SC.912.L.17.5: Analyze how population size is determined by births, deaths, immigration, emigration, and limiting factors (biotic and abiotic) that determine carrying capacity.
Population
In a biological sense a “Population” is a particular group of the same species that live in a particular area. Populations are continually fluctuating up and down. Generally populations increase and decrease due to a variety of biotic (living) and abiotic (non-living) factors. 4 key factors that effect population size are immigration, emigration, natality, and mortality.
4 Major Metrics Effecting Population Size
immigration: Organisms moving into an existing population, this causes the size of the group to increase. You can remember this easily because “immigration” begins with an “i” just like the word “into”, in immigration organisms are coming “into” the group.
emigration: Organisms leaving the population causes the population size to decrease. “Emigration” begins with the letter “E” just like the word “exit”.
natality: The rate at which new organisms are born into a population is called the natality, this causes the population to increase. Some species have high birth rates while others have lower birth rates, this is usually tied to how much time and resources a particular species invests in their children.
mortality: Mortality refers to the rate at which organisms in a particular population die off. This can be caused by a number of factors such as natural disasters, old age, pandemics, and predation.
Environmental Factors Effecting Population Size
Carrying Capacity
Environments support biological life. An environment has finite (limited) resources, due to this the environment cannot support infinite population growth. This limit on the size population that an environment can support is called “Carrying Capacity“.
Biotic Factors
Some of the factors that create the carrying capacity of the environment are Biotic (living) in origin. Some examples of biotic factors are things such as disease, and social interactions like predation.
Social Interactions
Social interactions between organisms can include those among a species and between different species. Predation or being preyed upon is a form of social interaction.
Organisms within a population vying for access to the same limited resources is also an example of “competition“. Since resources are limited some organisms who may be unfit for their environmental challenges will not live long enough to pass on their genes, altering the population size. The topic of competition and “natural selection” can be found here.
Symbiosis is the close relationship that forms between two species. When two organisms form a mutually beneficial relationship it is called “Mutualism“.
Conversely, “Parasitism” is when one organism is benefited from the relationship while one organism is directly harmed. Tapes worms are just one of many types of intestinal parasite commonly found in humans. The eggs of the parasite can be found in the feces of infected people and animals, this is one reason food preparers are required to wash their hands on the job. The eggs hatch in the intestines of the host, the worms attach themselves to the walls of the intestine and absorb nutrients from the food of the host. The eggs are released into the feces and the cycle will continue.
“Commensalism” is the form of symbiosis where one creature benefits while the other organisms is mostly unaffected by its actions. Dust mites that live on the surface of our skin consume dead skin that would fall off on its own, having largely no effect on humans. Dust mites form a commensalistic relationship with humans. Another example of this type of relationship is when large plants create shade for smaller plants to grow in.
Abiotic Factors
There are numerous abiotic or non-living factors that can effect a population’s size.
- Water
- Oxygen
- Carbon dioxide,
- Salinity
- Nutrient levels
- Temperature
- pH
- Sunlight
These abiotic factors can sometimes be effected by organisms. An example of this is photosynthetic algae, they create and release gaseous oxygen into bodies of water like lakes and ponds. When there is more oxygen present the bacteria in the water can increase their population. This leads to the bacteria consuming the oxygen in the water, if they consume too much the O2 level decreases and the CO2 level increases. This causes a chain reaction which leads to many fish and aquatic life dying. This process is triggered by an “Algal Bloom”, to learn more about this concept and how it effects Florida please check out our article here.
Growth Curves and Population Charts
The changes in populations can be tracked and graphed over time to create a line-graph. These graphs are incredibly important to properly craft public policy to manage wild life and ecosystems effectively. If a problem is identified early enough a change in environmental policies can be executed to mitigate or reverse the issue. A typical population growth curve will be comparing the population size on the “Y-axis” and the passage of time along the “X-axis”.
It should be noted that when a population exceeds the carrying capacity of its environment the population will have a sharp decline until it can get back to manageable levels.
The lines or “curve” on these charts will go up and down to reflect the changes in population over time and the behavior or trend of the line can be separated into a few key categories or “phases”. When a line is only steadily increasing we say that the population is experiencing “linear growth“. When a chart’s line is rapidly increasing and has become a positive upwards curve we say that the population is experiencing “exponential growth“.
In the figure above we can identify some dramatic trends in the growth curve. “Lag Phase” is when the population has just begun to be establish themselves and are remaining constant. The “exponential phase” is when the resources in the environment are still abundant and the population is able to rapidly increase. The “stationary phase” indicates the population has peaked and can no longer continue to grow, this may be due to exceeding the carrying capacity of their environment. The “Death Phase” indicates a rapid death or decline in the population size, this can occur until the population becomes “0” or simply returns to levels similar to the initial “lag phase”.
Life Pattern/History Charts
Type 1
These organisms tend to die off at the end of their lifespans. Humans are a prime example of a type 1 species. These organisms tend to focus their time and resources into only having a few offspring, we call this a “k-selection” strategy.
Type 2
Type 2 species are equally likely to die during all stages of their lifespan/development. Small insects are good examples of type 2 species and are equally likely to be eaten regardless of how old they are.
Type 3
Type 3 species experience most of their deaths at an early age when they are very vulnerable, only a few members of their species live to old age. These organisms practice the opposite strategy of k-selection, instead exhibiting “r-selection“. R-selection species emphasize quantity over quality when it comes to having children, they produce a large number of offspring at once and invest little resources in their survival and development. By having numerous babies they increase the likelihood that some of their offspring will survive to reproductive maturity.
| Life History Pattern | Description | Example |
| Type 1 | Organism typically survives its max lifespan | Humans, Elephants, Large mammals |
| Type 2 | Organism has equal chance of dying regardless of its age | House Fly, rodents, Small Birds |
| Type 3 | Organism is most likely to die early in life | Sea Turtles, Trees, Many types of Insect Larvae |
Succession
Primary Succession
When an environment is devoid (missing) of organisms and the first set of organisms move in and establish themselves it is called “Primary Succession“. This sometimes happens when a volcanic eruption occurs and a large amount of magma cools in the ocean. If this cooled and hardened magma solidifies and piles up in large enough quantities it will form a new island or land mass. Fertilized plant seeds could be blown to this new land mass or be dropped in the feces of birds. Once these seeds germinate and sprout, the new plant life will be able to establish a new ecosystem.
Once organisms have lived and died in this new environment enough they will form detritus or soil. This soil will allow new plants and organisms to move into the environment and establish their population on the island.
Secondary Succession
Once the appropriate composition of soil has been established new larger and more complex organisms are able to enter this environment. This can also occur when fires clear the existing plant life and animals from an ecosystem. The ashes from burnt plant and animal life help nutrients like nitrogen return to the soil. This nutrient rich soil allows new larger plants to grow, this concept is called “Secondary Succession“.
Ecosystem Services
Ecosystem services are the important services or products provided to humanity by nature for free. One example of this is the pollination of crops for agriculture. Wind, insects, and small animals provide pollination for the “angiosperm” plants we grow for food production. Once the plants are pollinated they become fertilized and produce fruits for us to harvest and serve the globe. If pollinators like bees suddenly disappeared we would still be able to produce fertilize plants but we would have to do it ourselves and this would be very time consuming and expensive. Certain plants and bacteria filter and remove excess nitrogen from the soil and water in a process called “nitrogen fixation”. Photosynthesis and oxygen production is an ecosystem services that allows us to breathe easy at night knowing there will always be more oxygen for our next breath.
