Chapter 36. Ecology
Recommended Articles: 【Biology】 Biology Table of Contents
1. Population
2. Community
3. Ecosystem
1. Population
⑴ Population: All individuals of a species living in a particular area.
⑵ Density fluctuations in populations
① Factors of increase: High birth rate, immigration
② Factors of decrease: High death rate, emigration
⑶ Survival of populations
① Human survivorship type (Type I): High death rate in old age
② Hydra survivorship curve (Type II)
③ Oyster survivorship curve (Type III): High death rate in youth
⑷ Population Structure
① Population structure: characteristics of density (abundance) and spatial distribution (dispersion)
② Measuring population size: census, mark–recapture, quadrat method, line-transect method, belt-transect method, etc.
○ Quadrat method: place a quadrat frame over the study area and count individuals within it.
Figure 1. Quadrat method
○ Density = number of individuals ÷ total area; A = 10/25, B = 4/25, C = 6/25
○ Relative density = number of individuals ÷ total number of individuals; A = 10/20, B = 4/20, C = 6/20
○ Frequency = number of quadrats in which the species occurs ÷ total number of quadrats; A = 6/25, B = 4/25, C = 6/25
○ Cover = area occupied by a species ÷ total area
○ Importance = sum of relative density, relative frequency, and relative cover
○ Species richness
○ Species diversity
○ Dominant species: the species with the greatest importance; i.e., a species with high biomass or many individuals that exerts major effects on the community
○ Keystone species: a species whose extinction would have the greatest impact on the ecosystem; i.e., one with relatively few individuals but a large ecological effect
○ Indicator species: a species found only (or characteristically) in a particular community
○ Rare species: a species with few individuals within the community
○ Foundation species: a species whose predator’s removal can lead to the disappearance of other species
○ Pioneer species: a species that creates or modifies habitat conditions in a new site (e.g., beavers)
③ Spatial distributions of populations
○ Clumped (aggregated) distribution: high density where resources are abundant
○ Uniform (regular) distribution: associated with territorial behavior
○ Random distribution: seen in nonsocial species with broad environmental tolerance; no specific drivers of pattern
④ Allee effect: the principle that populations have an optimal density
○ Very low densities can reduce growth or survival.
○ Relatedly, population size is proportional to habitat area.
⑸ Exponential Growth Model
① Growth rate (r): the percentage change in the size of a population over one year
○ r = birth rate − death rate
○ Birth and death rates are constants: the numbers of births and deaths are assumed to increase in proportion to population size.
② Population size (S) = S0(1 + r)n
○ S0: initial population size
○ n: number of years elapsed since the reference year
③ Limitation 1. Excessive population growth leads to high mortality: risk of extinction is high.
○ Density-dependent factors: resource shortages, increased disease, increased pollutants
○ Density-independent factors: events such as drought, extreme temperatures, and natural disasters
④ Limitation 2. There is a time lag between individual birth and reproduction → environmental effects do not appear immediately.
○ Demographic inertia: the time lag between when the birth rate declines and when population growth begins to decrease
⑹ Logistic Growth Model: addresses Limitation 1
① Assumes that the per-unit-time population growth rate is affected by population size and environmental resistance.
○ Carrying capacity (K): the maximum population the environment can support
○ Environmental resistance: for population size N, corresponds to 1 − K/N
○ ∴ dN/dt = rN(1 − N/K)
② r-selection: high population growth rate; small body size; many offspring; short generation time
○ Environmental resistance is close to zero, so growth roughly follows a J-shaped exponential model: density-independent growth
○ Features of the J shape: reproduction-oriented; short lifespan; small body size
③ K-selection: low population growth rate; large body size; few but high-quality offspring
○ As the population approaches carrying capacity, the growth rate nears zero.
○ Long lifespan and large body size; repeated reproduction; long developmental time; intense competition among individuals
○ K-selected organisms tend to reproduce at older ages; they have fewer offspring and populations are smaller.
