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Community Ecology

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Title: Community Ecology


1
Community Ecology
  • Reading Freeman, Chapter 50, 53

2
What is a community?
  • A community is an assemblage of plant and animal
    populations that live in a particular area or
    habitat.
  • Populations of the various species in a community
    interact and form a system with its own emergent
    properties.

3
Pattern vs. Process
  • Pattern is what we can easily observe directly -
    vegetation zonation, species lists, seasonal
    distribution of activity, and association of
    certain species.
  • Process gives rise to the pattern- herbivory,
    competition, predation risk, nutrient
    availability, patterns of disturbance, energy
    flow, history, and evolution.

4
  • Community ecology seeks to explain the underlying
    mechanisms that create, maintain, and determine
    the fate of biological communities. Typically,
    patterns are documented by observation, and used
    to generate hypotheses about processes, which are
    tested.
  • Not all science is experimental. Hypotheses
    tests can involve special observations, or
    experiments.

5
Emergent Properties of a Community
  • Scale
  • Spatial and Temporal Structure
  • Species Richness
  • Species Diversity
  • Trophic structure
  • Succession and Disturbance

6
  • Scale is the size of a community.
  • Provided that the area or habitat is well
    defined, a community can be a system of almost
    any size, from a drop of water, to a rotting log,
    to a forest, to the surface of the Pacific Ocean.

7
  • Spatial Structure is the way species are
    distributed relative to each other.
  • Some species provide a framework that creates
    habitats for other species. These species, in
    turn create habitats for others, etc.

8
  • Example Trees in a rainforest are stratified
    into several different levels, including a
    canopy, several understories, a ground level, and
    roots. Each level is the habitat of a distinct
    collection of species. Some places, such as the
    pools of water that collect at the base of tree
    branches, may harbor entire communities of their
    own.

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10
  • Temporal structure is the timing of the
    appearance and activity of species. Some
    communities, i.e., arctic tundra and the decay of
    a corpse, have pronounced temporal species, other
    communities have less.
  • Example Many desert plants and animals are
    dormant most of the year. They emerge, or
    germinate, in response to seasonal rains. Other
    plants stick around year round, having evolved
    adaptations to resist drought.

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12
  • Species Richness - is the number of species in a
    community. Clearly, the number of species we can
    observe is function of the area of the sample. It
    also is a function of who is looking. Thus,
    species richness is sensitive to sampling
    procedure

13
  • Diversity is the number of species in the
    community, and their relative abundances.
  • Species are not equally abundant, some species
    occur in large percentage of samples, others are
    poorly represented.
  • Some communities, such as tropical rainforests,
    are much more diverse than others, such as the
    great basin desert.
  • Species Diversity is often expressed using
    Simpsons diversity index D1-S (pi)2

14
Example Problem
  • A community contains the following species
  • Number of Individuals
  • Species A 104
  • Species B 71
  • Species C 19
  • Species D 5
  • Species E 3
  • What is the Simpson index value for this
    community?

15
Answer
  • Total Individuals (104197153)202
  • PA104/202.51 PB19/202.09
  • PC71/202.35 PD5/202.03PE3/202.02
  • D1-(.51)2(.09)2(.35)2(.03)2(.02)2
  • D1-.40.60

16
Clicker Question
  • In the example above, what was the species
    richness?
  • A. .60
  • B. 202 individuals
  • C. 5 species
  • D. .40
  • E. None of the above

17
Succession, Disturbance and Change
  • In terms of species and physical structure,
    communities change with time.
  • Ecological succession, the predictable change in
    species over time, as each new set of species
    modifies the environment to enable the
    establishment of other species, is virtually
    ubiquitous.

18
  • Example a sphagnum bog community may persist for
    only a few decades before the process of
    ecological succession changes transform it into
    the surrounding Black Spruce Forest.
  • A forest fire may destroy a large area of trees,
    clearing the way for a meadow. Eventually, the
    trees take over and the meadow is replaced.

