Organisms and Populations

Ecology (from Greek: oikos = house/home, logos = study): • The scientific study of the relationships between living organisms and their environment • Examines how organisms interact with each other (biotic interactions) and with their physical environment (abiotic factors) • A branch of biology at the interface of evolution, physiology, and environmental science Levels of ecological organisation (from smallest to largest): 1. Organism: Individual living entity and its adaptations to the environment 2. Population: Group of individuals of the same species in a defined area and time 3. Community: All populations of different species living and interacting in an area 4. Ecosystem: Community + its abiotic environment; includes energy flow and nutrient cycling 5. Biome: Large geographical area with similar climate, flora, and fauna 6. Biosphere: The global sum of all ecosystems; the zone of life on Earth Key areas of ecology: • Autecology: Study of individual species and their environment • Synecology (community ecology): Study of communities and ecosystems • Population ecology: Growth, distribution, dynamics of populations • Behavioural ecology: Evolutionary basis of animal behaviour • Landscape ecology: Spatial patterns and ecosystem processes Importance: • Basis for conservation biology and environmental management • Understanding biodiversity and ecosystem services • Addressing climate change, pollution, and habitat loss

Population: • A group of individuals of the same species living in a defined geographical area at a given time, who can interbreed and produce fertile offspring • The fundamental unit of ecological and evolutionary study Main attributes of a population: 1. Population size (N): • The total number of individuals in the population at a given time • Can range from a few individuals to billions • Measured by census, mark-recapture (Lincoln-Petersen method), or sampling 2. Population density: • Number of individuals per unit area or volume • Example: 200 fish per hectare, 500 bacteria per mL • More practical than absolute size • Can be measured by biomass or any other proxy 3. Birth rate (Natality): • Number of births per individual per unit time • b = B/N (B = total births, N = population size) • Increases population size 4. Death rate (Mortality): • Number of deaths per individual per unit time • d = D/N • Decreases population size 5. Age structure (Age distribution): • Proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive) • Indicates whether population is growing (young age structure), stable, or declining • Shown as age pyramid 6. Sex ratio: • Ratio of males to females in the population • Affects reproductive potential • Example: 1:1 in many sexually reproducing organisms 7. Growth rate (r): • Rate of change in population size per unit time • r = b – d (intrinsic rate of natural increase) 8. Spatial distribution: • How individuals are distributed in space: random, uniform, or clumped (aggregated)

Population growth: Change in population size over time due to births, deaths, immigration, and emigration. Formula: dN/dt = (b – d) × N = rN where r = intrinsic rate of natural increase, N = population size 1. Exponential Growth Model (J-shaped curve): • Occurs when resources are UNLIMITED and there is no competition • Population grows without any restraint • Also called unrestricted or Malthusian growth Equation: dN/dt = rN • Integrated form: Nt = N₀ × eʳᵗ (N₀ = initial population, Nt = population after time t, r = intrinsic rate of increase) Characteristics: • Population size increases rapidly → J-shaped curve • Growth rate increases as population grows (more individuals = more births) • Real-world examples: Bacterial growth in unlimited medium, reindeer on St. Matthew Island (introduced, grew from 29 to 6000 in 19 years, then crashed) Limitation: No real population grows exponentially forever; resources eventually limit growth 2. Logistic Growth Model (S-shaped/sigmoid curve): • Occurs when resources are LIMITED — more realistic model • Population growth slows as it approaches the environment's carrying capacity (K) • Also called Verhulst-Pearl logistic growth Equation: dN/dt = rN[(K–N)/K] where: • K = carrying capacity (maximum population the environment can sustain) • (K–N)/K = fraction of K still available = unutilised opportunity Phases: • Lag phase: Slow initial growth (few individuals) • Log phase: Accelerating growth • Deceleration phase: Growth slows as N approaches K • Plateau/Asymptote: Population stabilises at K Characteristics: • S-shaped (sigmoid) curve • Growth rate is maximum at N = K/2 • At N = K, growth rate = 0 (birth rate = death rate) • Intraspecific competition increases as N approaches K • More realistic — most natural populations show this pattern

