Chapter 53: Population Ecology – Study Notes

Population Ecology

Introduction to Population Ecology

Population ecology is the study of populations in relation to their environment, including environmental influences on population density and distribution, age structure, and variations in population size. Understanding population ecology is essential for conservation biology, resource management, and predicting changes in biodiversity.

• Population: A group of individuals of a single species living in the same general area.

• Population Dynamics: The study of how and why populations change in size and structure over time.

• Example: Tracking sea turtle nesting and hatchling survival to understand population trends.

Population Characteristics

Density and Dispersion

Population density refers to the number of individuals per unit area or volume, while dispersion describes the pattern of spacing among individuals within the boundaries of the population.

• Density: Influenced by births, deaths, immigration, and emigration.

• Dispersion Patterns:

  • Clumped: Individuals aggregate in patches (e.g., starfish on a rock).

  • Uniform: Evenly spaced, often due to territoriality (e.g., penguins nesting).

  • Random: Unpredictable spacing (e.g., dandelions blown by wind).

Measuring Population Size

Population size can be estimated using sampling techniques such as the mark-recapture method.

• Mark-Recapture Method: A sample of individuals is captured, marked, and released. Later, a second sample is captured, and the number of marked individuals is counted.

The estimated population size (N) is calculated as:

$N = \frac{sn}{x}$

• s: Number of individuals marked and released in the first sample

• n: Total number of individuals captured in the second sample

• x: Number of marked individuals recaptured in the second sample

Demography and Life Tables

Demography

Demography is the study of the vital statistics of populations and how they change over time, focusing on birth rates, death rates, and age structure.

• Life Table: Summarizes the survival and reproductive rates of individuals in specific age groups within a population.

• Cohort: A group of individuals of the same age, followed from birth to death.

Survivorship Curves

Survivorship curves graphically represent the number of individuals in a cohort surviving at each age.

• Type I: Low death rates during early and middle life, then an increase among older age groups (e.g., humans).

• Type II: Constant death rate over the organism's life span (e.g., squirrels).

• Type III: High death rates for the young, but lower death rates for survivors (e.g., oysters).

Reproductive Rates

Reproductive rates are estimated by determining the number of female offspring produced by females of different ages.

• Life tables and reproductive data are used to calculate population growth rates.

Population Growth Models

Exponential Growth Model

Exponential growth describes population increase under idealized, unlimited conditions. The rate of increase is constant, but the population accumulates more new individuals per unit time when it is large.

The exponential growth equation is:

$\frac{dN}{dt} = r_{max}N$

• N: Population size

• rmax: Maximum per capita rate of increase

Logistic Growth Model

The logistic model incorporates carrying capacity (K), the maximum population size the environment can support. As N approaches K, the growth rate slows.

The logistic growth equation is:

$\frac{dN}{dt} = r_{max}N\left(\frac{K-N}{K}\right)$

• K: Carrying capacity

• When N is small compared to K, growth is close to exponential.

• When N is large and resources are limited, growth slows.

Comparing Exponential and Logistic Growth

• Exponential growth results in a J-shaped curve.

• Logistic growth results in an S-shaped curve as the population reaches carrying capacity.

Life History Strategies

Life History Traits

Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism. These traits include age at first reproduction, frequency of reproduction, and number of offspring produced.

• Semelparity: One-time reproduction (e.g., salmon).

• Iteroparity: Repeated reproduction (e.g., humans, elephants).

Trade-offs and Life Histories

• Organisms face trade-offs between survival and reproduction, number and size of offspring, and parental care.

• r-selection: Selection for traits that maximize reproductive success in uncrowded environments (high growth rate, many offspring, little parental care).

• K-selection: Selection for traits that are sensitive to population density (few offspring, high parental care, stable environments).

Population Regulation

Density-Dependent Regulation

Density-dependent factors intensify as population size increases, regulating growth through mechanisms such as competition, disease, predation, territoriality, intrinsic factors, and toxic wastes.

• Competition for Resources: Increased competition reduces reproduction and survival.

• Disease: Transmission rates increase with density.

• Predation: Predators may focus on abundant prey.

• Territoriality: Limits population size when individuals claim exclusive areas.

• Intrinsic Factors: Physiological factors (e.g., stress) can reduce reproduction.

• Toxic Wastes: Accumulation can limit population size (e.g., yeast in wine vats).

Density-Independent Factors

• Factors such as weather, climate, and natural disasters affect populations regardless of density.

Population Dynamics and Human Population Growth

Population Dynamics

Population dynamics examines fluctuations in population size and the factors that cause them, including stability, fluctuation, and cycles (e.g., lynx and hare cycles).

Metapopulations

Metapopulations are groups of populations linked by immigration and emigration. Local populations may go extinct, but recolonization can occur from other patches.

Human Population Growth

• The human population grew rapidly in the 20th century but is now increasing more slowly.

• Growth rates vary by region due to differences in birth rates, death rates, and age structure.

Age Structure and Demographic Transition

• Age Structure: The relative number of individuals of each age in a population, often shown as a pyramid.

• Demographic Transition: The shift from high birth and death rates to low birth and death rates, associated with industrialization and improved living conditions.

Carrying Capacity and Ecological Footprint

• Carrying Capacity: The maximum population size that the environment can sustain.

• Ecological Footprint: The aggregate land and water area required by a person, city, or nation to produce all the resources it consumes and absorb all the waste it generates.

Summary Table: Exponential vs. Logistic Growth

Feature	Exponential Growth	Logistic Growth
Equation	$\frac{dN}{dt} = r_{max}N$	$\frac{dN}{dt} = r_{max}N\left(\frac{K-N}{K}\right)$
Curve Shape	J-shaped	S-shaped
Limiting Factors	None (ideal conditions)	Carrying capacity (K)
Population Size	Unlimited growth	Growth slows as N approaches K

Key Equations

• Population growth rate: $\Delta N = B - D$ Where $\Delta N$ is the change in population size, $B$ is the number of births, and $D$ is the number of deaths.

• Per capita change in population size: $\frac{\Delta N}{\Delta t} = rN$ Where $r$ is the per capita rate of increase.

• Exponential growth: $\frac{dN}{dt} = r_{max}N$

• Logistic growth: $\frac{dN}{dt} = r_{max}N\left(\frac{K-N}{K}\right)$

Summary

• Population ecology explores how populations grow, fluctuate, and are regulated by biotic and abiotic factors.

• Understanding population dynamics is crucial for conservation, resource management, and predicting human impacts on the environment.