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Chapter 2
ANIMAL ECOLOGY
Chapter opener 02
Ecology
Definition
• Ecology
– Study of the relationship of organisms to their
environment
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Ecology
• Necessary to understand the physiological and
behavioral mechanisms of organisms to understand
their ecological relationships
• Animals in nature coexist with others of the same
species as reproductive units are called populations
– Population has properties that cannot be
discovered by studying individuals alone
• Populations of many species live together in complex
communities
• The number of different species present in a
community is measured as species diversity
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Ecology
• Many species interact with each other such as predation,
parasitism, competition
• Predators obtain energy and nutrients by killing and
eating prey
• Parasites drive similar benefits but do not kill the host
• Parasitoid is a parasite that kills its host organism
• Competition occurs when resources become limited
• Mutualism occurs when both members of a pair of
species benefit from their interactions, usually avoiding
negative interactions with other species
• Larger units or ecosystems allow study of the community
and the physical environment
• Biosphere is the land, water, and atmosphere that
envelops the planet and supports all life on earth
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Ecology
Environment and the Niche
• Abiotic factors (nonliving)
– Space, energy forms including sunlight, heat, wind and
water currents, and the soil, air, water and chemicals
• Biotic factors (living)
– Include other organisms as food, or competitors,
predators, hosts or parasites
• Resources
– Environmental factors that an animal uses directly
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Ecology
• Some resources are expendable
– Food, once eaten, is no longer available and must be
continuously replenished
• Space is not consumed by being used and is
therefore nonexpendable
• Habitat
– Physical space where an animal lives and is defined by the
animal’s normal activity
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Ecology
• Niche
– The life requirements of an organism define its niche
– A niche includes the animal’s limits of temperature,
moisture, food, and other factors
– Addition of important factors such as salinity or pH
describes a complex multidimensional niche
– The niche of a species undergoes evolutionary changes
over successive generations
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Ecology
• Some animals are generalists
– Wide niches
– Can tolerate a wide range of salinity or eat a wide range of
foods
• Other animals are specialists
– Have narrow dietary requirements or limited tolerance to
temperature changes, etc.
• Fundamental niche
– Describes animal’s potential role to live within a wider
range of conditions
• Realized niche
– The narrower subset of suitable environments that an
animal actually experiences
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Hierarchy of Ecology
Populations
• Population Factors
– A population is a reproductively interactive group of
animals of a single species
– A geographically and genetically cohesive population that
is separable from other such populations is a deme
• Share a gene pool
– Movement among demes provides some evolutionary
cohesion among species
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Ecology
– Local environments may change unpredictably
• Can cause a local deme to become depleted or
eliminated
– Immigration
• Source of replacement among demes within a
region
– Interaction among demes in this manner called
metapopulation dynamics
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Ecology
• Survivorship Curves define the age pattern of a
population from birth to death of the last member
– Type I Survivorship Curve
• All individuals die at the same time, occurs rarely in nature
– Type II Survivorship Curve
• Rate of mortality as a proportion of survivors is constant across ages
– Characteristic of birds that care for their young
– Humans fall between Type I and Type II depending on nutrition and
medical care
– Type III Survivorship Curve
• Represents many species that produce huge numbers of young but
experience rapid and sustained mortality
• Explains the need for high reproductive output of many animals
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Ecology
• Semelparity is the condition in which an organism reproduces
only once during its life history
• Iteroparity is the condition in which an organism reproduces
mutiple cohorts of offsprings that may mature and reproduce
while their parents are still alive and reproductively active
• Populations of animals containing multiple coherts such as
robins, box turtles, and humans exhibit age structure
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Ecology
• Population growth
– Difference between rates of birth and death
• Populations have the ability to grow exponentially at the
intrinsic rate of increase (r)
• Forms a steeply rising curve
• Unrestricted growth is not prevalent in nature
• Growing population eventually exhausts food or space
– Planktonic blooms and locusts outbreaks both end when
resources are exhausted
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Ecology
• Among many resources, one will be depleted first
– Limiting resource
• The largest population that the limiting resource can
support is the carrying capacity (K)
• A population slows growth rate in response to
diminishing resources
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Ecology
• Carrying Capacity of the Human Species
– Humans have the longest record of exponential population
growth
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About 5 million around 8000 BC before the Agricultural Revolution
Human population rose to 500 million by 1650
1 billion by 1850
2 billion by 1927
4 billion by 1974
6 billion by October 1999
Will reach 8.9 billion by 2040
– Food production cannot keep pace with exponential
population growth indefinitely
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Ecology
• Extrinsic Limits to Growth
– Density-independent Factors
• Abiotic factors reduce populations by floods, fires,
storms and severe climate fluctuations
• These agents kill young or other members of a
population regardless of the size of population
• Cannot truly regulate population growth because they
are unrelated to population size
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Ecology
– Density-dependent Factors
• Biotic factors that respond to density of the population
– Include predators and parasites
• As a population increases in number and individuals live
closer together
– Effects of parasites and disease are more severe
• Competition between species for a common limiting
resource
– Lowers the effective carrying capacity for each
species
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Community Ecology
