Organisms and Populations

The One Thing

Organisms and populations are shaped by interactions between biotic and abiotic factors. Population size, structure, distribution, and survival change through births, deaths, migration, resource availability, environmental limits, and species interactions.

Reactions, Processes and Experiments

Key Terms

• Organism: An individual living being that carries out life processes such as nutrition, respiration, growth, and reproduction.
• Habitat: The physical place and environmental conditions in which an organism lives.
• Abiotic factors: Non-living environmental factors, including temperature, water, light, soil, pH, salinity, and humidity.
• Biotic factors: Living components of the environment, including plants, animals, microorganisms, predators, competitors, and parasites.
• Population: A group of individuals of the same species living in a particular geographical area and capable of interbreeding.
• Population density: The number of individuals of a population per unit area or volume. Biomass or percentage cover may be used when counting individuals is difficult.
• Natality: The number of individuals added to a population through births during a given period.
• Mortality: The number of individuals lost from a population through deaths during a given period.
• Immigration: The movement of individuals into a population from another area.
• Emigration: The movement of individuals out of a population to another area.
• Population growth: The change in population size over time due to births, deaths, immigration, and emigration.
• Exponential growth: Rapid population growth under unlimited resources, producing a J-shaped growth curve.
• Logistic growth: Population growth that slows as resources become limited and approaches the carrying capacity, producing an S-shaped curve.
• Carrying capacity: The maximum population size that an environment can support sustainably with its available resources.
• Environmental resistance: All limiting factors, such as shortage of food, disease, predation, and competition, that prevent unlimited population growth.
• Age pyramid: A diagram showing the proportions of individuals in pre-reproductive, reproductive, and post-reproductive age groups.
• Expanding population: A population with a large proportion of young individuals, usually represented by a broad-based age pyramid.
• Stable population: A population in which the pre-reproductive and reproductive groups are relatively balanced.
• Declining population: A population with a smaller proportion of young individuals than reproductive individuals, often shown by a narrow-based age pyramid.
• Predation: An interaction in which one organism, the predator, kills and eats another organism, the prey.
• Competition: An interaction in which organisms require the same limited resource, reducing the success of one or both organisms.
• Competitive exclusion: The principle that two species competing for exactly the same limiting resource cannot coexist indefinitely; the better competitor may eliminate the other locally.
• Resource partitioning: The division of resources by competing species in space, time, or method of use, allowing coexistence.
• Parasitism: An interaction in which a parasite benefits by obtaining nutrients from a host, while the host is harmed.
• Mutualism: An interaction in which both participating species benefit.
• Commensalism: An interaction in which one species benefits and the other is neither significantly helped nor harmed.
• Amensalism: An interaction in which one species is harmed while the other is unaffected.
• Adaptation: A structural, physiological, or behavioural feature that improves an organism's survival and reproduction in its environment.
• Homeostasis: The maintenance of relatively stable internal conditions despite changes in the external environment.
• Allen's rule: In closely related warm-blooded animals, body appendages tend to be shorter in colder climates and longer in warmer climates, helping regulate heat loss.
• Bergmann's rule: In many warm-blooded animals, individuals living in colder regions tend to have larger bodies, which reduces relative heat loss.

Easily Confused

• Abiotic factors vs biotic factors: Abiotic factors are non-living environmental conditions, whereas biotic factors are living components and their interactions.
• Natality vs immigration: Natality adds individuals through births, whereas immigration adds individuals moving in from another area.
• Mortality vs emigration: Mortality removes individuals through deaths, whereas emigration removes individuals moving out of the population.
• Exponential growth vs logistic growth: Exponential growth assumes unlimited resources and produces a J-shaped curve; logistic growth is limited by carrying capacity and produces an S-shaped curve.
• Population size vs population density: Population size is the total number of individuals; population density is the number per unit area or volume.
• Regulators vs conformers: Regulators maintain relatively stable internal conditions, whereas conformers undergo internal changes as external conditions change.
• Ectotherms vs endotherms: Ectotherms depend largely on external heat sources, whereas endotherms generate substantial internal heat through metabolism.
• Predation vs parasitism: A predator kills and eats its prey, whereas a parasite obtains nutrients from a host without the interaction necessarily involving immediate death.
• Competition vs predation: Competition involves organisms requiring the same limited resource; predation involves one organism killing and eating another.
• Competitive exclusion vs resource partitioning: Competitive exclusion predicts that species using exactly the same limiting resource cannot coexist indefinitely; resource partitioning allows coexistence by dividing resource use.
• Mutualism vs commensalism: Both species benefit in mutualism; only one benefits in commensalism, while the other is unaffected.
• Allen's rule vs Bergmann's rule: Allen's rule concerns appendage length and heat loss; Bergmann's rule concerns body size and relative heat loss.
• Population attributes vs individual features: Population attributes include density, natality, mortality, age distribution, sex ratio, and dispersion; individual features include age, size, and health.

What Gets Asked

• Population calculations: Questions may require use of $D=N/A$, $D=N/V$, or $\Delta N=(B+I)-(D+E)$. Marks are lost by reversing immigration and emigration or treating deaths as a population increase.
• Per capita rates and growth: Questions may test $b=B/N$, $d=D/N$, and $r=b-d$. A positive $r$ indicates growth, whereas a negative $r$ indicates decline.
• Growth curves and equations: Students may be asked to identify or explain exponential growth using $dN/dt=rN$ and $Nt=N0e^{(}rt)$, or logistic growth using $dN/dt=rN(K-N)/K$. The key distinction is unlimited resources and a J-shaped curve versus carrying capacity and an S-shaped curve.
• Population structure and measurement: Questions may involve density, age pyramids, dispersion, direct counting, sampling, percentage cover, biomass, or indirect signs. Marks are lost by confusing population attributes with features of individual organisms.
• Species interactions: Questions may require identification of competition, predation, parasitism, mutualism, commensalism, or amensalism from their effects on the interacting species. The interaction symbols must be matched correctly: mutualism (+,+), competition (-,-), predation (+,-), parasitism (+,-), commensalism (+,0), and amensalism (-,0).
• Adaptations and environmental responses: Questions may ask how temperature, water availability, thermoregulation, Allen's rule, Bergmann's rule, or natural selection affects survival. Marks are lost by describing adaptations as consciously produced rather than favoured through natural selection.