Unit 8 Beta

Ecology: the one-page sheet

8.1 Responses to the Environment

Organisms detect changes in their environment and respond with behavior and with changes in how their bodies work. Animals move toward or away from a stimulus (taxis) or change their speed and turning (kinesis); plants grow toward or away from light, gravity and touch (tropisms), using the hormone auxin. Many plants time flowering by night length (photoperiodism): short-day plants flower when the unbroken night is longer than a critical length, long-day plants when it is shorter, and phytochrome, switched on by red light and off by far-red, does the measuring. Circadian rhythms follow an internal clock of about 24 hours that light resets. Hibernation, estivation, dormancy and migration carry organisms through hard seasons. Animals communicate with visual, auditory, chemical and tactile signals, such as pheromones and the honeybee waggle dance. Some behavior is innate and some is learned. Cooperation and altruism toward relatives can spread by kin selection, because relatives share alleles, and courtship displays let animals choose mates.

Organisms respond to environmental changes with behavior (what an animal does) and with physiology (how its body works), such as flowering, hibernation and migration.
Taxis is movement aimed toward or away from a stimulus. In a kinesis an animal only speeds up or slows down, yet it still gathers where it slows down.
Plants grow toward or away from light, gravity or touch (tropisms). In photoperiodism a plant measures the night: short-day plants flower when the night is longer than a critical length; red light switches phytochrome on and far-red switches it off.
Circadian rhythms run on an internal clock of about 24 hours that light resets each day. Hibernation, estivation and migration carry animals through hard seasons.
Animals communicate with visual, auditory, chemical (pheromones) and tactile signals. Behavior may be innate or learned (habituation, imprinting, conditioning), and helping relatives can be favored by kin selection.

Receptors detect the change, such as phytochrome in a leaf or moisture receptors on an animal's body. Gene expression, growth or activity changes: a shoot tip makes flowers, a pill bug stops walking, a squirrel's metabolic rate drops. It ends up in better conditions or saves energy: on moist ground, flowering when pollinators fly, asleep through a winter with no food. Over generations, alleles behind well-timed responses become common, so most responses we see fit the organism's environment. The signal is a stimulus for other animals, so their behavior changes too: ants follow a trail, bees fly to food, a mate approaches. The relative, who shares many of its alleles, survives more often, so alleles for helping can spread (kin selection).

behavior: What an animal does in response to a stimulus, such as moving, calling, courting or hiding. Innate behavior appears in its full form the first time without practice; learned behavior changes with experience.
taxis: Movement directed toward or away from a stimulus, such as phototaxis (light) or chemotaxis (a chemical). A kinesis is different: the animal changes how fast it moves or how often it turns as conditions change, with no aim, and ends up gathering where it slows down.
tropism: A plant's growth toward or away from a stimulus from one direction: light (phototropism), gravity (gravitropism) or touch (thigmotropism). In a shoot lit from one side, the hormone auxin moves to the shaded side, where cells elongate more, so the shoot bends toward the light.
photoperiodism: A response to the relative lengths of day and night, such as flowering or the start of migration. Short-day plants flower when the night is longer than a critical length; long-day plants flower when it is shorter. Plants detect light with phytochrome, a pigment-protein that red light switches to its active form and far-red light switches back.
circadian rhythm: A circadian rhythm is a cycle of activity or body function with a period of about 24 hours, set by an internal biological clock. It keeps running in constant conditions, and daily light resets it to match the real day.
hibernation: A long period of low metabolic rate and low body temperature in winter, which saves energy when food is scarce. Torpor is a shorter drop of the same kind, estivation is a similar inactive state in hot, dry seasons, and dormancy is the resting state of seeds, buds and some animals.
seasonal migration: The regular, long-distance movement of animals between places used in different seasons, such as birds flying between summer breeding grounds and winter feeding grounds. Changing day length is a common trigger.
animal communication: A signal from one animal that changes the behavior of another. Signals can be visual (displays, colors), auditory (calls, songs), chemical (pheromones) or tactile (touch). The honeybee waggle dance tells nestmates the direction and distance of food.
innate and learned behavior: Innate behaviors (such as fixed action patterns, set sequences triggered by a simple cue) need no experience; learned behaviors change with experience. Kinds of learning: habituation (ignoring a harmless repeated stimulus), imprinting (learning during a short early period), classical conditioning (linking two stimuli) and operant conditioning (linking an action to its result).
cooperative behavior: Behavior in which individuals act together or one helps another. An altruistic act costs the helper but helps another. Kin selection explains altruism toward relatives: relatives share alleles, so helping them can pass on more copies of the helper's alleles (inclusive fitness).
courtship: Behavior that leads to mating, such as displays, songs and gifts. Courtship signals let individuals recognize their own species and let one sex choose among mates, so they are shaped by sexual selection.

