Ecology: Populations, Communities, Ecosystems and Conservation

Every organism you have met in this book lives somewhere, eats something, and is eaten, infected, crowded or helped by others. Ecology is the science of those relationships, scaled up from one population to the whole planet, and it explains why deserts sit where they do, why wolves can change a river, why lakes turn green, and why carbon from fossil fuels ends up in the ocean. It is also the science you will need to judge the biggest decisions of your lifetime about land, food, water, climate and the survival of species.

Section 1Ecology and the biosphere: climate and biomes

The scope of ecology

Ecology is the scientific study of the interactions between organisms and their environment. The environment includes abiotic (nonliving) factors such as temperature, water, light, soil and nutrients, and biotic (living) factors, meaning all the other organisms that affect an individual. Ecologists work at nested levels. Organismal ecology asks how an individual's anatomy, physiology and behavior fit its environment. Population ecology studies a population, a group of individuals of one species living in the same area, and asks what controls its size. Community ecology studies a community, all the populations of different species in an area, and how they interact. Ecosystem ecology adds the abiotic environment and follows energy flow and chemical cycling. Landscape ecology studies mosaics of connected ecosystems, and global ecology studies the biosphere, the sum of all the planet's ecosystems.

Ecology is an experimental and quantitative science, not a list of observations. Ecologists test hypotheses with field experiments (removing a predator from fenced plots), natural experiments (comparing islands of different sizes) and mathematical models. It also differs from environmentalism: ecology describes how nature works, while decisions about what to do with that knowledge involve values as well as science.

Why climate varies: sunlight and latitude

Climate is the long term pattern of temperature, precipitation, sunlight and wind in a region; weather is the short term condition. The single most important driver of global climate is that the sun heats the Earth unevenly. Near the equator, sunlight strikes the surface almost straight on, so a given amount of solar energy falls on a small area. Toward the poles, the same beam strikes at a low angle, spreads over a larger area and passes through more atmosphere, so each square meter receives less energy. The tropics are therefore warm year round, and the poles are cold.

Seasons exist because Earth's axis is tilted about 23.5 degrees relative to its orbit. As the planet orbits the sun, first the Northern and then the Southern Hemisphere tilts toward the sun, receiving more direct light and longer days. Seasons are not caused by changes in Earth's distance from the sun; in fact Earth is closest to the sun in early January, during the northern winter.

Global air circulation and precipitation

Uneven heating drives air movement. At the equator, intense heating warms the air, and warm air holds more water vapor. This moist air rises; as it rises it expands and cools, and cool air holds less water, so the vapor condenses and falls as heavy rain. That is why tropical rain forests lie near the equator. The now dry air flows toward the poles high in the atmosphere and sinks at about 30 degrees north and south latitude. Sinking air compresses and warms, absorbing moisture from the land, so the great deserts of the world (the Sahara, the Arabian, the Australian interior, the Sonoran) cluster near 30 degrees. Part of this air returns toward the equator along the surface, completing a loop called a Hadley cell. Similar circulation cells rise near 60 degrees, bringing moderate precipitation to temperate forests, and sink near the poles, which are therefore cold deserts.

Because Earth rotates, surface winds are deflected (the Coriolis effect), producing the easterly trade winds in the tropics and the westerlies in the temperate zones. These winds drive surface ocean currents, which carry heat from the tropics toward the poles and strongly shape regional climate. The Gulf Stream, for example, keeps western Europe far milder than places at the same latitude in eastern Canada.

Regional and local effects

Large bodies of water moderate climate because water has a high specific heat: it warms and cools slowly. Coastal areas therefore have milder winters and cooler summers than inland areas at the same latitude. In a Mediterranean climate, cool ocean currents produce mild wet winters and hot dry summers. Mountains also change climate. Air temperature falls roughly 6 °C for every 1,000 meters of elevation, so climbing a mountain is ecologically like traveling toward the poles. When moist air meets a mountain range, it rises, cools and drops its rain on the windward side; the descending air on the leeward side is warm and dry, creating a rain shadow. The deserts east of the Sierra Nevada and Cascade Ranges are rain shadow deserts. At the smallest scale, microclimates under a log, beneath a forest canopy or on a north facing slope differ enough to determine which organisms can live there.