Figure 2. Population growth models
(가): Exponential growth model, (나): Logistic Growth Model
⑺ Demographic transition: when the birth rate remains unchanged but the death rate declines, the growth rate increases
① Before the Industrial Revolution: most countries had both high birth and high death rates.
② After the Industrial Revolution: improved sanitation and advances in medicine reduced death rates.
③ The time lag between death rates and birth rates has a major impact on population size.
④ Many developing countries are in the demographic transition phase → implementation of family-planning policies.
⑤ Developed countries have passed through the transition and have low population growth rates → childcare/education welfare and immigration policies.
Figure 3. Demographic transition
2. Community
⑴ Community: the assemblage of all living populations inhabiting a given area.
⑵ Food web: the network of who-eats-whom relationships.
① Ecological niche (niche): the role an organism occupies in an ecosystem—the sum of the biotic and abiotic resources it uses.
○ Influenced by habitat and by diet/feeding mode.
○ A species’ fundamental niche and realized niche can differ depending on environmental conditions.
② Species within a community occupy unique ecological niches and interact with one another.
○ Ecotype: distinct forms that arise when the same species adapts to different environments.
③ Interactions in a community
○ Interaction 1. Mutualism (+/+): both interacting species benefit.
○ Examples: bees and flowering plants; large fishes and cleaner fish; ants and acacia trees.
○ Example: root-nodule bacteria in legumes.
○ Example: nutrient exchange between fungi and plant roots in mycorrhizae.
○ Interaction 2. Commensalism (+/0): only one species benefits, the other is unaffected.
○ Interaction 3. Predation (+/−): a predator consumes a prey organism.
○ Example: plant → insect → bird.
○ Lotka–Volterra model: when predator–prey relations are simple, population sizes oscillate.
Figure. 4. Lotka-Volterra model
○ Mathematical formulation: for ni (population size of the i-th population), ri (weight), and Ai←j (interaction coefficient representing how the j-th population affects the i-th population), dni/dt = ni ( ri + Σj Ai←j nj).
○ Interaction 4. Parasitism (+/−): a relationship in which a parasite obtains nutrients from its host.
○ Endoparasites: tapeworms, malaria, etc.
○ Ectoparasites: ticks, etc.
○ Interaction 5. Amensalism (0/−): a relationship in which only one species is harmed.
○ Interaction 6. Competition (−/−): competing for the same resources.
○ Example: mosquitoes, snails, and tadpoles compete for pond resources.
○ Supplement 1. Competitive Exclusion Principle
○ Two populations compete for limited resources; if the competition intensifies, competitive exclusion may occur.
○ Competitive exclusion: one species monopolizes food and space resources so that another species cannot establish.
○ Example: a strategy to prevent S. enteritidis from colonizing the chick’s intestine.
○ Chicks hatch with intestines essentially free of bacteria.
○ Untreated: few or no bacteria in the gut → S. enteritidis infection proceeds actively.
○ Ingestion of beneficial bacteria: large colonies of beneficial bacteria form on the inner intestinal surface.
○ Expose to Salmonella enteritidis.
○ Untreated: S. enteritidis infection proceeds actively.
○ Ingestion of beneficial bacteria: S. enteritidis fails to find space to form colonies and is excreted.
○ Supplement 2. Mimicry
○ Batesian mimicry: a palatable, harmless species imitates an unpalatable, harmful model.
○ Müllerian mimicry: two or more unpalatable species resemble each other, prompting predators to avoid all of them.
○ Supplement 3. Hamilton’s Rule
○ B: the benefit to the recipient, i.e., the expected number of offspring if the recipient survives.
○ C: the cost to the altruist, i.e., the reduction in offspring due to death (or other costs) from the altruistic act.
○ r: the coefficient of relatedness between individuals, i.e., the probability of sharing a particular allele.
○ Altruism is favored when B × r > C.
○ Supplement 4. Edge Effect
○ The edges of available habitat patches exhibit characteristics different from the interior.
○ Trait 1: at edges, the frequency of competition with adjacent habitats increases.
○ Trait 2: if the habitat is forest, wind exposure increases, temperature rises, humidity decreases, and light intensity increases.