19
  • Disturbances are events such as floods, fire,
    droughts, overgrazing, and human activity that
    damage communities, remove organisms from them,
    and alter resource availability.

20
Some Agents of Disturbance
  • Fire
  • Floods
  • Drought
  • Large Herbivores
  • Storms
  • Volcanoes
  • Human Activity

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22
Disturbance, Invasion, Succession
  • Disturbance creates opportunities for new species
    to invade an area and establish themselves.
  • These species modify the environment, and create
    opportunities for other species to invade. The
    new species eventually displace the original
    ones. Eventually, they modify the environment
    enough to allow a new series of invaders, which
    ultimately replace them, etc.

23
  • Invasion
  • Disturbance creates an ecological vacuum that can
    be filled from within, from outside, or both.
    For example, forest fires clear away old brush
    and open up the canopy, releasing nutrients into
    the soil at the same time. Seeds that survive
    the fire germinate and rapidly grow to take
    advantage of this opportunity. At the same time,
    wind-borne and animal-dispersed seeds germinate
    and seek to do the same thing.
  • The best invaders have good dispersal powers and
    many offspring, but they are often not the best
    competitors in the long run.

24
Succession
  • Disturbance of a community is usually followed by
    recovery, called ecological succession.
  • The sequence of succession is driven by the
    interactions among dispersal, ecological
    tolerances, and competitive ability.
  • Primary succession-the sequence of species on
    newly exposed landforms that have not previously
    been influenced by a community, e.g., areas
    exposed by glacial retreat.
  • Secondary succession occurs in cases which
    vegetation of an area has been partially or
    completely removed, but where soil, seeds, and
    spores remain.

25
  • Early in succession, species are generally
    excellent dispersers and good at tolerating harsh
    environments, but not the best interspecific
    competitors.
  • As ecological succession progresses, they are
    replaced with species which are superior
    competitors, (but not as good at dispersing and
    more specialized to deal with the
    microenvironments created by other species likely
    to be present with them).
  • Early species modify their environment in such a
    way as to make it possible for the next round of
    species. These, in turn, make their own
    replacement by superior competitors possible.

26
  • A climax community is a more or less permanent
    and final stage of a particular succession, often
    characteristic of a restricted area.
  • Climax communities are characterized by slow
    rates of change, compared with more dynamic,
    earlier stages.
  • They are dominated by species tolerant of
    competition for resources.

27
  • An Influential ecologist named F.E. Clements
    argued that communities work like an integrated
    machine. These closed communities had a
    predictable composition.
  • According to Clements, there was only one true
    climax in any given climatic region, which was
    the endpoint of all successions.
  • Other influential ecologists, including Gleason,
    hypothesized that random events determined the
    composition of communities.
  • He recognized that a single climatic area could
    contain a variety of specific climax types.

28
  • Evidence suggests that for many habitats, Gleason
    was right, many habitats never return to their
    original state after being disturbed beyond a
    certain point.
  • For example very severe forest fires have
    reduced spruce woodlands to a terrain of rocks,
    shrubs and forbs.

29
  • An incredibly rapid glacial retreat is occurring
    in Glacier Bay, Alaska. In just 200 years, a
    glacier that once filled the entire bay has
    retreated over 100km, exposing new landforms to
    primary succession.
  • Clements would have predicted that succession
    today would follow the sequence of ecological
    succession that has occurred in the past for
    other parts of Alaska.
  • In fact, three different successional patterns
    seem to be occurring at once, depending upon
    local conditions. Thus, Clements view of
    succession is somewhat of an oversimplification.

30
Are Climax Communities Real?
  • Succession can take a long time.
  • For example, old-field succession may require
    100-300 years to reach climax community. But in
    this time frame, the probability that a physical
    disturbance (fire, hurricane, flood) will occur
    becomes so high, the process of succession may
    never reach completion.

