populations chapter 42. chapter 42 populations key concepts 42.1 populations are patchy in space...

POPULATIONS

Chapter 42

Chapter 42 Populations Key Concepts 42.1 Populations Are Patchy in Space and

Dynamic over Time 42.2 Births Increase and Deaths Decrease

Population Size 42.3 Life Histories Determine Population Growth

Rates 42.4 Populations Grow Multiplicatively, but the

Multiplier Can Change 42.5 Immigration and Emigration Affect

Population Dynamics 42.6 Ecology Provides Tools for Conserving and

Managing Populations

Chapter 42 Opening Question

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

Populations and Abundance

Populations: groups of individuals of the same species

Humans have long been interested in understanding species abundance: To increase populations of species

that provide resources and food To decrease abundance of crop pests,

pathogens, etc.

Population density and size

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.

Distribution of Populations

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 42.1 Species Are Patchily Distributed on Several Spatial Scales

Distribution can change over time

Population densities are dynamic—they change over time.

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

Dispersion patterns

• Clumped- most common pattern; could be due to numerous factors: more suitable habitat, food source is unevenly distributed, more success when there are many individuals-pack of wolves, school of fish

• Uniform-most unusual pattern; territoriality contributes to this pattern, or competition for resources

• Random-if there are no competition factors or resources are relatively evenly distributed or if distribution is based on random factors-like seed dispersal by wind

Dispersion patterns

Births Increase and Deaths Decrease

Population Size Change in population size

depends on the number of births and deaths over a given length of time.

“Birth–death” or BD model of population change: DBNN tt 1

Births Increase and Deaths Decrease

Population Size Population growth rate (change in

size over one time interval):Nt1 Nt N (Nt Nt) B DB D

DBDB

tt

DB

T

N

1)1(

Population Estimates

Change in population size can be measured only for very small populations that can be counted, such as zoo animals.

To estimate growth rates, ecologists keep track of a sample of individuals over time.

Lincoln Peterson capture recapture method

Lincoln Peterson Population Estimate

• Quadrat 4m x 4m Littorina irrorata• N = M (n+1) / R+1• Where N is population size M is

original sample size• Second sample size is n • Recaptured snails  is • RN = 136 (109)/37 = 401 snails• 25 Littorina/m2)

Estimating Population Growth Rate

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 raterN

T

N

Predicting population change

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.

Life Histories Determine Population Growth Rates

Demography: study of processes influencing birth, death, and population growth rates

Life history: timing of key events such as growth and development, reproduction, and death during an average individual’s life Example: Life cycle of the black-

legged tick

opportunistic- “r” equilibrium “K” life history life history

• have short life spans• Often small bodied• rapid development to

reproductive maturity• many reproductive

periods / year• Many offspring • No parental care• larvae present in the

water during much or all of the year

• high death rates

• long life spans• Large bodied• relatively long

development time to reach reproductive maturity

• Few offspring• Extensive parental care• one or more reproductive

periods/year,• low death rates

Figure 42.3 Life History of the Black-Legged Tick

Life histories: r and K selected

• Life history is the birth, reproduction and death of organisms

• 3 factors affect the rate of increase (r ): # or reproductive periods, clutch size; maturation age

• R selected: opportunistic, early maturation, large clutches of small, independent individuals, no parental care (semelparity)

• K selected: equilibrium (K), few offspring, mature late, larger bodied, parental care (iteroparity)

• dN/dt = rmaxN((K-N)/K)• The graph of this equation shows an S-shaped

curve.

Fig. 52.11

Concept 42.3 Life Histories Determine Population Growth Rates

A life history shows the ages at which individuals make life cycle transitions and how many individuals do so successfully: Survivorship—fraction of individuals

that survive from birth to different life stages or ages

Fecundity—average number of offspring each individual produces at different life stages or ages

Table 42.1

Concept 42.3 Life Histories Determine Population Growth Rates

Survivorship can also be expressed as mortality: the fraction of individuals that do not survive from birth to a given stage or age.

Mortality = 1 – survivorship

Concept 42.3 Life Histories Determine Population Growth Rates

Survivorship and fecundity affect r. The higher the fecundity rate and survivorship, the higher r will be.

If reproduction shifts to earlier ages, r will increase as well.

