community ecology [compatibility mode] (1)

7/30/2019 Community Ecology [Compatibility Mode] (1)

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Community Ecology

MBA (DM)

What is a community?

Acommunity is an assemblage ofplant and animal populations thatlive in a particular area or habitat.

Populations of the various species ina community interact and form asystem with its own emergent

properties.

Pattern vs. Process

Pattern is what we can easily observedirectly - vegetation zonation, species lists,

seasonal distribution of activity, andassociation of certain species.

Process gives rise to the pattern-herbivory, competition, predation risk,nutrient availability, patterns of disturbance,energy flow, history, and evolution.

Community ecology seeks to explain theunderlying mechanisms that create, maintain, anddetermine the fate of biological communities.Typically, patterns are documented by observation,and used to generate hypotheses about processes,which are tested.

Not all science is experimental. Hypotheses testscan involve special observations, or experiments.


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Emergent Properties of a Community

Scale

Spatial and Temporal Structure

Species Richness

Species Diversity

Trophic structure

Succession and Disturbance

Scale is the size of a community.

Provided that the area or habitat is

well defined, a community can be a

system of almost any size, from adrop of water, to a rotting log, to a

forest, to the surface of the Indian

Ocean.

Spatial Structure is the way species are

distributed relative to each other.

Some species provide a framework thatcreates habitats for other species. Thesespecies, in turn create habitats for others,etc.

Example: Trees in a rainforest arestratified into several different levels,including a canopy, severalunderstories, a ground level, and roots.

Each level is the habitat of a distinctcollection of species. Some places,such as the pools of water that collectat the base of tree branches, mayharbor entire communities of theirown.


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Temporal structure is the timing of theappearance and activity of species. Somecommunities, i.e., arctic tundra and the decayof a corpse, have pronounced temporal species,other communities have less.

Example: Many desert plants and animals aredormant most of the year. They emerge, orgerminate, in response to seasonal rains. Otherplants stick around year round, having evolved

adaptations to resist drought.

Species Richness - is the number

of species in a community. Clearly,the number of species we can

observe is function of the area ofthe sample. It also is a function ofwho is looking. Thus, species

richness is sensitive to samplingprocedure


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Diversity is the number of species in thecommunity, and their relative abundances.

Species are not equally abundant, somespecies occur in large percentage of samples,others are poorly represented.

Some communities, such as tropicalrainforests, are much more diverse thanothers, such as the Thar desert.

Species Diversity is often expressed using

Simpsons diversity index: D=1- (pi)2

Example Problem

A community contains the following species:

Number of Individuals

Species A 104

Species B 71

Species C 19

Species D 5

Species E 3

What is the Simpson index value for thiscommunity?

Answer:

Total Individuals= (104+19+71+5+3)=202

PA=104/202=.51 PB=19/202=.09

PC=71/202=.35 PD=5/202=.03PE=3/202=.02

D=1-{(.51)2+(.09)2+(.35)2+(.03)2+(.02)2}

D=1-.40=.60

Clicker Question

In the example above, what was thespecies richness?

A. .60B. 202 individuals

C. 5 species

D. .40

E. None of the above


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Succession, Disturbance andChange

In terms of species and physicalstructure, communities change withtime. Ecological succession, the predictable change in species

over time, as each new set of species modifies theenvironment to enable the establishment of other species, isvirtually ubiquitous.

Example; a sphagnum bog community

may persist for only a few decades beforethe process of ecological succession

changes transform it into the surroundingBlack Spruce Forest.

A forest fire may destroy a large area of

trees, clearing the way for a meadow.Eventually, the trees take over and the

meadow is replaced.

Disturbances are events suchas floods, fire, droughts,

overgrazing, landslides, andhuman activity that damagecommunities, removeorganisms from them, and alterresource availability.

Some Agents of Disturbance

Fire

Floods

Drought Landslides

Storms

Volcanoes

Human Activity


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Disturbance creates opportunities for newspecies to invade an area and establishthemselves.

These species modify the environment, andcreate opportunities for other species to invade.The new species eventually displace theoriginal ones. Eventually, they modify the

environment enough to allow a new series ofinvaders, which ultimately replace them, etc.

