1. A Community Contains All Organisms in a Place that Live Together
1. Different Species Make Complex Adjustments to Community Living fig 24.1
2. Competition and Cooperation Play Key Roles

24.1 Competition shapes communities

1. Communities
1. Organisms Interrelate in Distinct Assemblages
1. Example: Redwood trees in Oregon fig 24.2
2. Certain individuals are more obvious in such collections
3. Other organisms are characteristic as well fig 24.3
1. Exist under conditions set by dominant species
2. Niches of organisms overlap one another

4. Organisms in communities share unique evolutionary history

2. Similar Communities Stretch Over Vast Areas
1. Organisms within them interact in similar manners
2. Organisms follow set patterns of distribution

2. The Niche and Competition
1. Description of a Niche
1. Sum total of all ways an organism uses resources of its environment
2. Includes space, food, temperature, conditions for mating, moisture
3. Also takes into account behavior at various seasons or times of day
4. Niche is not synonymous with habitat
1. Habitat is a place
2. Niche is a way of living

5. Description of Competition
6. Competition keeps an organism from using all of its niche
7. Struggle between two organisms to use same resources, if not enough for both
1. Interference competition: Fighting over resources
2. Exploitative competition: Consuming shared resources
3. Interspecific competition: Interactions between individuals of different species
1. Greatest between orgainsms that obtain food in same way
2. More acute between organisms that are similar

4. Intraspecific competition: Occurs between individuals of a single species

2. Realized Niche
1. Fundamental niche: Entire niche an organism is theoretically capable of using
2. Realized niche: Actual niche of an organism, in the presence of competitors
3. Example: Connell study of barnacle species fig 24.4
1. Investigated competitive interactions between two species
2. One species lives in shallower water, other in deeper water
3. In deeper zone, deep species always outcompeted shallow species
4. If deep species removed, shallow species inhabited deep regions
5. Deep species conversely could not survive in shallow waters
6. Fundamental niche of shallow species included niche of deeper species
7. Realized niche was narrower

4. Example: Two species of flour beetle
1. One species would always drive other species to become extinct
2. Small pieces of glass tubing in flour allows both to coexist
3. Shows that many organsisms can coexist in complex ecosystem

3. Gause and the Principle of Competitive Exclusion
1. Gause and His Classic Experiments with Paramecium
1. Three species grew well when grown in single culture
2. P. aurelia versus P. caudatum
1. caudatum always declined to extinction
2. aurelia grew faster than P. caudatum
3. Better utilized limited available resources

3. Formulation of principle of competitive exclusion
1. If two species are competiting for a resource
2. The one that uses the resource better will locally eliminate the other
3. No two species with the same niche can coexist

4. Competitive is not the inevitable outcome
1. Depends on fierceness of competition
2. Depends on degree of similarity between fundamental niches of competitors

2. Niche Overlap
1. Gause challenged P. caudatum with P. bursaria fig 24.5
1. Expected same result as before
2. Both species survived
1. P. caudatum dominated upper portion of culture tubes
2. P. bursaria favored growth in lower portion

3. Fundamental niche of both species was whole culture tube
4. Realized niche for each species was a different portion of the tube
5. Niches did not overlap too much

2. Also shows negative efffects of growth with competitor versus growth alone

3. Competitive Exclusion
1. Restatement of Gause's principle of competitive exclusion
2. No two species can occupy the same niche indefinitely
1. Coexist while competing for the same resources
2. One or more features of niche will always differ

3. Niche is a complex concept involving all environmental facets
1. Factors defining the niche are difficult to determine
2. Role of competitive exclusion more obvious when resources are drastically limited

4. Resource Partitioning
1. Persistent Competition Between Species Is Rare
1. One species drives out other, or natural selection reduces competition
2. Example: Five species of warblers
1. All five initially appeared to be competing for same resources
2. With closer observation, each feeds in different part of tree
3. Each species thus eats different subset of insects
4. Species not truly in competition
5. Subdivided the niche to avoid direct competition

2. Competition Also Avoided by Geographical Partitioning
1. Sympatric species live in same area
1. Avoid competition by living in different part of hanitat
2. Utilize diffferent food or other resources fig 24.6

2. Allopatric species do not live in same geographical area
1. Utilize same habiata locations, food resources
2. Not in competition, natural selection does not subdivide niche

3. Sympatric species exhibit greater morphological and behavioral differences than when allopatric.
1. Character displacement: Differences evident between sympatric species
1. Favored by natural selection too facilitate habitat partitioning
2. Thus reduces competition

