Population Ecology

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Population Ecology
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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 How Can We Manage Populations?

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54.1 How Do Ecologists Study Populations?

• A population consists of 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.

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54.1 How Do Ecologists Study Populations?

• 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.

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54.1 How Do Ecologists Study Populations?

• Population structure changes over time due to
  demographic events births, deaths, immigration,
  and emigration.
• These events create population dynamics. Study of
  these events is called demography.

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54.1 How Do Ecologists Study Populations?

• Population ecologists measure number and density
  of individuals, rates of demographic events, and
  locations of individuals.
• Individuals are often tagged or marked in some
  way to facilitate research.
• Tracking devices are also used. They may provide
  additional physiological and environmental data.

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Figure 54.1 By Their Marks You May Know Them
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54.1 How Do Ecologists Study Populations?

• Molecular markers are also used.
• Hydrogen isotopes have been used to determine
  where American redstarts molt during their
  migrations.
• Hydrogen isotopes in feathers reflect the
  latitude at which the feathers grew, because
  there is a strong latitudinal gradient of these
  isotopes in precipitation.

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Figure 54.2 Hydrogen Isotopes Tell Where
Migratory American Redstarts Molted Their Feathers
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54.1 How Do Ecologists Study Populations?

• Population density of terrestrial animals is
  usually measured per unit area for aquatic
  animals number per unit volume is used.
• Sometimes total mass of individuals or percentage
  of ground covered is used for density.

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54.1 How Do Ecologists Study Populations?

• Counting every individual in a population is
  often not possible. Ecologists use statistical
  methods to estimate population size from
  representative samples.
• For sedentary organisms, individuals in
  representative habitats can be counted, and the
  numbers extrapolated to the whole ecosystem.

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54.1 How Do Ecologists Study Populations?

• Estimating numbers of mobile animals involves
  capture and marking of some individuals, then
  capturing another sample of individuals.
• Proportion of marked individuals in the new
  sample is used to estimate population size

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54.1 How Do Ecologists Study Populations?

• Estimates of population size using this method
  are accurate only if the 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.

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54.1 How Do Ecologists Study Populations?

• Population densities can be used to estimate
  rates of demographic events.
• If these counts are made over many time
  intervals, we can determine how population
  density changes over time.

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54.1 How Do Ecologists Study Populations?

• A life table can be constructed by tracking a
  group of individuals born at the same time a
  cohort.
• Numbers that are still alive at later dates
  (survivorship) are determined.
• Some life tables include fecundity number of
  offspring produced in a time interval.

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Table 54.1 Life Table of the 1978 Cohort of the
Cactus Finch on Isla Daphne (Part 1)
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Table 54.1 Life Table of the 1978 Cohort of the
Cactus Finch on Isla Daphne (Part 2)
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54.1 How Do Ecologists Study Populations?

• 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.

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54.1 How Do Ecologists Study Populations?

• The data can be plotted to show survivorship in
  relation to age.
• Survivorship curves fall into three different
  general patterns.

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Figure 54.3 Survivorship Curves
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54.1 How Do Ecologists Study Populations?

• Age distribution reveals information about recent
  births and deaths.
• Example human population of the U.S.
• In the post WWII baby boom (19471964) average
  family size increased from 2.5 to 3.8 children.
• Individuals born at this time still constitute a
  dominant age class.

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Figure 54.4 Age Distributions Change over Time
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54.2 How Do Ecological Conditions Affect Life
Histories?

• An organisms life history describes how it
  allocates time and energy among the various
  activities throughout its life.
• Life histories can vary dramatically.
• For example, some animals have a single offspring
  per reproductive episode, some have many. Some
  species, such as salmon and agave reproduce only
  once and then die.

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Figure 54.5 Big Bang Reproduction
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54.2 How Do Ecological Conditions Affect Life
Histories?

• Life history traits influence populations that
  humans would like to manage.
• Black rockfish females continue to grow
  throughout their lives and 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.

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Figure 54.6 An Oil Droplet Is an Energy Kick Start
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54.2 How Do Ecological Conditions Affect Life
Histories?

• Intensive fishing off Oregon from 19961999
  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.

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54.2 How Do Ecological Conditions Affect Life
Histories?

• Ecological interactions influence the evolution
  of life histories.
• Influence of predation was tested on guppies in
  Trinidad. Where predator fish are excluded (by
  waterfalls) the 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.

