Chapter 46 Summary

Core Concepts Summary

46.1 A population consists of all the individuals of a given species that live and reproduce in a particular place.

A population is characterized by its size, range, and density. page 1004

Population size is the number of individuals at a particular time and place. page 1005

Population range is the total area in which a population lives. page 1005

Population density is the population size divided by its range. page 1005

Population size can be estimated by sampling and mark-and-recapture techniques. page 1006

46.2 Population size can increase or decrease over time.

Population size is influenced by births, deaths, immigration, and emigration. page 1007

The population growth rate is the change in population size, commonly expressed as the per capita rate of growth—the rate of growth (or decline) per individual in the initial population. page 1007

Initially, growth rates are often exponential, in which the number of individuals added to the population increases over time. page 1008

Population density often limits population growth because of decreasing resources. This limit defines the carrying capacity of the environment. page 1008

The growth curve of populations is commonly logistic, producing an S-shaped curve in which growth is exponential at first but tapers off as population size approaches the carrying capacity of the environment. page 1009

Population density can be limited by density-dependent factors, such as resources and predation, and by density-independent factors, such as drought and fire. page 1009

46.3 The age structure of a population helps ecologists understand past changes and predict future changes in population size.

The age structure of a population is the number of individuals of each age group and is often depicted as a histogram. page 1011

A growing population shows a pyramid-shaped age distribution, with the youngest classes more abundant than older classes, whereas a stable population shows an even distribution of age classes. page 1011

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Survivorship is the proportion of individuals from an initial cohort that survives to each successive age or stage of the life cycle. page 1011

Type I, Type II, and Type III survivorship curves correspond to mortality that occurs predominantly late in life, is even throughout life, or is concentrated early in life, respectively. page 1013

r-strategists are organisms that live in unpredictable environments, producing many young but investing relatively little in each offspring. page 1014

K-strategists live where resources are predictable; they produce few offspring but invest a lot in each. page 1014

r- and K-strategists represent extremes of a continuum of reproductive strategies. page 1014

The life history of an organism is the evolved pattern of resource investment in each stage of a given species’ lifetime. page 1014

Organisms often exhibit trade-offs between reproduction and other physiological functions. page 1014

46.4 The dynamics of populations are influenced by the colonization and extinction of smaller, interconnected populations that make up a metapopulation.

The size of local populations (patches) and the degree of connectedness between patches determines in part whether a given patch will persist or not. page 1015

An ecological island may be a true island surrounded by water or any habitat surrounded by uninhabitable areas. page 1016

The theory of island biogeography proposes that the number of species that can occupy an island depends on the size of the island and its distance from the mainland. page 1016

Species diversity on habitat islands reflects both the rate at which new species arrive and the rate at which colonist species become extinct. page 1019

Self-Assessment

  1. Name the four factors that affect population size.

    Self-Assessment 1 Answer

    A species’ population size is effectively determined by how many individuals enter into and exit a given region. Thus, population size depends on birth rates and immigration rates (individuals “entering” into a region), in addition to death rates and emigration rates (individuals “exiting” a region).

  2. Draw an exponential and a logistic growth curve, and explain what accounts for their different shapes.

    Self-Assessment 2 Answer

    The exponential growth curve represents a population with a constant per capita growth rate. At this stage, the environment can still support a growing number of individuals of one species. Neither the scarcity of resources nor overcrowding has limited the population size. Thus, the curve keeps extending upward on the right side of the graph. The straight line on the left side of the graph (representing a slight incline) has to do with a population having relatively few individuals to start. A handful of individuals can only produce so many offspring at once. However, if these individuals continue to reproduce, and so do their progeny (etc.), population size can rapidly increase.

    The logistic growth curve also contains a period of exponential growth; however, the curve levels off in a horizontal line on the right side of the graph. This population size (where the graph levels off) represents the carrying capacity of a given environment. At this point, the environment does not have enough resources to support any more members of a species. As a result, more individuals in a population die and the birth and death rates equilibrate, leading to a (relatively) constant population size over time.

