Population Ecology

1
Population Ecology
2
Population Ecology

• Populations in Space and Time
• Types of Ecological Interactions
• Fluctuations in Population Densities
• Population Fluctuations
• Variations in Species Ranges
• Managing Populations
• Regional and Global Processes Influence Local
  Population Dynamics

3
Populations in Space and Time

• The individuals of a species with a given area
  constitute a population.
• The distribution of the ages of individuals in a
  population and the way those individuals are
  distributed over the environment describe the
  population structure.
• Ecologists study population structure at
  different spatial scales, ranging from local
  subpopulations to entire species.
• The number of individuals of a species per unit
  of area (or volume) is its population density.

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Populations in Space and Time

• Ecologists are interested in population densities
  because dense populations often exert strong
  influences on their own members as well as on
  populations of other species.
• Density of terrestrial organisms is measured as
  number of individuals per unit area.
• Density of aquatic organisms is measured as
  individuals per unit volume.
• For some species such as plants, the biomass or
  percentage of ground covered may be a more useful
  measure of density than the number of individuals.

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Populations in Space and Time

• The structure of a population changes continually
  because of demographic eventsbirths, deaths,
  immigration, and emigration.
• Population dynamics is the change in population
  density through time and space.
• Demography is the study of birth, death, and
  movement rates that give rise to population
  dynamics.

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Populations in Space and Time

• Population dynamics can be represented by
• N1 N0 B D I E
• N1 number of individuals at time 1
• N0 number of individuals at time 0
• B number of individuals born between time 0 and
  time 1
• D number of individuals that died between time
  0 and time 1
• I number of individuals that immigrated
• E number of individuals that emigrated

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Populations in Space and Time

• Life table information can be used to predict
  future trends in populations.
• A cohort is a group of individuals that were born
  at the same time.
• A life table can be constructed by determining
  the number of individuals in a cohort that are
  still alive at specific times (the survivorship)
  and the number of offspring they produced in each
  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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Populations in Space and Time

• The life table for a cohort of the cactus finch
  on Isla Daphne in the Galápagos archipelago shows
  that mortality rates were initially high, leveled
  off, and again increased as the birds aged.
• Mortality rate also fluctuated through the years
  because survival depends upon seed production,
  and seed production is correlated with rainfall.

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Populations in Space and Time

• Survivorship curves in many populations fall into
  one of three patterns.
• In some populations (e.g., humans in the U.S.),
  most individuals survive for most of their
  potential life span and die at about the same
  age.
• In some (e.g., songbirds), the probability of
  surviving over the life span is the same once
  individuals are a few months old.
• In species that produce a large number of
  offspring and provide little parental care, high
  death rates for the young are followed by high
  survival rates during the middle of the life
  span.

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Figure 54.1 Survivorship Curves (Part 1)
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Figure 54.1 Survivorship Curves (Part 2)
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Populations in Space and Time

• The age distribution of individuals in a
  population reveals much about the recent history
  of births and deaths.
• For example, in the U.S., population size
  increased during the baby boom of the 1950s and
  again during the baby boom echo of the 1980s.
• Life tables can help us to understand why
  population densities change over time and to
  determine which groups should be the focus of
  efforts to save rare species.

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Figure 54.2 Age Distributions Change over Time
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Types of Ecological Interactions

• Species interactions fall into several
  categories.
• If both participants benefit from an interaction,
  the interaction is a mutualism (/ interaction).
• An example of mutualism is the association
  between plants and soil fungi called mycorrhizae,
  or between plants and nitrogen-fixing bacteria.
• Corals gain most of their energy from
  photosynthetic protists. The protists get
  nutrients when the corals digest animals.
• Termites have protists in their gut that digest
  cellulose they provide the protists, in turn,
  with nutrients.

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Types of Ecological Interactions

• If one participant benefits but the other is
  unaffected, the interaction is a commensalism
  (/0 interaction).
• Cattle egrets forage for insects near large
  mammals, and the movements of the large animal
  flush out insects, which the birds eat. The
  mammal does not gain or lose anything from this
  interaction.

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Figure 54.3 Commensalism Benefits One Partner
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Types of Ecological Interactions

• If one participant is harmed but the other is
  unaffected, the interaction is an amensalism (0/
  interaction).
• Trees and branches falling from trees damage
  smaller plants beneath them this is an example
  of amensalism.

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Types of Ecological Interactions

• One organism may benefit itself while harming
  another organism these interactions are called
  predatorprey and parasitehost interactions (/
  interactions).
• If two organisms use the same resources and those
  resources are insufficient for their combined
  needs, they are in competition (/ interaction).

