54 Population Population EcologyEcology

54 Population Ecology

• 54.1 How Do Ecologists Study Populations?54.1 How Do Ecologists Study Populations?

• 54.2 How Do Ecological Conditions Affect Life 54.2 How Do Ecological Conditions Affect Life Histories?Histories?

• 54.3 What Factors Influence Population 54.3 What Factors Influence Population Densities?Densities?

• 54.4 How Do Spatially Variable Environments Influence Population Dynamics?

• 54.5 How Can We Manage Populations?

What’s a population?What’s a population?

all the individuals of a species in a all the individuals of a species in a given area.given area.

Population structure describes the Population structure describes the age distribution of individuals, and age distribution of individuals, and how those individuals are spread how those individuals are spread over the environment.over the environment.

Photo 54.1 Rookery of northern elephant seals (Mirounga angustirostrus), Baja California.

Population Density?Population Density?

The number of individuals per unit The number of individuals per unit area or volume is the population area or volume is the population density.density.

Density has strong influence over how Density has strong influence over how individuals react with one another individuals react with one another and with populations of other and with populations of other species.species.

demographic events: births, deaths, demographic events: births, deaths, immigration, and emigrationimmigration, and emigration

Change the pop densityChange the pop density

demographydemography

Factors population ecologists study:Factors population ecologists study:

• # and density of individuals# and density of individualsTagging, markingTagging, marking

• rates of demographic eventsrates of demographic events

• locations of individualslocations of individualsTracking Tracking physiological & environmental dataphysiological & environmental data

Figure 54.1 By Their Marks You May Know Them

Figure 54.2 Hydrogen Isotopes Tell Where Migratory American Redstarts Molted Their FeathersFigure 54.2 Hydrogen Isotopes Tell Where Migratory American Redstarts Molted Their Feathers

What are molecular marker examples? Useful?What are molecular marker examples? Useful?

H isotopes -- where American H isotopes -- where American redstarts molt during migrationsredstarts molt during migrations

Isotopes in feathers reflect the latitude Isotopes in feathers reflect the latitude at which the feathers grewat which the feathers grew

strong latitudinal gradient of these strong latitudinal gradient of these isotopes in precipitationisotopes in precipitation

Population Estimates & Statistics to ExtrapolatePopulation Estimates & Statistics to Extrapolate

Mobile animals: capture, mark, recapture method

Proportion of marked individuals in the new sample is used to estimate population size:

N

n

n

m 1

2

2

Accurate?Accurate?

ONLY if marked individuals ONLY if marked individuals randomlyrandomly mix with the unmarked ones, and mix with the unmarked ones, and both are equally likely to be capturedboth are equally likely to be captured

Some animals learn to avoid traps, or Some animals learn to avoid traps, or learn that traps provide food and learn that traps provide food and become “trap-happy.”become “trap-happy.”

Equation for estimating N (pop size)

EIDBNN 01

Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 1)

LIFE TABLELIFE TABLE

life table: track a group of individuals born @ same time (cohort)

Survivorship: # still alive at later dates

fecundity: # offspring produced in a time interval

Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 2)

Life tables can be used to predict future trends.Life tables can be used to predict future trends.

For cactus finch, mortality rate was high during the first year, then dropped.

Mortality rate fluctuated year to year because the birds are dependent on seed production, which fluctuates with rainfall.

Figure 54.3 Survivorship Curves: survivorship in relation to age---3 MAIN PATTERNS!

Figure 54.4 Age Distributions (Age Structure Diagrams) Change over Time

average average family family size size

increased increased from 2.5 from 2.5

to 3.8 to 3.8 childrenchildren

5454.2 Life Histories

What’s a life history?

• describes how it allocates time and energy among the various activities throughout its life

• can vary dramatically

• single offspring per reproductive episode....or lots!

• Some reproduce only once and then die (salmon, agave)

Figure 54.5 Big Bang Reproduction

Figure 54.6 An Oil Droplet Is an Energy Kick Start

*Fishery Management**Fishery Management*

Black rockfish females continue to grow throughout their lives

large females produce many more eggs than small ones

Eggs from older females contain oil droplets that are food for developing fish, so offspring have a better chance for survival.

Cont.--

Intensive fishing off Oregon from 1996–1999 reduced average age of females from 9.5 to 6.5 years.

Age reduction decreased number of eggs produced and average growth rates of offspring.

Maintaining populations of this species may require no-fishing zones where females can grow to large sizes.

Local Red Drum ResearchLocal Red Drum Research

http://www.dnr.sc.gov/marine/mrri/insh_fish/reddrum/reddrumportal1.html

Guppies in Trinidad?Guppies in Trinidad?

Influence of predation

If predator fish are excluded (by waterfalls) fish have much lower mortality rates

When reared in the lab, guppies from the high predation site matured earlier, produced more eggs, and produced more offspring per brood.

5454.3 Population

Densities

Photo 54.4 Dogwinkles and barnacles in the intertidal zone.

Photo 54.5 Dense colony of purple sea urchins (Strongylocentrotus purpuratus).

Photo 54.6 Winter swarm of ladybugs (Hippodanina convergens).

dn dt equation...dn dt equation...

ΔN/Δt = rate of change of pop over time

r = net reproductive rate

Ndbt

Nr )(

rmax = intrinsic rate of increase

Nrt

Nmax

Is this intrinsic rate possible in nature?Is this intrinsic rate possible in nature?

YES!