8.2 Energy Flow Through Ecosystems

An ecosystem is a community of organisms together with its nonliving surroundings. Producers capture energy by photosynthesis or chemosynthesis; consumers get it by eating other organisms; decomposers break down dead matter. Gross primary productivity is the energy producers capture; net primary productivity, what is left after their own respiration, feeds everything else. At each trophic level most energy is released as heat by cellular respiration or passes to decomposers, so only about 10% is stored at the next level. Energy pyramids therefore narrow upward, top predators are few, and food chains are short. Endotherms spend more of their food on body heat than ectotherms, so they need more food and store less of it. Matter, unlike energy, cycles: carbon through photosynthesis, respiration, the oceans and fossil fuels; nitrogen through fixation, nitrification, ammonification and denitrification by bacteria; phosphorus from rock through living things and back to sediments; and water through evaporation, transpiration and precipitation.

Producers make food by photosynthesis or chemosynthesis. Consumers eat other organisms; decomposers break down dead matter. Each feeding step is a trophic level; linked food chains make a food web.
GPP is the energy producers capture; NPP = GPP − producer respiration, the energy available to consumers.
About 10% of the energy at one level is stored at the next (trophic efficiency); the rest becomes heat or goes to decomposers. That is why energy pyramids narrow upward.
Endotherms burn much of their food to make body heat, so they need more food and add less biomass per meal than ectotherms.
Matter cycles through biogeochemical cycles: carbon (photosynthesis, respiration, burning fossil fuels), nitrogen (fixation, nitrification, denitrification, all by bacteria), phosphorus (rock weathering, no gas phase) and water (evaporation, transpiration, precipitation).

Energy enters the ecosystem, which is gross primary productivity. What remains, net primary productivity, is the energy available to every consumer. Only about 10% of the energy stored at one level ends up stored at the next. Each level holds about a tenth of the energy of the one below, so top predators are few, and food chains rarely pass four or five levels. Carbon returns to the air as CO₂, and nitrogen and phosphorus return to soil and water as simple ions. Matter cycles through the ecosystem, but energy flows through it one way and must be supplied again by the sun.

