Population Ecology

Core Concepts

Population Characteristics

Population size (N) is the number of individuals. Populati on density is the number of individuals per unit area or volume. Population dispersion describes how individuals are distributed:

• Clumped: Individuals aggregate around resources or social groups (most common in nature — schooling fish, herding mammals, plants clustered where soil and moisture are favorable).

• Uniform: Evenly spaced, often due to territoriality or competition for resources (nesting seabirds, desert shrubs competing for water).

• Random: Position of each individual is independent of others (rare — occurs when resources are uniformly available and there are no strong social interactions).

Demography

Demography is the study of population statistics: birth rate (natality), death rate (mortality), immigration, emigration, age structure, and sex ratio. These factors determine whether a population grows, shrinks, or remains stable.

Survivorship curves plot the number of individuals surviving to each age. Three idealized types:

• Type I: High survival until old age, then rapid mortality. Large mammals (humans, elephants) with extensive parental care.

• Type II: Constant mortality rate at all ages. Many birds, some reptiles.

• Type III: Very high early mortality, but those surviving to adulthood have high survival probability. Many fish, marine invertebrates, trees — produce many offspring with little parental care.

Population Growth Models

Exponential growth: Occurs when resources are unlimited. The per capita growth rate (r) is constant at its maximum (r_max). The population grows by a constant proportion per unit time: dN/dt = rN This produces a J-shaped curve — slow initial growth followed by explosive increase. Exponential growth is unsustainable in the long term because resources are finite. It occurs when a population colonizes a new habitat, recovers from a crash, or is temporarily released from limiting factors.

Logistic growth: Incorporates carrying capacity (K) — the maximum population size an environment can sustain given available resources. As N approaches K, growth slows: dN/dt = rN[(K − N)/K] When N is small, the term [(K−N)/K] is close to 1, and growth approximates exponential. As N approaches K, the term approaches 0, and growth stops. This produces an S-shaped curve. The logistic model assumes: constant carrying capacity, linear effect of density on growth rate, and no time lags. Real populations may overshoot K (depleting resources) and then crash below K.

Carrying Capacity and Limiting Factors

Carrying capacity is determined by limiting resources — food, water, space, nesting sites, and other requirements. K is not permanently fixed; it can change as environmental conditions change, resources become more or less available, or the population’s resource-use efficiency evolves.

Population Regulation

Density-dependent factors: Their effect intensifies as population density increases. Examples: competition for resources, predation, disease transmission, accumulation of wastes. These factors tend to regulate populations around an equilibrium — as density rises, mortality increases or birth rate decreases, slowing growth.

Density-independent factors: Their effect is unrelated to population density. Examples: weather events (drought, flood, extreme temperatures), natural disasters (fire, volcanic eruption), and human-caused disturbances (habitat destruction). These factors can cause dramatic population fluctuations but do not regulate populations around an equilibrium.

Most populations are influenced by both types of factors. Density-dependent factors provide stabilizing regulation; density-independent factors introduce variability.

Population Cycles

Some populations exhibit regular cycles of abundance — snowshoe hares and lynx in the boreal forest, lemmings in the Arctic, some insects. These cycles may be driven by predator-prey dynamics, food availability, or intrinsic physiological changes.

Human Population Growth

The human population has grown exponentially over the past few centuries, from approximately 1 billion in 1800 to over 8 billion today. The growth rate has slowed (demographic transition) as societies undergo industrialization, education improves, and birth rates decline, but the absolute population continues to increase. The ecological footprint of human populations — resource consumption, habitat conversion, pollution — exceeds the planet’s capacity in many measures.

ELI-10

A population is a group of the same species living together. Populations grow when births plus immigrants outnumber deaths plus emigrants. They shrink when the opposite happens.

Populations do not just keep growing forever. They are limited by two kinds of controls:

The first kind gets stronger as the population gets more crowded. More rabbits mean less grass per rabbit, more fox attacks (because foxes find rabbits more easily), and more diseases spreading. These are density-dependent controls — they naturally push the population toward a stable level, like a thermostat.

The second kind hits regardless of how crowded the population is. A drought, a flood, or a fire kills a certain fraction regardless of density. These are density-independent controls — they cause unpredictable crashes but do not regulate populations around any particular level.

The maximum number of individuals an environment can support is the carrying capacity (K). Think of a restaurant: the carrying capacity is the number of seats. The restaurant can fill up to its seat count, but it cannot seat more than that (at least not without people standing in the aisles). If the restaurant suddenly gets more tables (the environment improves), K goes up. If half the kitchen burns down (a disturbance), K drops.