○ As a result, edge species and species from adjacent habitats are favored over interior specialists.
○ Supplement 5. Habitat Fragmentation
○ Definition: through human activities and other causes, habitats are progressively destroyed so that the remaining habitats become smaller and more isolated.
○ Outcome 1: only populations able to live in small habitats can persist.
○ Outcome 2: edge effects increase.
④ Roles within a community: producers, consumers (predators), decomposers
○ Producers: raise the energy level of the community; green plants that synthesize organic matter from inorganic substances.
○ Consumers: lower the energy level of the community; animals that obtain energy by consuming organic matter.
○ Decomposers: lower the energy level of the community; organisms that live by decomposing the carcasses and excreta of plants and animals, primarily microorganisms.
⑤ Keystone species: species that play a pivotal role in structuring the food web.
○ Example: sea stars feed on mussels; without the keystone sea star, mussel populations explode and disrupt the ecosystem.
⑥ Ecosystem energy flow: only ~10% at each trophic level is converted into biomass at the next level.
○ The amount of solar energy and the availability of water are key drivers of energy flow.
○ Biodiversity also influences energy flow.
○ Example: grasslands generate large amounts of biomass.
⑶ Nutrient Cycling: Recycling of inorganic nutrients through the food web
① Nutrient Efficiency: The ratio of energy transferred from one trophic level to the next, typically around 10%.
② Nitrogen Cycling
Figure 5. Nitrogen Cycling
○ Nitrogen Fixation and Others: Atmospheric nitrogen (N2) → Ammonia (NH3) → Ammonium (NH4+)
○ Case 1: High-energy fixation (lightning, meteorite)
○ Case 2: Nitrogen fixation (anaerobic conditions): Soil bacteria, rhizobia, azotobacter, cyanobacteria
○ Case 3: Nitrogen from corpses or waste products
○ Nitrification Process: Ammonium (NH4+) → Nitrite (NO2-) → Nitrate (NO3-)
○ Nitrosomonas: Ammonium (NH4+) → Nitrite (NO2-)
○ Nitrobacter: Nitrite (NO2-) → Nitrate (NO3-)
○ Chemical synthesis: Nitrosomonas and Nitrobacter synthesize sugar using energy released during oxidation of organic matter.
○ Denitrification Process: Nitrate (NO3-) → Nitrogen gas (N2)
○ Denitrifying bacteria: Nitrate (NO3-) → Nitrogen gas (N2). Anaerobic heterotrophs. In the respiratory electron transport chain, they use nitrite and nitrate as terminal electron acceptors instead of oxygen.
○ Plants can use either Ammonium (NH4+) or Nitrate (NO3-).
③ When phytoplankton increase, decomposers consume more oxygen to break down their dead biomass, leading to deterioration of water quality.
⑷ Extinction
① Extinction: when a species disappears completely.
② Measuring extinction rates
○ Extinction rate: the percentage of all species that go extinct in a given year.
○ Background extinction rate: the rate at which species disappear through normal evolutionary processes; 0.0001% per year.
○ Current extinction rate: 0.0037% per year, clearly suggesting the involvement of human activities.
③ Cause 1. Habitat destruction
○ Population growth increases pressure on natural areas.
○ Species–area curve: the number of species a natural area can support.
④ Cause 2. Habitat fragmentation
○ 1st. Reduces the area available to producers in the food chain.
○ 2nd. Top consumers need many calories and large home ranges, increasing environmental pressure.
○ 3rd. Decline in the population size of top consumers.
○ 4th. Inbreeding among top consumers accelerates extinction.
⑤ Cause 3. Invasive species
○ Invasive (non-native) species: species newly introduced through human activities.
○ Because they did not coevolve with local species, they often lack natural enemies and are destructive.
○ Examples: bluegill, American bullfrog, red-eared slider.
⑥ Cause 4. Overharvesting: using natural resources at a rate exceeding their reproductive capacity.
⑦ Cause 5. Pollution
○ Herbicides: threaten frogs and salamanders.
○ Use of agricultural fertilizers → eutrophication of waterways leading to algal overgrowth → oxygen depletion → mass fish die-offs.