2. Example: Darwin's finches fig 24.7
1. Have bills of same size when finches are allopatric
2. When species are sympatric they evolved beaks of different shape and size

24.2 Evolution often fosters cooperation

1. Coevolution and Symbiosis
1. Organisms Living Together Change and Adjust to One Another
1. Example: Flowering plants and pollinators fig 24.8
1. Plants evolve in relation to disoersal of gametes by animals
2. Animals evolve special traits to obtain resources from plants
3. Seeds have features to make them more dispersable

2. Interactions are examples of coevolution

2. Symbiosis Is Widespread
1. Symbiotic relationships involve two or more organisms
1. Often elaborate, generally permanent relationships
2. Carry potential for coevolution between organisms involved

2. Examples
1. Lichens = fungus and alga
2. Mycorrhizae = fungus and plant root
1. Fungus expidites plant's absorption of certain nutrients
2. Plant provides fungus with carbohydrates

3. Legumes = plant root and nitrogen-fixing bacteria
4. Leaf cutter ants and fungi
1. Ants cut leaves off plants in area
2. Collect bits underground, innoculate with fungi
3. Fungi specifically cultivated by ants, grow and reproduce
4. Ants feed on fungi
5. Example of symbiosis

3. Kinds of Symbiosis
1. Commensalism: One partner benefits, other neither benefits nor is harmed
2. Mutualism: Both participants benefit
3. Parasitism: One partner benefits, other is harmed, a kind of predation

2. Commensalism
1. Benefits One Species, Other Not Affected
1. Individuals of one species physically attached to individuals of another species
2. Examples
1. Epiphytic plants growing on other plants
2. Barnacles attached to marine animals

2. Examples of Commensalism
1. Sea anemones and clownfishes fig 24.9
1. Fish live among stinging tentacles of anemones
2. Tentacles paralyze other fish
3. Fish feed on detritus left by anemones

2. Certain birds clean parasites off grazing animals fig 24.10
1. Birds spend much time clinging to animals
2. Pick off parasites, other insects

3. When Is Commensalism Commensalism?
1. Difficult to ascertain if second partner benefits or not
2. Gray boundary between commensalism and mutualism

3. Mutualism
1. Relationship in Which Both Partners Benefit
1. Have fundamental importance in determining structure of biological communities
2. Spectacular examples between flowering plants and pollinators
3. Example: Ants and aphids fig 24.11
1. Aphids suck plant juices
2. Ants protect and herd aphids like cattle
3. Utilize aphid honeydew as food

2. Ants and Acacias
1. Acacia leaf stipules modified as pair of hollow thorns
1. Thorns inhabited by stinging ants
2. Thorns deter herbivores
3. Trees inhabited by ants produce food for them
1. Protein-rich Beltian bodies
2. Nectar at base of leaves

2. Ants and larvae protected by thorns of tree
3. Ants in return
1. Attack all other herbivores
2. Cut away branches of competing plants
3. Wastes provide source of nitrogenous fertilizer

4. Parasitism
1. Special Form of Symbiosis
1. Parasite much smaller than prey
2. Parasite in close association with prey
3. Harmful to prey organism, beneficial to parasite
4. Sometimes difficult to distinguish from predation

2. External Parasites
1. Ectoparasites feed on exterior surface of an organism
1. Example: Lice are parasites, live on bodies of vertebrates
2. Mosquitos are not considered parasites, interaction with host is brief

2. Parasitoids: Insects that lay eggs on living hosts
1. Common wasp behavior
2. Larvae feed on host, often kill it

3. Internal Parasites
1. Endoparasites are found inside the host
2. Some animal examples are readily identifiable, while others are not
1. Vertebrates have animal or protist parasites
2. Bacteria and viruses are not considered parasites

3. Internal parasites more specialized than external ones
1. More closely linked to host
2. Morphology and behavior more greatly modified over time
3. Bodily structure of parasite quite simplified

4. Brood Parasitism
1. Few birds lay eggs in nests of other species
2. Host parents raise baby as if it were their own
1. Parasite baby often requires greater investment in care
2. Reduces reproductive success of foster parents
3. Evolution favors recognition of parasite eggs with resulting rejection

24.3 Predators and their prey coevolve

1. Plant Defenses Against Herbivores
1. Predator-Prey Interactions
1. One organism uses the other for food
2. Plants attempt to limit being eaten by herbivores
3. Morphological defenses
1. Thorns, spines and prickles limit activities of browsers
2. Plant hairs with glandula, sticky tips
3. Deposition of silica toughens plant parts

