Sections

• definitions
• properties
• organism interactions
• competition
• predation
• symbiotic relationships
• disturbances
• succession

• definitions
• population
• all individuals of particular species living in same place at same time
• community
• all populations of organisms that live together & potentially interact in particular area at particular time
• ecosystem
• all communities of area & their interactions with each other & physical environment

Figure: Lion with kill in a grassland community

• Community properties
• community has own set of properties
• diversity
• prevalent forms of vegetation
• stability
• trophic structure

• Diversity
• variety of organism that make up a community
• has two components
• species richness
• total number of different species in community
• relative abundance of different species
• number of individuals of each of different species
• consider two communities, each made of up 4 species, A, B, C, D
• community 1 has 25A, 25B, 25C, 25D
• community 2 has 97A, 1B, 1C, 1D
• species richness
• same for both communities, each made up of 4 species
• relative abundance of different species
• relative abundance is very different

• Prevalent form of vegetation
• applies mainly to terrestrial communities
• two components
• types of dominant plants
• structure of dominant plants
• largely determines types of animals that will live in a community
• for example, consider deciduous trees of temperate deciduous forest versus coniferous trees of northern coniferous forest
• types of dominant plants
• are different
• structure of dominant plants
• vertical structure of forests is different

• Stability
• community’s ability to resist change
• return to its original species composition after being disturbed
• depends on
• type of community
• nature of disturbance

• Trophic structure
• feeding relationships among various species in community
• determines passage of energy and nutrients from autotrophs to heterotrophs

• Organism interactions
• populations of community are linked via
• interspecific interactions
• relationships between populations of different species of community
• can be considered based on
• affect interaction has on each species involved

• interspecific interactions
• types include
• competition
• predation/parasitism
• mutualism
• commensalism
• some types also considered "symbiotic"
• relationships between organisms of two different species that live together in relative permanent, close relationship
• parasitism
• mutualism
• commensalism
• competition
• detrimental to both species involved
• predation/parasitism
• beneficial to one species, detrimental to other species
• mutualism
• beneficial to both species
• commensalism
• beneficial to one species, other species unaffected

Table: Interspecific Interactions

• Interspecific interactions
• Competition
• may occur
• when a shared resource is limited
• between any 2 species that need same
• limiting resource or limiting factor
• shortage of which restricts success of species
• may be biotic or abiotic
• differ from species to species
• types
• interspecific competition
• between populations of two species
• intraspecific competition
• between members of same species

• between populations may result in
• reduction in density of one or both species
• local elimination of one of competitors
• is considered detrimental to both species involved
• though one will "win"
• neither will do as well as in absence of competitor

• restated
• struggle between two populations to utilize same resources
• when there is not enough of that resource to satisfy both
• studied by Russian ecologist G.F. Gause in 1934
• based on experiments in lab with 2 species of protists from genus Paramecium
• Paramecium aurelia
• Paramecium caudatum
• experiments by Gause
• lab experiments
• P. aurelia and P. caudatum were grown separately, in same conditions
• each grew rapidly, leveled off at carrying capacity
• when P. aurelia and P. caudatum were grown together
• P. caudatum was driven to extinction
• Gause concluded
• if two species are so similar that they compete for the same limiting resources
• then they can’t coexist in the same place
• one species will use resource more efficiently, gain competitive advantage
• eventually leading to local extinction of inferior competitor
• Gause restated his ideas as the
• competitive exclusion principle
• no two species can occupy the same ecological niche in the same place at the same time
• Competition and niche
• niche
• functional role of an organism in its surroundings
• sum total of organism’s use of resources of its habitat
• can be thought of as organism’s role in its community, its profession
• can be described in terms of a number of factors, such as
• space utilization
• food consumption
• temperature range
• moisture requirements
• niche
• is not synonymous with
• habitat
• space that organism inhabits
• is a pattern of living
• sometimes an organism cannot occupy its entire niche because someone else is using it
• competitive exclusion principle can be restated incorporating concept of niche:
• two species cannot exist in a community if their niches are identical
• ecologically similar species can coexist in a community if there are one or more significant differences in their niches
• classic test of competitive exclusion in field
• involved two species of barnacles attached to intertidal rocks on North Atlantic coast
• Balanus
• Chthalamus
• classic test of competitive exclusion
• natural situation
• Balanus lived on lower rocks, rarely exposed to atmosphere
• here, Balanus could always outcompete Chthalamus, crowding it off rocks
• Chthalamus lived higher up on rocks in shallower water that was frequently exposed to air due to tides
• classic test of competitive exclusion
• manipulated situation
• Balanus was removed from lower rocks
• Chthalamus could easily occupy the deeper zone
• indicating there was no physiological obstacle to it living in that zone
• classic test of competitive exclusion
• manipulated situation
• Balanus was physically placed in upper zone (where Chthalamus usually lived)
• it couldn’t survive
• apparently due to drying out in the air
• classic test of competitive exclusion
• conclusion
• Chthalamus
• fundamental niche included both zones
• realized niche was only upper zone
• Balanus
• fundamental niche was lower zone only
• realized niche was lower zone only
• competitive exclusion principle can be restated
• no 2 species with same niche can coexist
• no 2 species can occupy same niche indefinitely

