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43. Populations. Chapter 43 Populations. Key Concepts 43.1 Populations Are Patchy in Space and Dynamic over Time 43.2 Births Increase and Deaths Decrease Population Size 43.3 Life Histories Determine Population Growth Rates 43.4 Populations Grow Multiplicatively, but Not for Long.

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chapter 43 populations
Chapter 43 Populations
  • Key Concepts
  • 43.1 Populations Are Patchy in Space and Dynamic over Time
  • 43.2 Births Increase and Deaths Decrease Population Size
  • 43.3 Life Histories Determine Population Growth Rates
  • 43.4 Populations Grow Multiplicatively, but Not for Long
chapter 43 populations1
Chapter 43 Populations

Key Concepts

43.5 Extinction and Recolonization Affect Population Dynamics

43.6 Ecology Provides Tools for Managing Populations

chapter 43 opening question
Chapter 43 Opening Question

How does understanding the population ecology of disease vectors help us combat infectious diseases?

concept 43 1 populations are patchy in space and dynamic over time
Concept 43.1 Populations Are Patchy in Space and Dynamic over Time

Population—all the individuals of a species that interact with one another within a given area at a particular time.

Humans have long been interested in understanding species abundance:

to increase populations of species that provide resources and food and to conserve species for ethical and aesthetic reasons

to decrease abundance of crop pests, pathogens, etc.

concept 43 1 populations are patchy in space and dynamic over time1
Concept 43.1 Populations Are Patchy in Space and Dynamic over Time

Population density—number of individuals per unit of area or volume

Population size—total number of individuals in a population

Counting all individuals is usually not feasible; ecologists often measure density, then multiply by the area occupied by the population to get population size.

concept 43 1 populations are patchy in space and dynamic over time2
Concept 43.1 Populations Are Patchy in Space and Dynamic over Time

Abundance varies on several spatial scales.

Geographic range—region in which a species is found

Within the range, species may be restricted to specific environments or habitats.

Habitat patches are “islands” of suitable habitat separated by areas of unsuitable habitat.

concept 43 1 populations are patchy in space and dynamic over time3
Concept 43.1 Populations Are Patchy in Space and Dynamic over Time

Population densities are dynamic—they change over time.

Density of one species population may be related to density of other species populations.

concept 43 2 births increase and deaths decrease population size
Concept 43.2 Births Increase and Deaths Decrease Population Size

Change in population size depends on the number of births and deaths over a given time.

“Birth–death” or BD model of population change:

concept 43 2 births increase and deaths decrease population size1
Concept 43.2 Births Increase and Deaths Decrease Population Size

Population growth rate (how fast it is changing):

concept 43 2 births increase and deaths decrease population size2
Concept 43.2 Births Increase and Deaths Decrease Population Size

Per capita birth rate (b)—number of offspring an average individual produces

Per capita death rate (d)—average individual’s chance of dying

Per capita growth rate (r) = (b – d) = average individual’s contribution to total population growth rate

concept 43 2 births increase and deaths decrease population size3
Concept 43.2 Births Increase and Deaths Decrease Population Size

If b > d, then r > 0, and the population grows.

If b < d, then r < 0, and the population shrinks.

If b = d, then r = 0, and population size does not change.

concept 43 3 life histories determine population growth rates
Concept 43.3 Life Histories Determine Population Growth Rates

Life history—time course of growth and development, reproduction, and death during an average individual’s life

Life histories are quantitative descriptions of life cycles.

Example: the life cycle of the black-legged tick.

concept 43 3 life histories determine population growth rates1
Concept 43.3 Life Histories Determine Population Growth Rates

A life table shows ages at which individuals make life cycle transitions and how many individuals do so successfully.

Life tables have two types of information:

survivorship—fraction of individuals that survive from birth to different life stages or ages

fecundity—average number of offspring each individual produces at those life stages or ages

concept 43 3 life histories determine population growth rates2
Concept 43.3 Life Histories Determine Population Growth Rates

Life histories vary among species: how many and what types of developmental stages, age of first reproduction, frequency of reproduction, how many offspring they produce, and how long they live.

