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Analyzing GeographicDistributions

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Transplant Experiments

Moving individuals of a species to anunoccupied area and determining whether theycan survive and reproduce in the newenvironment

Some organisms can survive but cannotreproduce in an area; follow transplantexperiments for at least one generation!

Useful for determining whether the limitation ondistribution of a species results from theinaccessibility of an area

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A control is necessary to provideinformation on the effects of handling andtransplanting individual plants or animals.

Includes transplants done within thedistribution

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Transplant successful

The potential range of a species is largerthan its actual range.

Possibly cannot move into its potentialrange or lacks suitable means to do so;problems with dispersal

Can move into new areas, but does not dothat; study mechanism of habitatselection

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Transplant unsuccessful

Excluded by abiotic factors or biotic interactionwith other species

Negative effects of predators, parasites,competitors

Positive effect of interdependent species within

the actual range May be tested by conducting transplant

experiments with protective devices (caging)

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If other species do not set the limits on theactual range, some physical or chemicalfactors may!

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Disastrous when pests are introduced tonew areas

Indiscriminate transplanting of organismscontains all the seeds of ecologicaldisaster!

Most governments have stringent rulesprohibiting the importation of plants andanimals from other regions.

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DispersalMovement of Organisms

Ecological process affecting distributions

Of primary interest to ecologists andbiogeographers

Biogeography is the study of thegeographical distribution of life on Earth andthe rationale behind these distributions.

Biogeographers are interested in historicalchanges in distribution of animals and plants.

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Dispersal

One of the least studied aspects ofpopulation dynamics: the branch ofpopulation ecology concerned with factors

influencing the expansion, decline ormaintenance of populations

Dispersal can increase or decrease local

population densities. Immigration: moving in

Emigration: moving out

Migration: seasonal movements

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Modes of Dispersal

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Modes of Dispersal

Three modes of dispersal according toPielou (1979):

Diffusion

Jump dispersal

Secular dispersal

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Modes of DispersalDiffusion

Gradual movement of a population acrosshospitable terrain for a period of severalgenerations

Example: cane toad in Australia

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Cane Toad (Bufo marinus)

Native to Central and South America fromMexico to Brazil

Widely introduced in the 1930s to islands

in the Caribbean and the Pacific becauseit was believed to control scarab beetles,pests of sugar cane!

Brought to Queensland, Australia, in 1935,where it failed to control any insect pests;became a pest itself!

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Eat almost anything, but mainly insects

Reproduce prolifically; 8,000 to 35,000 eggstwice a year!

Toxic to many potential predators; some speciesof snakes are developing resistance to the toxin!

All forms of the toad poisonous, as they contain

a poison that causes cardiac arrest in humans Humans eating cane toad eggs have died from

the toxin!

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Cane toads have been moving west (fromQueensland) at ca. 40 km per year.

Individual marked toads have moved up to1.8 km per night, along roads (convenienthabitat corridors for their rapid spread).

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Modes of DispersalJump Dispersal

Movement of individual organisms acrosslarge distances followed by the successfulestablishment of a population in the new

area Occurs in a short period of time, during the

life of an individual

Usually occurs across unsuitable terrain Example: island colonization; human

introductions (assisted jump dispersal)

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Modes of DispersalSecular Dispersal

Occurs in evolutionary time and is rarely ofimmediate interest for ecologists working inecological time

If diffusion occurs in evolutionary time, thespecies undergoes extensive evolutionarychange in the process.

Geographic range of a secularly dispersing

species expands over geologic time, but naturalselection causes the migrants to diverge fromthe ancestral population.

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At the end of the Ice Age, as glaciers retreatedfrom Europe and North America, one of the mostspectacular colonizations occurred.

In 1899, Charles Reid, a British botanist, wasinterested in how trees re-colonized the BritishIsles after the Ice Age.

From the melting of the ice around 10,000 years

ago, till the British Isles were invaded by theRomans in AD 50, trees such as oak expandedtheir range 1000 km northward!

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Oaks mature at age 10 50 years, and seedsare dropped at an average of 30 meters fromparents.

If a tree produces a million seeds per generationover 300 generations and the seeds disperse 30meters with each generation, the simple diffusionmodel predicts a range extension of 36 km!

Reids Paradox: Observed large discrepancybetween the rapid rate of movement of trees re-colonizing areas at the end of the Ice Age andthe observed slow dispersal rate of tree seedsspreading by diffusion.