  3. Name two density-dependent and two density-independent factors that can limit the size of a population.

    Self-Assessment 3 Answer

    Density-dependent factors that affect a species’ population size include inter- and intraspecific competition for resources and space. For example, as resources are exhausted and competition intensifies, a population’s birth rate decreases but the death rate increases. However, competition isn’t the only density-dependent factor that influences population size. As a population grows and its density increases, diseases can also spread (increasing the death rate).

    Density-independent factors that affect population size include natural disasters (e.g., floods, droughts, hurricanes) and/or unexpected changes in temperature. Such occurrences can severely decrease the size of a population, regardless of how many individuals inhabit a given area.

  4. Draw a graph showing the age structure of a population that is growing rapidly and a graph of one that is not.

    Self-Assessment 4 Answer

  5. Plot a survivorship curve for a species with high rates of predation early in life and one for a species with high mortality late in life. Name the types of survivorship these species display.

    Self-Assessment 5 Answer

  6. Explain how r and K strategies relate to the predictability of the environment and in what kinds of environment each strategist would be more successful.

    Self-Assessment 6 Answer

    K- and r-strategists differ fundamentally in the number of offspring they produce, and in the amount of resources and energy they dedicate to their progeny. r-strategists produce several offspring, but do not invest much energy in the long-term raising or protection of their young. This is a particularly effective strategy for a species that inhabits an area where resources are limited, or for a species that has multiple predators. Thus, r-strategists tend to inhabit “unpredictable” environments, assuring that some of their progeny will survive due to the sheer number they produce.

    Conversely, K-strategists devote a large degree of energy and resources into raising their young, with the trade-off of only producing a few offspring at a time. This strategy is employed by species living in “predictable” environments, where resources are more readily available and predators are limited.

  7. Describe what is meant by a trade-off in physiological functions, and give an example.

    Self-Assessment 7 Answer

    A physiological trade-off occurs when an organism contributes more resources and/or energy to one particular function over another (i.e., in plants, producing more seeds while sacrificing defensive structures such as thorns).

  8. Give three examples of a habitat island, and explain what makes it an island.

    Self-Assessment 8 Answer

    A habitat island is a region that can support a given species, but is surrounded by environments that cannot support the same species. Thus, much like islands in the ocean, habitat islands are isolated areas. For example, a lake would be considered a habitat island for trout, as these fish could not survive in the forest surrounding the lake. Other examples of habitat islands include actual islands in the ocean (i.e., ants on an island can’t live in the surrounding ocean) or a valley surrounded by mountains (i.e., grasses able to inhabit the valley would not be able to survive at higher mountain elevations).

  9. Name factors that determine the diversity of species on a habitat island, and explain the relevance of these factors in managing the habitat of an endangered species.

    Self-Assessment 9 Answer

    The species diversity of a habitat island depends on the colonization and extinction rates of organisms inhabiting (or attempting to inhabit) this area. Initial colonization rates depend both on the size of a habitat island (more area means the potential to support a greater number of species) and its proximity to other habitat islands (from which organisms can migrate). In turn, extinction rates depend on the types of species colonizing a habitat island and their interactions with one another.

    Many of the factors that influence species diversity on a habitat island can also be taken into consideration during conservation efforts for an endangered species. Imagine that a conservationist wants to introduce members of an endangered species into a new environment. She must evaluate what other organisms have colonized this environment and the extent of human activity in this area. She must also determine if the size of this potential habitat is large enough to support the introduction of a new species. If few resources are available and/or the species will be heavily hunted (either by humans or other predators), this will likely result in the deaths of any endangered animals introduced into this environment. Furthermore, the conservationist has to assure that she introduces enough animals into the environment so that they will be able to find one another and mate (i.e., individuals must be in close proximity to one another). If steps are taken to assure that the above conditions are met, there is a good chance that members of this endangered species can take root and survive in this new habitat.