Life histories can vary within a species. For example, different human populations have different life expectancies and age of sexual maturity.

concept 43 3 life histories determine population growth rates3
Concept 43.3 Life Histories Determine Population Growth Rates

Individual organisms require resources (materials and energy) and physical conditions they can tolerate.

Rate at which an organism can acquire resources increases with the availability of the resources.

Examples: photosynthetic rate increases with sunlight intensity, or an animal’s rate of food intake increases with the density of food.

concept 43 3 life histories determine population growth rates4
Concept 43.3 Life Histories Determine Population Growth Rates

Principle of allocation—once an organism has acquired a unit of some resource, it can be used for only one function at a time: maintenance, foraging, growth, defense, or reproduction.

In stressful conditions, more resources go to maintaining homeostasis.

Once an organism has more resources than it needs for maintenance, it can allocate the excess to other functions.

concept 43 3 life histories determine population growth rates5
Concept 43.3 Life Histories Determine Population Growth Rates

In general, as average individuals in a population acquire more resources, the average fecundity, survivorship, and per capita growth rate increase.

concept 43 3 life histories determine population growth rates6
Concept 43.3 Life Histories Determine Population Growth Rates

Life-history tradeoffs—negative relationships among growth, reproduction, and survival

Example: investments in reproduction may be at the expense of adult survivorship or growth.

Environment is also a factor: if high mortality rates are likely, it makes sense to invest in early reproduction.

concept 43 3 life histories determine population growth rates7
Concept 43.3 Life Histories Determine Population Growth Rates

Species’ distributions reflect the effects of environment on per capita growth rates.

A study of temperature change in a lizard’s environment, combined with knowledge of its physiology and behavior, led to conclusions about how climate change may affect survivorship, fecundity, and distribution of these lizards.

concept 43 3 life histories determine population growth rates8
Concept 43.3 Life Histories Determine Population Growth Rates

Laboratory experiments have also shown the links between environmental conditions, life histories, and species distributions.

slide34
Figure 43.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions (Part 1)
slide35
Figure 43.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions (Part 2)
concept 43 4 populations grow multiplicatively but not for long
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

Population growth is multiplicative—an ever-larger number of individuals is added in each successive time period.

In additive growth, a constant number (rather than a constant multiple) is added in each time period.

concept 43 4 populations grow multiplicatively but not for long1
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

Charles Darwin was aware of the power of multiplicative growth:

“As more individuals are produced than can possibly survive, there must in every case be a struggle for existence.”

This ecological struggle for existence, fueled by multiplicative growth, drives natural selection and adaptation.

concept 43 4 populations grow multiplicatively but not for long2
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

Multiplicative growth has a constant doubling time.

The time it takes a population to double in size can be calculated if r is known.

concept 43 4 populations grow multiplicatively but not for long3
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

Populations do not grow multiplicatively for very long. Growth slows and reaches a more or less steady size:

concept 43 4 populations grow multiplicatively but not for long4
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

r decreases as the population becomes more crowded; r is density dependent.

As the population grows and becomes more crowded, birth rates tend to decrease and death rates tend to increase.

When r = 0, the population size stops changing—it reaches an equilibrium size called carrying capacity, or K.

concept 43 4 populations grow multiplicatively but not for long5
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

Spatial variation in environmental factors can result in variation of carrying capacity.

Temporal variation in environmental conditions may cause the population to fluctuate above and below the current carrying capacity.

Example: the rodents and ticks in Millbrook, New York.

concept 43 4 populations grow multiplicatively but not for long6
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

The human population is unique. It has grown at an ever-faster per capita rate, as indicated by steadily decreasing doubling times.

Technological advances have raised carrying capacity by increasing food production and improving health.

concept 43 4 populations grow multiplicatively but not for long7
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

In 1798 Thomas Malthus pointed out that the human population was growing multiplicatively, but its food supply was growing additively, and predicted that food shortages would limit human population growth.

His essay provided Charles Darwin with a mechanism for natural selection.