Terrestrial biomes

A biome is a major life zone characterized by its dominant vegetation (on land) or by its physical environment (in water). Terrestrial biomes are determined mainly by temperature and precipitation, and especially by their seasonal pattern; a climograph plots annual mean temperature against annual precipitation and shows where each biome falls. Biomes grade into one another through transition zones called ecotones, and disturbance such as fire, storms and human land use shapes them as much as climate does. Many grasslands and savannas, for example, are kept free of trees by periodic fires and grazing.

Aquatic biomes

Aquatic biomes cover about three quarters of the planet and are defined by salinity, depth, light and water movement. Water absorbs light quickly, so the upper photic zone has enough light for photosynthesis, while the deeper aphotic zone does not. The bottom surface is the benthic zone, home to organisms called the benthos, and much of the food that reaches the deep benthos is detritus sinking from above. In many lakes and oceans a narrow layer of rapid temperature change, the thermocline, separates warm surface water from cold deep water. In temperate lakes, the surface water cools in autumn and warms in spring until it matches the deep water, and wind then mixes the whole lake in a turnover that carries oxygen down and nutrients up.

• Lakes range from oligotrophic (deep, nutrient poor, clear, oxygen rich) to eutrophic (shallow, nutrient rich, cloudy with phytoplankton, often low in oxygen at depth).
• Wetlands (marshes, swamps, bogs) are among the most productive ecosystems, filter pollutants and buffer floods.
• Streams and rivers change from cold, clear, oxygen rich headwaters to warmer, murkier, nutrient rich lower reaches.
• Estuaries, where rivers meet the sea, have variable salinity and are nurseries for many fishes and shellfish.
• Intertidal zones are alternately covered and exposed by tides, so organisms there must tolerate drying and wave action.
• The oceanic pelagic zone is the open blue water, with plankton, fishes and whales; much of it is nutrient poor.
• Coral reefs are built by corals whose cells host photosynthetic dinoflagellate algae; they need warm, clear, shallow water and are extremely diverse.
• The marine benthic zone includes the deep sea floor, where hydrothermal vent communities rely on chemoautotrophic prokaryotes rather than sunlight.

Section 2Population ecology: growth and regulation

Density, dispersion and demography

Two basic properties describe a population. Density is the number of individuals per unit area or volume. Ecologists rarely count everyone; they sample plots or use the mark recapture method. In mark recapture, a researcher captures, marks and releases s individuals, later captures a second sample of n individuals, and counts the x marked ones among them. Assuming marked animals mix randomly, the population size N is estimated as N = s × n / x. If 50 fish are marked and a later sample of 40 contains 10 marked fish, N is about 50 × 40 / 10 = 200.

Dispersion is the spacing pattern of individuals. A clumped pattern, the most common, occurs where resources are patchy or where animals gain from groups (herds, schools). A uniform pattern usually results from direct interactions such as territoriality or chemical inhibition among plants. A random pattern, uncommon, occurs where individuals neither attract nor repel one another, like dandelions from windblown seeds.

Density changes through four processes: births and immigration add individuals; deaths and emigration remove them. Demography is the study of these vital rates. A life table follows a cohort, a group of individuals of the same age, from birth until all have died, recording survivorship and reproduction at each age. Plotting the number surviving against age on a logarithmic scale gives a survivorship curve.

Exponential growth

If immigration and emigration are ignored, the change in population size over a short time is births minus deaths. Expressed per individual, the per capita rate of increase r equals the per capita birth rate minus the per capita death rate. When r is constant, the population grows according to the model

dN/dt = r_maxN

where N is population size and r_max is the intrinsic rate of increase, the maximum per capita rate under ideal conditions. Because the growth rate is proportional to N, the larger the population, the more individuals are added per unit time, so the curve is J shaped. Exponential growth happens when a population is introduced to a new, resource rich environment or rebounds after a catastrophe. Elephants in Kruger National Park grew nearly exponentially for decades after hunting was banned; the whooping crane recovered the same way from about 15 birds. No population can grow exponentially for long, because resources run out.

