Population Ecology

Introduction to Population Ecology

Population ecology is the study of how and why the number of individuals in a population changes over time and space. It provides tools for understanding population size, factors affecting population growth, and strategies for managing species.

• Population:

• Population size: The total number of individuals in a population.

• Population density: The number of individuals per unit area or volume.

• Population structure: The age and sex distribution within a population.

Applications: Population ecology helps biologists study endangered species, control pests, and manage natural resources.

Key Parameters in Population Ecology

• Birth rate (b): The number of births per individual per unit time.

• Death rate (d): The number of deaths per individual per unit time.

• Immigration: The arrival of new individuals from other populations.

• Emigration: The departure of individuals to other populations.

Population size changes according to the balance of these parameters.

Life History Strategies and Trade-offs

Organisms face trade-offs in allocating limited resources between growth, reproduction, and survival. These trade-offs shape their life history strategies.

• Fecundity: The number of offspring produced by an individual.

• Survivorship: The probability of surviving to a particular age.

• Life-history continuum: Species can be placed on a spectrum from high fecundity/low survivorship to low fecundity/high survivorship.

Example: Mice have high fecundity and low survivorship, while elephants have low fecundity and high survivorship.

Population Growth

Basic Population Growth Model

The growth rate of a population is determined by the difference between birth and death rates, and can be influenced by immigration and emigration.

• Growth rate (r): The per capita rate of increase, calculated as:

• Change in population size (ΔN) over time (Δt):

• If immigration and emigration are negligible, population growth depends mainly on births and deaths.

Exponential Growth

Exponential growth occurs when resources are abundant and population size increases rapidly. The rate of increase (r) is constant and not limited by population density.

• Exponential growth equation:

• Intrinsic rate of increase (rmax): The maximum per capita growth rate under ideal conditions; species-specific and influenced by life history traits.

• Exponential growth is often observed when a population colonizes a new habitat or recovers after a catastrophe.

Example: Bacteria in a nutrient-rich flask or birds colonizing an uninhabited island.

Logistic Growth

Logistic growth occurs when population growth is limited by resources, leading to a decrease in growth rate as population size approaches the carrying capacity (K).

• Carrying capacity (K): The maximum number of individuals that a habitat can support over time.

• Logistic growth equation:

• Growth is rapid at first (when N is small), slows as resources become limited, and stops when N = K.

• Produces an S-shaped (sigmoidal) growth curve.

Density Dependence

Population growth can be regulated by density-dependent factors, which intensify as population density increases.

• Density-dependent factors: Competition for resources, disease, predation, and waste accumulation.

• Density-independent factors: Weather, natural disasters, and other factors not related to population density.

Example: As a population of birds on an island grows, competition for nesting sites increases, slowing population growth.

Stages of Logistic Growth

• Stage 1: Exponential growth (r is constant, density-independent).

• Stage 2: Growth rate declines as competition increases (density-dependent).

• Stage 3: Growth rate reaches zero at carrying capacity (N stabilizes).

Graphical Representation

Population growth curves can be used to model and predict changes in population size over time. Logistic growth curves show three distinct phases: rapid growth, slowing growth, and stabilization at carrying capacity.

Summary Table: Exponential vs. Logistic Growth

Additional info:

• Some content inferred from context and standard biology curriculum (e.g., definitions, equations, and examples).

• Tables reconstructed and expanded for clarity and completeness.