community (ecology): All the populations of different species that live and interact in one area, such as every plant, animal, fungus and microbe in a pond.
ecosystem: A community together with the nonliving parts of its environment, such as water, soil, air, light and temperature, linked by the flow of energy and the cycling of matter.
producer: An organism that makes its own organic molecules from inorganic ones, capturing energy from light (photosynthesis) or from chemical reactions (chemosynthesis). Producers, such as plants, algae and cyanobacteria, form the first trophic level.
chemosynthesis: Making organic molecules from carbon dioxide using energy released by oxidizing inorganic chemicals, such as hydrogen sulfide, instead of light. Chemoautotrophic bacteria and archaea are the producers of deep-sea hydrothermal vent communities.
consumer: An organism that gets energy and matter by eating other organisms. Primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Carnivores eat animals, and omnivores eat both plants and animals.
decomposer: An organism, mainly bacteria and fungi, that breaks down dead bodies and wastes (detritus) and releases their atoms as simple inorganic substances, such as carbon dioxide, ammonium and phosphate, that producers can use again. Detritivores, such as earthworms, eat detritus and help this breakdown.
trophic level: An organism's feeding position in an ecosystem, counted in steps from the producers: producers are the first level, primary consumers the second, and so on.
food chain: A food chain is one path of feeding from a producer through consumers, with arrows pointing from the food to the eater (the direction energy moves). A food web is all the linked food chains in a community.
trophic efficiency: The percentage of the energy stored in one trophic level that ends up stored in the next, often about 10% (the 10% rule), and commonly between 5 and 20%. The rest is released as heat by cellular respiration or goes to decomposers. An energy pyramid shows the falling energy at each level.
biomass: The total mass of living (or recently living) organic matter at a trophic level or in an area, usually given as dry mass per unit area.
primary productivity: The rate at which producers store energy in organic matter. Gross primary productivity (GPP) is the total captured by photosynthesis or chemosynthesis; net primary productivity (NPP) is what is left after the producers' own respiration (NPP = GPP − respiration), which is the energy available to consumers.
energy budget of endotherms and ectotherms: Endotherms (birds, mammals) use much of their food to make metabolic heat that holds body temperature, so they need more food per gram of body mass and store less of it as new biomass than ectotherms, whose body temperature follows their surroundings.
biogeochemical cycle: A biogeochemical cycle is the path an element or compound (carbon, nitrogen, phosphorus, water) takes as it moves between living things and nonliving reservoirs, the places where it is stored, such as the air, the oceans, soil and rock.
carbon cycle: The movement of carbon: photosynthesis takes CO₂ from the air into organic molecules; cellular respiration and burning return it; oceans absorb and release it; and some is stored for long periods in fossil fuels (coal, oil, gas) and rock. Carbon dioxide is a greenhouse gas that traps heat near Earth's surface.
nitrogen cycle: The movement of nitrogen: nitrogen-fixing bacteria turn N₂ from the air into ammonium (nitrogen fixation); nitrifying bacteria turn ammonium into nitrate (nitrification); plants take up both; decomposers release ammonium from dead matter (ammonification); and denitrifying bacteria in low-oxygen soil turn nitrate back into N₂ (denitrification).
phosphorus cycle: The movement of phosphorus, which has no important gas form: weathering of phosphate rock releases phosphate into soil and water, producers take it up, it passes through food chains, decomposers return it, and some settles into sediments that become rock again over very long times.
water cycle: The movement of water: evaporation from oceans and land and transpiration from plants carry water vapor into the air; it condenses and falls as precipitation; it runs off into rivers and oceans or soaks into the ground.

8.3 Population Ecology

A population is described by its density (individuals per unit area) and its dispersion (clumped, uniform or random). Demography follows births, deaths and migration: with no migration, dN/dt = B − D, and dividing by N gives the per capita growth rate. Under ideal conditions the per capita rate reaches r_max, the intrinsic rate of increase, and the population grows exponentially, dN/dt = r_max N: a constant fraction is added each period, so the number added keeps rising and the curve is J-shaped. Life history traits set r_max. r-selected species mature early and produce many small offspring with little care, so they grow and recover fast; K-selected species mature late and raise few offspring with much care. Semelparous species reproduce once; iteroparous species reproduce repeatedly. Survivorship curves show how survival changes with age: Type I species lose few until old age, Type II lose a constant fraction at every age, and Type III lose most of their young.

Population density is individuals per unit area; dispersion is how they are spaced: clumped, uniform or random.
Demography tracks births, deaths and migration. Formula sheet: dN/dt = B − D; per capita rate = (B − D) ÷ N.
Exponential growth: dN/dt = r_max N. A constant per capita rate times a growing N gives a J-shaped curve.
Life history: r-selected species (early, many, small offspring, little care) have a high r_max; K-selected species (late, few, large offspring, much care) a low one. Semelparity is breeding once; iteroparity is breeding many times.
Survivorship curves: Type I (most live to old age), Type II (same fraction dies at every age), Type III (most die young).