Logistic growth and carrying capacity

The carrying capacity, symbolized K, is the maximum population size that a particular environment can sustain. It is set by limiting resources such as food, water, nesting sites or shelter and can change as the environment changes. The logistic model adds this limit:

dN/dt = r_maxN (K − N)/K

The term (K − N)/K is the fraction of the carrying capacity still available. When N is small, it is close to 1 and growth is nearly exponential. As N approaches K, it approaches zero and growth slows to a stop. The result is an S shaped (sigmoid) curve. The number of individuals added per unit time is greatest at K/2, where the population is large enough to add many offspring but not yet crowded, which is why fisheries managers have tried to hold harvested stocks near half their carrying capacity (the idea behind maximum sustainable yield).

Real populations only approximate the logistic curve. Many overshoot K and then crash, or oscillate around it, because of time lags between a change in resources and the population's response. Some species have an Allee effect: at very low density individuals struggle to find mates or defend themselves, so small populations can decline even with plenty of resources, an important concern in conservation.

Life history traits and trade offs

An organism's life history is its schedule of reproduction and survival: the age of first reproduction, how often it breeds, and how many offspring it produces each time. Because energy and time are limited, life histories involve trade offs between reproduction and survival, and between offspring number and the care or resources invested in each. Some species are semelparous (big bang reproducers), breeding once and dying, such as Pacific salmon and agave plants; others are iteroparous, breeding repeatedly, such as most mammals and oak trees. Semelparity tends to be favored where adult survival is low or unpredictable.

Ecologists have described two broad strategies. r selection favors traits that maximize reproductive rate, common in unpredictable or newly disturbed environments where populations are often well below carrying capacity. K selection favors traits that help individuals compete and survive when populations are near carrying capacity. Most species fall somewhere between, and the scheme is a simplification, but it summarizes useful correlations.

Population regulation

What keeps a population from growing forever? Density independent factors affect the per capita birth or death rate regardless of density; frost, drought, floods and fires kill a similar fraction whether a population is sparse or dense. Density dependent factors change in strength as density changes, typically raising death rates or lowering birth rates as a population grows. Only density dependent factors produce true regulation, a negative feedback that pulls a population toward K. They include:

• Competition for resources: crowding reduces food per individual and lowers reproduction.
• Territoriality: limited territories cap the number of breeders.
• Disease: pathogens spread faster in dense populations.
• Predation: predators may switch to and concentrate on abundant prey.
• Toxic wastes: ethanol accumulation limits yeast in fermentation.
• Intrinsic factors: in crowded mice, stress hormones can delay maturation and suppress reproduction.

Some populations show regular boom and bust cycles. The snowshoe hare and lynx of northern forests rise and fall on a roughly 10 year cycle. Field experiments suggest the hare cycle is driven mainly by predation (with food supply playing a smaller role), and the lynx cycle follows the hares. A metapopulation is a network of local populations linked by migration, in which some patches go extinct and are recolonized; its persistence depends on that movement.

Human population growth

The human population grew slowly for most of history, then rapidly after the industrial revolution as death rates fell with better food, sanitation and medicine. It passed 8 billion people in the early 2020s. Global growth rates peaked in the 1960s at about 2 percent per year and have since fallen to under 1 percent, so the population is still growing, but more slowly. The demographic transition describes the shift from high birth and death rates to low birth and death rates as countries develop; death rates fall first, causing a period of rapid growth, and birth rates follow. Age structure diagrams show the proportion of people in each age class: a broad base predicts continued growth, while a column or narrow base predicts stability or decline. Humans' ecological footprint, the land and water area needed to supply a population's resources and absorb its wastes, varies more than tenfold between countries, so the effect of a population depends on consumption as well as numbers.

Section 3Community ecology: interactions and succession

Classifying interactions

Populations in a community affect one another through interspecific interactions. Ecologists classify them by their effect on each partner's survival and reproduction: positive (+), negative (−) or no effect (0).

Parasitism, mutualism and commensalism are forms of symbiosis, a close, long term association between species. Note that symbiosis does not by itself mean mutual benefit. Many relationships also shift with conditions; mycorrhizae can become parasitic when soil nutrients are abundant.

