The characteristics of populations introduced above – birth rate, death rate, and life expectancy – interact to form several basic strategies for survival. Insurance companies began investigations into life expectancies for various groups of people – males vs. females, for example – and compiled the data in life tables. Biologists plot these patterns through time in survivorship curves, which graph the number of all individuals still living (in powers of ten, on the Y-axis) for each age (on the X-axis). The three basic types of survivorship curves are illustrated in Figure below.
Figure 17.8
Survivorship curves correlate with strategies species use to adapt to various environments. Large organisms in relatively stable environments have few offspring but high levels of parental care; most individuals survive to old age (Type I). Smaller organisms in less stable environments produce many offspring but provide little parental care, and few survive to old age (Type III). Type II species show intermediate characteristics in response to a death rate which remains constant throughout life.
Species showing a Type I pattern have the highest survival rates, with most individuals living to old age. Many large animals, including humans, show this “late loss” pattern of survivorship; few offspring, high levels of parental care, and low “infant” death rates characterize Type I species. As we will see in a later lesson, human populations in rich countries fit this pattern more closely than do those in undeveloped countries.
Species with Type III survivorship patterns experience high death rates among offspring; relatively few survive to old age. Most plants and invertebrates and many fish show this “early loss” pattern. Parents invest most of the reproductive energy in high numbers of offspring to offset the high death rates, and little or no energy remains for parental care.
Species showing intermediate, Type II survivorship curves experience uniform death rates throughout their lives. Some birds and many asexual species show this “constant loss” pattern.
We’ll look at these strategies more closely in the next lesson as we study how populations grow and change: population dynamics.
Lesson Summary
Historic concern with overpopulation includes ancient Greek delay of marriage, Malthus’ predictions of a resource crisis, and Darwin’s use of exponential growth in his theory of natural selection.
A group lead by Julian Simon, cornucopians, believes that more people are better, because technology and innovation will solve population problems.
The study of the biology of natural populations can shed light on human population issues.
In biology, a population is a group of organisms of a single species living within a certain area.
Population size, the total number of individuals, is important for understanding endangered or threatened species, but population density is often more useful for comparing populations across time or space.
Minimum Viable Population (MVP) and Population Viability Analysis (PVA) use extensive field data to predict best management practices for a particular species in conservation biology.
Double bar graph population pyramids show proportions of males and females within age groups.
Population pyramids which have wide bases indicate high birth rates and probable population growth, and decreases from one age group to the next indicate death rates and (overall) life expectancy. Populations with narrow bases indicate low birth rates and shrinking populations, and those with bases roughly equal to peaks indicate stable populations and/or low death rates.
Delaying reproduction or increasing age to sexual maturity can decrease population growth rate, even if fertility levels remain the same.
Patchy habitat distribution results in patchy distribution of a population throughout its boundaries.
Dispersion of a population within its boundaries depends on intraspecies competition or cooperation.
Clumped distribution indicates social relationships or recent reproduction without dispersal.
Uniform distribution reveals competition among individuals for a limited resource.
Random distribution suggests little interaction among individuals.
Survivorship curves show the number of individuals which survive (on a power-of-ten scale) at each age level.
Large animals, which provide few offspring with high levels of parental care, experience low death rates and long average life expectancy – a Type I pattern. This pattern is typical for humans in rich/developed countries.
Among plants and many invertebrates which have many offspring but little or no parental care, offspring have high death rates and relatively low average life expectancy – a Type III pattern.
Some birds and many asexually reproducing species have constant death rates throughout life and intermediate average life expectancy – a Type II pattern.
Review Questions
Compare the cornucopian perspective on human population growth to the Malthus’ (sometimes called the Neo-Malthusian) view.
(If false, restate to make true.) Human concern about overpopulation is a recent phenomenon.
Define a biological population.
Define and compare the importance of population size vs. population density.
Explain how conservation biologists use Minimum Viable Population (MVP) and Population Viability Analysis (PVA).
How does patchy distribution differ from dispersion?
What types of information do population pyramids show? What kinds of inferences can you make using variations in population pyramid shape?
How does delaying reproduction affect population size, even if fertility remains constant?
Describe the three types of survivorship curves and the reproductive strategies they illustrate.
Apply what you have learned so far about population biology to your current understanding of human populations. Note: we will explore human populations in detail in a future lesson, so accept that your current understanding may be incomplete!