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Seed Dispersal

Tree seed dispersal may be mapped byputting out seed traps at differentdistances from the parent tree or by

mapping the location of seedlingsproduced by isolated trees.

S d Di l

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Seed DispersalCornus controversa, a deciduous tree

Mean seed dispersal = 6.7 meters

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Seed Dispersal

Average seed dispersal distances fortwelve species of temperate trees in theSouthern Appalachians estimated to be 4

34 meters

Too small to account for recolonization bysimple diffusion after the ice melted!

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Seed Dispersal

The answer to Reids paradox lies in haphazard,long range dispersal of seeds by winds andanimals!

Colonization rates are not driven by the meandispersal distance but by extreme dispersalevents!

Extreme dispersal events very difficult to record

because less than one in 10,000 seeds ends upbeing blown a long distance by the wind ortransported a great distance by an animal!

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Dispersal

May be affected by barriers: geographicfeatures that hinder or prevent dispersalacross it, producing isolation

Barriers not always the factor limitinggeographic ranges!

Introduced species may be unable to

survive in a new area!

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Dispersal

Humans have moved many speciesaround the globe during the past 200years, often with disastrous

consequences!

Failure to establish a species rarelystudied!

Accidental introductions only recordedwhen successful!

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Dispersal

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Dispersal

Continental bird introductions successful10 to 30% of the time!

Success rate generally higher on islands!

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Dispersal

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Dispersal

Tens Rule (Williamson and Fitter, 1996):The rule of thumb that 1 in 10 speciesimported into a country becomes

introduced, 1 in 10 of the introducedspecies become established and 1 in 10 ofthe established species become pests.

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Dispersal

According to the Tens Rule, the transition fromone state to another has a probability of ca. 10%(5 to 20%).

Rule does not apply to all taxonomic groups:

Vertebrates introduced between Europe and NorthAmerica Success at each step ca. 50%

Aquatic species in Europe 63% of introductionsbecome established.

Fewer than 10% of imported aquatic species become

introduced in Europe. Of the established non-native plants in the USA, 6

13% have invaded natural areas.

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Dispersal

Imported species are often successfulenough to encourage strong quarantineactions for all groups!

Species introduced by humans constitutespectacular examples of dispersal and itseffect on distribution!

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DispersalZebra Mussels (Dreissena polymorpha)

Fingernail sized mussel native to theCaspian Sea

Forms dense clusters on hard surfaces

and grows rapidly Discovered in 1988 in Lake St. Claire,

near Detroit, MI

Possibly introduced ca. 1985, by a shipfrom a freshwater port in Europe throughballast water

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Must have grown unnoticed for ca. 3years; noticed when they reached adensity of 750,000 individuals per square

meter!

Blocked water intakes of city watersystems, electrical power stations,

industrial facilities

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Rapid spread in river systems ofcentral USA

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Efficient filter feeders; positive effect onwater quality

Make the water clearer; increase thegrowth of rooted aquatic plants

Feed on phytoplankton, thus depressingpopulations of zooplankton

Smother native clam species by colonizingany available hard surface


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Why Dispersal?

Expanding populations

Climate change

Food availability

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Dispersal of Expanding Populations

Collared doves(Streptopeliadecaocto) spread from

Turkey into Europeafter 1900.

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Dispersal began suddenly:

Not influenced by humans

Took place in small jumps at around 45km/yr

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R id Ch i R

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Rapid Changes in Response toClimate Change

Organisms began to spread northwardabout 16,000 years ago following retreat ofglaciers and warming climate.

Evidence found in preserved pollen in lakesediments

Movement rate 100 - 400 m/yr

R id Ch i R

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Rapid Changes in Response toClimate Change

Di l i R Ch i

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Dispersal in Response to ChangingFood Supply

Holling (1959) observed numericalresponses to increased prey availability.

Increased prey density led to increased

density of predators.

Individuals move into new areas inresponse to higher prey densities.

Di l i R t Ch i

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Korpimaki and Norrdahl (1991) conducteda ten-year study on voles and theirpredators.

Di l i R t Ch i

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Dispersal in Rivers and Streams

One of the most distinctive features ofstream and river environments is current,the downstream flow of water.

Why doesnt the current eventually

wash all stream/river organisms out tothe sea?