○ Carbon dioxide: an air pollutant associated with climate change.
⑧ Cause 6. Inbreeding: the Great Irish Potato Famine (1847–1852) is a representative case.
○ At the time, the ‘Lumper’ variety, a South American import, was cultivated.
○ Extinction mechanism: positive feedback.
○ 1st. Small populations → inbreeding, genetic drift.
○ 2nd. Inbreeding and genetic drift → loss of genetic variation.
○ 3rd. Loss of genetic variation → reduced fitness and population adaptability.
○ 4th. Reduced fitness and adaptability → high mortality, low reproduction.
○ 5th. High mortality and low reproduction → even smaller populations.
○ Potato famine mechanism
○ 1st. Potatoes were propagated asexually → genetic diversity = 0.
○ 2nd. The late blight fungus arrived and potatoes rotted in the fields.
○ 3rd. Collapse (extinction) of the potato crop.
○ 4th. Of 9 million people, 1.5 million died and 1 million emigrated to North America.
3. Ecosystem
⑴ Ecosystems and Biomes
① Ecosystem: Abiotic environmental factors (light, temperature, water, air, soil) + biotic environmental factors.
② Biome: A typical ecosystem spread over a broad region.
○ A biome is a wide geographic area characterized by the dominant form(s) of vegetation.
③ Energy in ecosystems
○ Gross primary production (GPP): The total amount of organic matter produced by producers via photosynthesis; i.e., GPP = respiration + net primary production.
○ Net primary production (NPP): GPP minus the respiration of photosynthetic organisms.
○ NPP can be subdivided into biomass consumed by herbivores, plant mortality, leaf-litter (litterfall), and growth increment.
○ Ingestion by primary consumers: equal to the amount of producer biomass consumed.
○ The fate of primary consumers’ ingestion can be subdivided into respiration, excretion, carcass (mortality), and growth, etc.
○ Net community production (NCP): NPP minus heterotrophs’ respiration.
○ Production efficiency = growth ÷ ingestion.
○ Homeotherms (endotherms) have low production efficiency because much energy is used to maintain body temperature.
④ Trophic (ecological) pyramids
○ Pyramid of numbers: diagrams the number of individuals at each trophic level.
○ Pyramid of biomass: graphs biomass (dry mass); clearly reflects the food chain.
○ Pyramid of energy: shows energy flow and losses between trophic levels.
④ Ecosystem development (plant succession)
○ Stage 1. Succession: Changes in plant communities over time at the same site.
○ Stage 1-1. Primary succession: Succession starting where no plant community previously existed; lichens become the dominant species.
○ Stage 1-2. Secondary succession: Succession beginning after disturbance (e.g., wildfire) where soil nutrients and some community remnants remain; herbaceous plants become dominant.
○ Stage 2. Herbaceous plants → shrubs → trees.
○ Stage 3. Light-demanding forest: shade-intolerant species appear.
○ Definition: plants adapted to strong light.
○ Traits: high light-compensation point and high light-saturation point.
○ Example: pines.
○ Stage 4. Shade-tolerant forest: shade-adapted species appear.
○ Definition: plants adapted to weak light.
○ Traits: low light-compensation point and low light-saturation point.
○ Example: oaks.
○ Stage 5. Extreme: the final stage of ecosystem development.
○ Shade-intolerant species appear first, followed by shade-adapted species.
○ Species diversity increases in early stages and then decreases again later.
⑵ Terrestrial biomes: determined by vegetation types—forests, grasslands, deserts, and tundra
① Vegetation: plant cover that blankets the Earth’s surface; “climate → vegetation” (∴ depends on altitude and latitude)
② Forests: plant communities dominated by woody plants; cover ~1/3 of the land surface and hold ~70% of Earth’s biomass
②―① Tropical forest: intense solar radiation; 25–29 °C year-round; abundant rainfall (≥2,500 mm); equatorial to sub-equatorial zones
○ Very high biodiversity—e.g., up to 750 woody species per hectare → provides habitat for diverse organisms.
