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. 

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. 

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 footprint(definition:the area of biologically productive land and water needed to provide the resources for and assimilate the wastes from human activities.)that 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 capita(definition:per person)footprint for Canadians is more than the biocapacity (definition:the ability of a biologically productive area to provide resources and assimilate wastes.)of 5 Earths. Let’s explore some of the choices to reduce ecological footprint.

Describing Population

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. 

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. As you explore the following interactive, think if each method could be used to accurately determine human population.

3Tabs

Long Description

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:

Example

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 (definition:A cyclic response that can continue to amplify a stimulus.)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. 
    

   

   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.

As ecologists get to know a species often they uncover surprising behaviours. Elephants, for example, are more similar to humans than first impression. You can learn more about their human qualities in this persuasive article about recent conservation efforts in saving the African elephant. 

Density-dependent factors that are part of negative feedback on population help to keep a population from growing beyond its carrying capacity. When population growth is subject to a carrying capacity the population starts growing exponentially, but with time it starts to level off.

 In other words, the population growth changes depending on how close or far it is from the carrying capacity. This type of growth is called logistic growth.

The graph shown here is an idealized version of population growth that makes it easier to mathematically describe changes. In real life, however, the population growth curve is never so smooth. Using data from the furs caught by the Hudson’s Bay Company, Ecologists were able to recreate the population of hare (prey) and lynx (predators).

Indeed, the population growth curve here looks quite different from either exponential or logistic growth. Mathematically, we could describe this as sinusoidal growth, but, defining the period, maxima and minima of populations is difficult because of the many biotic and abiotic interactions not shown on the graph.