31
  • Increasing evidence suggests that some amount of
    disturbance and nonequilibrium resulting from
    disturbance is the norm for most communities.
  • One popular hypothesis is that communities are
    usually in a state of recovery from disturbance.
  • An area of habitat may form a patchwork of
    communities, each at different stages of
    ecological succession. Thus, disturbance and
    recovery potentially enable much greater
    biodiversity than is possible without disturbance.

32
Are biological communities real functional units?
  • Do communities have a tightly prescribed
    organization and composition, or are they merely
    a loose assemblage of species?
  • This is an unsolved problem in ecology.
  • Clements argued that communities are stable,
    functional units with a fixed composition-each
    integrated part needs the others. Every area
    should ultimately have the same species, given
    time.
  • Gleason argued that their composition is unstable
    and variable-they are more like assemblages of
    everything that can live together in one place

33
The Kiddie Pool Experiment
  • Jenkins and Buikema conducted an experiment to
    see whether artificial ponds would develop
    predictable assemblages of freshwater
    microorganisms.
  • -if this were the case, it would support the
    notion that communities are real, integrated
    units.
  • -They set up 12 identical ponds and filled them
    with sterile water. Came back in year to study
    the composition of the resulting communities.

34
  • Result-the ponds had very different compositions
    of species.
  • Accidents of dispersal, and different dispersal
    capabilities affected which species ended up in
    each pond.
  • The early arrival of certain competitors, and
    predators greatly affected the ability of later
    species to colonize later.
  • -Gleasons view was supported. Composition of
    communities is dictated largely by chance and
    history.

35
  • Trophic structure is the hierarchy of feeding.
    It describes who eats whom
  • (a trophic interaction is a transfer of energy
    i.e., eating, decomposing, obtaining energy via
    photosynthesis).
  • For every community, a diagram of trophic
    interactions called a food web.
  • Energy flows from the bottom to the top.

36
A Simple Food Web
Killer Whales
Sharks Harbor Seals
Yellowfin Tuna Mackerel Cod
Halibut

Zooplankton Unicellular Algae and Diatoms
37
Killer Whales Harbor Seals Mackerel Zooplankton
Phytoplankton
One path through a food web is a food chain.
38
  • The niche concept is very important in community
    ecology.
  • A niche is an organisms habitat and its way of
    making a living.
  • An organisms niche is reflected by its place in
    a food web i.e, what it eats, what it competes
    with, what eats it.
  • Each organism has the potential to create niches
    for others.

39
  • Keystone species are disproportionately important
    in communities.
  • Generally, keystone species act to maintain
    species diversity.
  • The extinction of a keystone species eliminates
    the niches of many other species.
  • Frequently, a keystone species modifies the
    environment in such a way that other organisms
    are able to live, in other cases, the keystone
    species is a predator that maintains diversity at
    a certain trophic level.

40
Examples of Keystone Species
  • California Sea Otters This species preys upon
    sea urchins, allowing kelp forests to become
    established.
  • Pisaster Starfish Grazing by Pisaster prevents
    the establishment of dense mussel beds, allowing
    other species to colonize rocks on the pacific
    coast
  • Mangrove trees Actually, many species of
    trees are called mangrove trees. Their seeds
    disperse in salt water. They take root and form
    a dense forest in saltwater shallows, allowing
    other species to thrive

41
Trophic Cascades
  • Species at one trophic level influence species at
    other levels the addition or subtraction of
    species affects the entire food web.
  • This causes positive effects for some species,
    and negative effects for others. This is called
    a trophic cascade. For instance, removing a
    secondary consumer might positively affect the
    primary consumers they feed upon, and negatively
    affect the producers that are food for primary
    consumers.