Competition and the niche

An organism's ecological niche is the full set of biotic and abiotic resources it uses and the conditions it tolerates: its role in the community, not just its address. Its fundamental niche is what it could occupy without competitors; its realized niche is the smaller portion it actually occupies in the presence of competitors. Joseph Connell showed this with two barnacles on Scottish rocky shores. Chthamalus lives on the upper rocks and Balanus lower down. When Connell removed Balanus, Chthamalus spread down into the lower zone, showing that its realized niche was restricted by competition. Removing Chthamalus did not let Balanus spread up, because Balanus cannot tolerate drying on the upper rocks.

The competitive exclusion principle, demonstrated by G. F. Gause with two species of Paramecium, states that two species competing for exactly the same limiting resources cannot coexist indefinitely; the slightly more efficient competitor eventually eliminates the other locally. Species with similar needs coexist only if their niches differ. Resource partitioning is the division of resources that allows this, such as several Anolis lizard species perching at different heights or in sun versus shade on the same trees. When two closely related species are compared, their traits often differ more where they live together (sympatric) than where they live apart (allopatric), a pattern called character displacement. The beak depths of Galápagos ground finches are the classic example.

Predation, herbivory and defenses

Predation selects for adaptations in both predator and prey. Predators have sharp senses, speed, claws, venom and camouflage. Prey defend themselves with behavior (hiding, fleeing, alarm calls, group defense) and with morphological and chemical defenses. Cryptic coloration (camouflage) makes prey hard to see. Aposematic coloration, bright warning color, advertises chemical defenses, as in poison dart frogs and monarch butterflies. In Batesian mimicry, a harmless species copies a harmful one (the nonvenomous scarlet kingsnake resembles the venomous coral snake). In Müllerian mimicry, two or more unpalatable species resemble each other, so predators learn to avoid all of them faster (many stinging bees and wasps share yellow and black bands). Plants defend against herbivores with thorns and with chemicals such as nicotine, morphine, tannins and the mustard oils that give spices their bite. The ongoing reciprocal adaptation between interacting species is coevolution.

Species diversity and community structure

Species diversity has two parts: species richness, the number of different species, and relative abundance, the proportion each species represents. A forest with 100 trees of five species split 20 each is more diverse than one with the same five species split 96, 1, 1, 1 and 1. Ecologists summarize both with indices such as the Shannon diversity index. Diverse communities tend to be more productive and more stable, and they are more resistant to invasive species.

Community structure depends heavily on feeding relationships. A food chain traces one path of energy transfer; a food web shows the many interlinked chains in a community. Food chains are usually short, rarely more than five links, largely because so much energy is lost at each step (Section 4).

Some species have outsized effects. Dominant species are the most abundant or have the greatest biomass, like sugar maples in some forests. A keystone species is not necessarily abundant but has a strong effect through its ecological role. Robert Paine removed the predatory sea star Pisaster from experimental stretches of rocky shore, and the mussels it ate took over, crowding out other species; richness dropped from more than 15 species to fewer than 5. Ecosystem engineers (foundation species) physically change the environment, as beavers do when they build dams that convert forest streams into ponds and wetlands.

Control can flow up or down the food web. In bottom up control, nutrient supply sets the abundance of producers, which set the abundance of herbivores, and so on up. In top down control, predators control herbivores, which control plants. A trophic cascade is a top down effect that alternates across levels. Where sea otters were hunted out of North Pacific kelp forests, sea urchins, their prey, exploded and grazed the kelp to bare "urchin barrens"; where otters returned, urchins declined and kelp forests recovered. Wolves reintroduced to Yellowstone are often cited as another case, although how strongly they changed riverside vegetation is still debated.

Disturbance and ecological succession

Communities are not static. Disturbances, events such as storms, fires, floods, droughts or human activities, change communities by removing organisms or altering resources. The intermediate disturbance hypothesis proposes that moderate levels of disturbance foster the greatest diversity: too little lets dominant competitors exclude others, too much allows only the hardiest, fast colonizing species to persist. Some ecosystems depend on disturbance; many pine forests and grasslands are maintained by periodic fire, and decades of fire suppression in such forests allowed fuel to build up, contributing to more severe wildfires.

