Populations, communities and ecosystems: core concepts
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Levels of organisation: population, community, ecosystem
A population contains all individuals of one species in a specified area and may vary in size and geographic extent. A community contains two or more populations of different species that coexist and interact in the same place. An ecosystem includes the community plus the abiotic environment; both biotic and abiotic factors determine ecosystem structure. These definitions align with standard course materials and practical guidance on sampling and levels of organisation . Population size and distribution depend on limiting factors such as food, space, predation, disease, temperature and water availability. Changes to any of these factors cause population shifts that cascade through the community and the wider ecosystem.
Interdependence and food relationships
All species in a community depend on others for food, shelter or environmental maintenance. Producers (photosynthetic organisms and some chemosynthetic bacteria) form the base of most food webs by converting inorganic energy into biomass. Consumers obtain energy by feeding on producers or other consumers. Decomposers break down dead material and return nutrients to the environment, completing energy and nutrient pathways . Food chains show linear feeding links; food webs show interconnected chains. Energy transfers between trophic levels are inefficient: a small fraction of biomass transfers to the next level and most energy is lost as heat and waste, which shapes pyramid structures and limits trophic length .
Adaptations and competition
Adaptations are structural, behavioural or physiological traits that increase an organism's chance of surviving and reproducing in its environment. Adaptations reduce vulnerability to predators, improve resource acquisition or enhance tolerance to abiotic extremes. Examples include insulating fat and small extremity size to reduce heat loss in cold climates or water-storage tissues in desert plants. Extremophiles show physiological adaptations to habitats with extreme pressure, temperature or chemistry fileciteturn0file6turn0file16. Competition occurs when organisms require the same limited resources. Intraspecific competition (within a species) and interspecific competition (between species) drive population dynamics and natural selection. Changes in resource availability cause winners and losers among competing organisms, altering community composition.
Sampling and estimating abundance
Sampling uses quadrats, transects and random or systematic selection to estimate the abundance and distribution of species without counting every individual. Quadrats record counts, percentage cover or species richness for a defined area and support comparisons across gradients of abiotic factors. Repeated samples and replicated transects increase confidence in trends and reduce sampling error fileciteturn0file2turn0file9. Appropriate sampling design reduces bias: random sampling avoids placement bias, while systematic sampling along a transect captures changes across an environmental gradient such as light or moisture.
Chemical cycling: carbon and other elements
The carbon cycle transfers carbon between atmosphere, organisms, soils, oceans and rocks through photosynthesis, respiration, decay and combustion. Photosynthesis converts atmospheric carbon dioxide into biomass. Respiration and combustion return carbon to the atmosphere as carbon dioxide. Decomposers break down dead organic matter and recycle carbon and nutrients into soil and the environment. Human activities that increase combustion and alter land use add extra carbon dioxide to the atmosphere and disturb natural balances . Other chemical cycles (for example nitrogen and water) also move elements between living organisms and the physical environment. Continuous cycling of chemicals sustains productivity by making essential elements available for uptake and growth.
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