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54.3 What Factors Influence Population Densities?

• All populations have the potential for explosive
  growth.
• Even when per capita growth rate remains
  constant, as population size increases, number of
  new individuals added per time unit increases
  exponential growth.

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54.3 What Factors Influence Population Densities?

• The term ?N/?t is the rate of change of the
  population over time.
• r is the net reproductive rate.

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54.3 What Factors Influence Population Densities?

• The highest possible value for r is rmax or the
  intrinsic rate of increase.
• It can be expressed as

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54.3 What Factors Influence Population Densities?

• Over short time periods, populations may grow at
  rates close to rmax.
• Example northern elephant seals that were hunted
  to near extinction populations grew
  exponentially on some islands after hunting was
  stopped.

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Figure 54.7 Exponential Population Growth (Part 1)
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Figure 54.7 Exponential Population Growth (Part 2)
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54.3 What Factors Influence Population Densities?

• Real populations cannot maintain exponential
  growth for long.
• Environmental limits cause birth rates to
  decrease and death rates to increase.
• The environmental carrying capacity (K) is the
  number of any particular species that can be
  supported in an environment.

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54.3 What Factors Influence Population Densities?

• Carrying capacity is determined by availability
  of resources such as food or shelter, plus
  factors such as diseases and parasites, and
  social interactions.
• Growth of a population usually slows when it
  nears carrying capacity.
• A graph of population size over time forms an
  S-shaped curve, and is known as logistic growth.

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Figure 54.8 Logistic Population Growth
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54.3 What Factors Influence Population Densities?

• Logistic growth can be modeled by adding a term
  for carrying capacity to the equation for
  population growth
• Growth stops when N K.

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54.3 What Factors Influence Population Densities?

• Per capita birth rates and death rates are
  influenced by density-dependent factors
• As population density increases, food supplies
  may be depleted, reducing amount of food
  available to individuals.
• Predators may be attracted to high densities of
  prey, increasing death rate.
• Diseases can spread more easily.

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54.3 What Factors Influence Population Densities?

• Other factors that influence populations are
  density-independent, such as weather-related
  phenomena.

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54.3 What Factors Influence Population Densities?

• Population density was studied in song sparrows
  on Mandarte Island off British Columbia, over 12
  years.
• Population size fluctuated significantly. Death
  rates were high during cold, snowy winters,
  regardless of population density.

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54.3 What Factors Influence Population Densities?

• Density-dependent factors were also important
• Number of breeding males was limited by
  territorial behavior.
• The more breeding females there were, the fewer
  offspring each one fledged.
• The more birds alive in autumn, the less chance
  that juveniles would survive the winter.

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Figure 54.9 Regulation of an Island Population of
Song Sparrows (Part 1)
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Figure 54.9 Regulation of an Island Population of
Song Sparrows (Part 2)
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Figure 54.9 Regulation of an Island Population of
Song Sparrows (Part 3)
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54.3 What Factors Influence Population Densities?

• In general, more stable population numbers are
  seen in species with long-lived individuals and
  low reproductive rates.
• For example, insect populations tend to fluctuate
  more than those of birds and mammals.
• Environmental factors can change carrying
  capacity for species.

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54.3 What Factors Influence Population Densities?

• Some years can be better for reproduction than
  others, and population density increases.
• Example whitefish in Lake Erie had a good year
  in 1944, and that cohort dominated for several
  years afterward.
• In a Wisconsin forest study in 1971, most black
  cherry trees had become established between 1931
  and 1941.

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Figure 54.10 Individuals Born during Years of
Good Reproduction May Dominate Populations (1)
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Figure 54.10 Individuals Born during Years of
Good Reproduction May Dominate Populations (2)
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54.3 What Factors Influence Population Densities?

• Population densities of species that depend on
  one resource are likely to fluctuate.
• Example in boreal forests, many birds and
  mammals eat conifer seeds. These trees reproduce
  synchronously and episodically.
• Mortality rates can be high in years with poor
  seed production.

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54.3 What Factors Influence Population Densities?

• Some species tend to be more common than others.
  Four factors have strong influence on the
  variation of population density among species
• Resource abundance
• Size of individuals
• Length of time a species has lived in an area
• Social organization

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54.3 What Factors Influence Population Densities?

• Species that use abundant resources generally
  reach higher population densities than those
  using scarce resources.
• Animals that eat plants are generally more
  abundant than animals that eat other animals.