After a disturbance, the species composition changes in a sequence called ecological succession. Primary succession begins in a virtually lifeless area where soil has not yet formed, such as new volcanic rock or the ground exposed by a retreating glacier. Prokaryotes, lichens and mosses arrive first as pioneer species, weathering rock and adding organic matter that slowly builds soil. Grasses, shrubs and finally trees follow over centuries. Secondary succession occurs where an existing community has been cleared but the soil remains, as after a fire or the abandonment of a farm field. It proceeds much faster, often from weeds to shrubs to forest within decades.

Early species can affect later ones in three ways. They may facilitate later arrivals by improving soil, as when nitrogen fixing alders enrich the soil for spruce at Glacier Bay, Alaska; they may inhibit them by competing; or they may have little effect (tolerance). Succession was once thought to end in a fixed climax community, but ecologists now recognize that continual disturbance means most communities are a shifting mosaic of patches at different stages.

Section 4Ecosystems: energy flow and nutrient cycles

Two laws, two processes

An ecosystem is a community plus its physical environment. Two processes tie it together: energy flow and chemical cycling. Energy enters most ecosystems as sunlight, is converted to chemical energy by autotrophs, passes to heterotrophs in food, and leaves as heat. Because the first law of thermodynamics says energy cannot be created or destroyed, and the second law says every transformation loses some energy as heat, energy moves one way through an ecosystem and must be continually replaced by the sun. Chemical elements, by contrast, are conserved (the law of conservation of mass) and cycle between the living and nonliving parts of the ecosystem.

Organisms are grouped into trophic levels by their main source of nutrition. Primary producers (autotrophs: plants, algae, cyanobacteria and, at vents, chemoautotrophs) make organic molecules. Primary consumers are herbivores; secondary consumers eat herbivores; tertiary consumers eat secondary consumers. Detritivores (decomposers), mainly prokaryotes and fungi along with earthworms and many insects, consume detritus, the dead organic matter of all levels. They release inorganic nutrients back to producers and are essential to every chemical cycle.

Primary production

Primary production is the amount of light energy converted to chemical energy by autotrophs during a given time. Gross primary production (GPP) is the total. Producers use part of it for their own respiration (R), and the rest is net primary production: NPP = GPP − R. NPP, typically about half of GPP, is the new biomass available to consumers, so it is the key measure. Only about 1 percent of the visible light reaching producers is converted to chemical energy.

Per unit area, the most productive ecosystems are tropical rain forests, estuaries, wetlands and coral reefs; the least productive are deserts, tundra and the open ocean. Because the open ocean is so vast, however, it contributes about as much to global NPP as all the land combined. Production is limited by different factors in different places. On land, temperature and moisture matter most, measured together as actual evapotranspiration. In the ocean and in lakes, production is usually limited by a limiting nutrient: often nitrogen in coastal seas, iron in parts of the open ocean, and phosphorus in many lakes. Experiments in which iron was added to patches of the Southern Ocean produced dramatic phytoplankton blooms.

Energy transfer and ecological pyramids

Consumers convert only a small fraction of the food they eat into their own new biomass. Of the energy in the plant material a caterpillar eats, roughly half passes out in feces, about a third is used in cellular respiration and lost as heat, and only about a sixth becomes caterpillar tissue. Endotherms, which spend much of their energy on keeping warm, convert an even smaller share. Across whole trophic levels, the trophic efficiency, the percentage of production transferred from one level to the next, ranges from about 5 to 20 percent and is commonly taught as 10 percent (the "10 percent rule"). Multiplying that loss over several steps explains why food chains are short and why top predators are rare. If producers make 10,000 kcal per square meter per year, primary consumers make about 1,000, secondary consumers about 100, and tertiary consumers about 10.

An energy pyramid shows these losses and always narrows upward. Biomass pyramids usually narrow too, but some aquatic ecosystems have inverted biomass pyramids: phytoplankton reproduce and are eaten so quickly that at any moment their standing biomass is smaller than that of the zooplankton that eat them, even though their production is far greater.

Biogeochemical cycles

Elements move among living organisms, the atmosphere, water, soil and rock in biogeochemical cycles. Each cycle has reservoirs, some available to organisms and some locked away (in rock or deep sediment), and processes that move elements between them. Gaseous cycles (carbon, nitrogen, water) are global; the phosphorus cycle has no significant gas phase and is more local.