2. Chemical Defenses
1. Produce secondary chemical compounds
1. Distinguish from primary chemical compounds
2. Primary compounds normally formed in metabolic pathways
3. Secondary compounds not formed in metabolic pathways
4. Chemicals are toxic, or disturb herbivore metabolism and/or development

2. Examples
1. Mustard family produces mustard oils
2. Milkweed/dogbane families produce milky sap containing cardiac glycosides

3. The Evolutionary Response of Herbivores
1. Some feed on restricted group of plants
1. Group frequently produces secondary compounds
1. Example: Cabbage butterflies fig 24.13
2. Example: Monarch butterflies and milkweed/dogbane

2. Evolution of plant/herbivore interaction (cabbage butterfly)
1. Plant evolves secondary compound (mustard oils), not eaten by herbivores
2. Herbivores (butterfly) evolve ability to break down compound
3. Herbivores lack competition from other herbivores
4. May evolve sense organs to specifically detect chemical

2. Animal Defenses Against Predators
1. Extra Benefits Derived From Ingesting Secondary Compounds
1. Monarch butterfly caterpillars concentrate and store cardiac glycosides
1. Stored through chrysalis stage, to adult, even adults' eggs
2. Cardiac glycosides protect Monarch from predators fig 24.14
3. Predator regurgitates ingested butterfly
4. Avoids conspicuously colored prey in future

2. Some predators acquire tolerance to chemical

2. Defensive Coloration
1. Insects feeding on milkweek are often brightly colored, advertize poisonous nature
2. Strategy called warning or aposematic coloration
3. Animals lacking defenses possess cryptic coloration
1. Blends in with surroundings, hides animal from predators fig 24.15
2. Camouflaged animals do not usually live in groups
3. If one discovered predator gains clues to presence of others

3. Chemical Defenses
1. Animals manufacture a variety of poisonous chemicals
1. Many arthropods use chemicals for defence and to kill prey
2. Marine animals and vertebrates have evolved various chemical defenses

2. Example: Poison dart frogs fig 24.16
1. Produce toxic alkaloids in mucous secreted from brightly colored skin
2. Only a few micrograms can kill a human
3. Over 200 alkaloids isolated from secretions
4. Many are important in neuromuscular research

3. Current investigation of other organisms for new anticancer drugs, antibiotics

3. Predator-Prey Cycles
1. Predation
1. One organism consumes another
2. Relationships between large carnivores and grazing animals
1. Moose and wolves on Isle Royale
2. Moose died of other causes, not regulated by wolf population fig 24.17

2. Refuges Promote Cycles
1. Predator may exterminate prey, having no food source it dies out fig 24.18
2. Provide refuges for the prey
1. Prey populations driven to low but recoverable numbers
2. Predator numbers subsequently decrease
3. Prey numbers recover
4. Predator numbers increase
5. Predator-prey populations will cycle

3. Cycles in Hare Populations: A Case Study
1. Population cycles characteristic of some small mammals
2. Sometimes stimulated by their predators
3. Example: North American snowshoe hare
1. Follows "ten year cycle"
2. Numbers fall ten to thirty fold in typicale cycle, may change as much as 100-fold

4. Two factors generate cycle: Food plants and predators
5. Food plants
1. Include willow and birch twigs
2. As hare density increases, twig density decreased
3. Hares forced to feed on high-fiber, low-quality food
4. Ensuing low birth rates, low juvinile survivorship, low growth rates
5. Hares spend more time search ing for food
6. With decrease in twig abundance, corresponding decrease in hare abundance
7. Two to three years required for twig quanitiy to recover

6. Predators
1. Key predator is Canada lynx
2. Shows "ten year cycle" tied to hare abundance cycle fig 24.19
3. As hare numbers increase, lynx numbers increase
4. When hare numbers decrease, lynx numbers decrease

7. Factors responsible for cycling determined through field experiment in 1992
1. Delete food effect (add food to region) and exclude predators
2. Hare numbers increase tenfold and stay there
3. Cycle retained if either factor allowed to operate alone
4. Thus both factors affect cycle

4. Predation Reduces Competition
1. Intricate interactions between predators and prey
1. Predators control levels of some species, survival of other enhanced
2. Predators greatly reduce competitive exclusion, reduce numbers of competing species