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Table 54.2 Types of Ecological Interactions
22
Factors Influencing Population Densities

• Species that use abundant resources often reach
  higher population densities than species that use
  scarce resources.
• Species with small individuals generally reach
  higher population densities than species with
  large individuals.
• This relationship can be demonstrated by a
  logarithmic plot of population density against
  body size for a variety of mammals worldwide.

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Figure 54.4 Population Density Decreases as Body
Size Increases
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Factors Influencing Population Densities

• Newly introduced species often reach high
  population densities.
• An example is species introduced into a region
  where their normal predators and diseases are
  absent.
• Zebra mussels whose larvae were carried from
  Europe in the ballast water of ships now occupy
  much of the Great Lakes and Mississippi River
  drainage.
• Complex social organizations (e.g., ants,
  termites, humans) may facilitate high densities.

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Figure 54.5 Introduced Zebra Mussels Have Spread
Rapidly
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Fluctuations in Population Densities

• If a single bacterium were allowed to grow and
  reproduce in an unlimited environment, explosive
  population growth would result.
• Within a month, the bacterial colony would weigh
  as much as the visible universe and would be
  expanding outward at the speed of light.
• But while populations do fluctuate in density,
  even the most dramatic fluctuations are less than
  what is theoretically possible.

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Fluctuations in Population Densities

• All populations have the potential for explosive
  growth because, as the number of individuals in
  the population increases, the number of new
  individuals added per unit of time accelerates,
  even if the rate per capita of population
  increase remains constant.
• If births and deaths occur continuously and at
  constant rates, a graph of the population size
  over time forms a J-shaped curve that describes a
  form of explosive growth called exponential
  growth.

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Figure 54.6 Exponential Population Growth (Part
1)
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Figure 54.6 Exponential Population Growth (Part
2)
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Fluctuations in Population Densities

• Exponential growth can be represented
  mathematically
• DN/Dt (b d)N
• DN the change in number of individuals
• Dt the change in time
• b the average per capita birth rate (includes
  immigrations)
• d the average per capita death rate (includes
  emigrations)

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Fluctuations in Population Densities

• The difference between per capita birth rate (b)
  and per capita death rate (d) is the net
  reproductive rate (r).
• When conditions are optimal, r is at its highest
  value (rmax), called the intrinsic rate of
  increase.
• rmax is characteristic for a species.
• The equation for population growth can be written
  
• D/Dt rmaxN

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Fluctuations in Population Densities

• For limited time periods, some populations may
  grow at rates close to rmax.
• Real populations do not grow exponentially for
  long because of environmental limitations.
• Environmental limitations include food, nest
  sites, shelter, disease, and predation.
• The carrying capacity of an environment (K) is
  the maximum number of individuals of a species it
  can support.
• Natural population growth more closely resembles
  an S-shaped curve.

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Figure 54.7 Logistic Population Growth
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Fluctuations in Population Densities

• The mathematical representation of this type of
  growth (logistic growth) is
• DN/Dt r(K N)/KN
• The equation for logistic growth indicates that
  the populations growth slows as it approaches
  its carrying capacity (K).
• Population growth stops when N K.

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Fluctuations in Population Densities

• Per capita birth and death rates usually
  fluctuate in response to population density that
  is, they are density-dependent.
• As a population increases in size, it may deplete
  its food supply, reducing the amount of food each
  individual gets. Poor nutrition may increase
  death rates and decrease birth rates.
• If predators are able to capture a larger
  proportion of the prey when prey density
  increases, the per capita death rate of the prey
  rises.
• Diseases, which may increase death rates, spread
  more easily in dense populations than in sparse
  populations.

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Fluctuations in Population Densities

• Factors that affect birth and death rates in a
  population independent of its density are said to
  be density-independent.
• For example, a severely cold winter may kill
  large numbers of a population regardless of its
  density.

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Fluctuations in Population Densities

• Fluctuations in population density are determined
  by all the factors acting on it.
• In a population of song sparrows, death rates are
  high during very cold winters regardless of
  population density (density-independent).
• However, the larger the number of breeding males
  (density-dependent), the larger the number that
  fail to gain territories and have little chance
  of reproducing.
• The larger the number of breeding females, the
  fewer offspring each female fledges. The more
  birds alive in the autumn, the poorer are the
  chances that juveniles born that year will
  survive the winter.

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Figure 54.8 Regulation of an Island Population
of Song Sparrows (Part 1)
39
Figure 54.8 Regulation of an Island Population
of Song Sparrows (Part 2)
40
Population Fluctuations

• A comparison between the cactus finch and the
  south polar skua shows that some populations
  fluctuate widely and others fluctuate remarkably
  little.
• Species with long-lived individuals that have low
  reproductive rates typically have more stable
  populations than species with short-lived
  individuals and high reproductive rates.
• Small, short-lived individuals generally are more
  vulnerable to environmental changes.