42
Top down vs. Bottom up
  • Most biological communities have both top-down
    and bottom-up effects on their structure and
    composition.
  • In a well known study of ponds by Matthew
    Leibold, it was demonstrated that the biomass of
    herbivores (zooplankton) was positively
    correlated to the biomass of producers (algae),
    indicating a top down effect.
  • He intentionally introduced fish to some ponds,
    The result was a decrease in zooplankton and
    increase in producers, indicating a top down
    effect.

43
Badly scanned from Rose and Mueller (2006)
44
Types of Interspecific Interactions
  • Effect on
    Effect on
  • Species 1
    Species 2
  • Neutralism 0 0
  • Competition - -
  • Commensalism 0
  • Amensalism - 0
  • Mutualism
  • Predation, -
  • Parasitism, Herbivory

45
Neutralism
  • Neutralism the most common type of interspecific
    interaction. Neither population affects the
    other. Any interactions that do occur are
    indirect or incidental.
  • Example the tarantulas living in a desert and
    the cacti living in a desert

46
Competition
  • Competition occurs when organisms in the same
    community seek the same limiting resource. This
    resource may be prey, water, light, nutrients,
    nest sites, etc.
  • Competition among members of the same species is
    intraspecific.
  • Competition among individuals of different
    species is interspecific.
  • Individuals experience both types of competition,
    but the relative importance of the two types of
    competition varies from population to population
    and species to species

47
Styles of Competition
  • Exploitation competition occurs when individuals
    use the same limiting resource or resources, thus
    depleting the amount available to others.
  • Interference competition occurs when individuals
    interfere with the foraging, survival, or
    reproduction of others, or directly prevent their
    physical establishment in a portion of a habitat.

48
Some specific types of competition
  • Consumptive competition
  • Preemptive competition
  • Overgrowth competition
  • Chemical composition
  • Territorial competition
  • Encounter competition

49
Example of Interference Competition
  • The confused flour beetle, Triboleum confusum,
    and the red flour beetle, Triboleum castaneum
    cannibalize the eggs of their own species as well
    as the other, thus interfering with the survival
    of potential competitors.
  • In mixed species cultures, one species always
    excludes the other. Which species prevails
    depends upon environmental conditions, chance,
    and the relative numbers of each species at the
    start of the experiment.

50
Outcomes of Competition
  • Exploitation competition may cause the exclusion
    of one species. For this to occur, one organism
    must require less of the limiting resource to
    survive. The dominant species must also reduce
    the quantity of the resource below some critical
    level where the other species is unable to
    replace its numbers by reproduction.
  • Exploitation does not always cause the exclusion
    of one species. They may coexist, with a decrease
    in their potential for growth. For this to
    occur, they must partition the resource.
  • Interference competition generally results in the
    exclusion of one of the two competitors.

51
The Competitive Exclusion Principle
  • Early in the twentieth century, two mathematical
    biologists, A.J. Lotka and V. Volterra developed
    a model of population growth to predict the
    outcome of competition.
  • Their models suggest that two species cannot
    compete for the same limiting resource for long.
    Even a minute reproductive advantage leads to the
    replacement of one species by the other.
  • This is called the competitive exclusion
    principal.

52
Evidence for Competitive Exclusion.
  • A famous experiment by the Russian ecologist,
    G.F. Gausse demonstrated that Paramecium aurellia
    outcompetes and displaces Paramecium caudatum in
    mixed laboratory cultures, apparently confirming
    the principle.
  • (Interestingly, this is not always the case.
    Later studies suggest that the particular strains
    involved affect the outcome of this interaction).

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54
Other experiments...
  • Subsequent laboratory studies on other organisms,
    have generally resulted in competitive
    exclusion, provided that the environment was
    simple enough.
  • Example Thomas Park showed that, via
    interference competition, the confused flour
    beetle and the red flower beetle would not
    coexist. One species always excluded the other.

55
Resource Partitioning
  • Species that share the same habitat and have
    similar needs frequently use resources in
    somewhat different ways - so that they do not
    come into direct competition for at least part of
    the limiting resource. This is called resource
    partitioning.

