# Lecture 25: Population Ecology
## General Biology II — Organismal, Evolution & Ecology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Define population ecology and describe the key characteristics used to describe populations
2. Distinguish between exponential and logistic population growth models and their assumptions
3. Explain the concept of carrying capacity and density-dependent regulation
4. Compare density-dependent and density-independent factors that affect population size
5. Interpret survivorship curves and life tables
6. Distinguish between r-selected and K-selected life history strategies
7. Apply demographic principles to human population growth and its implications
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## Lecture Content
### I. What Is a Population?
**Population** — a group of individuals of the same species living in the same area at the same time and capable of interbreeding. Population ecology studies the factors that influence population size, density, distribution, and growth over time. Key population characteristics: **Population size (N)** — total number of individuals. **Population density** — number of individuals per unit area or volume. **Dispersion (distribution) pattern** — how individuals are spaced within their habitat: Clumped — most common; individuals aggregate around resources, for protection, or due to social behavior (e.g., schooling fish, herding mammals) Uniform — evenly spaced; often results from territorial behavior or competition (e.g., nesting penguins, creosote bushes) Random — unpredictable spacing; occurs when resources are abundant and individuals do not interact strongly (e.g., dandelions in a field) **Age structure** — proportion of individuals in pre-reproductive, reproductive, and post-reproductive age classes. **Sex ratio** — ratio of males to females.
### II. Demography — Life Tables and Survivorship Curves
**Demography** — the study of vital statistics (birth rates, death rates, age structure) that influence population growth. **Life table** — an age-specific summary of survival and reproduction: Age class (x), number alive at start of age class (n_x), survivorship (l_x = proportion surviving from birth to age x), fecundity (m_x = average number of female offspring per female in age class x) Net reproductive rate (R_0) = sum of (l_x * m_x) — average number of female offspring per female over her lifetime; R_0 > 1 = population growing; R_0 = 1 = stable; R_0 < 1 = declining. **Survivorship curves** — plot of the proportion of individuals surviving to each age: **Type I** — low mortality early in life, high mortality in old age; high parental care (humans, large mammals, elephants) **Type II** — constant mortality rate throughout life (some birds, some lizards, rodents) **Type III** — very high mortality early in life, low mortality for survivors; many offspring, little parental care (oysters, sea turtles, many fish, most insects, plants).
A graph of survivorship curves with a logarithmic y-axis (proportion of survivors from 0.001 to 1.0) and a linear x-axis (percentage of maximum lifespan from 0 to 100). Three curves are plotted. Type I (labeled, with an icon of a human/elephant) starts high and remains high until late in life when it drops steeply, forming a convex curve. Type II (labeled, with an icon of a songbird) is a straight diagonal line from upper left to lower right, representing constant mortality. Type III (labeled, with an icon of an oyster/sea turtle) drops steeply early in life then levels off for the few survivors, forming a concave curve. Each curve is a different color and clearly labeled with representative organisms.
### III. Exponential Population Growth
Under ideal conditions (unlimited resources, no predation, no disease), populations grow exponentially. **Exponential growth model**: dN/dt = r_max * N. N = population size; t = time; r_max = intrinsic (maximum per capita) rate of increase = birth rate - death rate (under ideal conditions) dN/dt = rate of population change. Produces a **J-shaped curve** — growth accelerates as N increases; r_max depends on species biology: generation time, fecundity, age at first reproduction. Bacteria can double every 20 minutes (very high r_max) Elephants have generation times of ~25 years (low r_max) Exponential growth cannot continue indefinitely — resources become limiting. Observed in nature: populations colonizing new habitats, populations recovering after a crash, invasive species introductions, bacterial growth in fresh medium.
### IV. Logistic Population Growth
As a population grows, resources become scarce → birth rates decline and/or death rates increase → growth rate slows. **Carrying capacity (K)** — the maximum population size that the environment can sustain indefinitely given available resources (food, space, water, shelter) **Logistic growth model**: dN/dt = r_max * N * ((K - N) / K) The term (K - N)/K represents the fraction of carrying capacity remaining. When N is small relative to K: growth is approximately exponential. When N = K/2: population growth rate is at its maximum. When N approaches K: growth rate approaches zero (population stabilizes) When N > K: growth rate is negative (population declines back toward K) Produces an **S-shaped (sigmoidal) curve**. Assumptions of the logistic model: K is constant. Each individual added to the population has the same proportional effect on growth rate. No time lags in the effect of density on growth. In reality, populations often overshoot K, then oscillate around it, or exhibit more complex dynamics.
A side-by-side comparison of exponential and logistic growth models. Panel A (Exponential growth): A graph with population size (N) on the y-axis and time on the x-axis. The J-shaped curve rises steeply with no leveling off. The equation dN/dt = r_max * N is displayed above the curve. Annotations indicate that growth rate increases continuously as the population grows. Panel B (Logistic growth): A graph with the same axes. The S-shaped (sigmoidal) curve rises steeply at first, then inflects at N = K/2 (marked with a dashed horizontal line and labeled "maximum growth rate"), and levels off as it asymptotically approaches the carrying capacity K (shown as a horizontal dashed line at the top). The equation dN/dt = r_max * N * ((K - N) / K) is displayed above the curve. Three zones are annotated: "near-exponential growth" at low N, "decelerating growth" as N approaches K, and "equilibrium at K" where the curve plateaus. A small inset graph shows dN/dt (growth rate) versus N, with a parabola peaking at N = K/2 and returning to zero at N = K.
