Characteristics of Populations and Population Change

2. What is population density?

Population density is the relationship between the number of individuals of a population and the area or volume they occupy. For example, in 2001, the human population density of the United States (according to the World Bank) was 29.71 inhabitants per square kilometer, whereas China had a population density of 135.41 humans per square kilometer.

3. What is the population growth rate?

The population growth rate (PGR) is the percent change between the number of individuals in a population at two different times. Therefore, the population growth rate can be positive or negative.

4. What are the main factors that affect the growth of a population?

The main factors that make populations grow are births and immigration. The main factors that make populations decrease in number are deaths and emigration.

5. How different are the concepts of migration, emigration and immigration?

Migration is the movement of individuals of a species from one place to another. Emigration is migration seen as an the exit of individuals from one region (to another where they will settle permanently or temporarily). Immigration is migration seen as the settling in one region (permanently or temporarily) of individuals coming from another region. Therefore, individuals emigrate "from" and immigrate "to".

6. What are some examples of migratory animals?

Examples of migratory animals are: southern right whales from Antarctica, which reproduce on the Brazilian coast; migratory salmon that are born in the river, go to the sea and return to the river to reproduce and die; migratory birds from cold regions that spend the winter in tropical regions, etc.

• Population Ecology Review - Image Diversity: migratory animals

7. What is biotic potential?

Biotic potential is the capability for growth of a given population under hypothetical optimum conditions, in an environment without limiting factors for such growth. Under such conditions, the population tends to grow indefinitely. 

• Population Ecology Review - Image Diversity: biotic potential curve

8. What is the typical shape of a population growth curve? How can biotic potential be represented in the same way graphically?

A typical population growth curve (number of individuals over time, linear scale) has a sigmoidal shape. There is short and slow initial growth followed by fast and longer period of growth and once again a decrease in growth prior to the stabilization or equilibrium stage.

However, the population growth according to the biotic potential curve  is not sigmoidal; it is crescent-shaped and approaches infinity (there is neither a decreasing stage nor an equilibrium).

• Population Ecology Review - Image Diversity: population growth curve

9. What is environmental resistance?

Environmental resistance is the effect of limiting abiotic and biotic factors that prevent a population from growing as it would normally grow according to its biotic potential. In reality, each ecosystem is able to sustain a limited number of individuals of a given species.

Environmental resistance is an important concept in population ecology.

10. What are the main limiting factors in the growth of a population?

The factors that limit the growth of a population can be divided into biotic factors and abiotic factors. The main abiotic limiting factors are the availability of water and light and the availability of shelter. The main limiting biotic factors are population density and inharmonious (negative) ecological interactions (competition, predatism, parasitism, ammensalism). 

11. How do the availability of water and light and the climate affect the growth of a population?

The availability of water and light along with the climate are abiotic factors that limit the growth of a population. Since producers are responsible for the synthesis of organic material transferred along the food chains of an ecosystem, water and light affect the availability of food and a population cannot grow beyond the number of individuals the environment is able to feed. For example, in the desert, the biomass is relatively small and populations that live in this ecosystem are smaller (compared to the same species in environments with a large available biomass). The climate, including the temperature, affects population growth because excessive change in this factor, such as the occurrence of droughts or floods, may cause a significant decline in a population. Small climatic changes can also alter the photosynthesis rate and reduce the availability of food in the ecosystem.

12. How do populations of predators and prey vary in predatism?

Whenever a predator population increases, the prey population tends to decrease at first. Afterwards, the decrease in the prey population and the bigger population density of predators cause the predator population to decrease. The prey population then reverts the decreasing and begins to grow.

If variations in the size of populations occur at an unexpected intensity (different from the usual intensity of the ecological interaction), for example, due to ecological accidents that kill a large amount of prey, the prey-predator equilibrium is disrupted and both species may be harmed. The existence of the predator is sometimes fundamental for the survival of the prey population, since the absence of predatism favors the proliferation of the prey and, in some cases, when excessive proliferation creates a population size beyond the capacity of the ecosystem to sustain them, environmental damage occurs and the entire prey population is destroyed.

• Population Ecology Review - Image Diversity: predator x prey curve

13. What is the relationship between environmental resistance and population growth according to the biotic potential curve and the real population growth curve?

The difference between the real population growth curve (number of individuals x time) and population growth according to the biotic potential curve of a given population is a result of environmental resistance.

14. How different is the growth of a viral population according to its biotic potential from the growth of a bacterial population according to its biotic potential?

The growth curves of viruses and bacteria according to their biotic potential both present a positive exponential pattern. The difference between them is that over each time period, bacteria double their population, whereas the viral population multiplies dozens or hundreds of times. Therefore, the viral population growth curve has more intense growth. This happens because bacteria reproduce by binary division, with each cell generating two daughter cells, whereas each virus replicates generating dozens or even hundreds of new viruses.

15. What are age pyramids?

Age pyramids are graphical representations in the form of superposed rectangles which each represents the number of individuals included in age ranges into which a population is divided. Generally, the lower age ranges are closer to the bottom of the pyramid, always below the higher ranges, and the variable dimension that represents the number of individuals is the width (however, there are age pyramids in which the variable dimension is the height). 

• Population Ecology Review - Image Diversity: age pyramids

16. What analysis is provided by the study of human age pyramids?

The study of human age pyramids can provide the following types of analysis: the proportion of individuals at an economically active age; the proportion of the elderly (indicating the quality of pension and healthcare systems); the proportion of children and youth (indicating the need for job generation and educational services); the reproductive profile (shows the population growth tendency); the infant mortality rate (indicates the quality of the healthcare system, hygienic conditions, nutrition and poverty); life expectancy; etc.

It is possible to predict whether a population belongs to a rich and industrialized society or to a poor country, since the patterns of their age pyramids differ according to these conditions.

17. What are the main characteristics of the age pyramids of developed countries?

In a stabilized human population, the age pyramid has a narrower base, since the birth rate is not so high. The adult age ranges are generally wider than the infantile ranges, showing that, in practice, there is no population growth. There is a proportionally high number of older individuals, meaning that the quality of life is high and that the population has access to healthcare services and good nutrition. These are features of the age pyramids of developed countries.

• Population Ecology Review - Image Diversity: France age pyramid

18. What are the typical features of the age pyramids of underdeveloped countries?

The age pyramids of underdeveloped countries have characteristics related to the poverty of their populations, with a wider base and a narrow tip. The base age range, when much wider than the other levels, indicates a high birth rate. The levels just above the base may present a large reduction in poorer populations due to infant mortality. Ranges that represent youth are also wide, indicating future pressure on job and housing needs. The widths of the rectangles diminish as age increases to the tip, which represents the elderly, demonstrating difficult living conditions, precarious healthcare services and a short life expectancy.

• Population Ecology Review - Image Diversity: Kenya age pyramid