Disturbance, Invasion, Succession

Invasion:

Disturbance creates an ecological vacuum that canbe filled from within, from outside, or both. Forexample, forest fires clear away old brush and openup the canopy, releasing nutrients into the soil at thesame time. Seeds that survive the fire germinateand rapidly grow to take advantage of this

opportunity. At the same time, wind-borne andanimal-dispersed seeds germinate and seek to do thesame thing.

The best invaders have good dispersal powers andmany offspring, but they are often not the bestcompetitors in the long run.

Succession Disturbance of a community is usually

followed by recovery, called ecologicalsuccession.

The sequence of succession is driven by the

interactions among dispersal, ecologicaltolerances, and competitive ability.

Primary succession-the sequence of species onnewly exposed landforms that have not previouslybeen influenced by a community, e.g., areas exposedby glacial retreat.

Secondary succession occurs in cases which

vegetation of an area has been partially or completelyremoved, but where soil, seeds, and spores remain.


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Early in succession, species are generally excellentdispersers and good at tolerating harshenvironments, but not the best interspecificcompetitors.

As ecological succession progresses, they arereplaced with species which are superiorcompetitors, (but not as good at dispersing andmore specialized to deal with themicroenvironments created by other species likelyto be present with them).

Early species modify their environment in such away as to make it possible for the next round ofspecies. These, in turn, make their ownreplacement by superior competitors possible.

Aclimax community is a more

or less permanent and final stageof a particular succession, oftencharacteristic of a restricted area.

Climax communities are characterizedby slow rates of change, comparedwith more dynamic, earlier stages.

They are dominated by species

tolerant of competition for resources.

An Influential ecologist named F.E. Clementsargued that communities work like anintegrated machine. These closedcommunities had a predictable composition.

According to Clements, there was only one true

climax in any given climatic region, which wasthe endpoint of all successions.

Other influential ecologists, including Gleason,hypothesized that random events determinedthe composition of communities.

He recognized that a single climatic area couldcontain a variety of specific climax types.

Evidence suggests that for manyhabitats, Gleason was right, many

habitats never return to their original state

after being disturbed beyond a certainpoint.

For example; very severe forest fires havereduced spruce woodlands to a terrain ofrocks, shrubs and forbs.


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An incredibly rapid glacial retreat is occurring inGlacier Bay, Alaska. In just 200 years, a glacierthat once filled the entire bay has retreated over100km, exposing new landforms to primarysuccession.

Clements would have predicted that succession todaywould follow the sequence of ecological successionthat has occurred in the past for other parts ofAlaska.

In fact, three different successional patterns seem to

be occurring at once, depending upon localconditions. Thus, Clements view of succession issomewhat of an oversimplification.

Are Climax Communities Real?

Succession can take a long time. For example, old-field succession may require

100-300 years to reach climax community. Butin this time frame, the probability that aphysical disturbance (fire, hurricane, flood) willoccur becomes so high, the process ofsuccession may never reach completion.

Increasing evidence suggests that someamount of disturbance and nonequilibriumresulting from disturbance is the norm for mostcommunities.

One popular hypothesis is that communities areusually in a state of recovery from disturbance.

An area of habitat may form a patchwork ofcommunities, each at different stages of ecologicalsuccession. Thus, disturbance and recoverypotentially enable much greater biodiversity than ispossible without disturbance.

Are biological communities realfunctional units?

Do communities have a tightly prescribed organizationand composition, or are they merely a looseassemblage of species?

This is an unsolved problem in ecology.

Clements argued that communities are stable,functional units with a fixed composition-eachintegrated part needs the others. Every area shouldultimately have the same species, given time.

Gleason argued that their composition is unstable andvariable-they are more like assemblages of everythingthat can live together in one place


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The Kiddie Pool Experiment

Jenkins and Buikema conducted an experiment tosee whether artificial ponds would developpredictable assemblages of freshwatermicroorganisms.

-if this were the case, it would support the notionthat communities are real, integrated units.

-They set up 12 identical ponds and filled themwith sterile water. Came back in year to study the

composition of the resulting communities.

Result-the ponds had very different compositions ofspecies.

Accidents of dispersal, and different dispersalcapabilities affected which species ended up in eachpond.

The early arrival of certain competitors, andpredators greatly affected the ability of later speciesto colonize later.

-Gleasons view was supported. Composition ofcommunities is dictated largely by chance andhistory.