○ Rapid decomposition of organic matter and rapid re-uptake of nutrients by plants → low soil organic matter
○ Most mineral nutrients are locked in the vegetation; without heavy fertilization, soils are poor for agriculture.
○ Slash-and-burn fields last only 4–5 years; recovery takes decades.
○ Tropical communities are older than most other biotic communities → much longer periods for speciation → higher species diversity
○ In general, the greater the evaporation, the higher the species diversity.
②―② Temperate deciduous forest: ample water (750–2,500 mm) and light during the growing season but not in winter; ~23°–50° N/S
○ Dominant life form: broad-leaved trees
○ Typical succession: weeds → shrubs → broad-leaved trees → broad-leaved trees + shade plants
○ Regions: mid-latitudes of the Northern Hemisphere; Australia
○ Deciduous habit evolved to conserve water; as chlorophyll degrades, accessory pigments (e.g., tannins, anthocyanins, carotenoids, etc.) previously masked by sugars and chlorophyll produce autumn colors.
○ Shade-tolerant plants grow and flower rapidly during periods when light reaches the forest floor.
②―③ Coniferous (boreal/taiga) forest: winters to −50 °C, summers to ~20 °C; 300–700 mm precipitation; near the sub-polar zones (North America, Asia, Northern Europe)
○ Dominant taxa: pines and other conifers (e.g., Pinus, Abies)
○ The largest terrestrial biome on Earth.
○ Adapted to long, cold winters and humid summers.
○ Evergreen habit evolved: photosynthesis halts when tissues are frozen in winter; once thawed, existing chlorophyll allows photosynthesis to resume immediately.
②―④ Mediterranean-type shrubland (chaparral): spring 10–12 °C, summer ~30 °C; 300–500 mm annually (dry summers); found in the Mediterranean Basin, southern California, and southwestern Australia
○ Dominant life form: evergreen shrubs (e.g., olive, fig)
○ Long, dry summers and frequent fires → fire adaptations (e.g., seeds germinate after exposure to high heat.)
○ Example: Mediterranean area has hot, dry summers; has cool, wet winters.
③ Grasslands: 250–500 mm precipitation; 8–9-month dry season; dominated by herbaceous plants.
○ Example 1. Savanna: equatorial to sub-equatorial; tropical grassland
○ Warm throughout the year.
○ Example 2. Prairie: temperate grassland (e.g., South African veld, the Hungarian Puszta, the Pampas, central North American prairies)
○ Mean winter temperature below −10 °C; mean summer temperature ~30 °C
○ Dominant growth form: grasses with basal meristems that regrow after grazing.
○ Periodic fires in the dry season; vegetation recovers during the rainy season.
④ Deserts: mean 24–29 °C; <250 mm annual precipitation; commonly around ~30° N/S
○ Large diurnal temperature range: −30 to 50 °C
○ Habitat for annuals that complete their life cycle within 2–3 weeks during brief rainy periods.
○ Plant adaptations: waxy coatings, spines, water storage in columnar stems; adaptations to frequent fire
○ Animal adaptations (e.g., kangaroo rats): nocturnal and minimize water loss.
○ Example: kangaroo rats are active only at night and survive on water contained in seeds plus metabolic water from breaking down seed fats.
⑤ Tundra: winters below −30 °C; short summers with temperatures ≤10 °C
○ Examples: northern Norway; northern Canada
○ Plant growing season: ~50–60 days (about two months) in summer
○ Permafrost: frozen soil impedes drainage and restricts plant growth.
○ Strong winds and low temperatures favor prostrate (ground-hugging) growth forms.
○ Effect of global warming: some tundra areas are transitioning to coniferous forest.
○ Animal adaptations: fat storage, insulating fur/feathers, hibernation, migratory behavior
Figure 6. Terrestrial Ecosystems
⑶ Aquatic biomes (aquatic systems)
① Freshwater: salinity ≤ 0.1%
①―① Lakes and ponds (freshwater): bodies of water surrounded by land
○ May dry up seasonally and serve as key habitats for frogs and salamanders.