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Muller (1954, 1974) hypothesized thatpopulations are maintained via dynamicinterplay between downstream and

upstream dispersal. Colonization Cycle

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A wave of migrating snailsin the Rio Claro, Costa

Rica

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Close up ofmigrating snails

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Dispersal

Problems associated with studyingdispersal Most dispersals probably go unnoticed as

detailed distribution is known for so fewspecies; i.e. dispersal of individuals amongdifferent parts of a species range may beoccurring very often.

Organisms may disperse to a new area butnot colonize it because of biotic or physicalfactors.

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Dispersal

Genetic process affecting geographicdifferentiation

Colonization successful

Dispersal results in gene flow. Genetic structure of the population is affected.

Founder effect

Occurs when dispersing individuals are not a random

sample of the population

Newly established population genetically quite distinctfrom the original population!

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Dispersal

Mosquitoes (and other insectspecies) may be transportedlong distances by wind.

Studying the dispersal ofdisease-carrying mosquitoesimportant as the distances they

disperse determine wherecontrol work must be done! Malaria control zones in tropical

countries typically use a 2 kmbarrier surrounding humanhabitations as mosquitoes

rarely move that far! Salt marsh mosquitoes havebeen captured on oil rigs 74106 km away from shore inLouisiana and around 96 kminland in Australia!

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C l i ti d E ti ti

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Colonization and ExtinctionThe Case of Krakatau

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Colonization and Extinction

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Colonization and ExtinctionMarsupials

Terrestrial mammals other than bats do noteasily cross saltwater barriers.

Marsupials Kangaroos, Possums andOpossums became isolated in South America

and Australia early in the Tertiary (60 millionyears ago). New Zealand had no native marsupials or other

land mammals except for two species of bats atthe time the first Europeans arrived!

Of the placental mammals, only rodents andbats were able to colonize Australia beforehumans arrived!


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South America was isolated from NorthAmerica by a water gap across CentralAmerica through most of the Tertiary!

Became connected to North America ca.2million years ago! Mammals dispersed inboth directions!

Many South American mammals becamereplaced by North American ones.

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Difficult access, limited dispersal power ofdifferent species and adaptive radiationhas produced island faunas and floras of

unique nature! Plants and animals of Hawaii

The Galapagos islands

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Colonization and ExtinctionAntarctic Beech (Nothofagusspp.)

Nothofagusseeds heavy; poorly adaptedfor jump dispersal

Species probably spread slowly overland

by diffusion and have been stopped by thesea

Present distribution a byproduct of

continental drift!

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Habitat Selection

Some organisms choose not occupy all theirpotential range; they choose not to live in certainhabitats!

Habitat: part of the biosphere where a species

can live either temporarily or permanently Habitat selection is one of the most poorlyunderstood ecological processes!

Typically thought with respect to animals

Plants show habitat preferences, but cannotactively move from one habitat to another; seedsor spores arrive at a habitat by dispersal andmay survive or die!

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Habitat Selection

Assuming that animals cannot liveeverywhere, natural selection will favor thedevelopment of sensory systems that can

recognize suitable habitats! Look at places from the animals point of

view! Areas very similar to a human

observer may be very different to amosquito!

Habitat Selection

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Habitat SelectionAnopheline Mosquitoes

Important disease vectors Ecology well studied because of the practical

problems of malaria eradication

Each species usually associated with aparticular type of breeding sites

Large areas of water seem to be free ofAnophelesmosquitoes! Why?

The female mosquito selects the site in which to layits eggs!

Selection not only based on whether the water issuitable for larval growth and development!

Habitat Selection

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In southern India, Anopheles culicifacies, amalaria vector, does not occur in rice fieldsafter the plants grow to a height of 12

inches or more! Those older rice fieldssupport two other species of Anopheles.

If the eggs of Anopheles culicifaciesaretransplanted into old rice fields, the larvae

survived and produced a normal numberof adults!

Habitat Selection

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Absence of this species due to theselection of oviposition sites by females

Limiting factor is the physical barrier posed

by rice plants of a certain height

Vertical bamboo strips and glass rodsplaced vertically in small ponds also

deterred female mosquitoes from layingeggs!

Habitat Selection

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Female mosquitoes oviposit while flying.

Perform a hovering dance, never touchingthe water, but remaining ca. 5 10 cm

above it

Physical obstructions prevent femalemosquitoes from performing the dance

freely.

E l i f H bi P f

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Evolution of Habitat Preference

Natural selection favors individuals that use thehabitats in which the most progeny can beraised successfully.