Over short time periods, close to rmax

northern elephant seals were hunted to near extinction:

populations grew exponentially on some islands after hunting was stopped

Figure 54.7 Exponential Population Growth

But then the population reaches the ....But then the population reaches the ....

The environmental carrying capacity (K) # of any particular species that can

be supported in an environment

Environmental limits birth rates decrease, death rates increase

What determines K?

• availability of resources

• Food, shelter

• Diseases and parasites

• Social interactions

• S-shaped curve = logistic growth

Figure 54.8 Logistic Population Growth

Logistic growth model

Growth stops when N = K

NK

NKr

t

N

Density-dependent factors:Density-dependent factors:

• pop density increases food supplies may be depleted

• Predators may be attracted to high densities of prey, increasing death rate.

• Diseases can spread more easily.

Density-Independent Factors:Density-Independent Factors:

Weather-related phenomena

Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 1)

Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 2)

Figure 54.9 Regulation of an Island Population of Song Sparrows (Part 3)

Song sparrows on Mandarte Island

12 yr study

Population size fluctuated significantly.

Death rates high during cold, snowy winters, regardless of population density.

Density-dependent factors were also important:

• # breeding males limited by territorial behavior

• more breeding females fewer offspring each one fledged

• more birds alive in autumn less chance juveniles would survive the winter

Photo 54.7 Thorny tree swarms with locusts, Burkina Faso.

Photo 54.8 Adult Chinook salmon is released into holding tank at a hatchery in British Columbia.

General Trend:

More stable population # in species with long-lived individuals and low reproductive rates.

Insect pops fluctuate more than birds and mammals.

Environmental factors can change carrying capacity for species.

Figure 54.10 Individuals Born during Years of Good Reproduction May Dominate Populations (1)

Figure 54.10 Individuals Born during Years of Good Reproduction May Dominate Populations (2)

What if the pop dens depends on ONE resource?

likely to fluctuate

boreal forests: many birds and mammals eat conifer seeds.

trees reproduce synchronously and episodically

Mortality rates can be high in years with poor seed production.

4 Factors Strongly Influence Variation of Pop Density:

• Resource abundance

• Size of individuals

• Length of time a species has lived in an area

• Social organization

Animals that eat plants are generally more abundant than animals that eat

other animals.

WHY?

What about body size?

small body size higher pop densities

require less energy to survive vs larger

mammal species worldwide

Figure 54.11 Population Density Decreases as Body Size Increases

Oh, the invasive species!Oh, the invasive species!

No natural pathogens and predators very high population densities

Zebra mussels were introduced to the Great Lakes in 1985.

They spread rapidly and reached densities much higher than in their native Europe.

Figure 54.12 Introduced Zebra Mussels Have Spread Rapidly

Figure 54.13 The Last Refuge

Giant sequoias are restricted Giant sequoias are restricted to a few groves in the to a few groves in the

southern Sierra Nevada southern Sierra Nevada but Douglas firs are widespread and

abundant

54.5 How Can We Manage Populations?

Numbers of births and growth of individuals tend to be highest when population is below carrying capacity.

If humans wish to maximize the number of individuals harvested from a population, we should try to maintain it below carrying capacity.

Hunting seasons are established with this goal in mind.

54.5 How Can We Manage Populations?

In fast reproducing populations,

harvest rates can be high.

Growth rates of individuals are often density-dependent, so harvesting pre-reproductive individuals allows others to grow faster.

Some fish populations can be harvested on a sustained basis because

a few females can produce enough eggs to maintain the population!!!

54.5 How Can We Manage Populations?

Many fish have been overharvested and population sizes reduced.

Cod and haddock on Georges bank were so heavily exploited that fishing had to be stopped to allow populations to recover.

Figure 54.17 Overharvesting Can Reduce Fish Populations

54.5 How Can We Manage Populations?

Whaling has also resulted in declining populations.

Most whale populations have failed to recover.

Whales are large animals with slow reproductive rates. Many adults are needed to produce a small number of offspring.

54.5 How Can We Manage Populations?

The International Whaling Commission was established to guide recovery of whale populations.

Member countries voted to ban all commercial whaling, but some members now lobby to restore harvest of non-endangered species.

Lack of a market for whale meat may in the end cause the demise of commercial whaling.

54.5 How Can We Manage Populations?

Humans wish to decrease the size of populations of many pest species.

Reducing population numbers below carrying capacity stimulates higher birth rates and growth of the population.

A more effective approach is to remove the resources for the population, (e.g., making garbage unavailable for rats).

54.5 How Can We Manage Populations?

Humans introduce other species to control pests, such as the cactus moth to control Opuntia cacti in Australia.

Sometimes the introduced predator or parasite fails to control the pest; or worse, begins to attack other species.

54.5 How Can We Manage Populations?

Toads were introduced into Australia to control cane beetles in sugar cane fields.

The toads couldn’t reach the beetles high on the sugar cane plants, but have been an ecological disaster for other species.

They are poisonous, reproduce quickly, and outcompete native amphibians.

Figure 54.18 Biological Control Gone Awry

54.5 How Can We Manage Populations?

The size of the human population now contributes to most environmental problems.

Human social organization and specialization has allowed us to increase the carrying capacity for humans.

54.5 How Can We Manage Populations?

Earth’s current carrying capacity for humans

biosphere’s ability to absorb our by-products, especially CO2 from fossil fuels;

water availability

our willingness to cause extinction of other species to accommodate our increasing use of Earth’s resources.