56
  • Resource partitioning obviates competitive
    exclusion, allowing the coexistence of several
    species using the same limiting resource.
  • Resource partitioning could be an evolutionary
    response to interspecific competition, or it
    could simply be that competitive exclusion
    eliminates all situations where resource
    partitioning does not occur.

57
  • One of the best known cases of resource
    partitioning occurs among Caribbean anoles.
  • As many as five different species of anoles may
    exist in the same forest, but each stays
    restricted to a particular space some occupy
    tree canopies, some occupy trunks, some forage
    close to the ground.
  • When the brown anole was introduced to Florida
    from Cuba, it excluded the green anole from the
    trunks of trees and areas near the ground the
    green anole is now restricted to the canopies of
    treesthe resource (space, insects) has been
    partitioned among the two species
  • (for now at least, this interaction may not be
    stable in the long run because the species eat
    each others young).

58
Character Displacement
  • Sympatric populations of similar species
    frequently have differences in body structure
    relative to allopatric populations of the same
    species.
  • This tendency is called character displacement.
  • Character displacement is thought to be an
    evolutionary response to interspecific
    competition.

59
Example of Character Displacement
  • The best known case of character displacement
    occurs between the finches, Geospiza fuliginosa
    and Geospiza fortis, on the Galapagos islands.
  • When the two species occur together, G.
    fuliginosa has a much narrower beak that G
    fortis. Sympatric populations of G fuliginosa
    eats smaller seeds than G fortis they partition
    the resource.
  • When found on separate islands, both species have
    beaks of intermediate size, and exploit a wider
    variety of seeds.
  • These inter-population differences might have
    evolved in response to interspecific competition.

60
Competition and the Niche
  • An ecological niche can be thought of in terms of
    competition.
  • The fundamental niche is the set of resources
    and habitats an organism could theoretically use
    under ideal conditions.
  • The realized niche is the set of resources and
    habitats an organism actually used it is
    generally much more restricted due to
    interspecific competition (or predation.)

61
Two organisms cannot occupy exactly the same
niche.
  • This is sometimes called Gausses rule(although
    Gausse never put it exactly that way).
  • -Experiments by Gausse (Paramecium), Peter Frank
    (Daphnia), and Thomas Park (Triboleum) have
    confirmed it for simple laboratory scenarios.
  • -This creates a bit of a paradox, because so many
    species exist in nature using the same resources.
  • -The more complex environments found in nature
    may enable more resource partitioning.

62
Amensalism
  • Amensalism is when one species suffers and the
    other interacting species experiences no effect.
  • Example Redwood trees falling into the ocean
    become floating battering-rams during storms,
    killing large numbers of mussels and other
    inter-tidal organisms.

63
  • Allelopathy involves the production and release
    of chemical substances by one species that
    inhibit the growth of another. These secondary
    substances are chemicals produced by plants that
    seen to have no direct use in metabolism.
  • This same interaction can be seen as both
    amensalism, and extremely one-sided interference
    competition-in fact it is both.

64
Example Allelopathy in the California Chaparral
  • Black Walnut (Juglans nigra) trees excrete an
    antibiotic called juglone. Juglone is known to
    inhibit the growth of trees, shrubs, grasses, and
    herbs found growing near black walnut trees.
  • Certain species of shrubs, notably Salvia
    leucophylla (mint) and Artemisia californica
    (sagebrush) are known to produce allelopathic
    substances that accumulate in the soil during the
    dry season. These substances inhibit the
    germination and growth of grasses and herbs in an
    area up to 1 to 2 meters from the secreting
    plants.

