Download Chapter 53 - Canyon ISD

Chapter 53
Community Ecology
Community
• Any assemblage of populations in an area or
habitat
• Has a set of properties defined by its species
composition, with a structure determined by
the interactions between species
• Species richness: the number of species a
community contains
Individualistic Hypothesis
• H.A. Gleason in 1900
• Depicted a plant community as a chance
assemblage of species found in an area simply
because they happen to have the same abiotic
requirements
– Temperature, rainfall, and soil type
• Emphasizes studying single species
Interactive Hypothesis
• F.E. Clements in 1900
• Saw a community as an assemblage of closely
linked species, locked into association by
mandatory biotic interactions that cause the
community to function as an integrated unit
– A superorganism
• Emphasizes entire assemblages of species as the
essential units for understanding the
interrelationships and distribution of organisms
Rivet Model
• Suggests that most of the species in a
community are associated tightly with other
species in a web of life
– Reducing or increasing the abundance of one
species in a community will affect many other
species
• Not all the rivets in an airplane wing are
required to hold the wing together, but if you
take out the rivets one by one it would cause a
problem
Redundancy Model
• Most of the species in a community are not
tightly associated with one another, and the
web of life is very loose
• An increase or decrease in one species has
little effect on others
• Species are redundant
– If one predator disappears, another predatory
species in the community will take its place
Interspecific Interactions
• Relationships between different species of a
community
– Competition
– Predation
– Mutualism
– Commensalism
Competition
• Competitive Exclusion Principle
– 1934- Gause studied effects on interspecific
competition in the lab with two species of
Paramecium.
– Cultured protists under stable conditions with
constant amount of food added every day
• When grown separately, each population grew rapidly and
leveled off at its K
• When cultured together, one had the competitive edge in
obtaining food and drove the other to extinction
– Two species that are similar enough that they
compete for the same limiting resources cannot
coexist in the same place
Ecological Niche
• Sum total of a species’ use of the biotic and
abiotic resources in its environment
• If an organisms’ habitat it its address, the
niche is that habitat plus the organism’s
occupation
– Where it lives, what ranges it tolerates, what time
of day it is active, what it eats
• Two species cannot coexist in a community if
their niches are identical
Resource Partitioning
• Two possible outcomes of competition:
– Less competitive species will be driven to local
extinction
– One species may evolve enough through natural
selection to use a different set of resources
• Resource Partitioning
– Ex: Warblers
Character Displacement
• The tendency for characteristics to be more
divergent in sympatric populations of two
species than in allopatric populations of the
same two species
• Ex: Galapagos finches beak
– Enables two species to avoid competition by
feeding on seeds of different sizes and probably
represents an evolutionary outcome of past
competition
Predation
• Includes herbivory, parasitism, and carnivory
• Predator adaptations
– Acute senses that enable them to locate and
identify potential prey
– Claws, teeth, fangs, stingers, poison
– Heat-sensing, vibration-sensing organs
– Many herbivorous insects locate food by using
chemical sensors in their feet
Plant Defenses Against Herbivores
• Chemical toxins
– Strychnine from tropical vines
– Morphine from poppies
– Nicotine from tobacco
• Antipredator spines and thorns
• Could be distasteful to animals, but used as
spices for humans
– Cinnamon, cloves, and peppermint
• Some produce chemicals that imitate insect
hormones and cause abnormal development in
some insects that eat them
Animal Defenses Against Predators
• Hiding, escaping or defending themselves,
alarm calls
• Cryptic coloration: camouflage, is a passive
defense that makes them difficult to spot
Animal Defenses Against Predators
• Mechanical or chemical defense
– skunks and porcupines
– Poisonous toads
– Some acquire toxins from the food they eat
• Monarch butterflies store poison from milkweeds they
eat
Animal Defenses Against Predators
• Aposematic coloration: warning coloration for
organisms with chemical defenses
– Black with yellow or red stripes are unpalatable
animals
• Batesian mimicry: a harmless species mimics
a harmful model
– Larva of hawkmoth puffs up like a snake
• Even weaves back and forth and hisses
Animal Defenses Against Predators
• Mullerian mimicry: two unpalatable species
resembles each other
Animal Defenses Against Predators
• Predators will mimic others also
– Snapping turtles have tongues that resemble a
worm that lures in fish
Parasites and Pathogens as Predators
• Endoparasites: parasites that live within their
host
– Tapeworms and malarial parasites
• Ectoparasites: parasites that feed on the