○ Seasonal changes in wind and temperature drive nutrient cycling in lakes and ponds.
○ Eutrophication from fertilizers applied to nearby farmland and lawns causes algal blooms.
①―② Rivers and streams (freshwater): one-directional flowing watercourses
○ Headwaters: cold, clear, fast-flowing
○ Mid-reaches: warmer; algal growth increases, providing food for a wider array of animals.
○ Lower reaches: flow slowly toward the sea with high sediment loads; low light penetration reduces photosynthesis; decomposers increase; dissolved oxygen declines; benthic deposit-feeding animals increase.
①―③ Wetlands (freshwater): standing-water areas where aquatic plants grow
○ Difference from lakes/ponds: soils surrounding wetlands are also highly water-saturated.
○ As species-rich as tropical rainforests.
○ Functions: slow water flow to moderate flooding; filter toxic substances and sediments from water.
○ In the United States, more than 50% of wetlands have been lost or degraded.
② Marine: ~75% of Earth’s surface
②―① Oceans (marine): ~two-thirds of Earth’s surface; salinity ~3.4–3.5%
○ Example 1. Intertidal zone: water ebbs and floods with the tides; nutrient-rich.
○ Example 2. Abyssal plain: low temperatures, high pressure; unique biota at hydrothermal vents.
○ Among the least well-known biomes.
○ About 50% of atmospheric oxygen is produced by photosynthetic algae in the oceans.
○ Produces most freshwater via evaporation.
○ Marine biodiversity has declined by ~50% over the past 50 years due to overfishing.
②―② Coral reefs (marine): habitats built from the exoskeletons of corals
○ Occur on tropical continental shelves (<200 m depth) with warm, clear water.
○ The most diverse aquatic habitats; species density per unit area rivals that of tropical rainforests.
○ Corals filter organic matter and live in symbiosis with algae, making them sensitive to environmental change.
②―③ Estuaries (marine): where river freshwater enters the sea
○ Salinity varies widely: near-marine in dry periods, near-freshwater during rains
○ Nursery grounds for ~75% of commercially traded fish populations and for abundant shellfish.
○ Salt-marsh vegetation buffers against erosion and stabilizes shorelines.
○ Threatened by habitat loss and eutrophication from human activities.
⑷ Environmental Microbiology
⑸ Human habitats: humans have altered ~50% of Earth’s surface.
① Energy and natural resources
○ Energy use (e.g., fossil fuels): reliance on imported energy resources can dull awareness of environmental damage.
○ Natural resources: sustaining life requires substantial natural materials—food, housing, water, etc.
○ Ecological footprint: the land area required to support human activities, which is much larger than the area actually occupied.
② Waste production
○ Sewage pollution: factories discharge semi-solid waste as sludge and chemicals as industrial effluent → more advanced wastewater treatment in developed countries
○ Solid waste: most solid waste is stored in sanitary landfills (in developed countries) or open dumps (in developing countries).
③ Air pollution
○ Primary pollutants: emitted from burning fossil fuels such as coal and oil.
○ CO2, CH4: absorb infrared radiation and cause the greenhouse effect.
○ SO2, SO3: dissolve in water to form acids, producing acid rain; lichens are indicator species of acid rain.
○ N2O, NO, NO2: contribute to acid rain or act as secondary pollutants.
○ Hydrocarbons: precursors of secondary pollutants; irritate eyes and bronchi.
○ CFCs: deplete the ozone layer.
○ Fine particulate matter (PM)
○ Secondary pollutants: formed when primary pollutants undergo reactions under ultraviolet light.
○ Ozone, formaldehyde, PAN: cause respiratory diseases.
○ Smog episodes: involve formaldehyde, PAN, SO2
④ Water pollution
○ Dissolved Oxygen (DO)
○ Biochemical Oxygen Demand (BOD)
○ Chemical Oxygen Demand (COD)
⑤ Soil pollution and biomagnification
○ Soil acidification from chemical fertilizers and pesticides
○ Mercury (Minamata disease), cadmium (Itai-itai disease)
Input: 2020.10.25 19:34