The population size N changes at a rate dN/dt; with no migration, dN/dt = B − D. Dividing (B − D) by N gives the per capita growth rate; under ideal conditions it reaches its highest value, r_max. The more individuals there are, the more are added each period: growth speeds up even though the per capita rate stays the same. N plotted against time makes a J-shaped curve: exponential growth. Their numbers rebound quickly after a crash; species that breed late and have few young grow and recover slowly. Survivorship curves (Types I, II and III) show this pattern, which matches how much parents invest in each offspring.

population density: The number of individuals of a population per unit area or volume, such as 4 dandelions per square meter or 20 water fleas per liter.
dispersion pattern: How the individuals of a population are spaced within their area: clumped (in groups, often around patchy resources or in social groups), uniform (evenly spaced, often from territories or competition between neighbors) or random (no pattern).
demography: The study of how a population's size and makeup change through births, deaths, immigration and emigration. The birth rate and death rate are the numbers of births and deaths per unit time, often given per individual.
population growth rate: The population growth rate is the change in population size per unit time, dN/dt. With no migration, dN/dt = B − D, where B and D are the numbers of births and deaths per unit time. Dividing by N gives the per capita (per individual) growth rate.
intrinsic rate of increase: The intrinsic rate of increase, r_max, is the highest per capita growth rate a population can reach under ideal conditions, with plenty of resources: per capita births minus per capita deaths. It is high in species that reproduce early and often.
exponential growth: Population growth at a constant per capita rate, dN/dt = r_max N, so that more individuals are added in each period as N grows. Plotted against time, N makes a J-shaped curve. It happens only while resources are effectively unlimited.
life history: A species' pattern of when it starts to reproduce, how often, how many offspring it has and how much it invests in each. r-selected species mature early and produce many small offspring with little care; K-selected species mature late and produce few, well-cared-for offspring.
semelparity: Semelparity is reproducing once in a lifetime, usually with a huge effort, and then dying (Pacific salmon, agave, bamboo). Iteroparity is reproducing several or many times over a lifetime (oaks, humans, most birds).
survivorship curve: A survivorship curve plots how many of a group born at the same time are still alive at each age, usually per 1,000 on a log scale. Type I: most survive to old age. Type II: a constant fraction dies at every age. Type III: most die young. A life table lists the same survival data by age.

8.4 Effect of Density of Populations

No population grows exponentially for long. Limiting resources such as food, water, light and space set a carrying capacity, K: the largest population the environment can support over time. As N rises, intraspecific competition for those resources grows, so per capita births fall and deaths rise. Logistic growth captures this: dN/dt = r_max N (K − N)/K. The curve is S-shaped, growth is fastest at K/2, and it stops at K; a population above K shrinks. Density-dependent factors (competition, disease, wastes, predators) act harder in crowded populations and keep numbers near K. Density-independent factors such as frost, fire, floods and drought kill a similar fraction at any density and cause sudden drops. Linked prey and predator populations can rise and fall in regular population cycles, with the predator's peaks following the prey's.

A limiting resource (food, water, space, light, nutrients) caps growth. The largest population an environment can support over time is its carrying capacity, K.
Logistic growth: dN/dt = r_max N (K − N)/K. The (K − N)/K factor is the fraction of K still unused: near 1 when N is small, 0 at K, negative above K.
Growth is fastest at N = K/2; the curve is S-shaped.
Density-dependent factors (competition, disease, wastes, predators) act harder as density rises and hold N near K. Density-independent factors (weather, fire, floods) kill a similar fraction at any density.
Intraspecific competition is competition within a species. Linked prey and predator populations can rise and fall in population cycles, with the predator's peaks lagging the prey's.

The per capita growth rate is close to r_max, and the population grows almost exponentially. Intraspecific competition increases: individuals that lose get less food, find no nest site or grow less. The per capita birth rate falls and the per capita death rate rises: these factors are density-dependent. dN/dt = r_max N (K − N)/K: growth is fastest at K/2 and stops at K, giving an S-shaped curve. N falls back toward K, or crashes if the overcrowded population damaged its food supply and lowered K. It kills about the same fraction whatever the density (density-independent), so N drops suddenly, then regrows toward K.