### V. Factors Regulating Population Size
#### A. Density-Dependent Factors
Effects intensify as population density increases — provide negative feedback that limits growth. **Intraspecific competition** — competition among individuals of the same species for limited resources (food, space, mates, nesting sites) As density increases, per capita resource availability decreases → lower birth rates, higher death rates. **Predation** — predators may switch to a prey species when it becomes abundant; higher density = easier to find prey. **Disease and parasitism** — transmission rates increase at higher densities (more frequent contact) **Toxic waste accumulation** — metabolic waste builds up in dense populations (e.g., ethanol in yeast cultures) **Territoriality** — limits the number of breeding individuals in an area. **Intrinsic factors** — in some species, high density triggers physiological stress responses (elevated cortisol) → reduced reproduction, immune suppression.
#### B. Density-Independent Factors
Effects do not change with population density. Environmental disturbances: droughts, floods, fires, hurricanes, volcanic eruptions, extreme temperature events. Affect all individuals equally regardless of population size. Can cause sudden population crashes. Important especially in environments with unpredictable or extreme conditions.
### VI. Life History Strategies
**Life history** — the traits that affect an organism's schedule of reproduction and survival: age at first reproduction, number of reproductive events, number of offspring, parental care, lifespan. Trade-offs — organisms have limited energy and resources; investment in one trait comes at the cost of another: Number vs. size of offspring: many small offspring vs. few large offspring. Current vs. future reproduction: high investment now may reduce survival/future fecundity. Reproduction vs. survival: high reproductive effort may decrease lifespan. **r-selected species (opportunistic life history)**: Small body size, early maturity, many small offspring, little or no parental care, short lifespan. Thrive in unpredictable or disturbed environments; adapted for rapid population growth. Populations often well below K; regulated primarily by density-independent factors. Examples: insects, annual plants, bacteria, mice, dandelions. **K-selected species (equilibrial life history)**: Large body size, late maturity, few large offspring, extensive parental care, long lifespan. Thrive in stable, competitive environments; population size near K. Regulated primarily by density-dependent factors (competition) Examples: elephants, whales, primates, large trees, albatrosses; r-K selection is a continuum — most species fall somewhere in between. Modern life history theory focuses on specific trade-offs rather than the r-K dichotomy.
### VII. Human Population Growth
Human population has grown approximately exponentially for much of recent history: ~10,000 years ago: ~5 million; 1800: ~1 billion; 1930: ~2 billion; 1960: ~3 billion; 1999: ~6 billion; 2024: ~8 billion. Growth driven by: agricultural revolution, industrial revolution, improvements in medicine and sanitation, reduced death rates. **Demographic transition** — a shift from high birth and death rates to low birth and death rates as countries develop: Stage 1: pre-industrial — high birth and death rates; slow growth. Stage 2: transitional — death rates fall (improved medicine/sanitation); birth rates remain high; rapid growth. Stage 3: industrial — birth rates decline (urbanization, education, contraception); growth slows. Stage 4: post-industrial — low birth and death rates; population stabilizes or declines. **Age structure diagrams (population pyramids)** — show the distribution of age and sex in a population: Broad base (many young) = rapid growth (e.g., many sub-Saharan African countries) Roughly even distribution = slow growth or stable (e.g., United States) Narrow base (few young) = declining population (e.g., Japan, Italy) **Ecological footprint** — the total area of productive land and water required to support a population's resource consumption and waste absorption. Current global consumption exceeds Earth's biocapacity (ecological overshoot) Wealthier nations have disproportionately large ecological footprints per capita.
A three-panel figure on human population dynamics. Panel A: A graph of the human population over the last 12,000 years (x-axis: years from 10,000 BCE to 2025 CE; y-axis: population in billions from 0 to 8). The curve is nearly flat for most of history, then rises sharply beginning around 1800, illustrating near-exponential growth. Key milestones are labeled: agricultural revolution, industrial revolution, and modern medicine. Panel B: Three representative age-structure diagrams (population pyramids) side by side. The left pyramid has a very broad base and tapered top, labeled "rapid growth" (example: Nigeria). The middle pyramid has roughly even bars from bottom to top, labeled "slow growth" (example: United States). The right pyramid has a narrow base and a bulge in the middle-to-upper age classes, labeled "declining" (example: Japan). Males are on the left and females on the right of each pyramid, with age classes in 5-year increments on the vertical axis. Panel C: A bar chart showing ecological footprint per capita (in global hectares) for selected countries, with the global biocapacity per person marked as a horizontal dashed line. High-income countries (e.g., USA, Australia) exceed the biocapacity line by a large margin, while low-income countries fall below it.
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