Concept 42.3 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 42.3 Life Histories Determine Population Growth Rates

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

The rate at which an organism can acquire a resource increases with the availability of the resource. Examples: Photosynthetic rate increases

with sunlight intensity; an animal’s rate of food intake increases with the density of food

Figure 42.4 Resource Acquisition Increases with Resource Availability—Up to a Point

Concept 42.3 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, such as maintenance, 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.

The Principle of Allocation

Concept 42.3 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 42.3 Life Histories Determine Population Growth Rates

Life-history tradeoffs—negative relationships among growth, reproduction, and survival Example: A species that invests heavily

in growth early in life cannot simultaneously invest heavily in defense.

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

Concept 42.3 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.

Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 1)

Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 2)

Figure 42.6 Climate Warming Stresses Spiny Lizards (Part 3)

Concept 42.3 Life Histories Determine Population Growth Rates

Laboratory experiments have also shown the links between environmental conditions, life histories, and species distributions. Example: Quantifying life history traits

of two species of grain beetles in different temperature and humidity conditions explained distributions of these species in Australia.

Figure 42.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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, Chapter 42, p. 873 (2)

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

Multiplicative growth with a constant r has a constant doubling time.

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

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

K can be thought of as the number of individuals that a given environment can support indefinitely.

When population density reaches K, an average individual has just the amount of resources it needs to exactly replace itself.

When density <K, an average individual can more than replace itself; when density >K, the average individual has fewer resources than it needs to replace itself.

Figure 42.8 Per Capita Growth Rate Decreases with Population Density

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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 42.2 Population Densities Are Dynamic

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

Environmental changes affected fecundity of the Galápagos cactus ground finches: When females were 7 and 8 years old, they

produced no surviving young, and survivorship dropped.

Low food availability during these years resulted from a severe drought in 1985.

When the females were 5, a wet year produced abundant food and high fecundity.

Table 42.1

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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 42.9 Human Population Growth

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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

His essay provided Charles Darwin with a critical insight for the mechanism of natural selection.

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

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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 42.4 Populations Grow Multiplicatively, but the Multiplier Can Change

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.

Limits to growth

http://dieoff.org/page25.htm

Resource Consumption/Production

Figure 42.10 A Metapopulation Has Many Subpopulations

Concept 42.5 Immigration and Emigration AffectPopulation Dynamics

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

EDIBNN tt 1

Concept 42.5 Immigration and Emigration AffectPopulation Dynamics

In the BD model, populations are considered closed systems—no immigration or emigration.

In the BIDE model, subpopulations are considered open systems—individuals can move among them.

Concept 42.5 Immigration and Emigration AffectPopulation Dynamics

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

If dispersal is possible, individuals from other subpopulations can recolonize the patch and “rescue” the subpopulation from extinction.

Concept 42.5 Immigration and Emigration AffectPopulation Dynamics

Immigrants also contribute to genetic diversity within subpopulations.

This gene flow combats the genetic drift that can occur in a small population that reduces a species’ evolutionary potential.

Concept 42.5 Immigration and Emigration AffectPopulation Dynamics

In the metapopulation of Edith’s checkerspot butterfly, all but the largest subpopulation went extinct during a severe drought between 1975 and 1977.

In 1986, nine habitat patches were recolonized from the Morgan Hill subpopulation.

Patches closest to Morgan Hill were most likely to be recolonized because adult butterflies do not fly very far.

Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales

Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations

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

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 42.6 Ecology Provides Tools for Conserving and Managing Populations

Fisheries

Black rockfish grow throughout their life.

Older, larger females produce more eggs, and the eggs have larger oil droplets, which give the larvae a head start on growth.

Because fishermen prefer to catch big fish, intense fishing reduced the average age of female rockfish from 9.5 to 6.5 years.

Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations

These younger females were smaller, produced fewer eggs, and the larvae did not survive as well.

Population density rapidly declined.

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

Concept 42.6 Ecology Provides Tools for Conserving and Managing Populations

Reducing disease risk The black-legged tick’s life history

indicates that success of larvae in getting 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 42.6 Ecology Provides Tools for Conserving and Managing Populations

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

Largest patches can potentially have the largest populations and genetic diversity and are given priority.

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

Ability of the organism to disperse between patches is evaluated.

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction (Part 1)

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from

Extinction (Part 2)

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction (Part 3)

Concept 42.6 Ecology Provides Tools for Conserving and 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.

Figure 42.12 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.