1. Chapter 14: Population Ecology
Planet at capacity: patterns of population growth
Lectures by Mark Manteuffel, St. Louis Community College

2. 14.1–14.6 Population
ecology is the study of how populations
interact with their
environments.

3. These difficult questions are all part of ecology, a sub-discipline of biology defined as the study of the interactions between organisms and their environments. But don’t be fooled by the simple definition. Ecology encompasses a very large range of interactions and units of observation and is studied at different levels (Figure 14-2 From individuals to ecosystems). These include:
Individuals: How do individual organisms respond biochemically, physiologically, and behaviorally to their environment?
Populations: How do groups of interbreeding individuals change over time in terms of their growth rates, distributions, and genetic makeup?
Communities: How do the populations of different species within a locale interact with each other?
Ecosystems: At the highest level of organization within ecology, how do the living and non-living elements interact in a particular area, such as a forest, desert, or wetland?

4. When we are thinking about how many lobsters live off the coast of New England, we are analyzing…
Individuals
A population
A community
An ecosystem
Answer: 2

5. Population Density – Which area has the highest number
individuals?

6. —Charles Darwin, The Origin of Species
14.3 Populations can grow quickly for a while, but not forever. Why not?
There is no exception to the rule that every organic being naturally increases at so high a rate that, if not destroyed, the Earth would soon be covered by the progeny of a single pair.
—Charles Darwin, The Origin of Species

7. As an example of how a population rather than individual perspective is needed, consider the case of adaptations produced by natural selection. Genetic changes don’t actually occur within an individual. That is, no one giraffe ever evolved a larger neck. Instead, the genetic changes occurred within a population over time. As a consequence of differential reproductive success among the individuals of a population with different neck lengths, over time there come to be more individuals with longer necks. Birth rates, death rates, immigration and emigration rates, too, are features possessed not by individuals but by populations (Figure 14-3 A change in perspective).

8. Make a list of factors that affect population growth?

9. What would happen if more than two eggs survived
What would happen if more than two eggs survived? If the number were more than one offspring per individual, the population would grow and grow and grow until the Earth was covered with cod and elephants and every other species. But that hasn’t happened and it can’t. Let’s investigate why.
Figure 14-4 part 1 Unchecked growth.

10. With the same calculations, we could determine the population size for the next 10 or even 50 years—these numbers are plotted in Figure 14-4 part 2 (Unchecked growth).  When a population grows at a rate that is proportional to its current size—in other words, the bigger the population, the faster it grows—the growth is called exponential growth. The graph reveals that very quickly the size of a population growing exponentially becomes astronomical. In 10 years, our population would have grown from 500 individuals to In 50 years, it would reach almost 4 million. In fact, after 80 years, the population would pass one billion individuals. It’s clear that exponential population growth ends badly.

11. Take-home message 14.3
Populations tend to grow exponentially, but this growth is eventually limited.
Why is it limited?
What happens?
What will the growth curve look like?
But the world isn’t overrun with people or cod or elephants, so clearly exponential growth doesn’t occur for long. In the next section, we explore why, in the real world, populations cannot simply grow unchecked forever, and we add some realism to the exponential growth description.

12. 14. 4 A population’s growth is limited by its environment (a. k
14.4 A population’s growth is limited by its environment (a.k.a limiting factors)
Life gets harder for individuals when it gets crowded. Whether you are an insect, a plant, a small mammal, or a human, difficulties arise in conjunction with increasing competition for limited resources.

13. In particular, as population size increases, organisms experience:
Reduced food supplies due to competition
Diminished accessibility to places to live and breed due to competition
Increased incidence of parasites and disease, which can spread more easily when their hosts live at higher density
Increased predation risk as predator populations grow in response to the increased availability of their prey and also as the more densely packed prey become more visible.
Figure 14-5 Fighting over scarce resources.

14. Density-dependent Factors
The limitations on a population’s growth that are a consequence of population density
This ceiling on growth is the carrying capacity, K, of the environment.
The limitations on a population’s growth that are a consequence of population density—the number of individuals within a given area—are called density-dependent factors, and they cause more than discomfort. With increased density, a population’s growth is reduced as limited resources slow it down. This ceiling on growth is the carrying capacity, K, of the environment. And as a population size approaches the carrying capacity of the environment, death rate increases; migration rates increase (as individuals seek more hospitable places to live); and a reduction in birth rates usually occurs, too, as low food supplies give rise to poor nutrition, which, in turn, reduces fertility.