• fundamental niche
• niche of species in absence of competition
• as determined by maximum combination of tolerable environmental conditions
• realized niche
• portion of species’ fundamental niche that it can occupy in presence of competition
• fundamental niche versus realized niche
• examples
• anole lizards, Anolis sp.

Figure: Anolis distichus (left) and Anolis insolitus (right)

• two possible outcomes of competition between species having identical niches
• 1) less competitive species will be driven to local extinction
• loss of species at local level
• 2) one species may evolve to use a different set of resources
• known as resource partitioning

• resource partitioning
• differentiation of niches
• enables similar species to coexist in a community
• example
• Anolis lizards in Dominican Republic
• 7 species live in close proximity
• all feed on insects, other small arthropods
• competition is minimized because each species perches in a certain microhabitat

Figure: Resource partitioning in a group of lizards

• character displacement
• tendency for characteristics to be more divergent when two species live in same area than when same two species live in different areas
• example is two species of Galapagos finches, Geospiza fulginosa & G. fortis
• when they occur on different islands
• beak sizes are similar because they eat similar size seeds
• when they occur on same island
• beak sizes are different
• they eat different sized seeds to avoid competition

Figure: Character displacement: circumstantial evidence for competition in nature

• Predation
• definition & concept
• coevolution
• anti-predator defense mechanisms
• predator-prey interactions
• role in community diversity

• Predation
• interaction in which one species eats another
• predator
• the consumer in such interaction
• benefits
• prey
• the organism in such an interaction that is consumed
• does not benefit (is harmed)
• concept and terms can be applied to
• animal-animal interactions
• such as lion killing and eating antelope or other prey
• animal-plant interactions
• such as when an animal (bison, insect) eats part of a plant
• called herbivory
• parasitism
• concept and terms can be applied to
• parsitism
• one organism (parasite) lives in or on another organism (host), depends on host for nutrition
• also typically considered one of three types of symbiotic relationships
• predator-prey interactions can illustrate concept of
• coevolution
• concept that two or more species can reciprocally influence evolutionary direction of other
• adaptive responses of two species to one another
• anti-predator defense mechanisms
• needed because no species is entirely free from predation
• have evolved in every species
• in response to natural selection
• examples
• size, ability to flee, ability to hide, protective armor, noxious chemicals
• types
• Plant defenses against herbivores
• two major types
• morphological
• chemical
• morphological
• structural features that discourage browsing and feeding
• such as thorns, spines, prickles, plant hairs, deposits of silica in leaves
• chemical
• more crucial than morphological
• chemical compounds act by being toxic, repulsive, disrupting metabolism

• Animal defenses against herbivores
• major types
• mechanical
• chemical
• camouflage
• aposematic (warning) coloration
• deceptive coloration/appearance
• mimicry
• mechanical
• structural features such as quills, claws, shells, spines
• chemical
• venom in venomous animals, alkaloids in skin of poison-arrow frogs, malodorous spray of a skunk
• camouflage
• also known as cryptic coloration
• use of color/patterns that cause animals become less apparent to predators by blending in with their background
• a passive defense

• Figure: Camouflage: Poor-will (left), lizard (right)
  
• aposematic (warning) coloration
• often found in animals with effective chemical defenses
• warns predators that animal is toxic

• deceptive coloration/appearance
• a species comes to look like a larger animal or predator

Figure: Deceptive coloration: moth with "eyeballs"

