrogen. Perennial crops and cover crops keep roots in the soil all year.
Soil fertility: organic fertilizer (manure, compost) adds nutrients slowly and builds organic matter; liming raises the pH of acid soil; rotational grazing protects pasture.

water running downhill is slowed at each row, strip or step has more time to soak in and less energy to carry soil protect the soil from rain splash and wind the soil holds more water and nutrients, and yields can be kept up for decades

crop rotation: Growing different crops in a field in a planned order from year to year, for example corn, then soybeans, then wheat. It breaks pest and disease cycles and, when a legume is included, adds nitrogen to the soil.
contour plowing: Plowing and planting in rows that follow the contour lines of a slope (across it) rather than running up and down it. Each row acts as a small dam that slows runoff and catches soil.
terracing: Cutting a steep slope into a series of flat steps (terraces) for growing crops. Each step holds water and soil, shortening the distance water can run downhill.
windbreak: A row of trees or shrubs planted along the edge of a field to slow the wind, reducing wind erosion and the drying of soil and crops. Windbreaks can also give habitat and wood.
strip cropping: Planting alternating strips of different crops, often a row crop such as corn beside a dense cover crop such as hay or clover, across a slope. The dense strips slow runoff and catch soil washed from the row-crop strips.
no-till farming: Planting seeds directly into the leftover residue of the last crop without plowing. The residue protects the soil, so erosion falls, more water soaks in, and soil organic matter builds up; weeds are usually controlled with herbicides or cover crops instead.
organic fertilizer: Fertilizer from living things, such as manure, compost and plowed-in legumes. It releases nutrients slowly and adds organic matter that helps soil hold water and nutrients, but its nutrient content varies and is lower per kilogram than synthetic fertilizer.

5.16 Aquaculture

Aquaculture, raising fish and shellfish in ponds, tanks and net pens, supplies about half of the fish people eat and can ease pressure on wild stocks with high yields on little space. Open pens release concentrated waste, spread disease and parasites to wild fish, use antibiotics and lose escaped fish, and carnivorous species are fed wild fish. Closed tanks and plant-eating or filter-feeding species reduce these costs.

Aquaculture raises fish, shellfish and seaweed in ponds, tanks or net pens. It now supplies about half of the fish and shellfish people eat.
Benefits: a lot of food from a small area, less pressure on wild fish stocks, little fuel compared with fishing fleets, and fish that turn feed into body mass efficiently.
Costs: concentrated waste, disease and parasites that spread to wild fish, antibiotics, escaped fish, and wild fish caught to make feed for carnivorous farmed species. Shrimp ponds have replaced mangrove forests.
Lower-impact options: closed tanks with filters, farming plant-eating fish (tilapia, carp) and filter feeders (mussels, oysters) that need no feed.

their waste and uneaten feed fall to the sea floor decomposers use oxygen, so the water near the bottom loses oxygen wild fish swimming past the pens can be infected they compete or breed with wild fish and can weaken wild populations

aquaculture: Raising fish, shellfish or water plants in controlled conditions, such as ponds, tanks or net pens in the sea, for food. It now supplies about half of the fish and shellfish people eat.

5.17 Sustainable Forestry

Sustainable forestry harvests wood no faster than forests regrow while protecting soil, water and habitat. Selective cutting keeps most trees standing; reforestation replants cut land; prescribed burns remove fuel so wildfires stay cooler; and reusing and recycling wood lowers demand. Each method trades some cost or timber for a forest that keeps producing.

Sustainable forestry harvests wood no faster than the forest regrows and protects soil, water and habitat.
Selective cutting takes some trees and leaves most standing, so the forest keeps its cover, shade and habitat. It costs more than clearcutting.
Reforestation replants or lets trees regrow on cut land. Mixed native species restore more habitat than a single-species plantation.
Prescribed burns clear dead wood and brush to prevent hotter wildfires. Using recycled wood and paper, and protecting trees along streams, also help.

dead wood and brush build up on the forest floor a wildfire burns hotter and climbs into the treetops the forest takes much longer to recover later wildfires stay lower and cooler, and large trees survive

sustainable forestry: Managing forests so that wood is harvested no faster than the forest regrows, while soil, water, habitat and biodiversity are protected for the future. It uses methods such as selective cutting, reforestation, protecting streamside trees, prescribed burns and reusing or recycling wood.
selective cutting: Harvesting only some of the trees in a forest, often single mature trees or small groups, and leaving the rest standing. It keeps forest cover, shade and habitat and limits erosion, but costs more per log than clearcutting.
reforestation: Planting trees, or letting them regrow, on land where a forest was cut or burned. It restores habitat and carbon storage over decades; a plantation of one species restores far less biodiversity than a mixed native forest.
prescribed burn: A fire set on purpose by forest managers under safe weather conditions to burn dead wood, fallen branches and brush on the forest floor. It removes the fuel that would let a wildfire grow large and hot, and helps species adapted to fire.