Transcript Chapter 21-Community Ecology

Chapter 21: Community Ecology
21-1 Species Interactions
21-2 Properties of Communities
21-3 Succession
21-1 Species Interactions
I. Symbiosis (3 types, in addition to predation)
• A close, RELATIONSHIP between members of DIFFERENT species.
II. Predation (a heterotrophic property)
• Influences a FOOD WEB. (i.e., a BIOTIC regulator of population size)
(1) Predator
• Captures, KILLS, and consumes another individual (prey) for nutrition.
(2) Prey
• Becomes NUTRITION for another individual (predator).
(A) Predators, Prey, and Natural Selection
• Natural selection favors adaptations that IMPROVE executing predation
(offensive) AND evading predation (defensive).
Critical Thinking
(1) One species of butterfly develops a large, dark eyespot pattern on its
wings if it develops in a warm wet season. The same species of butterfly
will develop smaller, lighter eyespots if it develops in a dry, cool season.
What do you suppose could cause this phenotypic variation within the
same species?
(B) Mimicry (anti-predatory defense)
• Use of MORPHOLOGICAL deception to DETER predation.
(i.e., mimic is harmless; what it mimics IS dangerous or even deadly
mimicry that mimics a part of the environment is CAMOUFLAUGE.
(C) Plant-Herbivore Interactions
• Predation  EVIDENCE shows plants have EVOLVED adaptations to
PROTECT them from being eaten.
(1) Herbivores
• Deterred by PHYSICAL defenses  sharp thorns, spines, sticky hairs,
tough leaves, AND CHEMICALS.
(2) Secondary Compounds
• Chemicals from METABOLIC products can be poisonous, irritating, or
bad-tasting to DETER predation.
Toxic Examples include…
(1) Strychnine (leaves of plants of the genus Strychnos)
(2) Nicotine (leaves of tobacco plant)
(3) Poison ivy, Poison oak, Poison sumac
Medicinal Examples include…
(1) Morphine
(2) Taxol
(3) Quinine
III. Parasitism (NOT predation; 1st type of symbiosis)
• One member BENEFITS at EXPENSE of other, although typically NOT
resulting in immediate death.
(1) Parasite
• Evolved into TWO classes based upon their LOCATION within HOST.
(2) Host
• Source of NUTRITION, can be unaware of parasite’s presence.
(3) Ectoparasites
• External parasites, live on the host BUT do not enter the host’s body.
Examples include…
(1) Ticks, Fleas, and Lice
(2) Leeches and Lampreys
(3) Mosquitoes and Green-Head Flies
(4) Endoparasites
• Internal parasites, live INSIDE host’s body.
Examples include…
(1) Tapeworms, Hookworms, Pinworms (Pork, Beef, and Fish)
(2) Bacteria and Viruses (Lyme disease, Strep throat)
(3) Protozoans and Fungi (malaria, amoebic dysentery, ringworm)
(A) Evolution of Parasites and their Hosts
• Parasitism has FORCED evolution of defenses in HOSTS, including…
(1) Skin, Scales, Hair, Feathers (mechanical barriers)
(2) Tears, Saliva, and Mucus (chemical/mechanical barriers)
(3) Immune Cells (physiological response)
(B) Adaptations of Parasites
• Natural selection favors adaptations that ENHANCE a parasite to
safely EXPLOIT its host. Examples include…
(1) Anesthetic and Anticoagulant (mosquito, leech saliva)
(2) Skin Absorption of Nutrients (tapeworms)
(3) Razor sharp teeth-cut through scales (lamprey and leech)
IV. Competition (interspecific-b.o.s and intraspecific-b.s.s.)
• Results from niche OVERLAP—use of SAME limited resource by two or
more species (OR members of the same species)
NOTE: TYPICALLY one species will be able to USE a resource MORE
efficiently, leaving LESS of the resource available to other species.
An example…
Ex: Some species of plants leave TOXINS into the soil that prevent
individuals of OTHER species from GROWING nearby.
(A) Research Studies on Competition (e.g., competitive exclusion)
• Soviet ecologist (Gause)  studies with PARAMECIA cultured
separately AND in combination.
NOTE: Scottish ecologist (Connell) studied BARNACLES and noted that
although Chthamalus is capable of surviving in the lower intertidal zones,
it is crowded out by Semibalanus.