limiting resource: A limiting resource is a resource in short enough supply that it caps the growth of a population, such as food, water, nesting sites, light or soil nutrients. When it runs low, individuals compete for it.
carrying capacity: The largest population of a species that an environment can support over time with its available resources, written K. It can change when the resources change, for example in a drought year or when a habitat is damaged.
logistic growth: Population growth that slows as the population nears the carrying capacity: dN/dt = r_max N (K − N)/K. Plotted against time, N makes an S-shaped curve; growth is fastest at N = K/2 and stops at N = K.
density-dependent factor: A factor whose effect on per capita birth or death rates grows stronger as population density rises, such as competition for food, disease, the build-up of wastes and predators catching crowded prey. Density-dependent factors pull a population toward its carrying capacity.
density-independent factor: A factor that kills about the same fraction of a population, or changes its birth rate, whatever its density, such as frost, drought, fire, floods and storms. It can cause sudden drops unrelated to crowding.
intraspecific competition: Competition between individuals of the same species for the same limited resources. It increases with population density and is a major density-dependent factor.
population cycle: Regular rises and falls in population size over several years, as in snowshoe hares and lynx, where the predator's peaks follow the prey's. They arise from density-dependent links such as food supply and predators that respond to their prey's numbers after a delay.

8.5 Community Ecology

A community's structure is which species it contains and how many of each. Species richness counts the species; relative abundance gives each species' share of individuals; species diversity combines the two, and Simpson's diversity index measures it, from 0 for one species toward 1 for many evenly common ones. Interactions shape the structure. Interspecific competition harms both species and shrinks each species' fundamental niche to a smaller realized niche; species with identical needs cannot coexist (competitive exclusion), but resource partitioning lets similar species share. Predation and herbivory benefit one species and harm the other, and they favor prey defenses such as camouflage, warning coloration and mimicry. Symbioses range from mutualism (+/+) through commensalism (+/0) to parasitism (+/−). A change in a top predator can pass down a food web in a trophic cascade, alternately raising and lowering each level below: top-down control, as opposed to bottom-up control by nutrients and producers.

Species richness counts species; relative abundance is each species' share of all individuals. Species diversity combines both, and Simpson's diversity index measures it (0 = one species; near 1 = many, evenly common).
Interspecific competition (−/−) shrinks a species' fundamental niche to a realized niche. Species with identical needs cannot coexist (competitive exclusion); resource partitioning lets similar species share.
Predation and herbivory (+/−) shape prey numbers and favor prey defenses: camouflage, toxins, warning coloration, Batesian and Müllerian mimicry.
Symbioses: mutualism +/+, commensalism +/0, parasitism +/−.
A trophic cascade passes a change at the top of a food web down through each level (top-down control); nutrients and producers set limits from below (bottom-up control).

Each interaction can be scored for each partner: competition −/−, predation, herbivory and parasitism +/−, mutualism +/+, commensalism +/0. The species that uses it more efficiently grows faster, and the other declines: competitive exclusion. Each lives in a smaller realized niche, competition eases, and both can coexist (resource partitioning). Prey with camouflage, toxins, warning colors or mimicry survive and reproduce more, so prey defenses spread. Their prey change in the opposite direction, and the prey's food changes in the opposite direction again: a trophic cascade. Community structure is described by species richness, relative abundance and an index such as Simpson's diversity index.