15. “S” Shaped growth curve
When a population grows exponentially at first but its growth slows as the population size approaches the environment’s carrying capacity—an “S”-shaped growth curve—the population growth is called logistic growth. Logistic growth is a much better approximation of how populations grow in the real world than is exponential growth (Figure 14-6 Lack of resources limits growth). 

17. Density-independent Forces
Factors that strike populations without regard for the size of the population
Mostly weather-based
Density-independent forces can also knock a population down. These are factors that strike populations without regard for the size of the population, increasing the death rate or decreasing the number and rate of offspring that are produced. Density-independent forces are like “bad luck” limits to population growth. They are mostly weather-based, including calamities such as floods, earthquakes, fires, and lightning. They also include habitat destruction by other species, such as humans. The population hit may be at its carrying capacity or it may be in the initial stages of exponential growth. In either case, the density-independent force simply knocks down the population size. The population then resumes logistic growth.

18. In an environment in which these “bad luck” events repeatedly occur, a population might never have time to grow as high as the carrying capacity. Instead, it might perpetually be in a state of exponential growth, with periodic massive mortality events. The population’s growth would appear as a series of jagged curves as seen in Figure 14-7 (Events can limit population growth). 

19. How many people can Earth support?
Why does the answer keep increasing?
The growth of populations doesn’t always appear as a smooth “S”-shaped logistic growth curve. For some populations, particularly humans, the carrying capacity of an environment is not set in stone.

20. Consider that in 1883, an acre of farmland produced an average of 11
Consider that in 1883, an acre of farmland produced an average of 11.5 bushels of corn. By 1933 this was up to 19.5 bushels per acre. And by 1992 it had increased to 95 bushels per acre! How did this happen? The development of several agricultural technologies—including the use of vigorous hybrid varieties of corn, rich fertilizers, crop rotation, and effective pest management—have all made it possible to produce more and more food from the same amount of land. This is just one example of how the carrying capacity of an environment can be increased (Figure 14-8 Efficient crop production). Of course, at this same time, the carrying capacity has likely been decreased for many other species trying to live in the same environment as humans and their crops.

21. What does this data table tell you?
Logistic population growth
Exponential population growth
Population has reached carrying capacity.
1 and 3
2 and 3
Year Population (billion) 1 2 3
2024 (projected) 8
2045 (projected) 9
Answer: 2

22. Analyze this graph? What happened?

23. When the population reaches carrying capacity, it appears to persist indefinitely. How is that possible?
Number of births > number of deaths
Number of births = number of deaths
Number of births < number of deaths
None of the above
Answer: 2

24. 14.8 Populations can be described quantitatively in life tables and survivorship curves.24

25. Life Tables and Survivorship Curves
Allow biologists to predict an individual’s likelihood of either dying within a particular age interval or surviving the interval.
Biologists have learned some important lessons from insurance agents. Because insurance agents need to estimate how long an individual is likely to live, about 100 years ago they invented something called the life table. It ought to be called a death table, though, because in it they tally the number of people in a population within a certain age group, say 0 to 10 years old or 10 to 20 years old. They then count the number of individuals dying within that interval. From these numbers, they can predict an individual’s likelihood of either dying within the age interval or surviving the interval. For the insurance companies, their ability to make money hinges on making accurate life-span predictions. For biologists, a life table is like a quick window into the lives of the individuals of a population, from how long they’re likely to live to when they’ll reproduce and how many offspring they’ll produce. 25

26. Life Tables and Survivorship Curves
Survivorship curves Graphs of the proportion of individuals of a particular age that are alive in a population
From life tables, biologists can create survivorship curves, graphs of the proportion of individuals of a particular age that are alive in a population. Survivorship curves indicate an individual’s likelihood of surviving through an age interval and reveal, at a quick glance, a huge amount of information about a population, such as whether most offspring produced die shortly after birth (think back to the five million eggs produced by some fish) or whether most survive and are likely to live long lives (think about humans in the United States). 26