(1) Competitive Exclusion (i.e., an ecological principle of SELECTION)
• One species is ELIMINATED (i.e., OUT-COMPETED) for SAME limited
resource.
(B) Competition and Community Structure
• Competitors SEEK to evolve niche DIFFERENCES that DECREASE
competition.
Critical Thinking
(2) An aspiring community ecologist studies a community and finds no
evidence of competition. He concludes to his colleagues that competition
has never had an effect on the structure of the community. Do you believe
he is justified in his conclusion, why or why not?
(1) Character Displacement (i.e., an ecological principle of TRAITS)
• Evolution of ANTATOMICAL differences that REDUCE competition
between SIMILAR species.
(Ex: Finches of same habitat evolve DIFFERENT beak shapes to suit
different FOOD sources.)
(2) Resource Partitioning (i.e., an ecological principle of BEHAVIOR)
• When similar species attempt to COEXIST, each species uses ONLY
PART of the available resources.
Critical Thinking
(3a) Each bird is shown in the region of the tree
where it usually nests. Explain how three closely
related species of warbler are able to coexist in
the same tree. What phenomenon is demonstrated
by their coexistence?
(3b) What may happen if only a single species of
insect inhabited the spruce tree? Explain the
phenomenon that might be demonstrated in this
case.
(3c) Suppose the birds are seed-eating finches and
the tree produces large seeds and small seeds.
Each of the birds has either a small beak or a large
beak. Explain the evolutionary phenomenon
demonstrated in this case. Would a bird with a
medium-sized beak be able to coexist in the tree?
Why or why not?
V. Mutualism and Commensalism (2nd and 3rd types of symbiosis)
• Differ FROM parasitism in that NO member is NEGATIVELY affected.
(1) Mutualism (2nd type of symbiosis)
• BOTH members BENEFIT from association:
An example…
(1) Ant species and Shrub plant (Bull’s horn acacia)
Critical Thinking
(4) Some flowering plants are pollinated by only one pollinator. Explain why
this might be advantageous to the plant? Also, why could this specificity
possibly lead to extinction of the plant’s species?
(2) Pollinators
• WIDESPREAD mutualisms in terrestrial ecosystems.
(e.g., bees, butterflies, flies, beetles, bats, and birds)
(3) Commensalism (3rd type of symbiosis)
• One member BENEFITS and other is NEITHER helped NOR harmed.
An example…
(1) Cattle egrets (birds) feed on small insects forced out of their hiding
places by the Cape buffalo’s movement through the grasses in Tanzania.
21-2 Properties of Communities
I. Species Richness and Diversity
• What is the number of DIFFERENT species in a community? (RICHNESS)
• How UNIFORMLY is each species represented? (DIVERSITY).
Critical Thinking
(5) Two ecological communities exist:
(a) A freshwater lake has a total of 1,000 individuals and contains 25
different species. Each species has 40 individuals.
(b) A mountainous range ALSO has a total of 1,000 individuals and ALSO
contains 25 different species. Ten species have 15 members each, Twelve
species have 10 members each. One species has 30 members, one species
has 100 members and one species has 600 members.
* Which community has greater
species diversity, the lake OR the
mountainous range?
(A) Patterns of Species Richness (2 hypotheses)
• Species richness varies with DISTANCE from EQUATOR
(i.e., closer to the equator, the GREATER the richness. But why?)
• Hypothesis 1: TEMPERATE habitats are YOUNGER. (i.e., tropical
habitats were NOT disturbed by the ice ages, while habitats at latitudes
further NORTH and SOUTH were affected…Also the climate is MORE
stable in the tropics, stability is FAVORED).
• Hypothesis 2: Since plants can photosynthesize YEAR-ROUND in tropics,
there is MORE NRG available to support more SPECIES.
(B) The Species-Area Effect (e.g., for ISLAND habitats largely)
• LARGER areas typically contain MORE species than do SMALLER areas.
But WHY?
• Hypothesis: Larger geographical areas usually contain a GREATER
DIVERSITY of habitats and THUS can support GREATER species richness.
(C) Species Interactions and Species Richness
• Interactions among species can PROMOTE species richness;
(Ex: Sea star Pisaster and the mussel Mytilus.)