The logistic growth model is the S-shaped curve: slow growth at first, then rapid, then slowing as the population approaches its carrying capacity. Exponential growth is the J-shaped curve — the population doubling and redoubling without limits — which can only happen temporarily.

Different species have different survivorship patterns. Humans and elephants (Type I) — most survive to old age. Birds (Type II) — steady risk throughout life. Oysters and trees (Type III) — millions of babies, almost all die young, but the few survivors live long lives.

ELI Example

A population is like bacteria in a petri dish — at first, they multiply explosively (exponential growth). Then the sugar starts running out, wastes build up, and growth slows (logistic growth). Eventually the population levels off at the carrying capacity — the dish can feed only so many bacteria. If a lab tech adds more sugar (increases K), the population can grow again. If the dish dries out (density-independent disaster), the population crashes regardless of how many bacteria were in it.

Do Not Confuse

• Exponential vs. Logistic Growth: Exponential = J-shaped, no limit. Logistic = S-shaped, approaches carrying capacity (K).

• Density-Dependent vs. Density-Independent: Density-dependent = effect scales with population density (competition, disease). Density-independent = effect unrelated to density (weather, natural disasters).

• Population vs. Community: A population is one species. A community is all species in an area.

High-Yield Memory Anchors

• N changes by births (+), deaths (−), immigration (+), emigration (−).

• Exponential: dN/dt = rN (J-curve, unlimited). Logistic: dN/dt = rN[(K−N)/K] (S-curve, approaches K).

• Density-dependent = competition, predation, disease (thermostat). Density-independent = weather, disasters (random).

• Survivorship: Type I (late loss — humans), Type II (constant — birds), Type III (early loss — oysters).

• Dispersion: clumped (most common), uniform (territorial), random (rare).

Quick Check

Q1: A population is growing exponentially. Which of the following must be true?

A) The population has exceeded its carrying capacity

B) The per capita growth rate (r) is constant

C) Density-dependent factors are regulating the population

D) The population is near K

Q2: A deer population on an island grows according to the logistic model. For several years, the population hovers around 500 individuals. Then a hurricane kills 200 deer, reducing the population to 300. Predict what happens to the population over the next several years and explain why.

Q3: Compare how a density-dependent factor (e.g., a contagious disease) and a density-independent factor (e.g., a severe frost) would each affect a population of insects. How does each type of factor contribute to population regulation?

Quick Check Answers

A1: B. The per capita growth rate (r) is constant. Exponential growth occurs when r is constant at r_max and resources are effectively unlimited. The population has not reached K, and density-dependent factors are not constraining growth.

A2: After the hurricane reduces N from 500 to 300, the population is now well below K (500). Resources are abundant relative to the smaller population. The logistic model predicts that the population will grow rapidly (the [(K−N)/K] term is large — (500−300)/500 = 0.4) and return to approximately 500 over the next several years, following the upward portion of the S-curve. In reality, the population may overshoot 500 slightly before settling back, and if the hurricane damaged the habitat (reducing K), the new equilibrium may be lower.

A3: Density-dependent factor (contagious disease): At low insect density, transmission is rare and disease has minimal impact. As density increases, contact rates rise, transmission increases, and mortality from disease rises. This creates a negative feedback — high density → more disease → increased mortality → density declines → transmission slows. The disease tends to regulate the population around an equilibrium density. Density-independent factor (frost): A severe frost kills a similar proportion of insects regardless of whether the population is at high density or low density. The frost causes a population reduction, but it does not create a feedback loop — it does not regulate the population around any particular level. Frosts can cause erratic population fluctuations but do not provide stabilizing regulation. Both factors reduce population size, but only density-dependent factors provide the negative feedback that regulates populations around an equilibrium.

Chapter Summary

Population ecology studies the factors that determine population size, density, distribution, and growth. Exponential growth occurs when resources are unlimited; logistic growth incorporates carrying capacity (K) and density-dependent regulation. Populations are regulated by density-dependent factors (competition, predation, disease) and affected by density-independent factors (weather, disasters). Survivorship curves and age-structure diagrams provide demographic insights. Human population growth illustrates the tension between exponential increase and environmental limits.

Common Mistakes

• “Carrying capacity is a fixed number.” K varies with environmental conditions, resource availability, and technology. A drought lowers K. Fertilization raises K. K is dynamic, not static.

• “Populations always follow the logistic growth curve.” The logistic model is a simplification. Real populations may overshoot K, crash, fluctuate, or be affected by time lags and environmental variability. The model captures the essential concept of density-dependent growth limitation, not the exact trajectory.