Trophic structure is the hierarchy of feeding. Itdescribes who eats whom

(a trophic interaction is a transfer of energy: i.e.,

eating, decomposing, obtaining energy viaphotosynthesis).

For every community, a diagram of trophicinteractions called a food web.

Energy flows from the bottom to the top.

A Simple Food Web

Killer Whales

SharksHarbor Seals

Yellowfin Tuna

Mackerel Cod Halibut

Zooplankton

Unicellular Algae and Diatoms


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One paththrough a

food web is a

food chain.

Killer Whales

Harbor Seals

Mackerel

Zooplankton

Phytoplankton

The niche concept is very important in

community ecology.

Aniche is an organisms habitat and

its way of making a living.

An organisms niche is reflected by itsplace in a food web: i.e, what it eats,what it competes with, what eats it.

Each organism has the potential to

create niches for others.

Keystone species are disproportionately importantin communities.

Generally, keystone species act to maintain species

diversity. The extinction of a keystone species eliminates the

niches of many other species.

Frequently, a keystone species modifies theenvironment in such a way that other organisms areable to live, in other cases, the keystone species is apredator that maintains diversity at a certain trophiclevel.

Examples of Keystone Species

California Sea Otters: This species preys upon seaurchins, allowing kelp forests to become established.

Pisaster Starfish: Grazing by Pisaster prevents the

establishment of dense mussel beds, allowing otherspecies to colonize rocks on the pacific coast

Mangrove trees: Actually, many species of treesare called mangrove trees. Their seeds disperse insalt water. They take root and form a dense forest insaltwater shallows, allowing other species to thrive


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Trophic Cascades

Species at one trophic level influence species atother levels; the addition or subtraction of speciesaffects the entire food web.

This causes positive effects for some species, andnegative effects for others. This is called atrophic cascade. For instance, removing asecondary consumer might positively affect theprimary consumers they feed upon, and

negatively affect the producers that are food forprimary consumers.

Topdown vs. Bottom up

Most biological communities have both top-downand bottom-up effects on their structure andcomposition.

In a well known study of ponds by MatthewLeibold, it was demonstrated that the biomass ofherbivores (zooplankton) was positively correlatedto the biomass of producers (algae), indicating atop down effect.

He intentionally introduced fish to some ponds,The result was a decrease in zooplankton andincrease in producers, indicating a top downeffect.

Badly scanned from

Rose and Mueller (2006)

Types of InterspecificInteractions

Effect on Effect on

Species 1 Species 2

Neutralism 0 0

Competition - -

Commensalism + 0

Amensalism - 0

Mutualism + +

Predation, - +

Parasitism, Herbivory


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Neutralism

Neutralism the most common type ofinterspecific interaction. Neitherpopulation affects the other. Anyinteractions that do occur are indirect orincidental.

Example: the tarantulas living in adesert and the cacti living in a desert

Competition Competition occurs when organisms in the same

community seek the same limiting resource.This resource may be prey, water, light,nutrients, nest sites, etc.

Competition among members of the samespecies is intraspecific.

Competition among individuals of differentspecies is interspecific.

Individuals experience both types of

competition, but the relative importance of thetwo types of competition varies from populationto population and species to species

Styles of Competition

Exploitation competition occurswhen individuals use the same limitingresource or resources, thus depletingthe amount available to others.

Interference competition occurswhen individuals interfere with theforaging, survival, or reproduction ofothers, or directly prevent their physicalestablishment in a portion of a habitat.

Some specific types ofcompetition

Consumptive competition

Preemptive competition

Overgrowth competition Chemical composition

Territorial competition

Encounter competition


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Example of Interference Competition

The confused flour beetle, Triboleum confusum, andthe red flour beetle, Triboleum castaneumcannibalize the eggs of their own species as well asthe other, thus interfering with the survival ofpotential competitors.

In mixed species cultures, one species alwaysexcludes the other. Which species prevails dependsupon environmental conditions, chance, and therelative numbers of each species at the start of the

experiment.

Outcomes of Competition Exploitation competition may cause the exclusion of

one species. For this to occur, one organism mustrequire less of the limiting resource to survive. Thedominant species must also reduce the quantity of theresource below some critical level where the otherspecies is unable to replace its numbers byreproduction.

Exploitation does not always cause the exclusion ofone species. They may coexist, with a decrease intheir potential for growth. For this to occur, they mustpartition the resource.