NOTE: When the sea star Pisaster was REMOVED from a habitat where
the sea stars had PREYED on the mussel, Mytilus, the mussels CROWDED
OUT other species in the area. Predation by the sea star ON the mussel
population promoted diversity by CONTROLLING the superior
competitor—the mussel. (richness dropped from 15 to 8 during the study)
Critical Thinking
(6) Explain why it may be harder to measure the species diversity of a
community than it would be to measure the species richness.
(D) Community Stability (related to species richness)
• Species richness IMPROVES community stability when faced with
outbreaks of disease or disaster.
NOTE: The more LINKS BETWEEN species in a community, the GREATER
the STABILITY of THAT community.
21-3 Succession
I. Successional Changes in Communities
• Disturbances (i.e., fires, landslides, hurricanes, and floods) trigger a
sequence of ECOLOGICAL CHANGES in the community.
NOTE: Succession is the GRADUAL, sequential regrowth of a community in
an area.
Examples of Events…
Yellowstone Fire of 1988, Abandoned fields, Vacant lots,
Volcanic islands, and Glacier movement.
(1) Primary Succession (VERY slow, not as slow as evolution though)
• Development in an AREA that has NOT supported life previously
(e.g., BARE rock, a sand dune, or island formed by VOLCANIC eruptions).
(2) Secondary Succession (slightly faster)
• Sequential REPLACEMENT of species that follows DISRUPTION of a
PRE-existing community.
(3) Pioneer Species
• Species that predominate EARLY in succession—often SMALL, fastgrowing, AND fast-reproducing. (e.g., lichens, grasses, weeds)
(A) Primary Succession
• Minerals NECESSARY for plant growth are UNAVAILABLE.
Ex: Glaciers retreated from Canada about 12,000 years ago, leaving a
HUGE stretch of BARREN bedrock from which ALL the SOIL had been
scraped. (Canadian shield, through REPEATED freezing and thawing,
broke the ROCK into smaller pieces—LICHENS extract nutrients from
acids secreted and made soil that pioneer plants could use.)
(B) Secondary Succession (soil left INTACT for use)
• ~ 100 years for ORIGINAL ecosystem to return FROM a disturbance
(i.e., fire, landslide, storms, floods, abandoned farms).
Ex: After the Pacific Northwest wildfires  GRASSES and dandelions 
leads to SHRUBS followed up by small trees like dogwoods, until it finally
reaches the stage of a deciduous or coniferous FOREST.
II. The Complexity of Succession (varies with environment)
• Rate of succession  influenced by ABIOTIC and BIOTIC factors 
STAGING may be hard to PREDICT.
(1) Climax Community
• Point of STABILITY where species richness AND diversity remain
relatively CONSTANT from year to year.
Extra Slides AND Answers for Critical Thinking Questions
(1) The crysalis temperature regulates the gene for eyespot expression.
Natural selection favors this adaptation as a defense against predation.
Predators, such as lizards, prey on butterflies with large eyespots during
the dry season. The butterfly with small eyespots has a selective
advantage during the dry season.
(2) This conclusion is not valid. Past competition may have caused some
species to go extinct in the community and may have stimulated character
displacement and evolution of niche differences. All of these mechanisms
would reduce competition among members of the community, possibly to
the point where competition is no longer detectable.
(3)
(a) The 3 species of birds can coexist because they forage in different
areas of the tree. This is called resource partitioning.
(b) If all species had only one source of food, they would compete for that
limited resource. Eventually, one species would drive the others out
(competitive exclusion).
(c) The birds are demonstrating character displacement. They have
developed anatomical differences (beak size) that allow them to take
advantage of different resources (seeds). A bird with a medium-sized
beak probably would not survive. It would not be able to eat the large
seeds and would be out-competed for the small seeds.
(4) The plant’s pollen is more likely to be transported to a member of the
same species because the pollinator does not visit any other species. If
the pollinator becomes extinct, however, the plant may be doomed.
(5) Community A is more diverse because all species are equally important,
whereas community B is dominated by a single species.
(6) To calculate species diversity, an estimate is needed for the population
size of every species in the community. To calculate species richness, you
only need to know how many species are found there.
Revisiting Interdependence of Organisms
• Closely related species often display traits that are a result of
coevolution (the “arms-race”)
• NO organism exists isolated from biotic or abiotic factors.
Assessing Prior Knowledge
• Imagine that your neighborhood is overrun by deer. What factors do
you suppose could have caused this population explosion? And would these
factors qualify as density-dependent or density-independent? How could
your response to the deer population affect the overall community?