The carbon cycle

Photosynthesis by plants, algae and cyanobacteria removes CO2 from the atmosphere and ocean and builds it into organic molecules; cellular respiration by nearly all organisms returns it. These two fluxes are huge and roughly balanced. Much slower parts of the cycle store carbon for millions of years in limestone and in fossil fuels, the remains of ancient organisms. Burning fossil fuels moves carbon from that long term reservoir into the atmosphere far faster than natural processes remove it, and atmospheric CO2 has risen from about 280 parts per million before the industrial revolution to over 420 ppm today. Roughly a quarter of the extra CO2 dissolves in the ocean, forming carbonic acid and causing ocean acidification.

The nitrogen cycle

Nitrogen is abundant as N2 gas, but its triple bond makes it unusable by most organisms. It enters ecosystems mainly through nitrogen fixation, the conversion of N2 to ammonia (NH3) by certain prokaryotes, some free living and some, like Rhizobium, living in root nodules of legumes. Lightning fixes a little, and the industrial Haber Bosch process now fixes as much as all natural land processes combined. In soil, ammonia picks up a proton to become ammonium (NH4+). Ammonification by decomposers releases ammonium from dead organic matter. In nitrification, bacteria oxidize ammonium to nitrite (NO2−) and then nitrate (NO3−), the form most plants absorb most readily. Plants build the nitrogen into amino acids and nucleotides, which pass to consumers. Finally, under low oxygen conditions, denitrifying bacteria convert nitrate back to N2 (and some N2O, a greenhouse gas), closing the cycle.

The phosphorus and water cycles

Phosphorus has no important gaseous form. It weathers slowly from rock as phosphate (PO4³⁻), is absorbed by plants, passes through food webs and returns to soil through decomposition. Much eventually washes to the sea and settles into sediments, where it may take millions of years to be uplifted as rock again. Because it is released so slowly, phosphate often limits production in freshwater. When fertilizer, detergent or sewage adds phosphorus and nitrogen to lakes, algae bloom; when they die, decomposers consume the dissolved oxygen, killing fish. This process, cultural eutrophication, also creates seasonal dead zones in coastal waters such as the Gulf of Mexico.

In the water cycle, solar energy drives evaporation from oceans and transpiration from plants; water vapor condenses and falls as precipitation, then returns to the sea as runoff or seeps into groundwater aquifers. More water evaporates from the oceans than falls on them, so the excess is carried over land, where it supports terrestrial life before draining back.

Section 5Conservation biology and global change

Biodiversity at three levels

Biodiversity is the variety of life, and conservation biologists recognize it at three levels. Genetic diversity is the variation among individuals within a population and among populations of a species; it is the raw material for adaptation, and its loss leaves populations vulnerable to disease and environmental change. Species diversity is the variety of species in an ecosystem or across the planet. Scientists have named about 1.8 to 2 million species, and estimates of the true total, most of them insects and microbes, range far higher. Ecosystem diversity is the variety of ecosystems in the biosphere, each with its own interactions and services.

Species are going extinct far faster than the background rate seen in the fossil record, by many estimates hundreds to a thousand times faster, leading many biologists to describe the present as a sixth mass extinction. About one in four assessed mammal species and over 40 percent of assessed amphibians are threatened. Biodiversity matters for practical as well as ethical reasons. Many medicines come from wild organisms (the cancer drugs vincristine and vinblastine from the rosy periwinkle, aspirin's ancestor from willow bark), and wild relatives of crops carry genes for disease resistance. Ecosystems also provide ecosystem services: pollination, water purification, flood control, soil formation, carbon storage and fisheries, worth trillions of dollars a year. Many people also value biodiversity for its own sake, a feeling the biologist E. O. Wilson called biophilia.

The major threats

The main threats to biodiversity are often summarized with the acronym HIPPO.

Fragmentation breaks large habitats into small, isolated patches. Small patches hold fewer species, support smaller populations, and have a larger proportion of edge, where conditions such as light, wind, temperature and predation differ from the interior. Species that need deep forest interior suffer, while edge adapted species, such as cowbirds that lay their eggs in other birds' nests, increase. Pollutants can also undergo biological magnification: fat soluble toxins such as DDT, PCBs and methylmercury are not excreted, so they become more concentrated at each step up a food chain, reaching the highest levels in top predators such as eagles, tuna and orcas. Pregnant people are advised to limit certain predatory fish for this reason.

