8.3 Population Ecology

Skills you’ll gain in this topic:

• Describe the factors that affect population growth, such as birth and death rates.
• Explain density-dependent and density-independent factors in population changes.
• Use models to predict population growth patterns and stability.
• Connect population dynamics to available resources.
• Analyze how different environmental factors drive population changes.

What Controls Population Size and Growth

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 living in a hive, or a group of pine trees growing in a forest. 🐝🌲

Populations can vary in size, density, and distribution—characteristics subject to change when environmental changes occur, like resource availability and species interactions. They also vary in terms of genetic makeup, which can affect the ability of species to adapt to changing conditions.

In population ecology, the focus is on understanding the dynamics of the population and how a population is affected by various factors, such as competition, predation, and habitat availability, as these factors impact the health and growth of species. 

Understanding populations affects how humans approach conservation and management efforts in addition to informing how species interactions affect the overall functioning of ecosystems.  

Population Survival

A number of factors are important for a population to survive. These factors can be divided into two broad categories: biotic factors, which are related to the interactions between members of the population and other living organisms, and abiotic factors, which are related to the physical and chemical characteristics of the environment. Some key examples that are important for population survival include:

1. Resources: Populations need access to sufficient resources, such as food, water, oxygen, and shelter, in order to survive and reproduce. The availability of these resources can influence the size and distribution of a population.
2. Habitat: Populations need a suitable habitat in which to live and reproduce. Habitat loss and degradation can have significant impacts on population survival. 🌱
3. Competition: Populations within or between species may compete with each other for limited resources, including food, water, and mating partners. This competition can influence the survival and reproductive success of individual members of the population.
4. Predation: Populations may be preyed upon by other species, which can have significant impacts on their survival and reproduction.
5. Disease: Populations can be impacted by the spread of diseases, which can lead to a decrease in population size.
6. Climate: Populations may be influenced by changes in climate, such as changes in temperature, precipitation, and extreme weather events, which can affect the availability of resources and the suitability of the habitat for species. 🌧️

The relative importance of these factors varies depending on the species and the specific environment in which it lives.

Mathematical Equations 

Interactions and events within a population can be measured using a number of mathematical equations. Most simply, a population can be measured using the following equation for population growth: 

dN / dt = B - D

where:

dN is the change in population 

dt is the chage in time 

B is the birth rate 

D is the death rate 

This simple equation shows that the overall change in a population over time is equal to the number of births in that population minus the number of deaths in that population. 

Ex. In a population of iguanas, there are 42 births and 17 deaths over the past year. What is the change in population over the course of the year? 🦎

The overall change in population is +25, meaning that the population increased by 25 iguanas over the course of the year.

Exponential Growth

Exponential population growth is a type of population growth in which the number of individuals in a population increases geometrically at a constant rate over time. For exponential population growth to occur, some of the following conditions must be met:

1. There are no limiting factors: If a population has unlimited access to resources and suitable habitat, it can potentially grow exponentially.
2. The population has a high reproductive rate: Exponential population growth is more likely to occur in species that have a high reproductive rate, meaning they can produce a large number of offspring in a short period of time.
3. The population has a low mortality rate: If a population has a low mortality rate (i.e. a high survival rate), it has the potential to grow exponentially because the birth rate of new individuals in the population will be greater than the rate that current individuals die off.

Exponential population growth is not sustainable in the long term because eventually, the population will reach the carrying capacity of its environment, meaning it will no longer be able to continue growing at the same rate due to limited resources and other factors. However, in the short term, exponential population growth can have significant impacts on the environment and the availability of resources for other species.

There is another formula used to calculate exponential growth:

dN / dt = (r max) (N)

where: 

dN is the change in population size

dt is the change in time

r max is the maximum per capita growth rate of the population

N is the population size 

Ex. A population of 862 iguanas has a per capita growth rate of 0.05. What is the growth of the population after one year? What is the new population size? 🦎

The population grew by 43 iguanas in one year. The new population is 862 + 43, or 905 iguanas.

Image courtesy of Giphy.

Example of Exponential Growth

An example of exponential growth is the introduction of European rabbits to Australia in the 19th century. 🐇 Rabbits were originally brought to Australia for hunting purposes, but they quickly became an invasive species and spread across the continent due to their high reproductive rate. In general, female European rabbits can have up to six litters per year, with an average of four to six offspring per litter.

The exponential growth of the European rabbits created detrimental impacts on Australian ecosystems, including:

1. Competition with native species: The rabbits competed with native species for resources, such as food and habitat, which led to declines in the populations of many native species.
2. Habitat destruction: The rabbits caused significant damage to the landscape by burrowing and feeding on native vegetation, leading to habitat loss and degradation.
3. Economic impacts: The rabbits damaged crops and pastures, leading to losses for farmers and ranchers.

Despite efforts to limit the rabbit population, including fences and chemical control, exponential growth following the rabbit invasion has allowed the European rabbit to continue damaging Australia today. 

Logistic Growth

However, most population growth is logistic, characterized by an S-shaped curve, with a rapid increase in population size at first, followed by a slowing of the rate of population increase as the population approaches its carrying capacity. Logistic growth begins exponentially, but population growth is limited by resource availability, predation, and habitat quality. These limiting factors can act to decrease the rate of population increase as the population size increases, eventually leading to a stabilization of the population size at or near the carrying capacity of the environment, the number of individuals that an environment can support. 

For example, take the reintroduction of wolves to Yellowstone National Park in the mid-1990s. Predation serves as a limiting factor for population growth in the relationship between wolves and elk. Because wolves prey on elk for food, the elk population decreased after wolves returned to Yellowstone because they experienced predation pressure to check the size of their population. 🐺

Importantly, this relationship works in both directions. If the wolf population grows too large, there will not be enough elk to prey on. This means that the elk population exerts a bottom-up limiting factor on wolves while wolves provide a top-down limiting factor on elk.  

Learning Summary ✨

Population ecology is a subfield of ecology that focuses on the dynamics of species populations and how they interact with their environment. It involves the study of factors that affect the size and distribution of populations, as well as their survival and reproductive success. Some of the key concepts in population ecology include population growth and regulation, competition, predation, and mutualism. 

Population ecologists use a variety of tools and techniques, including field observations, experiments, and mathematical modeling, to understand the factors that influence population dynamics. Understanding how populations grow and change is valuable because it helps us to understand the factors that influence the abundance and distribution of species, which has important implications for conservation and management efforts.

Using mathematical equations, population growth and exponential growth are modelable. In combination with factors necessary for population survival, these equations help predict how populations may change over time.

Changes in populations are caused by various biotic and abiotic factors. Some of them include habitat and resource availability, interactions with other organisms in the environment, and the effects of climate and disease. These impacts will be further examined with upcoming topics on population density and community ecology. 

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Check out the AP Bio Unit 8 Replays or watch the 2021 Unit 8 Cram

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