# Environmental Science – Population Dynamics

Hello everyone! In the last Environmental Science talk, we began talking about sustainability. In this talk, we are going to be moving on to the topic of population dynamics.

Briefly, population dynamics encompass all the changes in specific or multiple populations, and relating the population to its environment. To understand this and dive deeper into the details of this topic, we first must understand what a population is. A population is a number of certain individuals within a species that occupy the same time and place as each other in their environment and includes several factors such as density, dispersion, size, and ages.

Population density consists of the number of individuals of the population per unit in the area of habitat or biome. For example, if there are 20 black bears in 10 square miles, then there are 2 black bears per square mile (Fun Fact – black bears each occupies around 10 square miles of territory in the wild). Some populations such as ants or mice have high population densities while some other animals like black bears have low population densities. Typically the trend we see is that prey occurs in large population densities while predators occur in smaller densities.

There are two types of density controls that affect populations: Density-Independent factors and Density-Dependent factors. Density-dependent factors have a bigger effect as the population density grows or diminishes such as predation, disease, and competition. Density-independent factors affect the size of the population regardless of its density being high or low such as habitat destruction, natural disasters, and human encroachment.

Population size is simply the size of the population of a species in a habitat. Sizes are usually counted through the tag and recapture methods, and change is estimated by the formula:

Population Change = (Natality+immigration) – (Mortality+emigration)

Natality stands for the number of births and mortality, number of deaths. For example, in a beaver population, there were initially 40 beavers, 15 newborns, 6 deaths, 2 immigrant beavers from another region, and 4 emigrants that left the habitat. The equation would be:

40+15+2=57; 57-(6+4)=47 beavers. There was a growth of 7 beavers.

Dispersion of population refers to how the population is spread out across their range or area of habitat. There are three forms of dispersion:

Now we will move onto the graphing of the populations. Two types of graphs for graphing populations of species are exponential and logistics. Logistic graphs are typically more realistic for they show a stabilization of population while exponential graphs do not, making it unreal because Earth does not have the resources and populations do not have the capability to keep growing without stopping.

Environmental resistance includes factors such as predation, lack of resources, competition, humans, natural disasters, etc that affect the population, stopping further growth. Space, food availability, territory are all also factors. This resistance usually effects the young of a species more than adults.

Typically when the population reaches carrying capacity it is not a smooth line as shown. There is typically an overshoot of the population where the population goes past the carrying capacity because of biotic potential, but then the population reduces in size under the carrying capacity due to death from lack of resources. This reduction is called dieback, and the excessive growth is an overshoot. Eventually, populations stabilize and come close the carrying capacity after the first overshoot and dieback. Overshoot and dieback curves at the carrying capacity can be stable, cyclic, irruptive, or irregular.

1. Cyclic
2. Stable
3. Irregular
4. Irruptive

(These occur at the population when it reaches the carrying capacity)

In the last section of this stem talk, I know its a long one :D, we will talk about different types of species.

R-selected species are opportunists or generalists (easily adaptable), have a lot of offspring but low survival rates, parents do not care about their children, reach reproduction cycles early in life, have high populations, low competition, shorter lifespan, and early succession. Examples of these include fish or insects.

K-selected species are specialists (not as adaptable, need a specific habitat), fewer offspring but are larger in size, have parental care to offspring, later reproductive ages, late succession, longer lifespan, and very competitive. Examples of these include lions, elephants, and humans.

*Succession – the order that species arrive into a new ecosystem or habitat*

Organisms can reproduce through sexual means, which requires two partners, one providing egg cells, and one with sperm, and asexual means which includes budding and only requires one parent. Asexual organisms are exact copies of their parents while sexual organisms are 50/50 of their parent’s traits.

This is a survivorship curve, showing the percentage of survivors as their age progresses:

Overall population dynamics is a very content heavy, but an easy to understand topic. I hope you found this talk very helpful! The next environmental science talk will be a short one on environmental impact and worldviews. Thank you!

-Written By: Neil D. 2/4/2019

Credit:

Living in the Environment –

G. TYLER MILLER, JR.
SCOTT E. SPOOLMAN

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