Why do species interactions often lead to co-evolution of the species involved? Give an example to illustrate your answer.
Summarize how ideas about ecological succession and climax communities have changed.
Further Reading / Supplemental Links
Robert Poulin and Serge Morand, Parasite Biodiversity. Smithsonian, 2005.
Mark Ross and David Reesor, Predator: Life and Death in the African Bush. Harry N. Abrams, Inc., 2007.
Bernhard Stadler and Tony Dixon, Mutualism: Ants and Their Insect Partners. Cambridge University Press, 2008.
Marlene Zuk, Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites that Make Us Who We Are. Harcourt, 2007.
http://www.sciencemag.org/cgi/content/abstract/292/5519/1115
Vocabulary
biological pest control
Deliberate introduction of a predator species into an area in order to control a pest species.
camouflage
Common adaptation in predator and prey species that involves disguise.
climax community
Final stage of ecological succession.
co-evolution
Evolution of interacting species in which each species is an important factor in the natural selection of the other species.
commensalism
Symbiotic relationship in which one species benefits while the other species is not affected.
community
Biotic component of an ecosystem.
competition
Relationship between organisms that strive for the same limited resources.
ecological succession
Process by which a whole community changes through time following a disturbance.
grazing
Type of predation in which the predator eats part of its prey but rarely kills it.
intraspecific competition
Competition between members of the same species.
interspecific competition
Competition between members of different species.
keystone species
Predator species that plays an important role in the community by controlling the prey population and, indirectly, the populations of many other species in the community.
mimicry
Using appearance to “mimic” another animal.
mutualism
Symbiotic relationship in which both species benefit.
parasitism
Symbiotic relationship in which one species (the parasite) benefits while the other species (the host) is harmed.
pioneer species
First colonizer species in an area undergoing ecological succession.
predation
Relationship in which members of one species (the predator) consume members of other species (the prey).
primary succession
Ecological succession that occurs in an area that has never been colonized before.
secondary succession
Ecological succession that occurs in a formerly inhabited area that was disturbed.
symbiosis
Close association between two species in which at least one species benefits.
true predation
Type of predation in which the predator kills its prey.
Points to Consider
The size and growth of populations in a community is influenced by species interactions. For example, predator-prey relationships control the population growth of both predator and prey species.
How would populations grow without these influences?
What other factors do you think might affect population growth?
What factors do you think may have affected the growth of the human population?
Chapter 17: Populations
Lesson 17.1: Characteristics of Populations
Lesson Objectives
Recognize that human concern about overpopulation dates to ancient Greek times.
Explain that Cornucopians believe that technology will solve population problems.
Connect the study of the biology of natural populations to better understand human population issues.
Define a biological population.
Give reasons why biologists study populations.
Compare the importance of population size to that of population density.
Explain how conservation biologists use Minimum Viable Population (MVP) and Population Viability Analysis (PVA).
Explain how patchy habitats influence the distribution of individuals within a population.
Define and explain the reasons for three patterns of dispersion within populations.
Describe how population pyramids show the age and sex structures of populations.
Interpret population pyramids to indicate populations’ birth and death rates and life expectancy.
Analyze the effect of age at maturity on population size.
Explain the structure and meaning of a generalized survivorship curve.
Compare and contrast the three basic types of survivorship curves.
Introduction
“Solving the population problem is not going to solve the problems of racism… of sexism… of religious intolerance… of war… of gross economic inequality—But if you don’t solve the population problem, you’re not going to solve any of those problems. Whatever problem you’re interested in, you’re not going to solve it unless you also solve the population problem. Whatever your cause, it’s a lost cause without population control.” --Paul Ehrlich, 1996
(From Paul Ehrlich and the Population Bomb, PBS video produced by Canadian biologist Dr. David Suzuki, April 26, 1996.)
What exactly is the population problem? How can it be solved?
Humans have shown concern for overpopulation since the Ancient Greeks built outposts for their expanding citizenship and delayed age of marriage for men to 30. In 1798, Thomas Malthus predicted that the human population would outgrow its food supply by the middle of the 19th century. That time arrived without a Malthusian crisis, but Charles Darwin nevertheless embraced Malthus’ ideas and made them the foundation of his own theory of evolution by natural selection. In a 1968 essay, The Tragedy of the Commons, Garrett Hardin exhorted humans to “relinquish their freedom to breed,” arguing in the journal, Science, that “the population problem has no technical solution,” but “requires a fundamental extension in morality." In 1979, the government of China instituted a “birth planning” policy, charging fines or “economic compensation fees” for families with more than one child. Others have opposing views, however. Julian Simon, professor of Business Administration and Senior Fellow at the Cato Institute, argued that The Ultimate Resource is population, because people and markets find solutions to any problems presented by overpopulation. A group known as cornucopians continues to promote the view that more is better.