species richness: The number of different species in a community, whatever their numbers.
relative abundance: The share of all individuals in a community that belong to one species, such as 30 of 120 birds (25%). Species evenness describes how equal these shares are: a community where every species is about equally common is very even.
species diversity: A measure of a community's variety that combines species richness (how many species) with relative abundance (how evenly individuals are spread among them). Two communities with the same species can differ in diversity.
Simpson's diversity index: Simpson's diversity index, D, on the formula sheet, measures species diversity from the counts of each species. It is the chance that two individuals picked at random belong to different species: 0 for a single species, closer to 1 for many evenly common species.
community structure: The makeup of a community: which species are present (species composition), how many of each, and how they interact, including their feeding relationships.
interspecific competition: Competition between individuals of different species that use the same limited resource, such as two bird species eating the same seeds. It lowers the growth and survival of both (−/−).
fundamental niche: A species' fundamental niche is the full range of conditions and resources it could use in the absence of other species; its realized niche is the smaller part it actually uses when competitors, predators and other species are present.
competitive exclusion principle: The principle that two species that need exactly the same limiting resources cannot coexist in the same place for long: the one that uses the resources more efficiently grows and reproduces faster, and the other dies out locally or shifts its niche.
resource partitioning: Species with similar needs dividing a resource, such as feeding at different heights in a tree, at different times or on different sizes of seed, so that they compete less and can coexist. Character displacement is the related evolution of greater differences, for example in beak size, where two species live together.
predation: An interaction in which a predator kills and eats prey (+/−). Predators and prey shape each other's numbers and traits, including prey defenses.
prey defense: A trait that protects prey from predators, such as camouflage, spines, toxins, warning coloration (bright colors that signal a toxic or dangerous animal) or mimicry. In Batesian mimicry a harmless species looks like a harmful one; in Müllerian mimicry two harmful species look alike.
herbivory: An animal eating parts of a plant or alga (+/−). Unlike most predators, a herbivore usually does not kill the plant. Plants defend themselves with thorns, tough leaves and toxic chemicals.
commensalism: A symbiosis in which one species benefits and the other is neither helped nor harmed (+/0), such as barnacles riding on a whale or cattle egrets catching insects stirred up by grazing cattle.
comparing symbioses: Comparing symbioses by their effects on the two partners: mutualism +/+ (both benefit), commensalism +/0 (one benefits, the other is unaffected) and parasitism +/− (one benefits, the other is harmed). Predation and herbivory are +/−, and competition is −/−.
parasitism: A symbiosis in which a parasite lives on or in a host and feeds on it, harming but usually not quickly killing it (+/−), such as a tick on a deer or a tapeworm in a gut.
trophic cascade: A chain of effects that runs down a food web when the top predator's numbers change: fewer predators mean more of their prey, fewer of the prey's food, and so on, alternating level by level. Control from the top of the food web is top-down control; control by the supply of nutrients and producers at the bottom is bottom-up control.

8.6 Biodiversity

Biodiversity is the variety of life at three levels: the alleles within species, the species in a community and the ecosystems in a region. Ecosystems with more species and more genetic variation tend to be more stable: they lose less when a drought, disease or other disruption strikes and recover faster, because some individuals and species tolerate the stress and take over the jobs of those that suffer. Species do not matter equally. A keystone species has an effect much larger than its abundance; a predatory sea star that eats mussels keeps the mussels from crowding out other species, and removing it lowers diversity. Foundation species such as kelp, corals and forest trees build the habitat others need, and ecosystem engineers such as beavers reshape it. Producers set the energy available to every level above. People depend on biodiversity for ecosystem services such as pollination, clean water and flood control. Simpson's diversity index puts a number on species diversity so that communities can be compared.

Biodiversity has three levels: genetic diversity, species diversity and ecosystem diversity.
Ecosystems with more species and more genetic variation tend to be more stable: they resist disruptions and recover faster (resilience), partly because some species can take over the jobs of others.
A keystone species has an effect far larger than its numbers; removing it can collapse community structure.
Foundation species build habitat by being abundant (kelp, corals, dominant trees); ecosystem engineers physically change it (beavers). Producers set what every level above can have.
Ecosystem services, such as pollination, clean water and flood control, depend on biodiversity. Simpson's index lets you compare diversity with numbers (skill: Simpson's diversity index).

Its individuals and species differ in how they cope with a drought, a disease or a cold winter. Tolerant ones survive and use the light, water and space the others leave, so some of each job in the ecosystem still gets done. Diverse ecosystems tend to be more stable and resilient than species-poor ones. The strong competitor cannot take over, so many other species keep room to live. The strong competitor excludes others, and diversity falls, though the keystone species itself was never abundant. Many other species depend on them, so losing them reshapes the whole community.

biodiversity: The variety of life in an area, at three levels: genetic diversity (the variety of alleles within species), species diversity (the number and evenness of species) and ecosystem diversity (the variety of ecosystems).
ecosystem diversity: The variety of ecosystems in a region, such as forests, wetlands, grasslands and streams, each with its own community and conditions. It is the broadest of the three levels of biodiversity.
ecosystem stability: An ecosystem's ability to keep functioning when conditions change: to resist a disruption (lose little) and to recover after it (resilience). More diverse communities, with more species and more genetic variation, tend to be more stable.
keystone species: A species whose effect on its community is far larger than its abundance or biomass would suggest, often a predator that holds down a strong competitor. Removing it changes community structure sharply, usually lowering diversity.
foundation species: A species that strongly shapes its community by being abundant or large and creating or forming the habitat, such as kelp, reef-building corals or the dominant trees of a forest. Ecosystem engineers, such as beavers, physically change the environment in ways that create habitat for others.
ecosystem services: Benefits people receive from ecosystems, such as pollination of crops, clean water filtered by wetlands and soils, flood control, fisheries, timber, carbon storage and recreation.