Further Reading / Supplemental Links
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookpopecol.html
http://www.geography.learnontheinternet.co.uk/topics/popn1.html
http://www.census.gov/ipc/www/idb/faq.html
http://www.biologicaldiversity.org/swcbd/species/orca/pva.pdf
http://nationalzoo.si.edu/ConservationAndScience/EndangeredSpecies/PopViability/default.cfm
Vocabulary
age at maturity
The age at which individuals (sometimes considered only for females) become able to reproduce.
age-sex structure
A graphical depiction of proportions of males and females across all age groups within a population; also depicted as a population pyramid.
birth rate (b)
The number of births within a population or subgroup per unit time; in human demography, the number of childbirths per 1000 people per year.
cornucopian
A person who believes that people and markets will find solutions to any problems presented by overpopulation.
death rate (d)
The number of deaths within a population or subgroup per unit time; in human demography, the number of deaths per 1000 people per year.
dispersion
The pattern of spacing among individuals within a population – clumped (clustered or grouped), uniform (evenly spaced), or random (no discernible pattern).
life expectancy
Average survival time for individuals within a population.
minimum viable population
The smallest number of individuals which can exist without extinction due to chance variations in reproduction, genetics, or environment.
overpopulation
A condition in which the number of individuals in a population exceeds the carrying capacity of their environment.
population
A group of organisms of a single species living within a certain area.
population density
The number of organisms per u
nit area or volume.
population viability analysis
A model of interaction between a species and the resources on which it depends used in conservation biology.
survivorship curve
Graph which shows the number of all individuals still living (in powers of 10, on the Y-axis) at each age (on the X-axis).
Points to Consider
Do you think Earth’s human population has a patchy distribution? Why or why not?
Do people show clumped, uniform, or random dispersion? Why?
How do you think birth rates compare with death rates in the human population? Predict the shape of a population pyramid for humans.
At this point in your study of population biology, do you consider yourself a Malthusian, following the ideas of Thomas Malthus, or a cornucopian?
Lesson 17.2: Population Dynamics
Lesson Objectives
Define population dynamics.
Describe exponential (J-curve) growth, and explain the conditions under which it occurs.
Explain Malthus’ ideas about human population growth and their significance to evolutionary theory.
Births and deaths: Balancing costs of reproduction and survival.
Clarify the relationship between population growth rate, birth rate, and death rate.
Compare trade-offs between survival and reproduction for altricial species to those of precocial and nest parasite species.
Describe the relationship between age at maturity and growth rate.
Analyze the equation for population growth rate.
Describe several means of dispersal, and its importance to population density.
Define migration and explain possible effects on population density and growth.
Compare nomadism, irruption, range expansion, and colonization in terms of their effects on population density.
Give examples of population growth patterns in nature.
Describe logistic (S-curve) growth, and explain the conditions under which it occurs.
Analyze the concept of carrying capacity in terms of population growth and resource availability.
Compare and contrast density-dependent and density-independent limiting factors.
Relate predator-prey cycles to density-dependent population control.
Compare and contrast the adaptations and environmental characteristic of r-selected species to those of K-selected species.
Introduction
Imagine a huge bowl of your favorite potato salad, ready for a picnic on a beautiful, hot, midsummer day. The cook was careful to prepare it under strictly sanitary conditions, using fresh eggs, clean organic vegetables, and new jars of mayonnaise and mustard. Familiar with food poisoning warnings, s/he was so thorough that only a single bacterium made it into that vast amount of food. While such a scenario is highly unrealistic without authentic canning, it will serve as an example as we begin our investigation of how populations change, or population dynamics. Because potato salad provides an ideal environment for bacterial growth, just as your mother may have warned, we can use this single bacterial cell in the potato salad to ask:
How Do Populations Grow Under Ideal Conditions?
Given food, warm temperatures, moisture, and oxygen, a single aerobic bacterial cell can grow and divide by binary fission to become two cells in about 20 minutes. The two new cells, still under those ideal conditions, can each repeat this performance, so that after 20 more minutes, four cells constitute the population. Given this modest doubling, how many bacteria do you predict will be happily feeding on potato salad after five hours at the picnic? After you’ve thought about this, compare your prediction with the “data” in Table below.
Table below: Like many populations under ideal conditions, bacteria show exponential or geometric growth. Each bacterium can undergo binary fission every 20 minutes. After 5 hours, a single bacterium can produce a population of 32,768 descendants.