2. Example: Marine sea star/bivalve interactions
1. Sea stars selectively prey on bivalves, prevent bivalves from monopolizing habitat
2. Remove sea stars from habitat, biodiversity drops substancially

3. Usually a mistake to artificially eliminate a major predator from a community
4. Results in decrease of not increase in biodiversity
5. Feedback systems control structure of natural communities
1. Predator's choice of prey depends on its relative abundance
2. Feeds on species A until it becomes rare, switches to species B

4. Mimicry
1. Batesian Mimicry
1. Related but unprotected species resemble protected ones
1. Unprotected specimen is the mimic
2. Protected specimen is the model

2. Unprotected must be fewer in number than protected species
1. Mimics must live with models
2. If in greater numbers, predators learn that most are edible

3. Most common examples include butterflies and moths
1. Predators use visual cues to hunt for prey, may use other cues as well
2. Models usually have caterpillars that feed on plants that produce toxins
3. Mimic caterpillars do not feed on such plants, aren't protected

4. Example: Viceroy butterfly fig 24.20
1. Viceroy resembles poisonous monarchs
2. Caterpillars feed on willow, cottonwoods, not thought to be distasteful
3. Batesian mimicry does not extend to caterpillar stage
4. Viceroy caterpillars camouflaged on leaves as bird droppings
5. Distasteful monarch caterpillar very conspicuous

2. Muellerian Mimicry
1. Unrelated, but protected species resemble one another
1. Examples include wasps and bees fig 24.21
2. Strengthens the distastefulness and provides a group defense

2. Behavior is imitated in both types of mimicry as well

24.4 Biodiversity promotes community stability

1. Community Structure
1. Some Ecosystems Are More Stable than Others
1. Biologically diverse ecosystems are more stable
2. More different kinds of organisms support a more complex web of interaction
3. More likely for an alternate niche to exist in the event of a disruption

2. Species Richness
1. Complexity of community depends on number of species and numbers of individuals
2. Species richness: Number of different species
3. Species rich communities are more complex
4. Relative differences among ecosystems
1. Forests have more species than grasslands
2. Tropical communitites have more than temperate ones
3. Fewer species inhabit harsh communities, deserts and polar tundras

3. Species Diversity
1. Communities also differ in relative abundance of species they contain
2. Some species are rare others are common
1. More diverse: Ten species with same number of individuals
2. Less diverse: Ten species, one with 91% of individuals, other nine with 1%

3. Species diversity: Weighted measure of species richness
4. Shannon diversity index (H)
1. For each species, determine proportion of individuals (P) it contributes to total
2. Each value of P multipled by its natural log
3. Producets of different species added together
4. H = [sum ofP (lnP) for all the species]

5. Comparison of deciduous forests in Indiana tbl 24.1
1. Both have same species richness
2. One is more diverse than other, due to more species being common within it

4. Factors Promoting Species Richness
1. Difficult to sort out importance of all factors
2. Ecosystem productivity
1. Species richness correlates with productivity fig 24.22
2. Example: Tilman at University of Minnesota
1. Burned, plowed, planted, tended series of experimental plots
2. Plots contained 1 to 24 native species
3. Monitored growth, accumulation of nitrogen from soil
4. The more species, the greater nitrogen uptake
5. Biodiversity promotes productivity

3. Spatial heterogeneity
1. When heterogenous, contain more kinds of variations
2. Accomodate more species by having more microhabitats
3. Species richness of animals reflects species richness of plants
4. Plants provide spatical heterogeniety to the animals
5. Example: Lizard/plant species in southwest fig 24.23

4. Climate
1. Role more difficult to assess
2. Might expect more species to exist where there is seasonal variation
3. Unpredictable climate changes might have opposite effect
4. Western mammal species increase as temperature range decreases fig 24.24

2. Biogeographic Patterns of Species Diversity
1. Species Diversity Cline
1. Increase in species richness from arctic to tropics
2. Gradient in number of species correlated with latitude fig 24.25

2. Why Are There More Species in the Tropics?
1. Evolutionary age
1. Tropics have existed over long, interrupted periods
2. Temperate regions have experienced glaciation
3. Greater age of tropics allows for evolution of more complex population interactions
4. Fosters greater variety of plants and animals
5. Long term stability of tropics now disputed, greatly exagerated

2. Higher productivity
1. Productivity due to greater solar radiation
2. Increases overall photosynthesis of plants
3. Field studies indicate greatest species riichness at intermediate productivity
4. Filtering of light in tropical forest may lead to variety of light environments

