Population Dynamics

Baby boomers. Generation X. Millennials. These are terms to define the most recent generations of humankind. This classification and labeling of human generations finds its roots in population ecology. Observing differences in population, and how they change over time, is essentially an exercise in population dynamics.

• First, we will look at the definition of population dynamics.
• After, we will talk about the characteristics of a population.
• Then, we will learn about models in population dynamics.
• After, we will discuss population dynamics in ecology.

Definition of Population Dynamics

What exactly is meant when using the term population dynamics in biology? A population is a convergence of individuals of the same species who live, interact, and reproduce in the same geographic region. The word dynamics means forces that produce change. Combine the two and we get population dynamics!

The definition of population dynamics is as follows.

Before we start our discussion on population dynamics, we should review some terminology to better visualize the interactions of a population with its surroundings (Fig. 1).

Population Characteristics

Population dynamics observes the changes in the static physical properties of populations. Here are the main observable characteristics of populations:

• Size

• Density

• Dispersion

• Sex distribution

• Age distribution

Size

Size is the count of individuals within a population. This count represents a specific and unique point in time. When observing population dynamics, we thus need at least two data points at time A and time B. In population ecology, the common symbol for population size is the uppercase letter 'N'.

Density

Density is the number of individuals of a population per region.

For example, a very dense population of squirrels may have one individual per square meter, whereas a more scattered population may have one individual per square kilometer.

The denser the population, the higher the level of competition. The multiple squirrels in the square meter may all have to tussle for the same acorn. Density is strongly correlated with biotic factors affecting population growth.

Dispersion

Dispersion describes the geographic distribution, or clustering, of the individuals in a population (Fig. 2).

This property is like density, but subtly distinct. For example, a woodpecker living in beech-maple forest lives in the entire forest, but may cluster around the location of maple trees because of their soft bark.

Sex distribution

Sex distribution is used to describe the quantity of individuals in a population who are male and female.

In population dynamics, counting individuals of each sex are in a population can help determine the maximum potential birth rate. By extrapolating the number of breeding females, population growth can be predicted.

Sex distribution is succinctly described by using a sex ratio, which is a fraction representing the number of individuals of one sex (e.g., females) to individuals of the other sex (e.g., males).

Age distribution

Age distribution is used to describe the quantity of individuals of a population within a specific age group, as sorted in classes, or cohorts. A cohort is a group of individuals in a population born within the same time frame. Population dynamics uses cohorts to observe whether a population is young or old, booming or busting.

Population Dynamics Models

Age-Structure Diagrams

During population growth studies, you may see age-structure diagrams. Age-structure diagrams (Fig. 3) are vertical histograms representing the cohorts of a population, separated by age group and sex.

Often, the shape of this model is indicative of a population growth trend. For example, a pyramid shaped age structure diagram shows a boom of young individuals, whereas another model that is narrowest at the base is indicative of an aging population with low population growth.

Survivorship Curve

Survivorship curves are another model which efficiently displays population dynamics. Survivorship curves plot a population's number of (surviving) individuals on the y-axis and the age of the individuals on the x-axis (Fig. 4).

Population growth in survivorship curves tend to follow one of three trends:

• Type I: A population with a high mortality rate in old age. Type I species are typically associated with strong parenting.

• Type II: A population with similar mortality throughout its life span.

• Type III: A population with a high mortality rate in young age. Type III species typically perform little to no parenting, preferring instead quantity over quality. Species that lay multiple eggs, like frog populations, are an example of a Type III species.

Growth Rate Curves

In population dynamics, there is a model of population growth rate that takes carrying capacity into consideration. The logistic growth curve (Fig. 6) is a population dynamics model with an “S-shaped” curve, where the carrying capacity (K) is represented as the population's ceiling, or maximum population growth rate.

In equation form, the exponential growth curve is expressed as such:

$\frac{dN}{dt}=rN\left[\frac{\left(K-N\right)}{K}\right]$

where N = size, t = time and r = growth rate, and K = carrying capacity.

Population Dynamics in Ecology

Population dynamics has many practical applications in ecology. Fish and game departments monitor populations to ensure healthy and fishing and hunting. Agricultural organizations track pest populations to protect their crops. Environmental protection groups ensure the survival of endangered, threatened or vulnerable species by observing shifts in population sizes.

For populations to be able to survive in their ecosystems, they have to adapt to the biotic and abiotic factors in their environment. When population and community density is high, then there is a lot of competition, and resources are scarce. Some species excel at winning when competing for resources. They spend a lot of energy doing so, and spend less energy reproducing. Thus, they experience slow population growth and rarely exceed the carrying capacity. A K-selected species carry fewer offspring and experiences slow population growth that follows the logistic growth curve. Elephants, tortoises, and bears are examples of K-selected species.

On the other hand, some species aren't good competitors. To compensate, these species have developed rapid population growth during periods of abundant resources, when competition is less necessary. These r-selected species carry large numbers of offspring and experience fast population growth in the shape of an exponential growth curve, exceeding the ecosystem's carrying capacity temporarily. The r-selected species typically experience population growth events known as booms, and then crashes. Swarms of mice during bountiful growing seasons or salmon spawning during the breeding season are examples of r-selected species.

Human Population Dynamics

The concepts of population dynamics extends to every species, even human populations! Demography is the application of population dynamics to the human species. In fact, demographics are extensively used in many other sciences, such as sociology, political science, anthropology, or geography. Demography uses its unique terminology when describing population dynamics.

Demography uses a demographic transition model (Fig. 7) to observe how human population growth patterns over long periods of time. Most often, these graphs are used to compare the human populations of two or more countries.

Population Dynamics Worksheet

Recall the following equations for population growth rates curves.

The two population dynamics models being represented by these equations are the exponential growth curve and the logistic growth curve, respectively. In this worksheet, we will explore how a population dynamics model can be extracted from population size data. For 12 weeks, a population dynamics scientist counted the population of two species of insects, Study Bugs and Smarter Bugs. The data is compiled in the table below. From this data, you will derive a population growth rate (r).