Populations

Populations

43

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 Populations

Key Concepts

• 43.5 Extinction and Recolonization Affect Population Dynamics

• 43.6 Ecology Provides Tools for Managing Populations

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

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.


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.


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.

Figure 43.1 Species Are Patchily Distributed on Several Spatial Scales (Part 1)

Figure 43.1 Species Are Patchily Distributed on Several Spatial Scales (Part 2)


Population densities are dynamic—they change over time.

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

Figure 43.2 Population Densities Are Dynamic and Interconnected

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:

DBNN tt 1


Population growth rate (how fast it is changing):

DBNNN tt 1

DBDB

tt

DB

T

N

1)1(


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

rNT

N


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

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.

Figure 43.3 Life History of the Black-Legged Tick


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

Table 43.1 Life Table for the 1978 Cohort of Cactus Ground Finch on Isla Daphne


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.


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.

Figure 43.4 Resource Acquisition Increases with Resource Availability (Part 1)


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.

Figure 43.5 The Principle of Allocation


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


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.


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.

Figure 43.6 Climate Warming Stresses Spiny Lizards (Part 1)

Figure 43.6 Climate Warming Stresses Spiny Lizards (Part 2)


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

Figure 43.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions (Part 1)

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

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.

In-Text Art, Ch. 43, p. 850


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.


Multiplicative growth has a constant doubling time.

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


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


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.

Figure 43.8 Per Capita Growth Rate Decreases with Population Density (Part 1)

Figure 43.8 Per Capita Growth Rate Decreases with Population Density (Part 2)


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.

Figure 43.2 Population Densities Are Dynamic and Interconnected


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.

Figure 43.9 Human Population Growth (Part 1)

Figure 43.9 Human Population Growth (Part 2)


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.


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.


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

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

Individuals may move in or out of subpopulations.


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

EDIBNN tt 1

Figure 43.10 A Metapopulation Has Many Subpopulations


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

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.


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.


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.


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.


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.


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.

Figure 43.11 Corridors Can Rescue Some Populations (Part 1)

Figure 43.11 Corridors Can Rescue Some Populations (Part 2)

Figure 43.12 A Corridor for Large Mammals

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 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.

Populations

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