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54.3 What Factors Influence Population Densities?

• Species with small body size generally reach
  higher population densities.
• Small individuals require less energy to survive
  than large ones.
• This is illustrated by mammal species worldwide.

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Figure 54.11 Population Density Decreases as Body
Size Increases
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54.3 What Factors Influence Population Densities?

• Species that are introduced to a new region where
  their normal pathogens and predators are absent
  can reach very high population densities.
• Example zebra mussels were introduced to the
  Great Lakes in 1985. They spread rapidly and
  reached densities much higher than in their
  native Europe.

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Figure 54.12 Introduced Zebra Mussels Have Spread
Rapidly
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54.3 What Factors Influence Population Densities?

• Complex social organization can lead to high
  population densities.
• Highly social species, such as ants, termites,
  and humans, can achieve very high densities.

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54.3 What Factors Influence Population Densities?

• The four factors do not explain all differences
  in species abundance.
• Douglas firs and giant sequoias have similar
  requirements, but Douglas firs are widespread and
  abundant, while giant sequoias are restricted to
  a few groves in the southern Sierra Nevada.
• Several desert pupfish species are found only in
  one spring in Death Valley.

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Figure 54.13 The Last Refuge
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54.3 What Factors Influence Population Densities?

• Species newly formed by polyploidy or founder
  events tend to have limited ranges and small
  population sizes.
• Species that are declining towards extinction,
  such as the giant sequoia, have shrinking ranges.
• Species that arise through a vicariant event
  typically have large population sizes.

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54.4 How Do Spatially Variable Environments
Influence Population Dynamics?

• Most populations are divided into separated
  subpopulations that live in habitat patches.
• The larger population to which the subpopulations
  belong is called the metapopulation.

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54.4 How Do Spatially Variable Environments
Influence Population Dynamics?

• Subpopulations are smaller, and more likely to
  undergo extinction from random environmental
  fluctuations.
• If individuals move frequently between
  subpopulations, it may prevent a declining
  population from going extinct the rescue effect.

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54.4 How Do Spatially Variable Environments
Influence Population Dynamics?

• Example bay checkerspot butterfly.
• Caterpillars feed on a few species of plants
  which grow on serpentine rock.
• In drought years the host plants die early,
  causing extinction of butterfly populations.
• The largest patch of suitable habitat was the
  source of individuals that recolonized the
  smaller patches.

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Figure 54.14 Metapopulation Dynamics
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54.4 How Do Spatially Variable Environments
Influence Population Dynamics?

• Metapopulation dynamics have also been studied in
  springtails.
• Habitat patches on rocks were manipulated. Small
  populations were more likely to go extinct than
  large ones.
• When habitat patches were connected by corridors,
  more species were able to maintain populations.

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Figure 54.15 Narrow Barriers Suffice to Separate
Arthropod Subpopulations (Part 1)
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Figure 54.15 Narrow Barriers Suffice to Separate
Arthropod Subpopulations (Part 2)
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54.4 How Do Spatially Variable Environments
Influence Population Dynamics?

• In migratory species, distant events in breeding
  or wintering ranges can be important, as well as
  stopover sites on migration routes.
• Example populations of three migratory bird
  species in a wood in England. Two species
  increased while one declined.

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Figure 54.16 Populations May Be Influenced by
Remote Events
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54.4 How Do Spatially Variable Environments
Influence Population Dynamics?

• Wood pigeons increased because of widespread
  adoption of canola as a crop provides abundant
  winter food.
• Garden warblers decreased because of drought in
  the wintering grounds in West Africa.
• The wood was no longer being cut for timber, and
  more holes were available for nesting of blue
  tits.

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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.

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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.

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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.

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Figure 54.17 Overharvesting Can Reduce Fish
Populations
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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.

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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.

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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).

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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.

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54.5 How Can We Manage Populations?

• Toads were introduced into Australia to control
  cane beetles in sugar cane fields.
• The toads couldnt 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.

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Figure 54.18 Biological Control Gone Awry
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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.

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54.5 How Can We Manage Populations?

• Earths current carrying capacity for humans is
  set in part by the biospheres ability to absorb
  our by-products, especially CO2 from fossil
  fuels also by water availability and our
  willingness to cause extinction of other species
  to accommodate our increasing use of Earths
  resources.