external surface of a host
– Mosquitoes and aphids
• Parasitoidism: lay eggs in living hosts, larva
feed on the host and kill it
– Small wasps
Mutualism
• Mutualism: interspecific interaction that
benefits both species
– Sometimes require the coevolution of adaptations
in both species
• Ex: cellulose-digesting bacteria in stomach of termites
– Most angiosperms have adaptations that attract
animals that function in pollination or dispersal
Commensalism
• Commensalism: an interaction between
species that benefits only one of the species
involved
– Sometimes involve one species obtaining food
that is inadvertently exposed by another
• Ex: cattle egrets and grazing animals
– Eat bugs flushed out by grazing
Trophic Structure
• Trophic structure: transfer of food energy
from its source in plants through herbivores to
carnivores and decomposers
– Food chain
• Four or five links, or trophic levels, make up a
food web
Food Webs
• It is not a chain because a given species may
weave into the chain at different levels
• Animals at each successive level tend to be
larger with each link (except parasites)
Length of a Food Chain
• Energetic Hypothesis: is the most widely
accepted reason
– Length is limited by the inefficiency of energy
transfer
• Dynamic stability hypothesis: long food
chains are less stable, fluctuations are
magnified at higher levels
Dominant Species
• Species that have the highest abundance or
highest biomass (sum weight of all individuals
in a population)
• Exert powerful control over the occurrence
and distribution of other species
• Species that are the most competitive in
exploiting nutrients become the dominant
species
Keystone Species
• Exert control by their ecological niches
• Figure out what it is by removing certain
species and looking at the effects
Bottom-Up Model
• Mineral nutrients (N) control the community
organization because the nutrients control the
plant (V) which control herbivores (H), which
control predator numbers (P)
–NVHP
• If you want to change the biomass, you have
to add more nutrients for the producers and
then all others will increase as well
Top-Down Model
• It is mainly predation that controls community
organization because predators control
herbivores, which control plants, which
control nutrient levels
–NVHP
• Also called the trophic cascade model
– Increasing predators will depress numbers on
lower levels
Biomanipulation
• Lake Vesijarvi in Finland
– Huge algae blooms due to toxic waste dumping
– Primary consumer fish (roach fish) were eating all
herbivorous zooplankton, which led to an increase
in algae
– They took away 1,018 tons of fish, and added fish
that prey on roach
– Algae went back to normal
Disturbance and Community Structure
• Disturbances: events such as storms, fire,
floods, drought, overgrazing, or human
activities that damage communities, remove
organisms from them, and alter resource
availability
• Humans are the most widespread agents of
disturbance
– Logging and clearing, mining, farming, overgrazing
– We currently use about 60% of Earth’s land
Ecological Succession
• Most apparent when a disturbance, such as a
large fire or volcanic eruption strips away the
existing vegetation
• Disturbed area may be colonized by new species
which are succeeded by other species =
ecological succession
• Primary succession: begins in a virtually lifeless
area where soil has not been formed
– Volcanic island or where a glacier has melted
– Bacteria  lichens and mosses  soil development
 grasses, shrubs and trees  community’s prevalent
vegetation
• Process takes hundreds or thousands of years
Secondary Succession
• Occurs where existing community has been
cleared by some disturbance that leaves the
soil intact
• Begins to return to something like its original
state
Processes Involved with Succession
• The early arrivals facilitate, or contribute to, the
appearance of the later species by making the
environment more favorable for the later species
– May make the soil more fertile
• The early species may inhibit establishment of
later species, so that the later species colonize
successfully in spite of, rather than because of,
the earlier species
• Early species may tolerate the later species but
do not help or hinder colonization
Processes Involved with Succession
• Increases soil quantity
– More detritus, decreased erosion because more plants
hold soil in place, provides more anchoring for plants
• Improved soil quality
– Soil gains organic matter, provides more nutrients for plant
growth
• Increase in water retained in soil
– Increased matter retains water, shading reduces
evaporation, more water is available for plants
• Higher humidity
– Caused by more transpiration
• Decrease in temperature
– Caused by shading
Biodiversity
• Species richness: total number of different
species
• Relative abundance: proportion of total
abundance that each species makes up
• Heterogeneity: measurement of biodiversity
that includes species richness and relative
abundance
Species-Area Curve
• The larger the area, the more species there
will be
– Larger areas offer a variety of different habitats
and microclimates