Malthus could not predict the effects of technology such as medical advances and the Green Revolution.

concept 43 4 populations grow multiplicatively but not for long8
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

Many believe that the human population has now overshot its carrying capacity for two reasons:

Technological advances and agriculture have depended on fossil fuels, a finite resource.

Climate change and ecosystem degradation have been a consequence of 20th century population expansion.

concept 43 4 populations grow multiplicatively but not for long9
Concept 43.4 Populations Grow Multiplicatively, but Not for Long

If the human population has indeed exceeded carrying capacity, ultimately it will decrease.

We can bring this about voluntarily if we continue to reduce per capita birth rate.

concept 43 5 extinction and recolonization affect population dynamics
Concept 43.5 Extinction and Recolonization Affect Population Dynamics

Regional populations (metapopulations) are made up of subpopulations in habitat patches.

Individuals may move in or out of subpopulations.

concept 43 5 extinction and recolonization affect population dynamics1
Concept 43.5 Extinction and Recolonization Affect Population Dynamics

The BIDE model of popultion growth adds the number of immigrants (I) and emigrants (E) to the BD growth model.

concept 43 5 extinction and recolonization affect population dynamics2
Concept 43.5 Extinction and Recolonization Affect Population Dynamics

Small subpopulations in habitat patches are vulnerable to environmental disturbances and chance events and may go extinct.

Individuals from other subpopulations can recolonize the patch if dispersal is possible.

concept 43 6 ecology provides tools for managing populations
Concept 43.6 Ecology Provides Tools for Managing Populations

Understanding life history strategies can be useful in managing other species.

Fisheries:

Black rockfish grow throughout their life. The number of eggs a female produces is proportional to her size. Older, larger females also produce eggs with oil droplets that give the larvae a head start on growth.

concept 43 6 ecology provides tools for managing populations1
Concept 43.6 Ecology Provides Tools for Managing Populations

Fishermen prefer to catch big fish. Intense fishing reduced the average age of female rockfish from 9.5 to 6.5 years.

These younger females were smaller, produced fewer eggs, and larvae didn’t survive as well.

Management may require no-fishing zones where some females can mature and reproduce.

concept 43 6 ecology provides tools for managing populations2
Concept 43.6 Ecology Provides Tools for Managing Populations

Reducing disease risk:

The black-legged tick’s life history indicates that success of larvae in obtaining a blood meal has greatest impact on the abundance of nymphs.

Thus, controlling the abundance of rodents that are hosts for the larvae is more effective in reducing tick populations than controlling the abundance of deer, the hosts for adults.

concept 43 6 ecology provides tools for managing populations3
Concept 43.6 Ecology Provides Tools for Managing Populations

Conserving endangered species:

Larvae of the endangered Edith’s checkerspot butterfly feed on two plant species found only on serpentine soils.

The two plant species are being suppressed by invasive non-native grasses.

Grazing by cattle can control the invasive grasses.

concept 43 6 ecology provides tools for managing populations4
Concept 43.6 Ecology Provides Tools for Managing Populations

Conservation plans begin with inventories of habitat and potential risks to the habitat.

Largest patches can potentially have the largest populations and are given priorty.

Quality (carrying capacity) of the patches is evaluated; ways to restore or maintain quality are developed.

Ability of the organism to disperse between patches is evaluated.

concept 43 6 ecology provides tools for managing populations5
Concept 43.6 Ecology Provides Tools for Managing Populations

For some species, a continuous corridor of habitat is needed to connect subpopulations and allow dispersal.

Dispersal corridors can be created by maintaining vegetation along roadsides, fence lines, or streams, or building bridges or underpasses that allow individuals to avoid roads or other barriers.

answer to opening question
Answer to Opening Question

By understanding the factors that control abundance and distribution of pathogens and their vectors, we can devise ways to control their abundance or avoid contact.

Black-legged ticks are vectors for the bacterium that causes Lyme disease.

For these ticks, abundance of hosts for larvae (rodents) determines tick abundance.

answer to opening question1
Answer to Opening Question

Rodent abundance depends on acorn availability.

Acorn production can be used to predict areas that are likely to become infested with ticks, and measures can be taken to minimize human contact.