Lesson 6 — r and K-Selected Life Strategies


Earth's Carrying Capacity

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Discuss the ramifications of your conclusions from the Population Growth Rates lab. How can we apply this information in the real world where human population growth is a pressing issue?

How fast is Earth's human population changing? Consider the graph below and check the World Population Clock.

Using growth rates, age structures, environmental state, and possible technologies, population scientists have predicted the three possible scenarios you see below. Currently, the estimated world population is 7.4 billion (July 2017).


World Population Projections


Source: European Environment Agency. CC BY License.


According to United Nations, the Earth's carrying capacity will level somewhere around 9 billion. By 2060, the estimated high, medium, and low projections are respectively: 11.1, 8.4, and 6.2 billion. Because the current population is over 7 billion, consider what each of these three scenarios might mean for human society. Changes in growth rate can be caused by a change in birth rate and death rate.



Source: United Nations. 2015.


Surprisingly, the world population is not increasing because of increasing birth rate. In fact, the global birth rate has decreased significantly in the past 20 years and it is expected to decline even further. Instead, the current increase in world population can be attributed to the increase in global life expectancy. Generally, people are living longer and staying healthier than they did 50 years ago. This is leading to an aging global population with a smaller number of younger population to support the elderly. The United Nations projects that, by the year 2100, one-third of the world population will be aged 60 years or above. In 2100, you may be part of the aging population of 3.2 billion people. What could this mean for the next generation, your children?


Self - Check 

  1. Life Strategies and Population Change

    Where on the population growth curve does natural selection begin to favour those with superior alleles? There are no winners without a competition. Competition begins only when scarcity of resources, waste accumulation, contagious disease, high predation and other density dependent factors become a daily fact of life. The organism's physical, chemical, and behavioral traits combine to keep it alive and reproducing when those around it are not. Use the following graph illustrating exponential and logistic growth to answer the questions. 



    Logistic (S) versus Exponential (J) growth curves



    1. Label the region on the logistic curve (a) where you would expect the greatest competition and, therefore, selection of favourable alleles. Support your answer.

    2. Label the region on the logistic graph (b) where you would expect the highest concentration of 'winners' (those with the most favourable alleles). Support your answer. 

    3. Where on this graph would you expect the predator-prey cycles discussed in a previous lesson? Label this location as (c). Support your answer.

    4. What would you expect to happen to the J-curve if it were extended? Why?


  2. Application of Growth Curves to Various Species

    1. For wildlife managers, deviations from expected growth patterns indicate problems. In lesson 3, you graphed the data from a population of caribou from 1910 to 1950 that showed an extended period of exponential growth followed by a crash. Considering what you know now about the characteristics of K-selected species, was the graph typical? Explain.

    2. According to the Human Population lab on the previous page, are Homo sapiens r or K selected? Does the world population growth curve support your decision? Explain.

    3. Consider the age-pyramid representations of growth rates in the DR Congo, Japan, and Sweden. Which of the countries shows exponential growth? Negative growth? Stationary or carrying capacity growth? 


      Source: CIA World Fact Book. 2015.  Public Domain

    4. Compare the slopes of the two graphs below. Both show world population growth rates. Why does the first graph appear to have a much steeper slope?






    1. During this period (a), environmental resistance is high due to the selecting effect of scarcity and other density dependent factors. Competition is eliminating the unfavourable alleles and increasing the frequency of favourable alleles.

    2. At (b), the population that exists when the curve hits carrying capacity is the survivors who bear the most favoured alleles. 

    3. At carrying capacity (c), the ratio of predators to prey is held in check by each other. High prey→high predators→low prey→low predators.

    4. Unlike environmental resistance seen in logistic growth, exponential growth occurs so rapidly that no 'weeding' of poor alleles occurs. The population jump in one generation is so great that the population suddenly is well above carrying capacity. Such scarcity occurs that virtually no reproduction occurs, and the population crashes.

    1. Caribou are K-selected (large, long life spans, nurture their young) and should show an S population growth curve. A short period of exponential growth flattens due to environmental resistance, followed by a relatively stable carrying capacity population.

    2. Homo sapiens are K-selected by definition, but our growth curve shows extended exponential growth typical of r-selected species. The best explanation is that technology has raised the carrying capacity of the environment to extend the period of exponential growth. In recent years, the slope of the curve is starting to flatten somewhat. 

    3. DR Congo shows exponential growth. Japan shows negative growth. The population growth of Sweden is stationary.

    4. The time axis is very compressed on the first graph; therefore, the slope is steeper. The first graph does not show future dates or computer projections of population changes.

Biology 30 © 2008  Alberta Education & its Collaborative Partners ~ Updated by ADLC 2019