Populations

Population

All members of the same species living in the same ecosystem or habitat at the same time.

Geographic range

The total area or spatial boundaries within which a population lives.

Habitat

The specific environment where an organism normally lives, characterized by biotic and abiotic features.

Population size (Nt)

The number of individuals of a species that occupy a given area or volume at a specified time.

Population density (D)

The number of individuals of the same species per unit area or unit volume of habitat.

Population density formula

\(D = \dfrac{N}{S}\) where N is number of individuals and S is area or volume.

Crude density

Population density measured across the entire habitat area, including unused space.

Ecological density

Population density measured per unit area actually used by the organisms.

Why crude density can mislead

It includes unused habitat, underestimating true density where organisms live and compete.

How body size affects density

Larger species require more space and resources, so they typically have lower densities.

Dispersion

The spatial pattern of individuals within a range; clumped, random, or uniform.

Clumped dispersion

Individuals grouped in patches due to patchy resources, social behavior, or limited dispersal.

Random dispersion

Individuals distributed unpredictably and independently when conditions are uniform and interactions are neutral.

Uniform dispersion

Individuals evenly spaced due to competition or territorial behavior.

Scale effect on dispersion

A pattern can appear random at small scales but clumped or uniform at larger scales; observation scale matters.

Recognition cue for clumped dispersion

Clusters around resource patches, parent plants, or social groups.

Recognition cue for uniform dispersion

Even spacing, territorial boundaries, or allelopathic zones.

Recognition cue for random dispersion

Unpredictable locations with no clear attraction or repulsion among individuals.

Food chain

A linear sequence showing energy flow from producers to consumers.

Food web

A network of interconnected food chains showing multiple feeding relationships.

Trophic level energy loss

Energy is lost as heat and through metabolism at each transfer, so less energy is available at higher levels.

10% rule implication

Approximately 10% of energy transfers to the next trophic level, limiting biomass and population size of predators.

Why energy pyramids are upright

Energy decreases at each trophic level, so higher levels cannot contain more energy than lower ones.

Carrying capacity (K)

The maximum population size an environment can sustain indefinitely without degrading resources.

Why carrying capacity changes

It varies with resource availability, environmental conditions, and human impacts.

Density-dependent factors

Factors whose effects increase with population density (competition, disease, predation).

Density-independent factors

Factors that affect populations regardless of density (fires, droughts, storms).

Example density-dependent factor

Food shortage intensifies as population density increases.

Example density-independent factor

Forest fire can reduce population regardless of density.

How density-dependent factors stabilize populations

They reduce reproduction or increase mortality as density rises, preventing runaway growth.

How density-independent factors cause crashes

They act regardless of density and can abruptly reduce population size.

Invasive species effect on natives

They compete for resources, alter habitats, and change trophic interactions, often reducing native carrying capacity.

Why invasives affect different niches

They can change habitat structure, nutrient cycles, or food webs, indirectly harming other species.

Population change formula

Population change = births + immigration − deaths − emigration.

Worked population change example

For 950 lemmings with births 255, deaths 103, immigration 6, emigration 9 → net change = 149; new population = 1,099.

Open vs closed population

Open allows immigration/emigration; closed changes only by births and deaths.

Why white‑tailed deer are open populations

Individuals move across regions, allowing immigration and emigration.

Exponential growth

Rapid increase when resources are abundant; growth rate proportional to population size (J‑shaped curve).

Logistic growth

Growth that slows and levels off as it approaches carrying capacity (S‑shaped curve).

Overshoot and crash

Population exceeds K, depletes resources, then suffers increased mortality and decline.

Why logistic is realistic

Natural limits (resources, space) prevent indefinite exponential growth.

Random sampling purpose

Estimate total population by sampling so each individual has equal chance of being counted.

How to extrapolate from 10% samples

Average individuals per sample × 10 (or scale by inverse of sample fraction).

Why increasing sample percent improves estimates

Larger sample sizes reduce sampling error and increase reliability.

When to use mark‑recapture

For mobile species where individuals move between samples, making quadrat counts inaccurate.

Mark‑recapture principle

Capture, mark, release, recapture; use proportion of marked in second sample to estimate total population.

Why monitor adults

Adults reflect reproductive potential and are easier to sample consistently for trend inference.

Difference geographic range vs habitat

Range is total area occupied; habitat is the specific environment where an organism lives.

Ecological niche

The role of a species in its environment, including resource use, interactions, and functional position.

Niche vs habitat

Habitat is where a species lives; niche is how it lives and interacts within that habitat.

Symbiosis

Close association between species; includes mutualism, parasitism, and commensalism.

Mutualism

Both species benefit from the interaction.

Parasitism

One species benefits at the expense of the host.

Commensalism

One species benefits while the other is unaffected.

Why compare populations across locations

To learn how environmental differences shape growth, density, survival, and adaptation.

How human activities affect populations

Habitat modification, hunting, introductions, pollution, and land‑use change alter resources and conditions.

Culling and selective harvest

Removing individuals to control population size and maintain sustainability or reduce conflicts.

Vertical farming definition

Growing plants in stacked indoor systems using hydroponics/aeroponics to produce food without horizontal farmland.

Vertical farming benefits

Year‑round production, reduced transport emissions, protection from climate variability, local food supply.

Vertical farming limitations

High energy/infrastructure costs and technical complexity for lighting and climate control.

Why dispersion matters for sampling

Clumped or uniform patterns require tailored sampling to avoid biased estimates.

How clumped dispersion biases sampling

Quadrats may miss or overcount clumps; randomization and more samples reduce bias.

How uniform dispersion biases sampling

Systematic sampling grids can align with spacing and produce errors; random sampling reduces bias.

Why small populations are vulnerable

Susceptible to stochastic events, genetic bottlenecks, and demographic fluctuations.

How population models aid management

They predict trends, evaluate scenarios, set quotas, and guide conservation actions.