Patterns in Species DiversityLecture 10Principles of EcologyCollege of Forestry, Guangxi UniversityEben Goodale

Lets discuss Terborgh et al. 2001

No class Saturday change syllabus again!

ReviewWe defined a community as a group of species living in the same area.We said a community has 3 characteristics:1) # species2) species have different relative abundances1 and 2 can be measured together by species diversity.Today we ask the question, Why?Why so many species?

TodaySpecies diversity varies over big spaces:Species diversity is higher in the tropics.Species diversity on islands of different sizes.Species diversity within a region:Heterogenous community theoryIntermediate disturbance hypothesisDoes species diversity matter?

The Science of BiogeographyAlfred Wallace (1823-1913)Actually co-discover of natural selection, but Darwin is given most credit.Wallace better remembered for his contributions to Biogeography.Biogeography is the study of the variation in species composition and diversity among geographic locations,Like Darwin, Wallace traveled widely, collecting animals as he went.

The Science of Biogeography

Wallaces Line: A line that runs through IndonesiaWallace discovered that the faunas on the two sides of this line very different.In fact fauna of Philippines closer to Africa (5500 km) than to New Guinea (750 km).

Just like Darwin discovered natural selectionBut didnt understand genetics, Wallacediscovered this line, but didnt understand the mechanism. What was it?

Plate tectonicsMovie of changes in continents

Plate tectonics

Plate tectonics: responsible for difference in large regions of globeExplains Wallaces line.Explains why different lineages of plants, animals in 6 regions of the world proposed by Wallace.Explains divergence like that seen in flightless birds.

Why are the tropics so diverse?Species diversity: a latitudinal patternPlantsBirds

Why are the tropics so diverse?They are more productiveThey are betterfitted to organismsbiologyBut this isnt all: Roede (1997) lists 28different hypotheses!Species diversity: a latitudinal patternTheyve been aroundfor longer.

Species diversity: a latitudinal pattern The mid-domain effect for anything thataffects organisms ranges, more organismswill be found in the middle of the gradientAnd the equator is at the middle of the world.Robert Colwell, UConnFrom Colwell and Lee 2000Essentially were coming down to explaining these broad trends by geometry.

Exceptions to species area curves-3 temperate forest zones about the same size But Asia has 6 X tree species of Europe and 3 X NAmerica. Why?

Exceptions to species area curvesAlps blocked the range movements of European trees!In a way too this is a geometric solution. Shows that length of timeImportant.

Exceptions to species-area curvesAnother exception is seabirds, which are more at high latitudes.Indeed oceans are more productive at high latitudes.Some evidence that productivity matters.Overall summary of tropical diversity: many theories that are not-exclusive, and good evidence for many of them.

Species diversity: other scalesGlobal diversity: all speciesRegional diversity: plate tectonics.Regional diversity: latitudeRegional diversity called It includes two components: Diversity: the # of species in any one community. Diversity: the change in what species you encounter as you move from community to community in the region.

Species-area curvesGeneral rule in ecology: the larger area you sample,the more species you will find. Adding diversity.

Species-area curve is different on continents vs. islandsIs Z higher or lowerfor islands?Z is higher because dispersalIs limited to islands.

What are islands?Islands in an aquatic matrix.Lakes in aterrestrial matrixMountains in an arid matrixlike Arizona

And what appears to be an islandchanges based on organismDispersal abilityvaries widely amongorganismsVery occasionally a lizard might raft across oceanSpores / seeds of some organismseverywhere

Theory of island biogeography.Robert MacArthurE.O. WilsonOriginally, atheoretical model, but has been usedfor applicationsfor how to buildreserves islands within areas of humandisturbance.1967

How do new species get to islands?They immigrate.

Number of species on islandImmigrationRateImportant note: this theory doesnt include speciation on the islands.(Species Per year)

What factors effect immigration? Number of species on islandImmigrationRateDistance to (or isolation of) islandSay this immigration lineis for a close islandwhat would the immigrationline be for a far island?(Species Per year)Say this immigration lineis for a small islandwhat would the immigrationline be for a large island?Size of islandFarther island, fewer immigrantsLarger island,bigger target

What factors effect immigration? Number of species on islandImmigrationRateOne more question: Should this be a straight line (linear)?Species have differentdispersal abilities, so ones that disperseeasily will come quickly.

(Species Per year)

How do species disappear from islands?ExtinctionNumber of speciesExtinctionRateWhat does thisgraph look like?(Species Per year)

What factors effect extinction?Size of islandNumber of speciesExtinctionRateThe larger the island,the bigger populationa species has.

The bigger populationa species has, the lesschance it will go extinct.

Hence, bigger islandshave lower extinction rates.OK, so lets say that thisgraph is for a small island.What would the graph fora large island look like?(Species Per year)

What factors effect extinction?Distance of islandNumber of speciesExtinctionRate(Species Per year)How about the distanceof an island?