Time (Hours and Minutes) Population Size (Number of Bacteria)
0 1
20 minutes 2
40 minutes 4
1 hour 8
1 hour 20 minutes 16
1 hour 40 minutes 32
2 hours 64
2 hours 20 minutes 128
2 hours 40 minutes 256
3 hours 512
3 hours 20 minutes 1024
3 hours 40 minutes 2048
4 hours 4096
4 hours 20 minutes 8192
4 hours 40 minutes 16,384
5 hours 32,768
(Source: CK-12 Foundation, License: CC-BY-SA)
Are you surprised? This phenomenal capacity for growth of living populations was first described by Thomas Robert Malthus in his 1798 Essay on the Principle of Population. Although Malthus focused on human populations, biologists have found that many populations are capable of this explosive reproduction, if provided with ideal conditions. This pattern of growth is exponential, or geometric growth: as the population grows larger, the rate of growth increases. If you have worked compound interest problems in math or played with numbers for estimating the interest in your savings account, you can compare the growth of a population under ideal conditions to the growth of a savings account under a constant rate of compound interest. The graph in Figure below, using potato salad bacterial “data,” shows the pattern of exponential growth: the population grows very slowly at first, but more and more rapidly as time passes.
Figure 17.9
Exponential or geometric growth is very slow at first, but accelerates as the population grows. Because rate of growth depends on population size, growth rate increases as population increases. Most populations have the ability to grow exponentially, but such growth usually occurs only under ideal conditions that are not found in nature. Note the J shape of the curve.
Of course, if bacterial populations always grew exponentially, they would long ago have covered the Earth many times over. While Thomas Malthus emphasized the importance of exponential growth on population, he also stated that ideal conditions do not often exist in nature. A basic limit for all life is energy. Growth, survival, and reproduction require energy. Because energy supplies are limited, organisms must “spend” them wisely. We will end this lesson with a much more realistic model of population growth and the implications of its limits, but first, let’s look more carefully at the characteristics of populations which allow them to grow.
Births and Deaths: Balancing Costs of Reproduction and Survival
The growth rate of a population is the change in population size per member of the population per unit of time. The symbol r denotes growth rate. Growth rate clearly depends on birth rate b, the number of births per individual within the population per unit of time, as well a death rate d, the number of deaths per individual per unit of time. The following equation calculates growth rate, according to our preliminary understanding:
r = b – d
growth rate = birth rate – death rate
If birth rate exceeds death rate, r is positive and the population grows. If death rate exceeds birth rate, r is negative and the population declines. And if birth rate and death rate are in equilibrium, growth rate is zero, and the population remains stable. In a stable population, each individual, on the average, produces one offspring which survives long enough to reproduce itself. Mere survival is not success in the game of life; natural selection requires that survivors reproduce. As Malthus realized, nearly all species have the potential to grow – to reproduce many more than just a single replacement offspring. However, species vary in the strategies they use to achieve reproductive success, making trade-offs between the energy and time “costs” of survival and those of reproduction. Age at first reproduction, frequency of reproduction, number of offspring, parental care, reproductive lifespan, and offspring death rate are some of the traits which build strategies for successful reproduction.
Analyzing extreme examples can help you understand the trade-offs species must make between survival and reproductive succ
ess. Let’s compare two groups of birds. Somewhat like precocious children who mature early, precocial birds run around to find their own food soon after hatching. Geese, ducks, and chickens use this strategy for raising their young (Figure below). Often living and nesting on the ground, precocial species are subject to high predation rates, so few survive long enough to reproduce. Therefore, those who do reproduce lay many eggs at once, and these eggs are large. The young emerge well-developed, ready to feed and escape predators soon after hatching. Precocial species invest a great deal of energy in a large number of offspring but do not spend much energy on parental care, because even though some offspring are likely to die, others will survive long enough to reproduce.
Figure 17.10
Geese and ducks use a strategy to ensure reproductive success. They invest a great deal of energy in a large number of large eggs, so that young are born well-developed and ready to fend for themselves almost immediately after hatching. Predation on goslings and ducklings is high, but this death rate is offset by a high birthrate. Overall, the population remains stable.
Contrast this precocial strategy with the opposite, altricial strategy used by robins and hummingbirds (Figure below). These birds hatch helpless and naked, completely unprepared for independent life. Parents invest little energy in just a few, small eggs; hummingbirds’ eggs are the smallest in the bird world, and average two per nest. However, survival of these offspring matters a great deal, because there are so few. So, parents build elaborate nests safely hidden in trees and invest a great deal of energy hunting for food around-the-clock until the young have developed enough to fledge and find food on their own.
CK-12 Biology I - Honors Page 75