27. Three distinct types of survivorship curves are given in Figure (Three types of survivorship).  The curves plot the percentage of individuals surviving at each age, across the entire range of ages seen for that species.
Type 1: At the top, in red, is the survivorship curve seen in most human populations. We have a very high probability of surviving during every age interval, until old age, then our risk of dying increases dramatically. Species with this shape of survivorship curve, including most large mammals, usually have a few features in common: They have few natural predators, so they are likely to live long lives; they tend to produce only a few offspring—after all, most of them will survive; and they invest significant time and effort in each of their offspring.
Type 2: In the middle, in blue, is a survivorship seen in many bird species, such as the robin, and small mammals, such as squirrels. A straight line, this survivorship curve indicates that the proportion alive in each interval drops at a steady, regular pace. In other words, the likelihood of dying in any age interval is the same, whether the bird is between ages 1 and 2 or 10 and 11.
Type 3: At the bottom, in green, is a survivorship curve in which most of the deaths occur in the youngest age groups. Common in most plant and insect species, as well as in many marine species such as oysters and fish, this survivorship curve describes populations in which the few individuals lucky enough to survive past the first few age intervals then become likely to live a much longer time. Species with this type of survivorship curve tend to produce very large numbers of offspring because most will not survive. They tend, however, not to provide much parental care, if any. A classic example might be a fish such as the mackerel. One female might produce one million eggs! Obviously, most of these (on average 999,998) must not survive to adulthood or else the planet would quickly be overrun with mackerel. 27

28. Type I Survivorship Curve
Species such as humans and other large mammals, which have fewer numbers of offspring but invest much time and energy in caring for their young, usually have a Type I survivorship curve. This relatively flat curve reflects low juvenile mortality, with most individuals living to old age.

29. Type II Survivorship Curve
A constant probability of dying at any age, shown by the Type II survivorship curve, is evident as a straight line with a constant slope that decreases over time toward zero. Certain lizards, perching birds, and rodents exhibit this type of survivorship curve

30. Type III Survivorship Curve
In some species that produce many offspring but provide little care for them (r-selected species), mortality is greatest among the youngest individuals. The Type III survivorship curve indicative of this life history is initially very steep, which is reflective of very high mortality among the young, but flattens out as those individuals who reach maturity survive for a relatively longer time; it is exhibited by animals such as many insects or shellfish

31. A population of cactus ground finches is characterized by a type II survivorship curve. Why are the probabilities of death in the life table not equal?
These numbers reflect a type I survivorship curve.
These numbers reflect a type III survivorship curve.
If one looks at the general trend of the data it best fits a type II survivorship curve.
Answer: 3

32. 14.13–14.15
The human
population is growing
rapidly. 32

33. Why is it bad news for young people today?
Age pyramids reveal much about a population.
What is the baby boom?
Why is it bad news for young people today?
The “baby boom.” It happened half a century ago, yet young people today may end up paying a price for it. What exactly was it and why does it still matter so much?
Beginning in the late 1940s, just after World War II, and continuing through the early 1960s, families in the United States had many more babies—about 30% more per month—than they would have had if they had followed the same pattern as their parents. Then the birth rates began to return to their earlier levels where they have remained ever since. As the babies from the baby boom years grew up, schools had to increase in size to accommodate all the children when they reached school age. Then the schools had to decrease in size once the baby boomers all graduated. Now, as these individuals approach retirement age, the question of how their retirement and health care needs will be met is one of the biggest issues facing society. 33

34. Describing Populations
In terms of the proportion of individuals from each age group
The population age distribution
Age groupings called cohorts
Populations often vary across space—dense in the cities and more sparse in rural areas. The baby boom illustrates that populations have an “age distribution” as well. Just as there may be more individuals in some areas and fewer in others, there may be more individuals in certain age groups and fewer in others.
Describing populations in terms of the proportion of individuals in each age group reveals interesting population features. A population can be divided into the percentages of individuals that are in specific age groupings, called cohorts, such as 0–4 years, 5–9 years, 10–14 years, and so on, in an “age pyramid.” People have radically different likelihoods of dying or reproducing, for instance, depending on their age. A 10-year-old isn’t going to produce any offspring, but a 30-year-old has a relatively high likelihood of reproducing. And a 10-year-old is less likely to die than an 80-year-old. 34

35. Around the world, countries vary tremendously in the age pyramids describing their populations. If two populations are the same size but have different age distributions, they will have some very different features.
Most industrialized countries are growing slowly or not at all; most of the population is middle-aged or old. In these countries, such as Norway, the age pyramid is more rectangular than pyramid-shaped. Because birth rates are low, the bottom of the pyramid is not very wide. And because death rates are low, too, the higher age classes in the pyramid don’t get significantly narrower. Instead, the cohorts remain about the same size all the way into the late sixties and seventies, at which point high mortality rates finally cause them to narrow. This gives the pyramid a more or less rectangular shape.
A more triangular shape is seen in the age pyramids of developing countries. Kenya, for example, has very high birth rates, reflected as a large base in its age pyramid. But high mortality rates, usually due to poor health care, cause a large and continuous reduction in the proportion of individuals in older age groups. In these countries, most of the population is in the younger age groups. 35

