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. It provides tools for understanding population size, the factors that affect population size, and the age-class distribution within populations. This knowledge is essential for managing threatened species and controlling pest populations.

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

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

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

• Population structure: The composition of a population, including age and sex distribution.

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 as a result of births, deaths, immigration, and emigration. For modeling purposes, immigration and emigration are often assumed to be zero, focusing attention on birth and death rates.

Life History Strategies and Trade-Offs

Life History Traits

Life history traits are characteristics that affect an organism's schedule of reproduction and survival. These include age at first reproduction, number and size of offspring, and lifespan. Trade-offs occur because resources are limited, so organisms must balance between maximizing reproduction (fecundity) and maximizing survival.

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

• Survivorship: The proportion of offspring that survive to a particular age.

• Trade-off: Allocation of resources to either growth, reproduction, or survival.

Life History Continuum

Additional info: This table summarizes the trade-offs between producing many small offspring with low survival versus few large offspring with high survival.

Population Growth Models

Quantifying Population Growth

The growth rate of a population is determined by the difference between the birth rate and the death rate. The rate of increase is denoted as r:

• r = b - d

The change in population size over a time interval is:

• Growth rate:

If , the population is growing; if , the population is shrinking.

Intrinsic Rate of Increase ()

The intrinsic rate of increase () is the maximum per capita growth rate of a population under ideal conditions (unlimited resources, minimal threats). It is species-specific and depends on life history traits.

• High : Bacteria, insects

• Low : Elephants, humans

Exponential Growth Model

Exponential growth occurs when resources are abundant and population size increases at a constant rate. The equation for exponential growth is:

Exponential growth is density-independent and is observed when a population colonizes a new habitat or recovers after a catastrophe.

• Example: Bacteria growing in a nutrient-rich flask, birds colonizing an uninhabited island.

Logistic Growth Model

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). The equation for logistic growth is:

Logistic growth produces an S-shaped curve, with three distinct stages:

1. Early growth is exponential (density-independent).

2. Growth rate declines as competition for resources increases (density-dependent).

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

• Carrying capacity (K): The maximum number of individuals that can be supported in a particular habitat over a period of time. K can vary depending on resource availability, space, water, soil quality, and other factors.

Stages of Logistic Growth

Density Dependence

Density-dependent factors cause the growth rate to slow as population density increases. These include competition for resources, predation, disease, and waste accumulation. In contrast, density-independent factors (such as weather or natural disasters) affect population growth regardless of density.

• Example: As a population of birds on an island grows, food becomes scarce, slowing population growth until it stabilizes at carrying capacity.

Graphical Representation of Population Growth

Population growth curves help visualize how populations change over time. Exponential growth produces a J-shaped curve, while logistic growth produces an S-shaped curve. The transition from density-independent to density-dependent growth is marked by a slowing of the growth rate as resources become limited.

• Stage 1: Rapid, exponential growth (abundant resources).

• Stage 2: Growth slows as competition increases.

• Stage 3: Population stabilizes at carrying capacity (K).

Summary Table: Exponential vs. Logistic Growth

Additional info: The notes also reference a graph of temperature versus latitude, which is a common ecological pattern. Descriptions and explanations of such patterns are important in ecology for understanding how environmental variables affect species distributions.