Conserving populations

Conservation biology integrates ecology, evolution, genetics and social science. One approach focuses on small populations. A small population is vulnerable to an extinction vortex: inbreeding and genetic drift reduce genetic diversity, which lowers fitness and reproduction, which shrinks the population further. The greater prairie chicken in Illinois fell to fewer than 50 birds with very low egg hatching rates; when biologists brought in birds from larger populations elsewhere, genetic variation and hatching success recovered. Conservation planners estimate a minimum viable population, the smallest size at which a species can likely sustain itself, and they pay attention to effective population size, the number of individuals that actually breed, which is often far smaller than the total count.

A second approach focuses on declining populations: identify why a population is declining, test the hypotheses, and remove the cause. The red cockaded woodpecker of the southeastern United States needs mature pine forest with little undergrowth for its nest cavities. When managers restored such habitat with controlled burns and drilled starter cavities, populations began to recover. Laws such as the U.S. Endangered Species Act and international agreements on wildlife trade protect listed species and their habitats.

Conserving landscapes and ecosystems

Increasingly, conservation targets whole landscapes. Biodiversity hot spots are relatively small areas with many endemic species (species found nowhere else) and many threatened species; together, hot spots covering a small percentage of land hold a large fraction of the world's plant and vertebrate species. Movement corridors, strips of habitat connecting isolated patches, allow migration and gene flow; highway underpasses and overpasses let animals cross roads safely. Protected areas and reserves are most effective when they are large enough to sustain populations of wide ranging species and when they are managed together with the surrounding human landscape. Restoration ecology repairs damaged ecosystems, using bioremediation (organisms that detoxify pollutants, such as bacteria that clean up oil) and biological augmentation (organisms that add essential materials, such as nitrogen fixing plants on mine spoils).

Reserve design draws on the equilibrium model of island biogeography, which predicts the number of species on an island (or a habitat patch) as a balance between immigration and extinction. Larger islands have lower extinction rates, and islands near a mainland source have higher immigration rates, so large, near islands hold the most species. Habitat fragments in a sea of farmland or cities behave like islands.

Climate change

Earth's surface is kept habitable by the greenhouse effect. Sunlight passes through the atmosphere and warms the surface, which radiates infrared energy; greenhouse gases such as CO2, methane (CH4), nitrous oxide (N2O) and water vapor absorb much of that infrared and re radiate part of it back, warming the lower atmosphere. Without any greenhouse effect, Earth's average surface temperature would be about −18 °C instead of about 15 °C. Human activities, chiefly burning fossil fuels, deforestation and agriculture, have increased greenhouse gas concentrations, strengthening the effect. Average global surface temperature has risen by about 1.1 to 1.3 °C since the late 1800s, with the most warming at high northern latitudes.

The biological effects are already visible. Species ranges are shifting toward the poles and upslope, and some mountain and polar species have nowhere left to go. The timing of seasonal events, or phenology, is changing: many plants flower earlier, which can mismatch them with pollinators, and some migratory birds now arrive after their insect food has peaked. Warmer oceans cause coral bleaching, in which corals expel their photosynthetic algae and may starve, while ocean acidification makes it harder for corals, mollusks and some plankton to build calcium carbonate shells and skeletons. Thawing permafrost releases stored carbon, a positive feedback that amplifies warming. Droughts, heat waves and fire seasons are becoming more intense in many regions.

Responses fall into two categories. Mitigation reduces the cause: shifting to renewable and nuclear energy, improving efficiency, protecting and restoring forests and wetlands that store carbon, and cutting methane and nitrous oxide from agriculture. Adaptation reduces harm: building corridors so species can track shifting climates, assisting migration of vulnerable species, breeding heat and drought tolerant crops, and protecting coasts. Other global changes, such as stratospheric ozone depletion by chlorofluorocarbons, show that coordinated international action works: after CFCs were phased out under a global agreement, the ozone layer has begun to recover.

Words worth knowing