Unit 3: Populations

Population

A group of individuals of the same species living in the same area at the same time.

Generalist species

A species that can use a wide variety of resources and survive in many different environments; typically has a broad tolerance range.

Specialist species

A species that uses a narrow set of resources or thrives under a narrow set of environmental conditions; often vulnerable if its specific habitat/resource disappears.

Tolerance range

The range of environmental conditions (e.g., temperature, salinity, moisture) a species can survive and reproduce in.

Niche breadth

How wide or narrow a variety of resources and environmental conditions a species can use or tolerate (broad in generalists, narrow in specialists).

Invasive species

A non-native species that spreads successfully in a new area; often succeeds because it reproduces quickly and tolerates a range of conditions (generalist-like traits).

Population dispersal pattern

How individuals in a population are distributed across space over time.

Clumped dispersion

A dispersal pattern where individuals cluster in patches; common when resources are patchy or when group living provides benefits (e.g., safety, social behavior).

Random dispersion

A dispersal pattern where individuals are spread unpredictably; occurs when resources are relatively consistent and individuals interact little (e.g., wind-dispersed plants like dandelions).

Uniform dispersion

A dispersal pattern where individuals are evenly spaced, often due to competition or territorial behavior that maximizes distance between individuals.

Biotic potential

The maximum reproductive capacity of an organism under optimum environmental conditions.

Environmental resistance

Any factors that inhibit population increase (e.g., limited resources, disease, predation, unsuitable habitat, harsh weather).

Life history strategy

How an organism allocates limited energy to growth, survival, and reproduction (often discussed using r-selected vs. K-selected trends).

r-selected species (r-strategists)

Species that produce many offspring, mature early, invest little parental care, and have short lifespans; populations can grow rapidly but fluctuate widely and may crash when conditions worsen.

K-selected species (K-strategists)

Species that produce fewer offspring, mature slowly, invest more parental care, and live longer; populations tend to stabilize near carrying capacity and are often regulated by density-dependent factors.

Intrinsic rate of increase (r)

The maximum potential population growth rate under ideal conditions (from population growth models).

Carrying capacity (K)

The maximum population size an environment can sustain long term given available resources and conditions; can change over time as environments change or degrade.

Limiting factor

Any resource or environmental condition that limits the abundance, distribution, and/or growth of a population.

Liebig’s law of the minimum

Even if many factors are favorable, the factor in shortest supply (least favorable) determines growth, abundance, or distribution.

Density-dependent limiting factor

A limiting factor whose effects increase as population density increases (e.g., competition, disease transmission, predation, parasitism).

Density-independent limiting factor

A limiting factor whose effects are not related to population density (e.g., drought, floods, hurricanes, temperature extremes, many forms of pollution).

Bottom-up control

Population regulation driven mainly by resource availability (food, nutrients, habitat); if resources increase, populations can increase.

Top-down control

Population regulation driven mainly by predators, parasites, or disease; predation pressure can keep prey populations low even if resources are plentiful.

Positive feedback loop

A feedback loop that amplifies change; an increase in one part of a system leads to changes that further increase the original change.

Negative feedback loop

A feedback loop that promotes stability; changes (often via limiting factors) counteract growth and help populations fluctuate around carrying capacity.

Overshoot

When a population temporarily exceeds carrying capacity.

Dieback

A population crash that can follow overshoot when resources are depleted or habitat damage occurs.

Exponential growth

Growth where population increase is proportional to current population size under ideal conditions with abundant resources (often produces a J-shaped curve).

Logistic growth

Growth that starts exponential but slows as the population approaches carrying capacity due to limiting resources (often produces an S-shaped curve).

Doubling time

The time required for a population growing approximately exponentially at a constant rate to double in size.

Rule of 70

An approximation for doubling time: doubling time ≈ 70 ÷ growth rate (%), assuming exponential growth at an approximately constant rate.

Survivorship curve

A graph showing the proportion of individuals in a cohort that survive as they age; indicates when mortality is most likely across the lifespan.

Type I survivorship curve

High survival through early/middle life with a steep drop in old age; associated with fewer offspring and higher parental care (e.g., humans, elephants).

Type II survivorship curve

Roughly constant mortality across ages; the curve declines steadily (e.g., many songbirds, rodents).

Type III survivorship curve

Very high early-life mortality with much higher survival for individuals that reach later life stages; associated with many offspring and little parental care (e.g., sea turtles, many fish, oysters).

Crude birth rate (CBR)

Number of births per 1,000 people per year; “crude” because it does not adjust for age structure.

Crude death rate (CDR)

Number of deaths per 1,000 people per year; “crude” because it does not adjust for age structure.

Infant mortality rate (IMR)

Number of deaths of infants under age 1 per 1,000 live births per year; strongly linked to sanitation, nutrition, healthcare, and maternal health.

Immigration

The number of individuals entering a population; increases local population size.

Emigration

The number of individuals leaving a population; decreases local population size.

Human population growth rate (%) (CBR/CDR method)

Growth rate (%) = (CBR − CDR) ÷ 10 (and if including migration: (CBR + immigration rate − CDR − emigration rate) ÷ 10).

Total fertility rate (TFR)

The average number of children a woman is expected to have over her lifetime given current age-specific birth rates; often clearer than CBR for family size.

Replacement-level fertility

The TFR needed to keep population size stable long term (ignoring migration); typically slightly above 2 (often ~2.1) because not all children reach reproductive age.

Age-structure diagram (population pyramid)

A diagram showing the distribution of a population across age groups (often separated by sex); used to predict future growth trends and dependency pressures.

Population momentum

Continued population growth after fertility declines because a large cohort is entering reproductive age.

Demographic Transition Model (DTM)

A model describing how birth and death rates change as a country develops from a pre-industrial to an industrialized economy; it is a model, not a law.

DTM Stage 1 (Pre-Industrial / High Stationary)

High birth and high death rates, so growth is low; mortality is high due to limited healthcare, poor living conditions, and food scarcity.

DTM Stage 2 (Transitional / Early Expanding)

Death rates drop due to sanitation, clean water, vaccinations, medical advances, and education, while birth rates remain high for a time; rapid population growth results.

DTM Stage 3 (Industrial / Late Expanding)

Birth rates decline (often due to urbanization, higher costs of children, female education/employment, contraception access, and retirement safety nets) while death rates stay low; growth slows.

DTM Stage 4 (Post-Industrial / Low Stationary)

Low birth and low death rates; population growth is near zero when birth and death rates are roughly equal, and standard of living is typically higher.