The growth of population rate depends on birth rates and death rates and also migration. Now, what will happen to the growth of a population when resources are unlimited? Let’s find the facts here.
Charles Darwin on his theory of natural selection was influenced by the English clergyman Thomas Malthus. In 1798, Malthus published a book which stated that populations with unlimited natural resources grow very rapidly after which population growth decreases as resources become drained. This accelerating pattern of increasing population size is known as exponential growth.
To know about exponential growth, you are able to see bacteria. They are prokaryotes which reproduce by prokaryotic fission. This division needs about an hour for a lot of bacterial species. Let say that 1000 bacteria are put in a big flask with an unlimited supply of nutrients. Then, after an hour, you will be able to see that there will be one round of division with each organism dividing once and it results in 2000 organisms. In the next hour, each of the 2000 organisms will double so that they produce 4000. So, in the next 3 hour, you will be able to see that there are 8000 bacteria in the flask. The number of bacteria will increase again and again since there are unlimited nutrients.
So, the important concept of exponential growth is that the rate of population growth, the number of organisms which are added in each reproductive generation is accelerating. It is increasing at a bigger rate. Then, after a day and 24 of these cycles, the population will be increased from 1000 to more than 16 billion. When the size of the population which is N is plotted over time, then it will produce a J-shaped growth curve.
The example of the bacteria is not representative of the real world where resources are limited. Even some bacteria will die when they experiment and not reproduce, lowering the rate of the growth. So, when we calculate the rate of growth of a population, the rate of death is subtracted from the birth rate.
What Happens To The Population Growth When Resources Are Limited?
From the example of bacteria, we are able to state that when resources are unlimited, the populations show exponential growth and it makes the curve have a J-shape. However, when resources are limited, the populations show logistic growth. In the logistic growth, the expansion of population decreases as resources become scarce. It levels off when the carrying capacity of the environment is reached and it makes the curve to be in an S-shape.
Key Points of Exponential Growth and Logistic Growth
Here are some key points of exponential growth and logistic growth.
- Exponential population growth will be able to happen in an ideal environment. It is when resources are unlimited.
- In such an environment competition will not be found to place limits on a geometric rate of growth.
- At first, the growth of population will be slow. It is because there is a shortage of reproducing individuals which may be widely spreaded.
- When the numbers of the population increase the rate of growth will also increase and it results in an exponential (J-shaped) curve.
- This maximal growth rate for a population which is given is known as its biotic potential.
- You are able to see exponential growth in populations which are very small or in regions that are newly colonised by a species.
- Logistic population growth will be able to happen when the numbers of the population start to approach a finite carrying capacity.
- The carrying capacity is the maximum number of a species where it is able to be sustainably supported by the environment.
- When the carrying capacity is approached by a population, environmental resistance happens, and it will slow the rate of growth.
- It effects in a curve of sigmoidal (S-shaped) growth which plateaus at the carrying capacity (denoted by κ).
- Eventually, logistic growth will be able to be seen in any stable population which occupies a geographic space which is fixed.
Factor Which Determines Carrying Capacity
So what are the factors which determine carrying capacity? It is important for you to know that basically, any kind of resource which is important to the survival of a species can act as a limit. Let’s take an example. For plants, some key resources for them are water, sunlight, nutrients and the space to grow. How about animals? For them, the important resources are water, food, shelter and nesting space. If the quantity of these resources is limited, it will result in competition between members of the similar population or intraspecific competition.
The competition between the same population for getting resources may not be able to affect populations which are well below their carrying capacity, the resources are so many and all of the individuals are able to get what they need. But, if the size of the population increases, the competition will be tight. Besides, the accumulation of waste products will be able to reduce the carrying capacity of an environment.
Environmental conditions which are able to control the rate is called limiting factors where a process can happen.
- Density-dependent or density-independent factors can determine the growth of population.
The environmental factors of density dependent are influenced by the relative size of a population.
- These factors are the numbers of predators, the availability of food and other resources and also the spread of pathogenic diseases.
The environmental factors of density independent are not influenced by the relative size of a population.
- These factors are the change of climate and weather, and also the occurrence of natural disasters such as earthquakes.
Examples of Populations Factors
Below, you are able to see the examples of population factors.
Density Dependent Factors include PANDA. What are they?
- Availability of resources such as water and shelter
- Nutrient supply such as food source
- Disease/ pathogenic spread
- Accumulation of wastes
Density Independent Factors include PAW. What are they?
- Phenomenon such as natural disasters
- Abiotic factors such as CO2 levels, and temperature
- Weather conditions such as storms, floods and many more