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Population Ecology. 54 Population Ecology. 54.1 How Do Ecologists Study Populations? 54.2 How Do Ecological Conditions Affect Life Histories? 54.3 What Factors Influence Population Densities? 54.4 How Do Spatially Variable Environments Influence Population Dynamics?

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54 population ecology
54 Population Ecology
  • 54.1 How Do Ecologists Study Populations?
  • 54.2 How Do Ecological Conditions Affect Life Histories?
  • 54.3 What Factors Influence Population Densities?
  • 54.4 How Do Spatially Variable Environments Influence Population Dynamics?
  • 54.5 HowCan We Manage Populations?
what s a population
What’s a population?

all the individuals of a species in a given area.

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

population density
Population Density?

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

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

slide6
demographic events: births, deaths, immigration, and emigration

Change the pop density

demography

factors population ecologists study
Factors population ecologists study:

# and density of individuals

Tagging, marking

rates of demographic events

locations of individuals

Tracking

physiological & environmental data

what are molecular marker examples useful

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

What are molecular marker examples? Useful?

H isotopes -- where American redstarts molt during migrations

Isotopes in feathers reflect the latitude at which the feathers grew

strong latitudinal gradient of these isotopes in precipitation

population estimates statistics to extrapolate
Population Estimates & Statistics to Extrapolate

Mobile animals: capture, mark, recapture method

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

accurate
Accurate?

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

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

life table
LIFE 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

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.

slide20

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

average family size increased from 2.5 to 3.8 children

what s a life history
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)

slide24

Figure 54.6 An Oil Droplet Is an Energy Kick Start

Why are wildlife

biologists interested?

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 abetter chance for survival.

slide26
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 research
Local 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.

photo 54 5 dense colony of purple sea urchins strongylocentrotus purpuratus
Photo 54.5 Dense colony of purple sea urchins (Strongylocentrotus purpuratus).
dn dt equation
dn dt equation...

ΔN/Δt = rate of change of pop over time

r = net reproductive rate

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

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
What determines K?

availability of resources

Food, shelter

Diseases and parasites

Social interactions

S-shaped curve = logistic growth

logistic growth model
Logistic growth model

Growth stops when N = K

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

song sparrows on mandarte island
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
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 8 adult chinook salmon is released into holding tank at a hatchery in british columbia
Photo 54.8 Adult Chinook salmon is released into holding tank at a hatchery in British Columbia.
general trend
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.

slide51

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

slide52

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

what if the pop dens depends on one resource
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
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

what about body size
What about body size?

small body size  higher pop densities

require less energy to survive vs larger

mammal species worldwide

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.

slide60

Figure 54.13 The Last Refuge

Giant sequoias are restricted to a few groves in the southern Sierra Nevada but Douglas firs are widespread and abundant

54 5 how can we manage populations
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 populations1
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 populations2
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.

54 5 how can we manage populations3
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 populations4
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 populations5
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 populations6
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 populations7
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.

54 5 how can we manage populations8
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 populations9
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.