41
Figure 54.9 Population Sizes May Be Stable or
Highly Variable
42
Population Fluctuations

• Episodic reproduction can generate fluctuations.
• In Lake Erie, 1944 was such an excellent year for
  reproduction of whitefish that they dominated
  catches in the lake for several years.
• Most of the black cherry trees in a Wisconsin
  forest in 1971 had become established between 30
  and 40 years earlier.

43
Figure 54.10 Individuals Born During Years of
Good Reproduction May Dominate Populations (1)
44
Figure 54.10 Individuals Born During Years of
Good Reproduction May Dominate Populations (2)
45
Population Fluctuations

• Densities of populations that depend on limited
  resources fluctuate more than those that use a
  greater variety of resources.
• The cactus finch populations fluctuate with the
  annual production of seeds that they eat.
• Many northern coniferous trees reproduce
  synchronously and episodically. There are years
  of massive production and years with little seed
  production. Populations of birds and mammals
  that depend on the seeds fluctuate also.

46
Population Fluctuations

• Predatorprey interactions generate fluctuations
  because predator population growth lags behind
  growth in prey and the two populations oscillate.
• When prey is scarce, its predator is scarce.
• When prey becomes plentiful again, the predator
  population will increase in a staggered fashion.

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Population Fluctuations

• Changes in population density among small mammals
  and their predators living at high latitudes are
  the best-known examples of predatorprey
  interactions.
• Experiments with Canada lynx and snowshoe hares
  revealed that the oscillating cycle of their
  populations was driven by both predation and food
  supply for the hares.

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Figure 54.11 Hare and Lynx Populations Cycle in
Nature (Part 1)
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Figure 54.11 Hare and Lynx Populations Cycle in
Nature (Part 2)
50
Figure 54.12 Prey Population Cycles May Have
Multiple Causes (Part 1)
51
Figure 54.12 Prey Population Cycles May Have
Multiple Causes (Part 2)
52
Population Fluctuations

• Subpopulations are found when suitable habitat
  occurs in separated patches.
• Each subpopulation has a probability of birth
  (colonization) and death (extinction).
• Subpopulations are more prone to extinction since
  they are typically smaller than the population as
  a whole and more vulnerable to local
  disturbances.
• If individuals frequently move between
  subpopulations, immigrants may prevent declining
  subpopulations from becoming extinct, a process
  known as the rescue effect.

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Population Fluctuations

• The bay checkerspot butterfly provides an example
  of the dynamics of a divided population.
• The larvae of this butterfly feed on only a few
  species of annual plants in a small area of
  California the largest patch supports thousands
  of butterflies.
• During drought years, most plants die early in
  the spring, and several subpopulations on small
  patches become extinct.
• The largest patch then disperses individuals to
  recolonize the smaller patches.

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Figure 54.13 Subpopulation Dynamics
55
Population Fluctuations

• In experiments with springtails and mites,
  scientists created isolated patches of the
  animals habitat.
• The number of species present declined 40 (rarer
  species declined more than common ones), showing
  that small, isolated populations are more likely
  to become extinct than larger ones.

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Figure 54.14 Narrow Barriers Suffice to Separate
Arthropod Subpopulations (Part 1)
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Population Fluctuations

• In a second experiment, similar patches were
  connected by corridors of moss that were either
  intact or disrupted by a small barrier.
• Patches connected by unbroken corridors contained
  more species a year later than the discontinuous
  corridors, showing that even a small barrier was
  enough to reduce the rescue effect.

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Figure 54.14 Narrow Barriers Suffice to Separate
Arthropod Subpopulations (Part 2)
59
Variations in Species Ranges

• Factors contributing to variation in geographic
  ranges of species include speciation processes,
  dispersal abilities, and interactions with other
  species.
• Speciation processes influence range sizes
• A species that arises by polyploidy inevitably
  begins with a very small range.
• Species that arise through founder events also
  have small ranges.
• Species that arise via allopatric speciation
  begin with large ranges.
• As a species declines toward extinction, the
  range shrinks until it vanishes.

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Figure 54.15 The Last Refuge
61
Variations in Species Ranges

• Dispersal abilities restrict geographic ranges.
• As the experiments with arthropods in moss
  patches show, even small barriers may prevent
  some species from colonizing an area.
• Therefore, the absence of many species from an
  area may be due simply to failure to get there.