Figure: Batesian mimicry

• mimicry
• "copycat" adaptation in which one species mimics appearance of another
• species that lacks a defense comes to resemble a species that has a defense
• mimicry, two types of mimicry
• Batesian mimicry
• undefended species mimics defended species
• undefended species must be rare in the area
• flower fly (no stinger) mimics a honey bee (with stinger), predators avoid both

• Figure: Batesian mimicry
  
• Mullerian mimicry
• two defended species in community come to resemble each other
• each species gains advantage because predators learn more quickly to avoid both
• cuckoo bee and yellow jacket

Figure: Müllerian mimicry: Cuckoo bee (left), yellow jacket (right)

• predator-prey interactions
• predators rarely drive prey to extinction because
• natural communities are complex
• predators themselves are often preyed upon
• predators can switch to alternative food sources
• defense mechanisms of prey can be successful
• occur in virtually all communities
• may or may not generate cycles in both populations
• determined by two factors
• population growth of prey in absence of predator
• relationship between prey population size & amount of prey eaten by average predator (known as functional response)
• probability of predator-prey cycles occurring (all else being equal) increases
• when prey exhibit little density dependence
• predators functional response increases rapidly as prey density increases
• functional response
• relationship between prey population size and amount of prey eaten by an average predator
• affected by
• population densities
• search time
• capture/subduing time
• consuming time
• digestion time
• three types
• type 1
• type 2
• type 3
• Communities:
  
• type 1
• prey consumption rises linearly to a plateau
• characteristic of filter feeders
• at high concentrations of prey, predation rate is "maxed out"
• predator "processes" prey as fast as it can, reaches plateau
• type 2
• prey consumption rises asymptotically a plateau
• characteristic of invertebrates
• as prey density increases, predation increases at slower and slower rate
• prey are dense enough that predator doesn’t have to spend time searching, only handling
• type 3
• prey consumption is a sigmoid (S-shaped) function of prey density
• at low prey densities, greater proportion of search effort is "wasted" (unsuccessful)
• triggers more attempts which then increase success rate
• plateau is eventually reached time of predator is dominated by handling time
• predator-prey cycles
• characterized by
• sharp increases in numbers followed by
• seemingly periodic crashes
• classic example is snowshoe hare and Canadian lynx cycle

Figure: snowshoe hare and lynx

• explained by two hypotheses
• top-down control
• bottom-up control
• top-down control hypothesis
• lynx prey on hare
• reduces hare population
• fewer hares supports fewer lynxes
• causes periodic reduction in lynx population
• lag-time, offset from hare reduction
• reduced numbers of predators (lynx) allows population of prey (hare) to recover and increase
• increased numbers of prey (hare) support increased numbers of predators and lynx population increases
• cycle continues
• doubt has been cast on this
• snowshoe hares have been found to exhibit similar 10-year "boom-or-bust" cycles on islands where lynx are absent
• leading to 2nd hypothesis
• bottom-up control
• bottom-up control hypothesis
• rather than cycle being driven by predator at top
• might be driven by food source of prey (hare) at bottom
• reduction in quantity or quality of food source (plants) of hare leads to crash of hare population
• fewer hare support fewer predators and lynx population crashes
• reduction in hare population gives plant population time to recover
• increased plant population supports more hares and hare population increases
• increased hare population supports more lynx and lynx population increases
• cycle continues, driven by plant availability

Figure : Population cycles of the snowshoe hare and lynx

• role in community diversity
• predator-prey relationships can help maintain community diversity
• some species have more central roles in community or ecosystem than do others
• keystone species
• not most abundant species in community
• exerts control on community structure not by its numbers but by its ecological niche
• reduces density of strongest competitors in community
• keystone species, example
• sea star (Pisaster ochraceous) of rocky intertidal zone of Washington state
• feeds preferentially on mussels, will also eat other invertebrates
• removal sea star resulted in explosion of mussel population
• mussels monopolized space and excluded other invertebrates
• community became less diverse

Figure: Testing a keystone predator hypothesis

Figure : Testing a keystone predator hypothesis

• keystone species, example
• sea otters in North Pacific
• declines in their populations (possibly due to killer whales) have resulted in destruction of kelp forests
• sea otters feed on sea urchins who feed on kelp
• in absence of sea otters, sea urchin populations explode and decimate kelp forests

