8.3 Population Ecology

What is a Population?
A population is defined as a group of the same species living in the same area. Example populations include humans residing in Seattle, a colony of bees in a hive, or a group of pine trees in a forest. 🐝🌲

Populations can vary in size, density, and distribution—characteristics that change based on resource availability and environmental conditions. In population ecology, we study how populations grow, interact, and adapt to their environments.

Factors Affecting Population Survival
To survive, a population must have access to essential resources such as food, water, oxygen, and shelter. Populations are influenced by several biotic and abiotic factors:

Resources: Access to food, water, and shelter determines survival.
Habitat: A suitable living environment is crucial for reproduction and growth.
Competition: Both intra- and interspecific competition can influence resource availability.
Predation: Predators play a major role in controlling population sizes.
Disease: Spread of disease can lead to population decline.
Climate: Temperature, precipitation, and extreme weather events can affect resources and habitat suitability. 🌧️

The importance of each factor varies depending on the species and environment in question.

Mathematical Models in Population Ecology
Population changes are measured using mathematical models. The basic equation for population growth is:

dN / dt = B – D

Where:

dN: Change in population size.
dt: Change in time.
B: Birth rate.
D: Death rate.

This equation helps calculate whether a population is growing or shrinking based on birth and death rates. For instance, if there are 42 births and 17 deaths in a population of iguanas over the past year, the population has grown by 25 iguanas. 🐊

Exponential Growth
Exponential growth occurs when a population grows without constraints, leading to a rapid, geometric increase in size. It is described by:

dN / dt = (r max) (N)

Where:

dN: Change in population size.
dt: Change in time.
r max: Maximum per capita growth rate of the population.
N: Population size.

For example, a population of 862 iguanas with a growth rate of 0.05 grows by 43 individuals in one year, reaching 905 iguanas.

Exponential growth is unsustainable in the long term, as resources become limited and environmental pressures increase.

Example: The Rabbit Invasion of Australia
An example of exponential growth is the introduction of European rabbits to Australia in the 19th century. Initially introduced for hunting, rabbits reproduced rapidly due to a lack of natural predators. 🐇 This led to massive ecological damage, including:

Competition with native species for resources.
Habitat destruction due to overgrazing.
Economic impacts on farmers and ranchers.

Despite various control methods, rabbits continue to pose challenges to Australia’s ecosystems.

Logistic Growth
Most population growth follows a logistic model, characterized by an S-shaped curve. This growth starts rapidly but eventually stabilizes at the environment’s carrying capacity—the maximum number of individuals an environment can sustain.

A well-known example is the reintroduction of wolves to Yellowstone National Park. Wolves prey on elk, and their predation acts as a limiting factor for elk populations. Likewise, if wolf numbers grow too high, elk populations decrease, limiting the wolves’ food supply. This bottom-up and top-down regulation keeps both populations in check. 🐺

Learning Summary
Population ecology is the study of species populations, their dynamics, and interactions with the environment. Factors such as resource availability, competition, and predation influence population growth and survival. Ecologists use mathematical equations to model changes, helping predict population trends and inform conservation efforts.

Understanding population dynamics is crucial for managing ecosystems and protecting biodiversity. Upcoming topics will further explore how population changes impact community ecology and density-related factors.