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Variations in Species Ranges

• Predators may eliminate their prey in some places
  but not in others.
• In ponds on islands in Lake Superior, chorus
  frogs are found in only some of the habitats that
  seem suitable for them.
• The tadpoles have three major predators
  salamander larvae, dragonfly nymphs, and dytiscid
  beetles.
• Experiments indicated that dragonfly nymphs were
  able to eat all sizes of tadpole and when these
  nymphs were present, the pond lacked tadpoles.

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Figure 54.16 Predators Exclude Prey from Some
Habitats (Part 1)
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Figure 54.16 Predators Exclude Prey from Some
Habitats (Part 2)
65
Variations in Species Ranges

• Competition may restrict species ranges.
• Two species of barnacles live on North Atlantic
  seashores, but as adults, one species lives
  higher in the intertidal zone than the other,
  with little overlap between the two (a phenomenon
  called intertidal zonation).
• If one of the species is removed experimentally,
  the vertical range of the other species becomes
  greater.
• The higher-zone barnacle outcompetes the other
  because it is more hardy when exposed to air in
  the lower zone, the other barnacle is able to
  smother or crush higher-zone intruders.

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Figure 54.17 Competition Restricts the
Intertidal Ranges of Barnacles
67
Variations in Species Ranges

• Plants and sessile animals compete for space
  mobile animals compete for food.
• In order to control scale insects in Southern
  California, a parasitic wasp species was
  introduced.
• The first wasp introduced failed to control the
  insect scales.
• Then a second wasp with a higher reproductive
  rate was introduced.
• The second wasp displaced the first wasp within a
  decade.

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Managing Populations

• A general principle of population dynamics is
  that the total number of births and the growth
  rates of individuals tend to be highest when a
  population is well below its carrying capacity.
• If we wish to maximize the number of individuals
  that can be harvested from a population, that
  population should be managed so that its
  population is far below its carrying capacity.
• Hunting seasons are established with this
  objective in mind.

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Managing Populations

• Populations with high reproductive capacities can
  sustain their growth despite a high rate of
  harvest.
• Fish are an example of a population with high
  reproductive capacity.
• Many fish populations can be harvested heavily
  for many years because only a modest number of
  females must survive to reproductive age to
  produce the eggs needed to maintain the
  population.
• However, any specieseven those with high
  reproductive capacitycan be overharvested.

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Managing Populations

• The whaling industry engaged in excessive
  harvests that almost caused the extinction of
  blue whales.
• Management of whale populations is difficult
  because they reproduce at a low rate.
• Since whales are distributed worldwide, their
  management is dependent on cooperative action by
  all whaling nations (which is difficult to
  achieve).

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Figure 54.18 Overexploitation of Whales (Part 1)
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Figure 54.18 Overexploitation of Whales (Part 2)
73
Managing Populations

• To reduce the size of populations of undesirable
  species, removal of resources is more effective
  than large-scale killing.
• By removing resources, the species will have a
  reduced carrying capacity and therefore lower
  numbers.
• Killing large numbers of the species would simply
  reduce them to a population size that grows more
  rapidly to reach its carrying capacity.
• Conversely, if a rare species is to be preserved,
  the most important step usually is to provide it
  with suitable habitat.

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Managing Populations

• Humans have introduced many species to new
  habitats outside their native ranges.
• Natural predators or environmental factors that
  keep the introduced species in check in its
  native surroundings are often absent, and
  population explosions can occur.
• Opuntia cactus was introduced into Australia and
  became a pest in grazing land. A moth whose
  larvae eat Opuntia was then introduced as a
  method of biological control.

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Figure 54.19 Biological Control of a Pest
76
Managing Populations

• For many thousands of years, Earths carrying
  capacity for humans was set at a low level by
  food and water supplies and by disease.
• The domestication of plants and animals, improved
  agriculture, mining, use of fossil fuels, and
  modern medicine have contributed to a staggering
  increase in human population.
• Earths carrying capacity is currently limited by
  its ability to absorb the by-products of fossil
  fuel consumption (especially CO2), by water
  availability, and by whether we are willing to
  cause the extinction of millions of other species
  to accommodate our use of Earths resources.

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Figure 54.20 Human Population Growth
78
Regional and Global ProcessesInfluence Local
Population Dynamics

• Local population dynamics are often influenced
  both by local events and by and remote events.
• From 1950 1980, populations of three species of
  birds in England changed dramatically.
• The population of wood pigeons increased because
  of the widespread cultivation of oilseed rape, a
  food source.
• Garden warblers declined to two pairs because of
  a severe drought in their wintering grounds in
  West Africa.
• The population of blue tits increased because of
  local events an end to the cutting of trees and
  therefore a greater availability of nesting sites.

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Figure 54.21 Populations May Be Influenced by
Remote Events