Interference competition generally results in theexclusion of one of the two competitors.

The Competitive Exclusion Principle Early in the twentieth century, two mathematical

biologists, A.J. Lotka and V. Volterra developed amodel of population growth to predict theoutcome of competition.

Their models suggest that two species cannotcompete for the same limiting resource for long.Even a minute reproductive advantage leads tothe replacement of one species by the other.

This is called the competitive exclusionprincipal.

Evidence for Competitive Exclusion.

A famous experiment by the Russian ecologist,G.F. Gausse demonstrated that Parameciumaurelliaoutcompetes and displaces Parameciumcaudatumin mixed laboratory cultures,

apparently confirming the principle. (Interestingly, this is not always the case. Later

studies suggest that the particular strains involvedaffect the outcome of this interaction).


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Other experiments...

Subsequent laboratory studies on otherorganisms, have generally resulted incompetitive exclusion, provided that theenvironment was simple enough.

Example: Thomas Park showed that,via interference competition, theconfused flour beetle and the red flowerbeetle would not coexist. One speciesalways excluded the other.

Resource Partitioning

Species that share the samehabitat and have similar needs

frequently use resources insomewhat different ways - so thatthey do not come into direct

competition for at least part of the

limiting resource. This is calledresource partitioning.

Resource partitioning obviates competitiveexclusion, allowing the coexistence ofseveral species using the same limitingresource.

Resource partitioning could be an

evolutionary response to interspecificcompetition, or it could simply be thatcompetitive exclusion eliminates allsituations where resource partitioning doesnot occur.


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One of the best known cases of resourcepartitioning occurs among Caribbean anoles.

As many as five different species of anoles mayexist in the same forest, but each stays restricted toa particular space: some occupy tree canopies,some occupy trunks, some forage close to theground.

When the brown anole was introduced to Floridafrom Cuba, it excluded the green anole from thetrunks of trees and areas near the ground: thegreen anole is now restricted to the canopies of

trees:the resource (space, insects) has beenpartitioned among the two species

(for now at least, this interaction may not be stable in thelong run because the species eat each others young).

Character Displacement Sympatric (speciation that occurs in the

absence of geographic isolation) populations ofsimilar species frequently have differences inbody structure relative to allopatric (speciationresulting from geographical separation from acommon ancestor) populations of the samespecies.

This tendency is called characterdisplacement.

Character displacement is thought to be anevolutionary response to interspecificcom etition.

Example of Character Displacement

The best known case of character displacement occursbetween the finches, Geospiza fuliginosaandGeospiza fortis, on the Galapagos islands.

When the two species occur together, G. fuliginosa

has a much narrower beak that G fortis. Sympatricpopulations ofG fuliginosaeats smaller seeds than Gfortis: they partition the resource.

When found on separate islands, both species havebeaks of intermediate size, and exploit a wider varietyof seeds.

These inter-population differences might have evolvedin response to interspecific competition.

Competition and the Niche

An ecological niche can be thought of in terms ofcompetition.

The fundamental niche is the set of resources andhabitats an organism could theoretically use under

ideal conditions.

The realized niche is the set of resources andhabitats an organism actually used: it is generallymuch more restricted due to interspecific competition(or predation.)


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Two organisms cannot occupyexactly the same niche.

This is sometimes called Gausses

rule(although Gausse never put it exactlythat way).-Experiments by Gausse (Paramecium), Peter Frank(Daphnia), and Thomas Park (Triboleum) have

confirmed it for simple laboratory scenarios.

-This creates a bit of a paradox, because so many

species exist in nature using the same resources.

-The more complex environments found in naturemay enable more resource partitioning.

Amensalism

Amensalism is when one speciessuffers and the other interacting speciesexperiences no effect.

Example: Redwood trees falling intothe ocean become floating battering-rams during storms, killing largenumbers of mussels and other inter-tidal organisms.

Allelopathy involves the productionand release of chemical substances byone species that inhibit the growth ofanother. These secondary

substances are chemicals produced byplants that seen to have no direct usein metabolism.

This same interaction can be seen asboth amensalism, and extremely one-sided interference competition-in fact itis both.