Natural selection may act directly upon the

behaviors that result in habitat choice or it mayselect for individuals that have the capacity tolearn which habitat is appropriate

Populations in marginal habitats may besustained only by a net outflow of individualsfrom preferred habitats.

E l i f H bi P f

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Directional selection for specifichabitat features that increase the

probability of successful nesting

In birds, survivaland reproductiondepends on nest

site choices! May be the basis

for nest sitepreference!

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Evolution of Habitat PreferenceBlue-Winged Teal (Anas discors)

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Evolution of Habitat PreferenceFretwell 1972

Individuals free tomove into anyhabitat without anyconstraint; Ideal

Free Distribution

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Evolution of Habitat PreferenceFretwell 1972

Model predicts that when density is high,both good and poor habitats will haveequal suitabilities but different densities!

Individuals will be crowded in the mostsuitable habitat and occur at lowerdensities in the less suitable habitats.

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E l ti f H bit t P f

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Problems arise whenever habitats change. We create and destroy new habitats; some

species have responded by invading andcolonizing our habitats (rats, mosquitoes).

We do not know how rapidly organisms canchange the genetic and behavioral machinerythat results in habitat selection!

Organisms with fixed, genetically programmedhabitat selection may require considerable timeto evolve the necessary machinery that willenable them to select a new suitable habitat.

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E ploitati e Interactions

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Exploitative Interactions

Exploitation: Interaction betweenpopulations that enhances fitness of oneindividual while reducing fitness of the

exploited individual Predators kill and consume other organisms. Parasites live on host tissue and reduce host

fitness, but do not generally kill the host.

Parasitoid is an insect larva that consumesthe host.

Pathogens induce disease.

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Limitation by Predators

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Work on intertidal invertebrates hasprovided classic examples of the influenceof predation on distribution; example:

Kitching and Ebling (1967) Studies conducted at Lough Ine, an arm of

the sea on the south coast of Ireland


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yKitching and Ebling 1967

The common mussel (Mytilus edulis) Widespread species on exposed rocky coasts

in southern Ireland and throughout the world

Small mussels (less than 25 mm long)abundant on the exposed rocky Atlantic coastbut rare or absent within Lough Ine and moreprotected parts of the coast

Large mussels (30 70 mm long) abundant inthe northern end of the lough!


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Transferred pieces of rock with Mytilusattached from various parts of the lough toothers

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Small Mytilusdisappeared quickly whentransplanted anywhere in Lough Ine butdid not disappear if transplanted to the

open coast. The rapid loss suggests predators are

responsible.


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Large

mussels


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Large Mytilusnaturally occur in thesouthwestern part of the lough.

Disappeared rapidly from most stations,

except places where they occurrednaturally

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Situation possibly more complex;exploitative interactions weave populationsinto a web of relationships that defies easy

generalization. Predator may feed on a variety of prey

species.

Each prey species may in turn be fed onby several predatory species.


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Prey may restrict distribution of itspredator!

If the prey is to restrict the predators

range, the predator must be veryspecialized and feed on one or twospecies of prey only.

Specialist or monophagous predator

(most common among insects notvertebrates)

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Disease and Parasitism

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Native Bird Fauna of Hawaii

Native birds common atelevation >1500 m

Introduced birdscommon at elevation

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Malaria parasite mostcommon at intermediateelevations


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The main malaria vector, the mosquito Culexquinquefasciatus, is most common in lowland areas.

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Extinction of native bird fauna of Hawaii intwo pulses

After 1900:

Extinction due to avian malaria Birds that went extinct at this time lived in the mid-

elevation forests where malaria parasites weremost prevalent.

Native birds retreated to high elevation forestswhere mosquitoes are rare.

Effect of climate change?

Allelopathy

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Allelopathy

Some organisms, particularly plants, maybe limited in local distribution by poisonsor antibiotics, also called allelopathic

agents. The action of penicillin among

microorganisms is a classic!

Allelopathy

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Allelopathy

Soil sickness due to toxic secretions fromplants

For a piece of ground continuously planted

in one crop, the yields decrease andcannot be improved by additional fertilizer!

Detrimental effects of plants growing with

one another grass and apple trees!

Allelopathy

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Allelopathy

Allelopathy

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Allelopathy

Wheat has good potential for weed controlin agricultural landscapes.

Wheat seedlings reduce root growth in the

annual ryegrass (a common weed).

