POPULATION DYNAMICS [CSEC BIOLOGY]

SYLLABUS REFERENCE  

[B7.1] discuss the factors that affect the growth and survival of populations including human populations; 

INTRODUCTION

 This is a Lionfish - Pterois volitans (red lionfish) and Pterois miles (devil firefish).

The following is a short video about it. 

It is an invasive species, currently wreaking havoc in the Caribbean Sea and the Atlantic Ocean. It is an interesting insight into the growth of species in natural environments. The Lionfish is an example of what happens to a population when there are very few factors restricting its growth. On the other hand, the species is a very effective predator against several native Caribbean species, to the point of threatening their very existence. 

This article explores the factors that influence the growth of natural populations, and explore how human populations are affected differently. 

A Reminder... 

POPULATION
A group of organisms of the same species that occupy the same habitat at any one time. 

POPULATION GROWTH CURVES 

Population growth is simply the increase in the number of individuals over time. No matter how favorable a habitat may be, a species' population does not increase forever. The change in population size over time can be represented by a population growth curve. 

Here's an example, tracing the growth of algae in a pond. 

This is also known as a sigmoid (S-shaped) growth curve. 

These are the phases of a population growth curve, as shown in figure above. 

• [a] Lag Phase: a set number of organisms migrate to a new area; a period of little growth. The new population of algae grows slowly at first. Although they reproduce, the total number of plants does not increase very quickly, at first. 
• [b] Log Phase: Organisms reproduce, so numbers increase greatly. At first, there are no limiting factors, so the population begins to grow at a rapid rate, steadily doubling over time. 
• [Between b & c] Transitional Phase: Growth rate begins to slow down as competition for resources increases. 
• [c] Stationary Phase: Little to no growth; births and deaths help to level off the numbers. After a while, there is not enough oxygen in the water to sustain all the algae. This causes reproduction rates to slow to a point where there is no increase in numbers. 
• [d] Death Phase: As oxygen runs out, more algae die off and the number in the population drops. 

FACTORS RESTRICTING POPULATION GROWTH

These are main environmental factors that slow, stabilize or cause population growth decline. 

They work by putting pressure on the processes that add or remove individuals from a population, as shown below. 

Now let's take a look at those factors in some detail:

There are some other factors that restrict population growth:

Migration 

Organisms migrate in search of food and to increase their chances of survival. The numbers in the population could decrease as the organisms may not return. 

This is an important factor when considering human populations in specific countries and geographic regions. It also has had a significant effect on Caribbean populations over the years. 

Click Here for an Interactive Version of this Chart 

Disasters 

This can reduce the size of a population, as they often cause loss of habitat. Organisms that survive the disaster may still die due to lack of food, or they may migrate. 

There are two main types of disasters:

• Natural (hurricanes, earthquakes, volcanic eruptions) 
• Human-made (oil spills and fires) 

THE HUMAN POPULATION - GLOBAL TRENDS 

The human population was subject to the same checks and balances as other populations for thousands of years. However, since the 19th century, that has changed. The global population saw a dramatic increase and it has been accelerating since then. 

As a result, the human population growth curve is J-shaped, instead of S-shaped. There is no stabilisation stage, yet. 

Interestingly, some experts predict a stabilisation at 12 billion in 2200. A lot can happen before then, though. 

Click Here for an Interactive Version of this Chart 

The factors that led to this drastic change were as follows:

• Improved food supplies 
• Increased food production 
• Better facilities for storage and transporting foods. 
• A better understanding of how disease is caused
• Many infectious diseases have been controlled 
• Improved sanitation 
• Humans do not have any predators. 

As mammals, humans have the same needs as other animals. We need space, food, and water. Humans are also susceptible to diseases that can be fata. 

Also, although we have few natural predators, humans often kill each other in times of war. Water and conflict lead to an increase in the death rate. They can also lead to a decline of breaths, as family life is disrupted, while food and water sources as blocked or damaged. 

SUMMARY 

The size of a species' population is constantly changing over time and is balanced between birth and death rates, as well as rates of immigration vs emigration. This balance is affected by various environmental factors, such as predation, resource availability, and diseases. 

Human populations are similar to that of other species, in terms of their basic needs for food, space, etc. However, since the 19th century, humans have found ways to overcome some of these restrictive factors. The result is a rapidly increasing global population, with no stabilisation phase in sight so far. 

Their population growth curves reflect this difference. A typical growth curve for most species is sigmoid (S-shaped). The growth curve for the human population is J-shaped. 

Human populations are still vulnerable to various factors, and some that are particular to this species - namely war and conflict. Wherever it occurs, it causes drastic changes in the local population, due to increased death rate and migration (refugees), and a drop in birth rate. 

FURTHER READING

• Population Growth Patterns 
• The Lionfish and Their Impact 
• Invasive Species