65
Commensalism
  • Commensalism is an interspecific interaction
    where one species benefits and the other is
    unaffected.
  • Commensalisms are ubiquitous in nature birds
    nesting in trees are commensal.
  • Commensal organisms frequently live in the nests,
    or on the bodies, of the other species.
  • Examples of Commensalism
  • Ant colonies harbor rove beetles as commensals.
    These beetles mimic the ants behavior, and pass
    as ants. They eat detritus and dead ants.
  • Anemonefish live within the tentacles of
    anemones. They have specialized mucus membranes
    that render them immune to the anemones stings.
    They gain protection by living in this way.

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Mutualism
  • Mutualism in an interspecific interaction between
    two species that benefits both members.
  • Populations of each species grow, survive and/or
    reproduce at a higher rate in the presence of the
    other species.
  • Mutualisms are widespread in nature, and occur
    among many different types of organisms.

68
Examples of Mutualism
  • Most rooting plants have mutualistic associations
    with fungal mychorrhizae. Mychorrhizae increase
    the capability of plant roots to absorb
    nutrients. In return, the host provides support
    and a supply of carbohydrates.
  • Many corals have endosymbiotic organisms called
    zooxanthellae (usually a dinoflagellate). These
    mutualists provide the corals with carbohydrates
    via photosynthesis. In return, they receive a
    relatively protected habitat from the body of the
    coral.

69
Mutualistic Symbiosis
  • Mutualistic Symbiosis is a type of mutualism in
    which individuals interact physically, or even
    live within the body of the other mutualist.
    Frequently, the relationship is essential for the
    survival of at least one member.
  • Example Lichens are a fungal-algal symbiosis
    (that frequently includes a third member, a
    cyanobacterium.) The mass of fungal hyphae
    provides a protected habitat for the algae, and
    takes up water and nutrients for the algae. In
    return, the algae (and cynaobacteria) provide
    carbohydrates as a source of energy for the
    fungus.

70
Facultative vs. Obligate Mutualisms
  • Facultative Mutualisms are not essential for the
    survival of either species. Individuals of each
    species engage in mutualism when the other
    species is present.
  • Obligate mutualisms are essential for the
    survival of one or both species.

71
Other Examples of Mutualisms
  • Flowering plants and pollinators. (both
    facultative and obligate)
  • Parasitoid wasps and polydna viruses. (obligate)
  • Ants and aphids. (facultative)
  • Termites and endosymbiotic protozoa. (obligate)
  • Humans and domestic animals. (mostly facultative,
    some obligate)

72
Predation, Parasitism, Herbivory
  • Predators, parasites, parasitoids, and herbivores
    obtain food at the expense of their hosts or
    prey.

73
  • Predators tend to be larger than their prey, and
    consume many prey during their lifetimes.

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75
  • Parasites and pathogens are smaller than their
    host. Parasites may have one or many hosts
    during their lifetime. Pathogens are parasitic
    microbes-many generations may live within the
    same host. Parasites consume their host either
    from the inside (endoparasites) or from the
    outside (ectoparasites).

76
  • Parasitoids hunt their prey like predators, but
    lay their eggs within the body of a host, where
    they develop like parasites.

77
  • Herbibores are animals that eat plants. This
    interaction may resemble predation, or parasitism.

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79
Predator-Prey and Parasite-Host Coevolution
  • The relationships between predator and prey, and
    parasites and hosts, have coevolved over long
    periods of time.

80
  • About 50 years ago, an evolutionary biologist
    named J.B.S. Haldane suggested that the
    interaction between parasite and host (or
    predator and prey) should resemble an
    evolutionary arms race
  • First a parasite (or predator) evolves a trait
    that allows it to attack its host (or prey).
  • Next, natural selection favors host individuals
    that are able to defend themselves against the
    new trait.
  • As the frequency of resistant host individuals
    increases, there is natural selection for
    parasites with novel traits to subvert the host
    defenses.
  • This process continues as long as both species
    survive.
  • Recent data on Plasmodium, the cause of malaria,
    support this model.

