Positive and Negative Factors in Population Growth

Population growth is a balance between positive and negative factors. For example, natality and immigration lead to increases in population, whereas mortality and emigration lead to decreases. The energy available to individuals in a population leads to different survival outcomes. If there is an abundance of energy available then birth rates and possibly immigration rates increase. For most species, though, the energy they utilize is limited to its impact on metabolic processes. Autotrophs grow better with an abundance of light energy while heterotrophs benefit from an abundance of organic sources of nutrition.

Human populations, on the other hand, utilize a greater variety of energy sources in our food systems. The following video summarizes some of the changes in sources of energy over time.

With each invention we were able to sustain a greater number of people. With the world population currently over 7.5 billion people (check Worldometers for the exact number) there is concern over how much higher the population will grow. Could a planet with 12 billion people be on the horizon? Watch this next video to see if this prediction is realistic.

How do small steps lead to changes?

Using specific details from the video on overpopulation, explain how the balance in positive and negative population growth factors affect how human population number changes.

Natural populations are similarly affected by positive and negative factors affecting population growth. For example, salmon populations in key rivers in British Columbia have seen a decline over recent decades. The causes are manyfold and interconnected. A quick Google search for Fraser River salmon decline will show you just some of the causes.

Energy in Human Food Chains

One invention that altered energy sources is cooking. Some animals can use tools to capture more food, but only humans cook their food prior to eating it. The advantages to this behaviour, which takes both time and added energy, are described in the following video.

How does energy move in food systems?

Using specific details from this video, suggest how human food systems might have been different if cooking was never invented?

With each invention in energy sources there have been consequences to the environment that balance the benefits to human food systems. The invention of agriculture, for example, changed ecosystems from grasslands, forests and estuaries to pastures, cropland and paddocks. This and other consequences to the natural environment have lead people to question our impact on our world. All species have an ecological footprintthat is a product of population size, the intensity that they take resources from their environment, and the waste they produce. It’s sobering to realize that the per capitafootprint for Canadians is more than the biocapacity of 5 Earths. Let’s explore some of the choices to reduce ecological footprint.

How does energy move in food systems?

Ecological footprint is the product of three factors:

  • population size,
  • intensity of resource extraction,
  • intensity of waste production.

Each of these factors puts strain on our food systems. Search for a strategy human populations or individuals can make to reduce each of the three factors that contribute to our ecological footprint. How would this help our food systems to be more effective at producing quality food?

Share these answers with the class in the comment section below.  Which one of your strategies do you think would have the highest impact for postive change?

 

Describing Populations

When it comes to describing changes in population we must first be able to describe it at a specific point in time. The size, mobility, distribution, and behaviour of individuals in a population as well as the area and accessibility of the habitat are all considerations in how the size of the population can be determined.

A graphic showing three boxes each with text above and below the box. The first box is brown, showing 25 white circles in 3 groups of 8 to 9 circles. This box is labeled “Clumped”. Below this box the text reads: “Organisms are clustered together in groups. This may reflect a patchy distribution of resources in the environment. This is the most common pattern of population dispersion.” The second box is blue, showing 25 white circles in no obvious pattern. This box is labeled “Random”. Below this box the text reads: “Organisms have an unpredictable distribution. This is typical of species in which individuals do not interact strongly.” The last box is green, showing 21 circles in 5 rows of 4 to 5 circles. This box is labeled “Uniform”. Below this box the text reads: “Organisms are evenly spaced over the area they occupy. This is typical of species in which individuals compete for a scarce environmental resource, such as water in a desert.

 

 

 

Population distribution is often categorized in one of three patterns.  by CK12 used under NonCommercial CC BY-NC

 

These patterns of distribution are useful in describing natural populations as well as human populations. Think about which distribution pattern best describes human populations.

There are three common methods used by population ecologists to determine the size of a population. (Interactive has been taken out)

Look up the following and use the question below to frame your ideas…

  • Counting and Random Sampling
  • Quadrat / Random Sampling
  • Mark Recapture

How does understanding change?

How could different population distribution affect the accuracy and reliability of these three methods of determining population size?

Determining population size and density is a useful first step in determining population dynamics. For example, when Ecologists study the effect of human ecological footprint on natural populations they start by looking at changes in population number for different species. Is there a way to see if our efforts to reduce our ecological footprint is working? The Living Planet Index is a UN-recognized measure of the trend in the population changes for over 3,600 different vertebrate species in all biogeographic regions around the world. You can see which species close to where you live are part of this important study of biodiversity loss using data from the Zoological Society of London and World Wildlife Foundation.