8.7 Disruptions to Ecosystems

Ecosystems are disrupted by human activity and by natural events. Habitat loss and fragmentation leave too little space for many species. Invasive species spread when they escape the predators, parasites and competitors of their home range and can outcompete or eat native species. Overharvesting removes individuals faster than populations replace them. Pollution adds harmful substances: excess nitrate and phosphate cause eutrophication, in which algal blooms die, decomposers use up dissolved oxygen and dead zones form; persistent, fat-soluble toxins such as mercury, DDT and PCBs biomagnify, reaching their highest concentrations in top predators. CO₂ from burning fossil fuels and clearing forests drives climate change, which shifts ranges and the timing of seasonal events, and ocean acidification, which leaves less carbonate for shells and corals. Natural disturbances such as volcanic eruptions, hurricanes, fires and El Niño events also change ecosystems suddenly. Diverse ecosystems tend to resist and recover from these disruptions better.

Human impact lowers biodiversity through habitat loss, invasive species, overharvesting, pollution and climate change.
Invasive species spread when they escape the predators, parasites and competitors of their home range.
Eutrophication: extra nitrate and phosphate cause an algal bloom; decomposers of the dead algae use up O₂, creating a dead zone.
Biomagnification: persistent, fat-soluble toxins (mercury, DDT, PCBs) reach their highest concentrations in top predators.
Extra CO₂ warms the climate and causes ocean acidification, which makes shells harder to build. Natural disturbances (eruptions, El Niño, fires, storms) also cause sudden change.

Algae, which were limited by those nutrients, grow and multiply fast: an algal bloom. Dead algae sink into deeper water. They use dissolved oxygen faster than it can mix down from the surface. Fish swim away or die, and animals that cannot leave suffocate: a dead zone. Biodiversity falls until nutrient inputs drop and oxygen returns.

invasive species: An invasive species is a non-native (introduced) species that spreads rapidly in a new region and harms native species or the ecosystem, often because it has escaped the predators, parasites and competitors that held it in check in its home range.
habitat loss: The destruction or breaking up of the places species live, for example by clearing forests for farms or building on wetlands. Habitat fragmentation leaves small, isolated patches that support only small populations and have more edge.
overharvesting: Killing or removing individuals of a species faster than the population can replace them, as in overfishing or overhunting. Species that mature late and have few offspring are hit hardest.
pollution: Harmful substances or energy added to the environment, such as pesticides, heavy metals, plastics, excess nutrients, or heat from power plants. A pollutant is any such substance.
climate change: Long-term change in Earth's climate. Today it is driven mainly by greenhouse gases, chiefly CO₂ from burning fossil fuels and clearing forests, which trap more heat (global warming), shifting temperatures, rainfall and the timing of seasons.
ocean acidification: The fall in seawater pH as the oceans absorb more CO₂ from the air. CO₂ reacts with water to form carbonic acid, and the extra H⁺ ions lower the amount of carbonate available to animals that build shells and skeletons, such as corals, oysters and some plankton.
eutrophication: Over-enrichment of water with nutrients, mainly nitrate and phosphate from fertilizer runoff and sewage. It causes algal blooms; when the algae die, decomposers use up dissolved oxygen, which can kill fish and create dead zones.
biomagnification: The rise in concentration of a persistent, fat-soluble toxin, such as mercury, DDT or PCBs, at each higher trophic level. Each consumer eats many times its own mass of food and stores the toxin instead of excreting it. Bioaccumulation is the build-up of a toxin in one organism over its lifetime.
natural disturbance: An event not caused by people that suddenly changes an ecosystem, such as a volcanic eruption, a hurricane, a wildfire started by lightning, a flood, or an El Niño event, in which warm surface water spreads across the tropical Pacific and shifts weather and ocean food supplies.
human impact: The effects of human (anthropogenic) activity on ecosystems, including habitat loss, invasive species, overharvesting, pollution and climate change, which together are lowering biodiversity worldwide.