Allelopathy

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Allelopathy

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Competition

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Competition

Concept so familiar in capitalist societies! All organisms utilize resources. All resources have finite limits. Eventually, at least one resource becomes

limiting and individuals that are unable toacquire necessary quantities of that resource willdie.

In ecology, competition refers to a negative

interaction between two species that use thesame type of resources and live in the same sortof places.

Competition

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Competition

Competition among animals usually overfood

Competition among plants usually over

light, nutrients, water, pollinators Two species do not need to be closelyrelated in order to be involved incompetition.

Ants, birds and rodents may compete forseeds in a desert.

Types of Competition

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Interference or Contest Competition Direct aggressive interaction between individuals

Organisms limit access to a necessary resource.

Organisms harm one another in the process.

The resource may or may not be in short supply.Example: Territories

Scramble or Resource Competition Direct competition for a resource in short supply

All members have access to a limiting resource. Resources are divided among all individuals even if no

individual obtains enough resources to survive.


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Intraspecificcompetition Competition between members of the same

species

Interspecific competition Competition between individuals of two

different species

Reduces fitness of both

Competition

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Competition

One indication of competition is the widerdistribution of a species in the absence ofanother species.

If competition is suspected to affect thelocal distribution of species, two questionsmust be answered: Does competition occur between these

species? Removal Experiment What are the resources for which competition

occurs?

CompetitionSt d lli C i M t lid

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Stream-dwelling Carnivorous Mustelids

American mink introduced to UK for fur farmingca. 1900

Eurasian otter native

Mink numbers increased dramatically in 19501980; expanded their geographic range andthreatened a number of native species

Mink numbers dropped as of 1985, with fewer

sites occupied by minks perhaps competitionwith otters?


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In 1999, 17 otters released into an areaoccupied by minks

Experimental and control populations

monitored for two years


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Competition forspace?

Otters larger andexclude minks by

direct aggression!

Competition

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When two species compete for resources,one will always be better than the other ingathering or utilizing the resource that isscarce.

Unless the weak competitor evolves, it willbe eliminated! Avoid the superior competitor by selecting a

different part of the habitat Avoid the superior competitor by making a

change in the diet (diet shift)

Competition

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p

Crossbills are finches that have curvedcrossed tips on the mandibles.

Extract seeds from conifer cones by lateral

movements of the lower jaw

Competition

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p

White-winged crossbill smallest crossbill;small bill; feeds mainly on larch seeds (inrelatively soft cones)

Competition

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p

Medium-sized common crossbill eatsspruce seeds.

Competition

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p

Larger parrot crossbill feeds on the hardcones of Scotch pine!

Competition

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p

Dietary differences not necessarilypreserved when the species live inisolation

Medium sized common crossbill evolved aScottish subspecies that has a large billand feeds on pine seeds and an Asiatic

subspecies that has a small bill and feedson larch seeds!

Competition

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p

Current distributions may be the outcomeof competition over evolutionary time; theghost of the competition past

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Abiotic Factors Limiting Distribution

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Temperature and moisture are the twomain limiting factors to the distribution oflife on Earth.

Enormous body of literature addressingtheir effects on organisms!

Climate and Vegetation

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g

The vegetation of any site depends on theareas climate.

Climatic factors, mainly temperature and

moisture, are the main factors controllingthe distribution of vegetation.

Gradients of Vegetation in NorthAmerica

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America

East --------------------------------------------------------------------------------West

South------------------------------------------------------------------------------North


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Geographers classify climate based onvegetation.

Native vegetation a meteorological

instrument capable of measuring all theintegrated climatic elements!

Climate and VegetationTerrestrial Biomes

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Terrestrial Biomes

Major divisions of the terrestrialenvironment

Distinguished primarily by their

predominant plants (and animals) Associated with particular climates

Have different natural histories

Example: desert, tropical rain forest


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Terrestrial Biomes

Distribution of biomesacross the globeinfluenced by globalclimate, particularly

geographic variationsin temperature andprecipitation

Tropical Rain Forest at Sunrise

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Climate and VegetationEffect of Latitude and Altitude

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Effect of Latitude and Altitude


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Terrestrial Biomes

Climate and VegetationWhittaker 1975

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Whittaker 1975


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Whittaker 1975

Very low precipitation (0 50 mm): trees cannotbe supported; arid shrublands and deserts

Low precipitation (50 150 mm): small trees;low density of trees; woodlands and savannas

Relatively low precipitation (150 250 mm):drought; deciduous trees to tolerate drought

Precipitation > 250 mm: Broad leaf and

evergreen trees


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Whittaker 1975

Warm temperature (20 to 30 degrees):Broad leaf and evergreen trees

Cool temperature (5 to 20 degrees):Deciduous trees that lose their leaves inwinter

Low temperature (-5 5 degrees): Needleleafed evergreen trees

Very low temperature (Less than -5degrees): Trees cannot grow.


