Ecology (IB)

• Species: A group of organisms that can interbreed to produce fertile offspring
• Population: A group of organisms of the same species living in the same area at the same time
• Community: All the populations of different species in an area
• Ecosystem: A community of organisms and their abiotic environment
• Biome: A large geographical area with a distinct climate and set of organisms
• Biosphere: All the ecosystems on Earth

Autotrophs and Heterotrophs

• Autotrophs (producers): synthesise organic molecules from inorganic substances (photosynthesis or chemosynthesis)
• Heterotrophs (consumers): obtain organic molecules by consuming other organisms
  • Herbivores (primary consumers), carnivores (secondary/tertiary consumers), omnivores, detritivores, saprotrophs (decomposers)

Food Chains and Webs

• Energy enters through photosynthesis (light → chemical energy)
• Transferred through trophic levels: producer → primary → secondary → tertiary consumer
• Energy is lost at each level as heat (respiration), faeces, and urine
• Only about 10–20% of energy is transferred between trophic levels

Energy Pyramids

• Pyramids of energy show energy flow per unit area per unit time ($\text{kJ m}^{-2} \text{yr}^{-1}$)
• Always pyramid-shaped (cannot be inverted)
• Pyramids of numbers or biomass can sometimes be inverted (e.g., one tree supporting many insects)

Productivity

• GPP = total energy fixed by photosynthesis
• NPP = GPP − respiration = energy available to consumers
• NPP determines the biomass available for the next trophic level

Carbon cycles between the atmosphere, organisms, oceans, and lithosphere:

• Photosynthesis: $CO_2$ → organic carbon (glucose)
• Respiration: organic carbon → $CO_2$ (all organisms)
• Combustion: fossil fuels/wood → $CO_2$
• Decomposition: dead organisms → $CO_2$ (by saprotrophs)
• Fossilisation: carbon stored in fossil fuels and sedimentary rocks
• Ocean exchange: $CO_2$ dissolves in oceans; used by coral, shells ($CaCO_3$)

Greenhouse Effect

• Greenhouse gases ($CO_2$, $CH_4$, $H_2O$, $N_2O$) absorb and re-emit infrared radiation
• This traps heat in the atmosphere, keeping Earth warm enough for life
• Enhanced greenhouse effect: increased greenhouse gas concentrations → global warming

Evidence for Climate Change

• Rising global temperatures (instrumental records)
• Shrinking glaciers and ice caps
• Rising sea levels
• Changes in species distribution
• Ice core data (trapped gas bubbles show historical $CO_2$ and temperature correlation)
• Coral bleaching

Consequences of Climate Change

• Habitat loss (polar regions, coral reefs, coastal areas)
• Changes in species distribution and migration patterns
• Increased extreme weather events
• Threats to food security (crop failures)
• Ocean acidification (CO₂ + H₂O → carbonic acid)

Estimating Population Size

• Quadrats: random sampling for sessile organisms; estimate = mean × (total area / quadrat area)
• Capture-mark-recapture (Lincoln Index): for mobile animals $N = \frac{n_1 \times n_2}{n_3}$ Where $n_1$ = first capture marked, $n_2$ = second capture total, $n_3$ = recaptured marked

Population Growth

• Exponential growth (J-curve): unlimited resources
• Logistic growth (S-curve): limited resources → population reaches carrying capacity ($K$)
• Phases: lag → exponential → transitional → plateau (at $K$)

Factors Affecting Population

• Density-dependent: competition, predation, disease (intensify with population density)
• Density-independent: natural disasters, climate events

Examples

• Mutualism: Rhizobium bacteria in legume root nodules (nitrogen fixation ↔ sugars)
• Parasitism: Tapeworm in human intestine
• Competition: Red vs grey squirrels in the UK

$\chi^2 = \sum \frac{(O - E)^2}{E}$

• Tests whether observed results differ significantly from expected
• Degrees of freedom = number of categories − 1
• If $\chi^2$ > critical value at $p = 0.05$ → reject null hypothesis (significant difference)

$D = 1 - \sum \left(\frac{n}{N}\right)^2$

• $D$ ranges from 0 (no diversity) to 1 (maximum diversity)
• Used to quantify biodiversity in different habitats

Question: In a mark-recapture study, 50 beetles were caught, marked, and released. A week later, 60 were caught, of which 15 were marked. Estimate the population size. (2 marks)

Solution:

$N = \frac{n_1 \times n_2}{n_3} = \frac{50 \times 60}{15} = \frac{3000}{15} = 200 \text{ beetles}$

• Ecosystems include communities + abiotic environment; energy flows from producers through trophic levels.
• Energy is lost as heat at each level; only ~10–20% transfers. NPP = GPP − R.
• The carbon cycle involves photosynthesis, respiration, combustion, decomposition, and fossil formation.
• Enhanced greenhouse effect from increased CO₂/CH₄ causes global warming and climate change.
• Populations grow exponentially then stabilise at carrying capacity; estimated by quadrats or mark-recapture.