Interactions Among Populations in
Communities
• In a community, populations of different species interact
• The number of species that share a habitat
– Known as species diversity
• Species interactions may benefit or harm the species involved
• In a predator-prey interaction
– Predator benefits and the prey is harmed
• Parasitism benefits the parasite and harms the host
• Herbivory benefits the animal and harms the plant
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Community Ecology
• In commensalism
– One species benefits and the other neither benefits nor is
harmed
• Some mutualistic relationships become obligatory
mutualism
– Neither can survive without the other
• Competition between two species
– Reduces the fitness of both
– Asymmetric competition (amensalism) affects one species
less than the other
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Community Ecology
Competition and Character Displacement
• Competition occurs when two or more species share a
limiting resource
• If resource is not in short supply
– Sharing the resource does not demonstrate competition
• Niche overlap
– Portion of the niche’s resources that are shared by two or
more species
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Community Ecology
• Competitive exclusion
– Principle that no two species will occupy the same niche
for a long time
– Eventually one will exclude the other
• To coexist, two species can specialize by partitioning
a shared resource
• Specialization involves character displacement
– Differences in organismal morphology or behavior related
to exploitation of a resource
– They do not compete with each other directly
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Community Ecology
• Where two species coexisted
– Competition between them led to evolutionary
displacement to diminish the competition
• When two or more species reduce niche overlap to
share the same general resources, they form a guild
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Community Ecology
Predators and Parasites
• Many animals and plants are in co-evolutionary
relationships
– Each in a race with the other
• If a predator relies primarily on a single prey species
– Populations tend to fluctuate cyclically with each other
• Predator-prey relationship
– Led to development of mimicry
• Harmless species mimic models that have toxins or
stings
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Community Ecology
• Another mimicry complex consists of many different
species, all with noxious or toxic factors, that evolve
to resemble each other
• A keystone species is so pervasive to a community
that its absence drastically changes the entire
community
• Keystone species reduce competition and allow more
species to coexist on the same resource
• Periodic natural disturbances also allow more species
to coexist in diverse communities
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Community Ecology
• Ectoparasites
– Host provides nutrition from its body and aids dispersal of
parasite
• Endoparasites
– Lost ability to choose habitats
– Must have tremendous reproductive output to ensure
offspring will reach another host
• Coevolution between parasite and host is expected to
generate an increasingly benign, less virulent
relationship if host organisms are uncommon and/or
difficult for a parasite to infest
– Death of the host also ends or shortens a parasite’s life
– Exceptions occur when alternative hosts are available
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Ecosystems
Trophic Levels
• Primary producers
– green plants or algae, fix and store energy from sunlight
• Herbivores
– First level of consumers that eat plants
• Carnivores
– Feed on herbivores or other carnivores
• Decomposers
– Mainly bacteria and fungi
– Break dead organic matter into mineral components for
reuse by plants to start the cycle over again
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Ecosystems
Energy Flow and Productivity
• Every organism has an energy budget and must
obtain enough energy to grow, reproduce, etc.
• Gross productivity (Pg)
– Total energy assimilated or taken in
– Some used to maintain metabolism
• Net productivity (Pn)
– Energy stored in the animal’s tissue as biomass
– Available for growth of the animal and for reproduction
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Ecosystems
• Energy is limited and can be represented as
Pn = Pg - R where R is respiration
• Energy budget of every animal is finite
• Much energy is lost when it is transferred between
trophic levels in food webs
• More than 90% of the energy in an animal’s food is
lost as heat
• Less than 10% is stored as biomass
• Each trophic level contains only 10% of the energy of
trophic level below it
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Ecosystems
• Ecological Pyramids
– Eltonian pyramid
• Based on numbers of organisms at each trophic level
• Does not indicate mass of organisms at each level
– Pyramid of biomass
• Total bulk or “standing crop” of organisms at each trophic
level
• Energy pyramids
– Depicts rate of energy flow between levels
– Never inverted
• Amount of energy transferred from each level is less than what
entered it
– Gives best overall picture of community structure because it is
based on production
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Ecosystems
Life Without the Sun
• From 1977 to 1979, dense communities were first
discovered on sea floor thermal vents
• Producers in the vent communities are
chemoautotrophic bacteria that oxidize hydrogen
sulfide
• Tubeworms and bivalve molluscs form trophic
communities that rely on this non-photosynthetic
source of nutrients
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Ecosystems
Nutrient Cycles
• Decomposers feed on remains of animals and plants
and on fecal material and return substances to the
ecosystem
• Biogeochemical cycles involve exchanges between
living organisms, rocks, air and water
• Continuous input of energy from sun keeps nutrients
flowing and the ecosystem functioning
• Synthetic compounds challenge nature’s nutrient
cycling
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Ecosystems
• Pesticides in food webs
– May be concentrated as they travel up through succeeding
trophic levels
– Kills non-target species
– Some chemicals remain in the environment for long
periods of time
• Genetic engineering of crop plants aims to improve
resistance to pests and lessen the need for chemical
pesticides
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Extinction and Biodiversity
Biodiversity
• Rates of speciation slightly exceed rates of extinction
• Approximately 99% of all species that have ever lived
are extinct
• Extinction events killing at least 5% of existing
species have occurred almost continuously
throughout geological time
• Human activity clearly has induced numerous species
extinctions
• Humans must avoid making the present time rival
the great extinction crises of earth’s geological
history
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