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Some geographers have tried to set climateboundaries independent of vegetation. Thornthwaite 1948: Climatic classification based on

balance between precipitation and potentialevapotranspiration (the amount of water lost from

the ground by evaporation and from the plants bytranspiration in the presence of unlimited watersupply)

Potential evapotranspiration difficult to measure; maybe computed as a function of temperature!

Actual evapotranspiration: The actual amount ofwater lost by plants (usually a standard crop) throughevapotranspiration given the precipitation

Temperature and Moisture asLimiting Factors

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Limiting Factors

Two options for organisms living in acertain habitat:

Tolerate climatic conditions as they are

Escape through evolutionary adaptations


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Limiting Factors

Temperature and moisture limit thedistribution of a species through theireffect on

survival. reproduction.

development of young organisms.

interaction with other organisms (competitors,predators, parasites) near the limits oftemperature and moisture tolerance.


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Limiting Factors

Plants and animals respond differently to agiven environmental variable at differentstages of their life cycle.

Mean temperature or precipitation will notalways be correlated with limits ofdistribution even if temperature and/or

moisture are the limiting factors.


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Limiting Factors

To show that temperature or moisture limits thedistribution of an organism:

Determine which phase of the life cycle is mostsensitive to temperature or moisture.

Identify the physiological tolerance range for this lifecycle phase.

Show that the temperature or moisture range in themicroclimate where the organism lives is permissible

for sites within the normal geographic range andlethal for sites outside it.


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Limiting Factors

Range limits of animals more likely to beset by temperature; few geographicdistributions of animals are likely to be set

directly by precipitation. Moisture is more likely to be of direct

importance for plants than for animals.


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Limiting Factors

Different plants vary greatly in their ability towithstand water shortages. Drought tolerant plants

Improve water uptake by the roots (deeper roots ormore branched roots)

Reduce water loss by stomatal closure, reduction ofleaf surface area and prevention of cuticularrespiration (thick cuticle)

Store water

Xerophytes (plants that live in dry areas) showmany of these special adaptations for avoidingwater loss.

Distributions of Plants along aMoisture Temperature Gradient

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Moisture Temperature Gradient

Enceliaspecies distributions correspond tovariations in temperature and precipitation.


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o stu e e pe atu e G ad e t


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p

Each species occupies a distinctivemicroenvironment.

Uplandslopes

Ephemeralstreamchannelsand desertwashes


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Distributions of Barnacles Along anIntertidal Exposure Gradient

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Organisms living in an intertidal zone haveevolved different degrees of resistance todrying.

Barnacles show distinctive patterns ofzonation within intertidal zones.


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Connell (1961) found Chthamalus stellatusis restricted to upper levels while Balanusbalanoidesis limited to middle and lower

levels.


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Distributional boundaries are very clear in the intertidal zones of rocky coastlines.


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Balanusappears to be more vulnerable todesiccation, excluding it from the upperintertidal zone.

Chthamalusadults appear to be excludedfrom lower areas by competition withBalanus.

Interaction Between Temperatureand Moisture

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Connell (1961) discovered interspecificcompetition in barnacles. Balanusplays arole in determining lower limit of

Chthamaluswithin intertidal zone. Interspecific competition did not however

account for all observed patterns.

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Interaction Between Temperatureand Moisture

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Light as a Limiting Factor

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Great diversity of photosyntheticresponses to variations in light intensityamong plants

Some reach maximal photosyntheticactivity at one quarter full sunlight.

Others never reach a maximumphotosynthetic activity; increasing

photosynthetic rate as light intensityincreases

Light as a Limiting Factor

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Shade tolerant plants: can live and grow inthe shade of other plants

Shade intolerant plants: cannot surviveand grow in the shade of another plant;they require open areas for survival.

Shade tolerance is a complex of traits thatis not fixed for each species and varies

with plant age microclimate and

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