Organisms and Populations

The One Thing

Organisms survive and reproduce through adaptations to particular habitats, while population size and distribution are determined by demographic processes, environmental limits, and interactions among species.

Reactions, Processes and Experiments

Key Terms

• Habitat: The place and environmental conditions where an organism normally lives.
• Adaptation: A heritable feature or adjustment that helps an organism survive and reproduce in its environment.
• Acclimatisation: A temporary, reversible adjustment made by an individual in response to environmental changes.
• Population: A group of individuals of the same species living in a particular area at a given time and capable of interbreeding.
• Population density: The number of individuals of a population per unit area or volume; it may also be measured using biomass or percentage cover.
• Natality: The number of births added to a population during a given period.
• Mortality: The number of deaths occurring in a population during a given period.
• Immigration: Movement of individuals into a population from another area.
• Emigration: Movement of individuals out of a population to another area.
• Sex ratio: The proportion or ratio of males to females in a population.
• Age distribution: The proportion of individuals in different age groups, commonly shown as prereproductive, reproductive, and post-reproductive groups.
• 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.
• Population interactions: Relationships between populations of different species that influence their survival, growth, or reproduction.
• Predation: An interaction in which one organism, the predator, captures and consumes another organism, the prey.
• Competition: An interaction in which organisms use the same limited resource, reducing the benefit available to one or both participants.
• Parasitism: An interaction in which a parasite obtains nourishment from a host, benefiting while usually harming the host.
• Commensalism: An interaction in which one species benefits and the other is neither significantly helped nor harmed.
• Mutualism: An interaction in which both participating species benefit.
• Amensalism: An interaction in which one species is harmed while the other is not significantly affected.
• Gause's competitive exclusion principle: Two closely related species competing for exactly the same limited resources cannot coexist indefinitely in the same niche.
• Allen's rule: In mammals living in colder climates, body extremities such as ears and limbs tend to be shorter, reducing heat loss.
• Bergmann's rule: In many birds and mammals, individuals in colder regions tend to have larger bodies, which reduces relative surface area and heat loss.
• Counter-current heat exchange: A heat-conserving mechanism in which warm arterial blood transfers heat to cooler venous blood returning from the body surface.
• Abiotic factor: A non-living component of the environment, including temperature, water, light, soil, or nutrients.
• Environmental resistance: The combined effects of limiting environmental factors that prevent unlimited population growth.
• Intrinsic rate of natural increase ($r$): The difference between birth rate and death rate in the exponential growth equation.
• Carrying capacity ($K$): The population size that an environment can support sustainably with its available resources.
• Interaction notation: A system in which $+$ means benefit, $-$ means harm, and $0$ means no significant effect.

Easily Confused

• Adaptation and acclimatisation: An adaptation is a heritable feature or adjustment shaped by natural selection, whereas acclimatisation is temporary and reversible within an individual.
• Immigration and emigration: Immigration is movement into a population; emigration is movement out of a population.
• Natality and mortality: Natality adds individuals through births; mortality removes individuals through deaths.
• Exponential and logistic growth: Exponential growth assumes effectively unlimited resources and produces a J-shaped curve; logistic growth includes environmental resistance and produces an S-shaped curve approaching carrying capacity.
• Population density and population size: Population density is the number of individuals per unit area or volume, whereas population size is the total number of individuals.
• Predation and parasitism: A predator captures and consumes prey, whereas a parasite obtains nourishment from a host and usually remains associated with it.
• Commensalism and mutualism: In commensalism one species benefits and the other is not significantly affected; in mutualism both species benefit.
• Competition and predation: Competition involves organisms using the same limited resource and is $-/-$; predation involves one organism consuming another and is $+/-$.
• Allen's rule and Bergmann's rule: Allen's rule concerns shorter body extremities in colder climates; Bergmann's rule concerns larger overall body size in colder regions.
• Abiotic factors and population interactions: Abiotic factors are non-living environmental influences, whereas population interactions occur between organisms or populations.

What Gets Asked

• Calculating population change: Questions may require use of $N(t)=N(0)+B+I-D-E$. Marks are lost by assigning births, deaths, immigration, or emigration the wrong sign.
• Calculating demographic rates: Questions may ask for per capita birth rate or per capita death rate. The relevant number of births or deaths must be divided by the average population size during the stated period.
• Interpreting growth equations and curves: Questions may distinguish $dN/dt=rN$ and $N_t=N_0{e}^{rt}$ from $dN/dt=rN(K-N)/K$. The common error is associating exponential growth with an S-shaped curve or logistic growth with a J-shaped curve.
• Interpreting age pyramids: A broad base indicates an expanding population, a roughly bell-shaped pyramid indicates a stable population, and a narrow base or urn shape indicates a declining population. The base must not be confused with the reproductive or post-reproductive groups.
• Classifying species interactions: Questions may use interaction notation. Predation and parasitism are $+/-$, competition is $-/-$, commensalism is $+/0$, mutualism is $+/+$, and amensalism is $-/0$.
• Applying named adaptations and ecological rules: Questions may ask about kangaroo rats, desert plants, Allen's rule, Bergmann's rule, or counter-current heat exchange. Marks are lost by describing a temporary acclimatisation as a heritable adaptation or by confusing shorter extremities under Allen's rule with larger body size under Bergmann's rule.