Ecology — Populations, Ecosystems and Homeostasis

Ecology is the study of how organisms interact with each other and their environment. It operates at multiple levels of organisation: Individual (organism): Physiological and behavioural responses to the environment. Adaptation is the process by which populations become suited to their environments through natural selection. Population: All individuals of the same species in a given area. Population ecology studies how populations change over time — birth rates, death rates, immigration, emigration. Population size N changes as: dN/dt = B − D + I − E. Community: All populations of different species in a given area. Community ecology studies species interactions: predation, competition, symbiosis (mutualism, commensalism, parasitism). Ecosystem: A community plus its abiotic (non-living) environment. Ecosystems include energy flow (sunlight → producers → consumers → decomposers) and nutrient cycling (carbon, nitrogen, phosphorus, water cycles). Biosphere: The sum of all ecosystems on Earth — all living organisms and their environments from the deep ocean floor to the upper atmosphere. Habitat vs Niche: Habitat: where an organism lives (the address). Niche: the role an organism plays in its ecosystem (the occupation) — including diet, behaviour, habitat use, interactions. No two species can occupy exactly the same niche in the same area for long (competitive exclusion principle).

Populations grow, fluctuate, and sometimes decline according to predictable patterns. Exponential growth: When resources are unlimited, populations grow exponentially: dN/dt = rN, where r is the intrinsic rate of natural increase. Population size doubles repeatedly at a constant interval. Shown by: bacteria in a new culture, invasive species arriving in a new habitat. Logistic growth: As populations grow, resources become limiting. Growth rate slows as N approaches the carrying capacity (K) — the maximum population size the environment can sustain: dN/dt = rN × (K−N)/K. The resulting S-shaped (sigmoidal) growth curve characterises most real populations. Density-dependent limiting factors: Competition for food, water, space. Predation (increases with prey density). Disease (spreads more easily at high density). Intraspecific competition (within species). These factors increase mortality or decrease reproduction as population grows — regulating population size. Density-independent limiting factors: Climate (drought, frost), natural disasters (floods, fires), pollution. Affect population regardless of density. Life history strategies: r-selected species: short-lived, reproduce early and often, produce many small offspring, little parental care. Adapted to unpredictable environments. Examples: mice, insects, annual plants. K-selected species: long-lived, reproduce late, produce few large offspring, significant parental investment. Adapted to stable environments near carrying capacity. Examples: elephants, humans, oak trees. Age structure: The proportion of a population at different age classes affects future population growth. A population with many young individuals will grow; one with many old individuals will decline.

Species in a community interact in multiple ways, categorised by effect on each species (+, −, or 0): Competition (−/−): Both species negatively affected. Interspecific competition (between species) leads to competitive exclusion (one species eliminates the other) or resource partitioning (species evolve to use different resources, coexisting). Character displacement: competing species evolve to become more different when sympatric. Predation (+/−): Predator benefits, prey is harmed. Drives co-evolution — prey evolves defences, predators evolve to overcome them. Population cycles: Lotka-Volterra equations describe coupled oscillations of predator-prey populations (hare-lynx cycle). Symbiosis: Mutualism (+/+): both species benefit. Mycorrhizal fungi and plant roots (fungus gets carbohydrates, plant gets water and minerals). Gut microbiome and humans. Nitrogen-fixing bacteria in legume root nodules. Commensalism (+/0): one benefits, other unaffected. Barnacles on whales. Cattle egrets following large mammals. Parasitism (+/−): parasite benefits, host harmed. Different from predation — parasite usually doesn't kill the host immediately. Malaria parasite (Plasmodium), tapeworms, HIV. Keystone species: A species with disproportionately large effects on its ecosystem relative to its abundance. Sea otters (eat urchins, which destroy kelp forests). Wolves reintroduced to Yellowstone triggered a trophic cascade — reduced elk grazing, allowed riverside vegetation to recover, changed river flow. Ecological succession: Progressive change in community composition over time. Primary succession: colonisation of bare rock (lichens → mosses → grasses → shrubs → trees). Secondary succession: recovery after disturbance (faster, soil already present). Climax community: the stable endpoint.

Ecosystems require continuous energy input but recycle matter. Energy flow: Energy flows one way through ecosystems — it cannot be recycled. Primary producers (plants, algae) capture solar energy via photosynthesis. Primary consumers eat producers. Secondary consumers eat primary consumers. Each trophic level contains only ~10% of the energy of the level below (10% rule) — the rest is lost as heat (second law of thermodynamics). This is why food chains rarely exceed 4–5 levels. Gross primary production (GPP): total photosynthesis. Net primary production (NPP): GPP − plant respiration = energy available to primary consumers. Carbon cycle: Carbon cycles between atmosphere, biosphere, oceans, and geological reservoirs. Photosynthesis removes CO₂; respiration, decomposition, and combustion release it. The burning of fossil fuels has increased atmospheric CO₂ from ~280 ppm (pre-industrial) to >420 ppm (2024), driving climate change. Nitrogen cycle: Nitrogen (N₂) comprises 78% of air but most organisms can't use it directly. Nitrogen-fixing bacteria (Rhizobium in legume root nodules, free-living Azotobacter) convert N₂ to ammonia (NH₃). Nitrification converts NH₃ to nitrites then nitrates (used by plants). Denitrification converts nitrates back to N₂. Medical relevance: Bioaccumulation and biomagnification: persistent pollutants (DDT, mercury, PCBs) accumulate in organisms and concentrate up the food chain (biomagnification). Methylmercury in large ocean fish (tuna, swordfish) — public health advice limits consumption, especially in pregnant women. Zoonoses: infectious diseases transmitted from animals to humans (60% of emerging infectious diseases). Ebola (bats), HIV (chimpanzees), influenza (birds/pigs), SARS-CoV-2 (likely bats → intermediate host → humans). Ecological disruption increases human-animal contact, driving emergence.

Biodiversity: Biodiversity encompasses genetic diversity (variation within species), species diversity (number and evenness of species), and ecosystem diversity (variety of habitats and communities). Biodiversity is important for: Ecosystem services: provisioning (food, water, medicine), regulating (climate, disease, pollination), cultural (recreation, aesthetics). Ecosystem stability: diverse ecosystems are more resistant to disturbance (redundancy of function). Pharmaceutical discovery: ~50% of drugs are derived from or inspired by natural compounds. Aspirin from willow bark, penicillin from Penicillium mould, taxol from Pacific yew tree bark. Biodiversity crisis: Earth is experiencing its sixth mass extinction — the current extinction rate is estimated at 100–1,000 times the natural background rate. Primary causes (HIPPO): Habitat destruction, Invasive species, Pollution, Population (human), Overexploitation. Conservation medicine: The intersection of ecology, conservation, and medicine. Emerging infectious diseases at the human-animal-ecosystem interface. Deforestation brings humans into contact with reservoir species. Bushmeat hunting. Biodiversity loss may increase disease transmission (dilution effect — diverse ecosystems dilute pathogens). Climate change and health: Rising temperatures expand the range of vector-borne diseases (malaria, dengue, Lyme disease). Increased extreme weather events. Food and water insecurity. Displacement and conflict. Climate change is recognised as the greatest threat to global health in the 21st century.