Dynamic Populations

An important thing to remember in ecology is that ecosystems are constantly changing. When ecologists think about conserving natural populations they are thinking in terms of a range of population size rather than a specific size. Individuals within a species are constantly interacting with each other, intraspecifically, with members of other species, interspecifically, and with the abiotic environment. The effect of these interactions is changes in population growth. The rate of population growth, mathematically shown as r, is a value of the percent change in population, described as a decimal. Average growth rate can be simply calculated in the following way:

Average Growth Rate = (change in population) / (change in time)

 Example

A graph showing time, without units, on the x-axis and population density on the y-axis. Data points are shown as dots and a curve of best fit traces a smooth curve in the shape of a slanted letter “S”. The curve increases slowly from just above to origin to around a population density of 50 by time 10. The curve then increases quickly to a population density of around 260 by time 20. Finally, the curve levels off to a population density of around 270 by time 30. From time 0 to 15 the curve closely follows the curve. From time 15 to 30 the data points are further from the curve, above and below it.

 

 

The growth of Paramecium caudatum protist in a sample of pond water over time. Each day the population density is measured in the number of protist cells counted per cm3 of water. by Nature.com

Using your best judgement to read the graph, you can make the following calculation:

Max. population = 280 cells/cm3
Min. population = 5 cells/cm3
Starting time = 0 d
End of growth time = 21 d

Average Growth Rate = (280 – 5)/(21- 0) = 13 cells/cm3/d

 

Population Dynamics

 

Interactions can be broadly divided into two categories:
  1. Density-independent factors are typically abiotic factors that affect population size, either increasing or decreasing it. Examples include weather and climate, natural disasters and human interventions in natural habitats. As you explore this section, think if density-dependent and density-independent factors similarly affect human populations.
  2. Density-dependent factors, on the other hand, are typically biotic factors that can increase or decrease a population size. Because their effect on population growth increases with population size, these factors have the effect of feeding back on themselves.For example, the Allee effect is a concern for small or spread out populations. Certain density-dependent factors can amplify their effects on populations. For the Allee effect, as the population gets smaller the ability for individuals to find mates in order to reproduce decreases leading to a further reduction in population size. We call this type of feedback positive feedback even though the effect is to lower the population because the population size is moving away from a healthy population size. In this instance, the Allee effect can lead to species extinction.Another density-dependent factor that contributes positive feedback is the number of reproducing individuals. As the population increases the ability for individuals to find mates in order to reproduce increases leading to a further increase in population. After a few generations, the population grows exponentially. Examples of this type of population growth include plants and animals reproducing in springtime, and bacterial infections.
     
    Seven circles, each labeled in order from Day 0 to Day 6. In the first circle, representing Day 0, one oblong unicellular organism is seen. In the second circle, representing Day 1, two similar looking unicellular organisms are seen close to each other. The remaining circles each show a random distribution of similar unicellular organisms. The number of organisms increase from 4 on Day 2 to 64 on Day 6.

     

     

    Under the right conditions, Paramecium caudatum can double in number every day. The population is sampled daily and viewed under a microscope.  by Nature.com

    Other density-dependent factors work in the opposite way: they help to bring population growth back towards a healthy size. This is described as negative feedback. In the example of feedback that we saw earlier, in Unit 2, Activity 5 on cellular respiration, an enzyme was inhibited by a buildup of products from a metabolic pathway. Feedback inhibition is an example of negative feedback. Many interspecific interactions are also examples of negative feedback. Some of these interactions are summarized in this video.

     

All these interactions involve a benefit to at least one of the species. For mutualism, both species benefit and allow for positive population growth, while for commensalism, the second species experiences no benefit or harm. The population does not change because of this interaction. Parasitism, however, harms the second species for the benefit of the first.

Three other important interactions involve harm to at least one population of individuals: predation, herbivory, and competition. Of these, competition poses the greatest threat to both populations as it ultimately harms both populations more than if they didn’t interact with each other. Since evolution is driven by survival of the fittest, natural selection favours individuals that can avoid and minimize the negative effects of these three biotic interactions.

Human activities affect natural populations, often in a negative way. Conservation efforts work to stop, or at least slow down, these negative effects. In essence, conservation is about saving the natural world as we know it. The ecologists that get involved in conservation do so for personal reasons, as we can see in this video.