Figure: Sea otters as keystone predators in the North Pacific

• Symbiotic relationships
• interaction between two or more species that live together in close proximity (on, in, very near) in relatively permanent relationships
• three types
• parasitism (also considered predation)
• commensalism
• mutualism

• parasitism
• one organism (parasite) lives in or on another organism (host)
• parasite is generally smaller than host
• one species benefits (parasite) and other species is harmed (host)
• can be viewed as type of predator-prey relationship
• organism that is "preyed" upon doesn’t necessarily die
• examples
• tapeworms, bloodflukes, apicomplexans, nematodes, leeches

Figure: The two-host life history of Plasmodium, the apicomplexan that causes malaria

Figure: The life history of a blood fluke, Schistosoma mansoni

Figure: Anatomy of a tapeworm

Figure: Parasite nematode, Trichinella spiralis

Figure: Parasitic behavior: A female Nasonia vitripennis laying a clutch of eggs into the pupa of a blowfly (Phormia regina)

• mutualism
• both species benefit from relationship
• example
• ants and acacia trees
• tree provides protein-rich structures, sugar, housing
• ants provide protection to tree from other insects

Figure: Mutualism: bacterial "headlights"

Figure : Mutualism between acacia trees and ants

• commensalism
• one species benefits and other species is not significantly affected (neither benefits nor is harmed)
• few true cases probably exist
• unlikely that one of species is truly unaffected
• example
• tropical fish living among tentacles of sea anemone gain protection and eat scraps from the anemone’s food

Figure: Commensalism between a bird and mammal

• Disturbances
• events that alter a community and usually remove organisms from it
• affect all communities
• frequency and severity vary from community to community
• can have positive or negative affects
• types include
• storms
• fire
• floods
• droughts
• overgrazing
• human activities

Figure: Storm disturbance to coral reef communities: Heron Island Reef in Australia

Figure: Storm disturbance to coral reef communities

Figure: Routine disturbance in a grassland community

Figure: Environmental patchiness caused by small-scale disturbances: A fallen tree

Figure: Patchiness and recovery following a large-scale disturbance

Figure: Large-scale disturbance: Mount St. Helens

Figure: Forest fire

• ecological succession
• process of community change that results from disturbance
• predictable transition in species composition over time following a disturbance
• ultimately producing a relatively stable, long-lasting community called
• climax community

• relatively stable, long lasting
• complex and interrelated community
• specific types that occurs depends on climate, soil type
• in some areas, the climax community never occurs

• climax community vs. successional community
• maintains mix of species for long time vs. temporary
• tends to have many specialized niches vs. generalized niches
• have more types of organisms vs, fewer types of organisms
• tend to recycle nutrients, maintain constant biomass vs. accumulate large amounts of material

• ecological succession
• concept that communities proceed through a series of regular, predictable changes in structure over time
• occurs because activities of organisms cause changes in their surroundings
• making local environment suitable for other kinds of organisms
• pace and direction affected by several factors
• two different kinds are recognized
• primary succession
• secondary succession

• primary succession
• begins with
• total lack of organisms and bare mineral surfaces or water
• less frequently observed
• usually takes very long time
• due to lack of soil and few nutrients for plants
• examples; new volcanic islands, rubble left by retreating glacier

• factors that determine rate of succession and kind of climax community
• type of substrate
• rock, sand, clay
• types of spores, seeds, vegetative structures of plants
• climate

• challenging because bare rock or soil is inhospitable place for organisms to live
• collection of organisms that can become established and survive is called
• pioneer community
• lichens
• common members
• small, slow growing, mutualistic
• each step in process from pioneer community to climax community is called
• successional stage or seral stage
• entire process is called a sere
• aquatic primary succession
• except for oceans, most aquatic ecosystems are considered temporary
• will eventually be replaced by a terrestrial ecosystem
• aquatic ecosystems receive continual input of soil and organic particles
• resulting in gradual filling of shallow bodies of water

• as sediments accumulate, different types of plants can eventually become established
• wet soil will form
• grasses will become established
• more sediments will be trapped
• secondary succession
• more commonly observed
• proceeds more rapidly
• begins with destruction or disturbance of existing ecosystem
• some soil present
• some seeds or roots from which plants can begin growing

Figure: Secondary succession on land

Fig.

Figure: Alders and cottonwoods covering the hillsides

Figure: Spruce coming into the alder and cottonwood forest

Figure: Spruce and hemlock forest