Example: Allelopathy in the California Chaparral

Black Walnut (Juglans nigra) trees excrete anantibiotic called juglone. Juglone is known to inhibitthe growth of trees, shrubs, grasses, and herbsfound growing near black walnut trees.

Certain species of shrubs, notably Salvia leucophylla(mint) andArtemisia californica(sagebrush) areknown to produce allelopathic substances thataccumulate in the soil during the dry season. Thesesubstances inhibit the germination and growth ofgrasses and herbs in an area up to 1 to 2 metersfrom the secreting plants.


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Commensalism Commensalism is an interspecific interaction where one

species benefits and the other is unaffected.

Commensalisms are ubiquitous in nature: birds nesting intrees are commensal.

Commensal organisms frequently live in the nests, or on thebodies, of the other species.

Examples of Commensalism:

Ant colonies harbor rove beetles as commensals. Thesebeetles mimic the ants behavior, and pass as ants. They eatdetritus and dead ants.

Anemonefish live within the tentacles of anemones. They

have specialized mucus membranes that render themimmune to the anemones stings. They gain protection byliving in this way.

Mutualism

Mutualism in an interspecific interactionbetween two species that benefits bothmembers.

Populations of each species grow, surviveand/or reproduce at a higher rate in thepresence of the other species.

Mutualisms are widespread in nature, andoccur among many different types oforganisms.

Examples of Mutualism Most rooting plants have mutualistic associations

with fungal mychorrhizae. Mychorrhizae increasethe capability of plant roots to absorb nutrients. Inreturn, the host provides support and a supply ofcarbohydrates.

Many corals have endosymbiotic organisms calledzooxanthellae (usually a dinoflagellate). Thesemutualists provide the corals with carbohydratesvia photosynthesis. In return, they receive arelatively protected habitat from the body of thecoral.


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Mutualistic Symbiosis Mutualistic Symbiosis is a type of mutualism

in which individuals interact physically, or evenlive within the body of the other mutualist.Frequently, the relationship is essential for thesurvival of at least one member.

Example: Lichens are a fungal-algal symbiosis (thatfrequently includes a third member, acyanobacterium.) The mass of fungal hyphaeprovides a protected habitat for the algae, and takes

up water and nutrients for the algae. In return, thealgae (and cynaobacteria) provide carbohydrates as asource of energy for the fungus.

Facultative vs. Obligate Mutualisms

Facultative Mutualisms are notessential for the survival of eitherspecies. Individuals of each speciesengage in mutualism when the otherspecies is present.

Obligate mutualisms are essential forthe survival of one or both species.

Other Examples of Mutualisms

Flowering plants and pollinators. (bothfacultative and obligate)

Parasitoid wasps and polydna viruses.

(obligate)Ants and aphids. (facultative)

Termites and endosymbiotic protozoa.(obligate)

Humans and domestic animals. (mostlyfacultative, some obligate)

Predation, Parasitism,Herbivory

Predators, parasites, parasitoids, and

herbivores obtain food at the expense of theirhosts or prey.


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Predators tend to be larger than

their prey, and consume many preyduring their lifetimes.

Parasites and pathogens are

smaller than their host. Parasitesmay have one or many hosts

during their lifetime. Pathogensare parasitic microbes-manygenerations may live within the

same host. Parasites consumetheir host either from the inside

(endoparasites) or from theoutside (ectoparasites).

Parasitoids hunt their prey likepredators, but lay their eggs within thebody of a host, where they develop likeparasites.


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Herbibores are animals that eatplants. This interaction may resemblepredation, or parasitism.

Predator-Prey and Parasite-HostCoevolution

The relationships between predatorand prey, and parasites and hosts,have coevolved over long periods oftime.

About 50 years ago, an evolutionary biologist namedJ.B.S. Haldane suggested that the interactionbetween parasite and host (or predator and prey)should resemble an evolutionary arms race:

First a parasite (or predator) evolves a trait that allows it toattack its host (or prey).

Next, natural selection favors host individuals that are able to

defend themselves against the new trait. As the frequency of resistant host individuals increases, there

is natural selection for parasites with novel traits to subvertthe host defenses.

This process continues as long as both species survive.

Recent data on Plasmodium, the cause of malaria, supportthis model.


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Example of Parasite-Host Coevolution

The common milkweed,Asclepias syriacahas leavesthat contain cardiac glycosides: they are verypoisonous to most herbivores. This renders themvirtually immune to herbivory by most species.