36. Notice the “bulge” in the U. S. age pyramid
Notice the “bulge” in the U.S. age pyramid. The shape of the age pyramid in the United States has economists worried that the social security system (including Social Security and Medicare) will not be able to offer older citizens sufficient benefits in the next 10–30 years. Because of the unusually large number of babies born about 50–65 years ago, an unusually large number of people are now reaching retirement age. Since the baby boomers were born, there haven’t been any years with such a large cohort. This means that the current numbers of working individuals who contribute to the social security system are not sufficient to cover the payouts promised to the large number of retirees, and the baby boomers will be expensive to support as they reach retirement. 36

37. Rapid Growth, Slow Growth, Negative Growth

38. Which country has the lowest proportion of its population between 60 and 64 years old?
Norway
Kenya
United States
Answer: 2
UNITED STATES

39. 14.14 As less-developed countries become more developed, a demographic transition often occurs.
Many countries have undergone industrialization, and in the process their patterns of population growth follow common paths marked first by periods of faster growth and later by slow growth. The sequence of changes is remarkably consistent.
Start with a pre-industrial country. Such countries usually have high birth rates and high death rates. These result from poor and inefficient systems of food production and distribution along with a lack of reliable medical care. As industrialization begins, both food production and medical care improve. These improvements inevitably lead to a reduction in the death rate. The birth rate, however, remains relatively high and so the country’s population grows rapidly. 39

40. As industrialization continues, further changes occur
As industrialization continues, further changes occur. Most importantly, a higher standard of living is attained. This results from higher levels of education and employment and, in conjunction with the improved health care, finally causes a reduction in the birth rate. The new, lower birth rate then slows the population’s growth. The progression from:
high birth rates and high death rates to (slow population growth)
high birth rates and low death rates to (fast population growth)
low birth rates and low death rates (slow population growth)
is called the demographic transition (Figure With industrialization, death rates drop and, later, birth rates drop too). 40

41. Why is there a difference?
Population growth is alarmingly slow in Sweden and alarmingly fast in Mexico.
Why is there a difference?
The demographic transition can take decades to complete, so it is not always easy to identify it as it occurs. A survey of countries around the world, though, reveals many at different points on the transition.
In the past few decades, the demographic transition has been completed in Japan, Australia, the United States, Canada, and most of Europe, leading to a slowing of population growth. In Mexico, Brazil, southeast Asia, and most of Africa, on the other hand, the transition has not been completed and population growth is still dangerously fast. 41

42. Take-home message 14.14
The demographic transition tends to occur with the industrialization of countries.
It is characterized by an initial reduction in the death rate, later followed by a reduction in the birth rate.
The demographic transition illustrates how health, wealth, and education can lead to a reduction in the birth rate without direct government interventions. 42

43. Humans are a phenomenally successful species. The numbers don’t lie
Humans are a phenomenally successful species. The numbers don’t lie. There are more than six billion people alive today (Figure World population: a slow start, but rapidly picking up steam).  And current birth rates exceed death rates by so much that we add 80 million more people to the total each year. But for all of our success, the laws of physics and chemistry still apply. We all need food to eat for energy and we need space to live. We need a variety of other resources, too, and we also need the capability of processing and storing all of the waste products we generate. Because of these limits to perpetual population growth, we may in fact become victims of our own success.
This we know for certain: Human population growth cannot continue forever at the current rate. Like every other species, our environment has a carrying capacity beyond which the population cannot be maintained. The question is: How high can it go? What is our carrying capacity? This, unfortunately, is a difficult question to answer—although not for a lack of trying. For more than 300 years, biologists have been making estimates, starting when Anton von Leeuwenhoek (the inventor of the microscope) made an estimate of just over 13 billion people. The median of all the old as well as the new estimates is just over 10 billion, and the United Nations conservatively suggests that it is somewhere between 7 and 11 billion. 43

44. At which point is the population experiencing the maximum rate of both birth and death?
Circle 1
Circle 2
Circle 3
Answer: 1

45. Which point on the graph below might represent a population after a long period of industrialization?
Circle 1
Circle 2
Circle 3
Answer: 3