3. Predictability
1. Climates are stable and predictable
2. Unchanging environment may encourage specialization, reduce competition
3. Produces larger number of specialized species in tropics

4. Predation
1. More intense in tropics
2. Could reduce importance of competition
3. Permits niche overlap, promote species richness

5. Spatial heterogeniety
1. Promotes species richness
2. Provides great number of microhabitats, foster larger number of species

3. Island Biogeography
1. Oceanic Islands as Natural Laboratories
1. Examine factors that promote species richness
2. Examined by MacArthur and Wilson
3. Number of species related to size of island

2. The Equilibrium Model
1. Species constantly dispersed to islands
1. Islands accumulate more and more species
2. Other species lost to extinction

2. When number of species filled to capacity, no new ones estabilshed without extinction
1. Assumes characteristic equilibrium number fig 24.26a
2. Identity of species may change

3. Island species rcihness is a dynamic equilibrium between colonization and extinction
1. Dependent on island size and distance from mainland
2. Small islands have higher rate of extinction
3. Less colonization in islands farther from mainland
4. Small islands far from mainland have fewest species fig 24.26b
5. Large islands closer to mainland have more species

4. Example: Asian Pacific bird species comparison fig 24.26c
1. Positive correlation of species richness with island size
2. Negative correlation of species richness and distance from mainland

3. Testing the Equilibrium Model
1. Test on small mangrove islands off Florida Keys
1. Examined arthropod populations of islands
2. Larger islands had more species, smaller islands had fewer species

2. Fumigated islands to kill all arthropods fig 24.27
3. Islands recolonized, assumed steady-state within one year
1. Capacity of island set equilibrium number of species
2. Chance determined exact identity of species

4. Ecological Succession
1. Succession
1. Ecosystems have a tendency to change from simple to complex
2. Cleared land becomes occupied by larger and more diverse plants
3. Small pond becomes filled with vegetation encroaching from the edges

2. Secondary Succession
1. Occurs in areas once exhibiting life but disturbed in some manner
2. Frequently initiated by humans
3. Also result from fires or by abandoning agricultural fields

3. Primary Succession
1. Occurs in areas devoid of all life
1. New volcanic islands
2. Areas after retreat of glaciers fig 24.28

2. Xerarch succession occurs on land
1. Example: Glacial moraines fig 24.29
2. Lichens grow first, acid secretions break down rock
3. Mosses colonize pockets of soil, build up nutrients
4. Next colonizers include alder shrubs
5. Finally form dense spruce forests

3. Hydrarch succession occurs in open water
1. Oligotrophic lake is poor in nutrients
2. Eutrophic lake is rich in nutrients
3. Oligotrophic lake may become eutrophic through accumulation of organic matter

4. Climax vegetation (climax community)
1. Characteristic vegetation may be associated with climate of region
2. Term no longer useful as once presumed
1. Climates keep changing
2. Process of succession is very slow
3. Nature of a region's vegetation affected by human activities

4. Why Succession Happens
1. Tolerance
1. Early successional stages characterized by weedy r-selected species
2. Do not compete well in estabilshed communities
3. Tolerant of harsh, abiotic conditions of barren areas

2. Facilitation
1. Weedy species introduce changes that favor less-weedy species
2. In glacer example
1. Mosses fix nitrogen to allow alder survival
2. Alders lower soil pH, allow for invasion of spruce

3. Inhibition
1. Changes from one species favor growth of other species
2. May also inhibit growth of first species
3. Example: Alders do not grow well in acidic soil that they produce

4. With maturation of ecosystems
1. More K-selected species replace r-selected ones
2. Greater species richness in mature ecosystems than in immature ecosystems
3. Increase in total biomass, decrease in net productivity
4. Earlier stages more productive than later ones
5. Agricultural systems not allowed to mature, productivity kept high

5. Preserving Biodiversity
1. The Role of Disturbance
1. Disturbances interrupt succession of plant communities
1. Include forest fires, drought, floods
2. Also includes animal disruptions like gypsy moth destruction
3. Also includes overgrazing of forests or pastures

2. Human activity is greatest disruption

2. Keystone Species
1. Some communities are more resistant to disturbances
2. Keystone species: Interacts in critaical ways with other elements of ecosystem fig 24.30
3. Loss of keystone species may affect many other organisms

3. The Importance of Biodiversity
1. Must preserve natural ecosystems and their biological diversity
2. Locate organisms of most use, save them from extinction
3. Loss of diversity causes loss of knowledge, potential loss of human prosperity