Figure 17.1
The Chinese government mandates population control by charging economic compensation fees for families with more than one child.
Would you support a law forbidding you to marry until a certain age? Do you know how such a law would affect population growth? Would you limit the size of all families to one child (Figure above)? Do you believe families should welcome as many children as possible? Should these decisions be regulated by law, or by individual choice? Clearly, the “population problem” reaches beyond biology to economics, law, morality, and religion. Although the latter subjects are beyond the scope of this text, the study of population biology can shed some light on human population issues. Let’s look at what biologists have learned about natural populations. Later, we will look more closely at human populations, and compare them to populations in nature.
Measuring Populations
In biology, a population is a group of organisms of a single species living within a certain area. Ecologists study populations because they directly share a common gene pool. Unlike the species as a whole, members of a population form an interbreeding unit. Natural selection acts on individuals within pop
ulations, so the gene pool reflects the interaction between a population and its environment.
Biologists study populations to determine their health or stability, asking questions such as:
Is a certain population of endangered grizzly bears growing, stable, or declining?
Is an introduced species such as the zebra mussel or purple loosestrife growing in numbers?
Are native populations declining because of an introduced species?
What factors affect the growth, stability, or decline of a threatened population?
The first step in characterizing the health of a population is measuring its size. If you are studying the population of purple loosestrife plants on your block , you can probably count each individual to obtain an accurate measure of the population’s size. However, measuring the population of loosestrife plants in your county would require sampling techniques, such as counting the plants in several randomly chosen small plots and then multiplying the average by the total area of your county. For secretive, highly mobile, or rare species, traps, motion-detecting cameras, or signs such as nests, burrows, tracks, or droppings allow estimates of population size.
Figure 17.2
Purple loosestrife plant populations show patchiness due to uneven distribution of their wetland habitats, and clumped dispersion, due to local variation in soils.
Two problems with absolute size lead ecologists to describe populations in other terms. First, because your county may not be the same size as others, the total number of individuals is less meaningful than the population density of individuals – the number of individuals per unit area or volume. Ecologists use population densities more often for comparisons over space or time, although total number is still important for threatened or endangered species.
Concern about threatened and endangered species has led conservationists to attempt to define minimal viable population size for some species. A species’ MVP is the smallest number of individuals which can exist without extinction due to random catastrophic variations in environmental (temperature, rainfall), reproduction (birth rates or age-sex structure), or genetic diversity. In 1978, Mark Shaffer incorporated an estimate for grizzly bear MPV into the first Population Viability Analysis (PVA), a model of interaction between a species and the resources on which it depends. PVAs are species-specific, and require a great deal of field data for accurate computer modeling of population dynamics. PVAs can predict the probability of extinction, focus conservation efforts, and guide plans for sustainable management.
Patterns in Populations I: in Space (or Patterns in Space)
A second problem in measuring population size relates to the distribution of individuals within the population’s boundaries. If your county has extensive wetlands in the southern half, but very few in the north, a countywide population density estimate of purple loosestrife, which grows primarily in shallow freshwater pond edges, marshes, and fens, would be misleading (Figure above). Patchy habitat – scattered suitable areas within population boundaries – inevitably leads to a patchy distribution of individuals within a population. On a smaller scale, plants within even a single wetland area may be clumped or clustered (grouped), due to soil conditions or gathering for reproduction. The characteristic pattern of spacing of individuals within a population is dispersion (Figure below). Clumped dispersion is most common, but species that compete intensely, such as cactus for water in a desert, show uniform, or evenly spaced, dispersion.
Figure 17.3
Populations of cacti in the desert, such as this group of cholla, show uniform, or even, dispersion due to fierce competition for water. The diagrams to the right show nearly uniform (top), random (middle), and clumped (bottom) dispersion patterns.
Other species, whose individuals do not interact strongly, show a random, or unpredictable, distribution. Useful measures of population density must take into account both patchiness of habitat and dispersion of individual organisms within the population’s boundaries.
