Ecology & Environment

Organisms, environment and adaptation

Ecology studies relationships among organisms and their physical and biological surroundings. Abiotic factors include temperature, water, light and soil characteristics. Biotic factors include competitors, predators, parasites and mutualists. An organism’s habitat is where it lives, while its niche includes its ecological role and resource use.

Adaptations can be structural, physiological or behavioural. Thick cuticles can reduce water loss, physiological regulation can stabilise internal conditions, and behaviour can reduce exposure to environmental extremes. Acclimatisation is an adjustment within an individual’s lifetime; evolutionary adaptation involves heritable change across generations.

• A limiting factor restricts performance even when other conditions are favourable.
• Tolerance ranges differ between species and life stages.
• Population properties such as density and age structure describe a group rather than a single organism.

Population growth and regulation

Population size changes through births, deaths, immigration and emigration. Exponential growth is represented by dN/dt = rN when resources do not impose a limiting effect in the model. The per-capita rate r determines how rapidly a population grows under the stated conditions.

Logistic growth adds a carrying capacity K: dN/dt = rN(1 − N/K). Growth slows as population size approaches K. Carrying capacity depends on resource availability and environmental conditions; it is not an unchanging number for every habitat.

• In the simple logistic model, the absolute growth rate is greatest at N = K/2.
• Density-dependent factors include competition and many disease effects. Weather events can act independently of density.
• An age pyramid distinguishes pre-reproductive, reproductive and post-reproductive groups and helps interpret growth potential.

Compare growth at different densities

If r = 0.2 per year, N = 100 and K = 500, the logistic model predicts dN/dt = 0.2 × 100 × (1 − 100/500) = 16 individuals per year at that instant. With the same r and N, exponential growth predicts 20. The logistic resource term reduces growth.

Interactions among species

Interactions can be summarised by their effects on the participants, while remembering that effects may depend on conditions. Competition occurs when organisms use a limiting shared resource. Predation and parasitism benefit one participant at the expense of another. Mutualism benefits both partners, and commensalism benefits one with little detectable effect on the other.

• Resource partitioning can reduce competition by separating resource use in space, time or type.
• Predators can influence prey abundance and community diversity.
• A parasite often depends on a host for an extended period; a predator typically kills and consumes prey.

Ecosystem structure and productivity

An ecosystem contains interacting organisms and their abiotic environment. Producers fix energy into organic material; consumers obtain it by feeding; decomposers break down dead organic matter. Food chains describe selected feeding links, while food webs represent interconnected pathways.

Gross primary productivity is the rate at which producers fix energy or carbon into organic material. Net primary productivity is what remains after the producers’ respiration: NPP = GPP − R. NPP provides biomass that can support consumers and decomposers.

Calculate available primary production

If GPP is 2,400 units of energy per square metre per year and producer respiration uses 1,000 units in the same units, NPP is 1,400. Keep area and time units consistent, and do not subtract consumer respiration when calculating producer NPP.

Energy flow and ecological pyramids

Energy enters many ecosystems through sunlight, passes through trophic transfers and is progressively dissipated as heat. Energy is not recycled in the way chemical nutrients are. Only part of the energy available at one trophic level is incorporated into biomass at the next.

The often-used 10% transfer rule is a simplified average for practice problems, not an exact constant for every ecosystem. A pyramid of energy is upright when measured over comparable areas and time periods because each transfer reduces usable energy. Pyramids of numbers and standing biomass can be inverted.

• A small standing biomass of rapidly reproducing phytoplankton can support a larger standing biomass of consumers.
• Organisms with mixed diets may occupy more than one trophic level.
• Food webs provide alternative pathways; a change in one population can have direct and indirect effects.

Decomposition, nutrient cycles and succession

Decomposition includes fragmentation, leaching, microbial catabolism, humification and mineralisation. Warm, suitably moist and oxygenated conditions often favour breakdown, while chemical composition also matters. Lignin-rich material can decompose more slowly than easily metabolised material.

Carbon cycles among the atmosphere, organisms, soils and water through fixation, respiration, decomposition and other processes. Nitrogen transformations include fixation, ammonification, nitrification and denitrification. Unlike energy flow, elements are repeatedly reused in biological and geochemical pathways.

Ecological succession describes changes in community composition over time. Primary succession begins where developed soil is absent; secondary succession occurs where soil or other biological legacies remain after disturbance. The trajectory depends on species interactions, dispersal, resources and continued disturbance.

Biodiversity and conservation

Biodiversity includes genetic variation within species, species diversity and ecosystem diversity. Habitat loss and fragmentation, overexploitation, invasive species and co-extinctions can reduce diversity. Conservation protects both organisms and the ecological relationships that sustain them.

In situ conservation protects populations within natural habitats, for example through protected areas and habitat management. Ex situ conservation maintains organisms or genetic material outside their natural habitat, such as seed banks and captive breeding programmes. The approaches can support one another.

• Endemism describes restriction to a particular geographic region; it is not identical to rarity or threatened status.
• A biodiversity hotspot combines high endemism with substantial habitat loss under defined criteria.
• Conservation planning considers habitat connectivity, genetic variation and functioning ecological processes, not just species counts.