Population interactions between two species (+, −, or 0 for each): (a) Mutualism (+/+): • Both species benefit; often obligatory • Examples: – Mycorrhizae: Fungi + plant roots → fungus gets carbohydrates; plant gets minerals – Lichens: Fungi + algae/cyanobacteria → fungus gets food; alga gets protection – Rhizobium + legumes: Bacteria fix nitrogen; plant provides shelter and carbon – Fig tree + fig wasp: Wasp pollinates fig; wasp lays eggs in fig — obligate mutualism – Orchid Ophrys + bee: Flower mimics female bee for pollination – Clownfish + sea anemone: Fish protected by anemone tentacles; fish removes parasites from anemone (b) Competition (−/−): • Both species are harmed — compete for same limited resources • Interspecific competition: between different species • Resource partitioning reduces competition (e.g., birds using different parts of same tree) • Gause's Competitive Exclusion Principle: Two species competing for identical resources cannot coexist indefinitely • Examples: – Flamingos + fishes competing for zooplankton in a lake – Different warblers partitioning tree zones (MacArthur's warblers) – Abingdon tortoise going extinct due to competition with goats (Galapagos) (c) Predation (+/−): • Predator benefits; prey is harmed • Predator is usually larger and kills prey • Essential ecological role: controls prey population, maintains ecosystem stability • Examples: – Tiger eating deer – Lynx–hare population cycles (classic example) – Plants as 'prey': herbivory (caterpillar eating plant leaves) (d) Parasitism (+/−): • Parasite benefits at the expense of host; host is harmed but usually not killed immediately • Parasite usually smaller, lives on or in host • Co-evolution of parasite and host virulence • Examples: – Lice on humans; head lice, body lice – Plasmodium (malaria parasite) in human RBCs – Tapeworm (Taenia) in human intestine – Cuscuta (dodder) — parasitic plant on host plants – Brood parasitism: Cuckoo lays eggs in other birds' nests (e) Commensalism (+/0): • One species benefits; the other is unaffected • Examples: – Cattle egret and cattle: Bird follows cattle, eats insects disturbed by grazing; cattle unaffected – Barnacles on whale: Barnacles get transportation and food access; whale unaffected – Orchids (epiphytes) on tree: Orchid gets support and light; tree unaffected – Sea anemone on hermit crab's shell (generally) – Sharks and remora fish (f) Amensalism (0/−): • One species is harmed; the other is unaffected • Also called antibiosis when chemicals are involved • Examples: – Penicillium produces penicillin → kills nearby bacteria (bacteria harmed; Penicillium unaffected) – Large trees shade out small plants underneath (allelopathy) – Allelopathy: Black walnut tree releases juglone, inhibiting growth of nearby plants

Desert animals have evolved remarkable adaptations to conserve water in arid environments: 1. Behavioural adaptations: • Nocturnal activity: Active at night (cooler temperatures) → reduces water loss from evaporation and panting • Burrowing: Stay underground during the day where it is cooler and more humid • Estivation (summer dormancy): Some animals become dormant during extreme heat/drought • Reduced physical activity during hottest periods 2. Morphological (structural) adaptations: • Kangaroo rat: – No sweat glands – Long nasal passages with countercurrent heat exchange — cool, moist air in nose condenses water vapour from exhaled breath → reduces respiratory water loss – Very concentrated urine and dry faeces – Obtain water entirely from metabolic water (water produced during cellular respiration of food) • Camel: – Hump stores fat (not water) — fat oxidation produces metabolic water – Can lose up to 30% body weight as water loss without ill effects – Can drink 100+ litres of water quickly when available – Elliptical red blood cells resist crenation during dehydration – Concentrated urine; dry faeces 3. Physiological adaptations: • Metabolic water: Oxidation of food produces water — H₂ from fats + O₂ → H₂O – Fat produces most metabolic water (9g H₂O per 10g fat oxidised) • Highly concentrated urine: Excrete very little water; kidney with very long loop of Henle • Dry faeces with almost no water • Tolerance of high body temperatures → reduce panting/sweating • Torpor during extreme conditions 4. Reptile adaptations: • Thick, impermeable scales prevent evaporative water loss • Excrete nitrogenous waste as uric acid (solid) — minimal water used vs urea or ammonia 5. Desert insects: • Waxy cuticle impermeable to water • Spiracles (breathing pores) kept closed except when needed