Monarch butterfly larvae have evolved the ability totolerate these toxins, and sequester them within theirbodies. They are important specialist herbivores ofmilkweeds.

These sequestered compounds serve the additional

purpose of making monarch larvae virtually inedible tovertebrate predators.

Predator-Prey Population Dynamics

Predation may be a density-dependent mortality factor tothe host population-and prey may represent a limitingresource to predators.

The degree of prey mortality is a function of the density ofthe predator population.

The density of the prey population, in turn, affects the birthand death rates of the predator population.

i.e, when prey become particularly common, predatorsincrease in numbers until prey die back due to increasedpredation, this, in turn, inhibits the growth of prey.

Typically, there is a time lag effect.

There is often a dynamic balancebetween predators and prey that is

necessary for the stability of bothpopulations.

Feedback mechanisms may controlthe densities of both species.

Example of Regulation of Host Population by aHerbivore

In the 19th century, prickly pear cactus, Opuntia sp. wasintroduced into Australia from South America. Because noAustralian predator species existed to control the populationsize of this cactus, it quickly expanded throughout millions ofacres of grazing land.

The presence of the prickly pear cactus excluded cattle andsheep from grazing vegetation and caused a substantialeconomic hardship to farmers.

A method of control of the prickly pear cactus was initiatedwith the introduction ofCactoblastis cactorum, a cactuseating moth from Argentina, in 1925. By 1930, densities ofthe prickly pear cactus were significantly reduced.


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Sometimes predator species can drive their prey tolocalized extinction.

If there are no alternate prey, the predator then goes extinct.

If the environment is coarse grained (habitat or landscapein which vagility of an animal sp. is low relative to the size ofthe patch), this makes the habitat available for recolonizationby the prey species.

Example: The parasitic wasp Dieratiella rapaeis a veryefficient parasitoid. One female can oviposit into severalhundred aphids during its lifetime. Frequently, aphids aredriven locally extinct and the adults must search for newpatches when they emerge. Once the aphid and the host are

gone, the host plants may become re-infested with aphids.

In other cases, there are alternateprey to support the predator andthe prey is permanently excluded.

Example: Freshwater fish such as

bluegills and yellow perchfrequently exclude smallinvertebrates such as Daphnia

pulexfrom ponds. The fish thenswitch to other prey such as insects

larvae.

The time-lag effect may lead to predator-prey oscillations.

Most predators do not respond instantaneously to theavailability of prey and adjust their reproductionaccordingly.

If predator populations grow faster than prey

populations, they may overshoot the number of preythat are able to support them

This leads to a rapid decline in the prey, followed by arapid decline in the predator.

Once the predator becomes rare, the prey populationmay begin growing again.

This pattern is called a predator-prey oscillation.

Cycles in the population dynamics of the snowshoe hareand its predator the Canadian lynx (redrawn from

MacLulich 1937). Note that percent mortality is an elusivemeasure, it may, or may not, be useful since mortalityvaries with environment and time.


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In the 1920s, A. J. Lotka (1925) and V. Volterra (1926)devised mathematical models representing host/preyinteraction.

The Lotka-Volterra curve assumes that prey destruction is afunction not only of natural enemy numbers, but also of preydensity, i.e., related to the chance of encounter.

This model predicts the predator-prey oscil lations sometimesseen in nature. Populations of prey and predator werepredicted to flucuate in a regular manner (Volterra termedthis "the law of periodic cycle").

Lotka-Volterra model is an oversimplification of reality. Innature, many different factors affect the densities ofpredators and their prey.

Interme iate Distur anceHypothesisOften applies well to succession

disturbance-mediated

coexistence

Disturbance frequency

Humans can cause both high and

intermediate disturbance

EarlySuccession

LateSuccession Mid-Succession

Algae species in a stream fit the IDH

Figure 20.8Algalspecies

div

ersityH


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Summary ofdisturbance & succession

Biodiversity increases with time, overall, insuccession.

Moderate disturbance can provide the greatestbiodiversity because it prevents competitiveexclusion.

Successions endpoint depends on the climateultimately (Biodiversity varies with the climate).

The endpoint of succession is ecological stability

because the community structure persists throughtime. [biodiversity and ecological stability arerelated]

community ecology [compatibility mode] (1)

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