81
Example of Parasite-Host Coevolution
  • The common milkweed, Asclepias syriaca has leaves
    that contain cardiac glycosides they are very
    poisonous to most herbivores. This renders them
    virtually immune to herbivory by most species.
  • Monarch butterfly larvae have evolved the ability
    to tolerate these toxins, and sequester them
    within their bodies. They are important
    specialist hervivores of milkweeds.
  • These sequestered compounds serve the additional
    purpose of making monarch larvae virtually
    inedible to vertebrate predators.

82
Predator-Prey Population Dynamics
  • Predation may be a density-dependent mortality
    factor to the host population-and prey may
    represent a limiting resource to predators.
  • The degree of prey mortality is a function of the
    density of the predator population.
  • The density of the prey population, in turn,
    affects the birth and death rates of the predator
    population.
  • i.e, when prey become particularly common,
    predators increase in numbers until prey die back
    due to increased predation, this, in turn,
    inhibits the growth of prey.
  • Typically, there is a time lag effect.

83
  • There is often a dynamic balance between
    predators and prey that is necessary for the
    stability of both populations.
  • Feedback mechanisms may control the densities of
    both species.

84
Example of Regulation of Host Population by a
Herbivore
  • In the 19th century, prickly pear cactus, Opuntia
    sp. was introduced into Australia from South
    America. Because no Australian predator species
    existed to control the population size of this
    cactus, it quickly expanded throughout millions
    of acres of grazing land.
  • The presence of the prickly pear cactus excluded
    cattle and sheep from grazing vegetation and
    caused a substantial economic hardship to
    farmers.
  • A method of control of the prickly pear cactus
    was initiated with the introduction of
    Cactoblastis cactorum, a cactus eating moth from
    Argentina, in 1925. By 1930, densities of the
    prickly pear cactus were significantly reduced.

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  • Sometimes predator species can drive their prey
    to localized extinction.
  • If there are no alternate prey, the predator then
    goes extinct.
  • If the environment is coarse grained, this makes
    the habitat available for recolonization by the
    prey species.
  • Example The parasitic wasp Dieratiella rapae is
    a very efficient parasitoid. One female can
    oviposit into several hundred aphids during its
    lifetime. Frequently, aphids are driven locally
    extinct and the adults must search for new
    patches when they emerge. Once the aphid and the
    host are gone, the host plants may become
    re-infested with aphids.

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  • In other cases, there are alternate prey to
    support the predator and the prey is permanently
    excluded.
  • Example Freshwater fish such as bluegills and
    yellow perch frequently exclude small
    invertebrates such as Daphnia pulex from ponds.
    The fish then switch to other prey such as
    insects larvae.

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The time-lag effect may lead to predator-prey
oscillations.
  • Most predators do not respond instantaneously to
    the availability of prey and adjust their
    reproduction accordingly.
  • If predator populations grow faster than prey
    populations, they may overshoot the number of
    prey that are able to support them
  • This leads to a rapid decline in the prey,
    followed by a rapid decline in the predator.
  • Once the predator becomes rare, the prey
    population may begin growing again.
  • This pattern is called a predator-prey
    oscillation.

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Cycles in the population dynamics of the snowshoe
hare and its predator the Canadian lynx (redrawn
from MacLulich 1937). Note that percent mortality
is an elusive measure, it may, or may not, be
useful since mortality varies with environment
and time.
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  • In the 1920s, A. J. Lotka (1925) and V. Volterra
    (1926) devised mathematical models representing
    host/prey interaction.
  • The Lotka-Volterra curve assumes that prey
    destruction is a function not only of natural
    enemy numbers, but also of prey density, i.e.,
    related to the chance of encounter.
  • This model predicts the predator-prey
    oscillations sometimes seen in nature.
    Populations of prey and predator were predicted
    to flucuate in a regular manner (Volterra termed
    this "the law of periodic cycle").
  • Lotka-Volterra model is an oversimplification of
    reality. In nature, many different factors
    affect the densities of predators and their prey.

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