Age-Sex Structure of a Population
Figure 17.4
Grizzly bear populations include adults up to 25 or 30 years old, capable of reproducing, and young immature bears under 6 years old. Healthy populations include roughly equal proportions of each age group.
Density and dispersion describe a population’s size, but size is not everything. Consider three populations of endangered grizzly bears, each containing one individual per 20 km2, and a total of 100 individuals in 2,000 km2. These populations are “equal” with respect to size. One population, however, has 50 immature (non-reproducing young) bears and 50 adult bears able to reproduce. A second population has the same number of immature and adult bears, but of the 50 adults, 45 are male. The third population has 30 immature bears and 70 bears of reproductive age. Which population is healthiest (Figure above)?
The answer is not simple, but age and sex differences between populations are significant indicators of health. Biologists concerned about a population’s future study age and sex within the population and then graph the results to show the age-sex structure as a population pyramid, although the result does not always resemble a pyramid. The X-axis in this double bar graph indicates percentage of the population, with males to the left and females to the right. The Y-axis indicates age groups from birth to old age.
Figure 17.5
A generalized age-sex structure or population pyramid shows the proportion of males and females (X-axis) at each age level (Y-axis). This example shows a slightly higher proportion of females compared to males, and a much higher proportion of young individuals compared to old.
The population in the generalized example (Figure above) contains a large proportion of young individuals, suggesting a relatively high birth rate (number of births per individual within the population per unit time). The bars narrow at each age interval, showing that a significant number of individuals die at every age. This relatively high death rate (number of deaths per individual within the population per unit time) indicates a short life expectancy, or average survival time for an individual. Note the slightly greater proportion of females compared to males at each age level. Careful study could determine whether the cause for this imbalance is the ratio of female to male births, or higher death rates for males throughout a shorter lifespan. You will learn in a later lesson that this pattern is characteristic of human populations in less developed countries.
Figure 17.6
Age structures can reveal a populations health. Type I, with most individuals below reproductive age, often indicates a growing population. Type II, with roughly equal proportions of the population at each age level, indicates a stable population. Type III, with more individuals at (or above!) reproductive age than young, describes a declining population.
A population’s age structure may reveal its health (Figure above). A growing population (Type I) usually has more young individuals than adults at beyond reproductive age. A stable population (Type II) often has roughly equal numbers of young members and adults. A declining population shows more adults and fewer young (Type III). Sex structure may also affect the health of a population. Sex determination in sea turtles, for example, is temperature-dependent; lower egg incubation temperatures produce males, while temperatures as little as 1-2oC higher produce females (Figure below). Some biologists predict that climate change may result in sea turtle sex structure shifts toward females, which could further endanger already threatened species. Continued monitoring of age-sex structures among sea turtles might be able to detect such changes before they become irreversible.
Figure 17.7
The sex of a sea turtle is determined by the temperature at which it develops males in cooler temperatures, and females in temperatures as little as 1-2C warmer. Climate change may threaten natural sex ratios. Such changes would be reflected in changing age-sex structure pyramids.
Although it is not shown in population pyramids, an important factor affecting population size is the age at which individuals become able to reprod
uce (Table below). Recall that age at maturity (when reproduction becomes possible) was the factor that even ancient Greeks recognized could affect population growth, when they prohibited marriage for males under the age of 30. We will return to this relationship in a later lesson, but for now, try to grasp it intuitively: if a person delays reproduction until age 30 and then has one child each year for two years, his or her fertility is 2. A person who has two children, one each year, beginning at age 20 also has a fertility of 2. Assume that these four children are born in the same two-year period, and that each offspring reproduces two children at the same age as his/her parent did. Sixty years after the initial four childbirths, the “delayed reproduction” individual will have 2 X 2 X 2 = 8 descendants. However, the early reproducing family will have 2 X 2 X 2 X 2 = 16 offspring – double the population increase of the first family. Do you think this could this be one way to slow human population growth?
Number of Offspring Produced Over Time Age at First Reproduction Initial Reproduction 20 years later 30 years later 40 years later 60 years later
20 years Generation 1: 2 offspring Generation 2: 4 offspring Generation 3: 8 offspring Generation 4: 16
30 years Generation 1: 2 offspring Generation 2: 4 offspring Generation 3: 8
Patterns in Populations Through Time
CK-12 Biology I - Honors Page 74