If its close, more immigrants will comeand there will be fewerextinctions.So if we plot a farther island on thisplot it goes where?

What factors effect extinction?ExtinctionNumber of speciesExtinctionRateNow is this graph linear?As there are more andmore species, interspecificcompetition will increase, so curve will look like(Species Per year)

Theory of Island BiogeographyPostulates that for every island, there will be some number of species where immigration will be balanced by extinction.(Species Per year)Species on islandextinctionimmigrationEquilibriumPointImportant point:equilibrium doesnot mean that thesame organisms will be there yearafter yearidentity of specieswill change, butnumber stay the same

Theory of Island Biogeography(Species Per year)Species on islandSay this is the graph for asmall islandWhats the graph for a large island?Same result for a closeisland

Exercise(Species Per year)Species on islandThe curves in blue shows aisland that is ______ than thecurves in black.

Farther or smallerFarther or largerCloser or smallerCloser or larger

Theory of Island BiogeographyGo through this figure yourself, and makesure you understand it

Testing the theory:observationPlant species on Galapagos Islands

Teting the theory: experimentationMany observational studies show that island area and isolation are major factors influencing species number.But correlation does not prove causation.Best to do an experiment.

Experimental islandsSimberloff and Wilson (1969) fumigate islands and wipe out allinsects on them, then watch the number of insects that return

Experimental islandsFar islandClose island

Experimental islandsThe Simberloff (1976)took some islandsand made themsmaller

Island-biogeography theory applied to fragmentation and conservationFragmentation is a process of making islands. People have used island biogeography to develop sets of reserves. Also used it to predict how many species might go extinct given a certain amount of fragmentation.

A really large scale experimentBiological Dynamics of Forest Fragments Project (BDFFP).Started in 1979 by Thomas Lovejoy.Made experimental fragments in Amazonian Brazil.

As expected, size of island a big factorSmaller fragments much more hit by extinction.Also interesting findings show that different kinds of animals and plants have very different kinds of responses.From Stouffer et al. 2011

TodaySpecies diversity varies over big spaces:Species diversity is higher in the tropics.Species diversity on islands of different sizes.Species diversity within a region:Heterogenous community theoryIntermediate disturbance hypothesisDoes species diversity matter?

OK, so communities differ in species diversity but why?Heterogeneous environmental theory.First developed by Robert MacArthurCame out of the idea of niches, and the competitive exclusion theoryMacArthur generally worked with birds

Robert MacArthurStudent of HutchinsonDied at age of 42 in 1972

Heterogeneous Environments:Opportunities for SpecializationRemember that eachspecies has a niche Its occupation.And no two species can have the same nichecompetitive exclusion principle (Gauses experiments)We talked about this when talking about competition.In these birds thatMacArthur studiedeach species uses differentpart of the tree.

So it makes sense thatthe higher the canopyof forests, the morespecies there will be.This was MacArthursfirst result (1958)

Further experiments demonstrate that vertical heterogeneity increases species diversityMacArthur hung a ropevertically from the top of the canopy and measuredhow many times vegetationhit the rope. From this he calculatedvertical profiles of differentforests, and compared themin their diversity (# of different layers and evennessof layers). MacArthur and MacArthur 1961

Further experiments demonstrate that vertical heterogeneity increases species diversityMacArthur and MacArthur 1961He found that the more diversethe vertical vegetationthe more speciesof birds were presentBasically the idea is that the More diverse an area is in vegetation,The more niches can be fit into it =The more species.

Heterogeneous environment theory: plants vs. animalsFor animals, the environment thats important is usually structure of vegetation.What about for plants?Different plants may use different types of soil nutrients.And we know that soil resources can be variable in space, suggesting they can help explain plant diversity.

Variable conditions can also retain diversityUnder unvarying conditions, competitive exclusion occurs. But disruptive processes such as predation and disturbance can alter the course of competition.Remember a sea star kept a dominant competitor from getting too abundant.

Variable conditions can also retain diversityOr a sea palm on a rocky tidal short that was a poor competitor, but could colonize areas of high disturbance.Hence, variable environments can maintain species co-existence

Intermediate Disturbance HypothesisThis idea leads to another hypothesis explaining diversity: the intermediate disturbance hypothesis.Very disturbed environments will have low diversity.So will very stable environments.Diversity will peak somewhere in the middle.But both could exist in the middle

Intermediate Disturbance HypothesisIntermediate DisturbanceHypothesis tested by Wayne Sousa(1979), on rock tidal communities.

Intermediate Disturbance HypothesisDisturbance in streamsPrairie dog disturbance

TodaySpecies diversity varies over big spaces:Species diversity is higher in the tropics.Species diversity on islands of different sizes.Species diversity within a region:Heterogenous community theoryIntermediate disturbance hypothesisDoes species diversity matter?

Stability and species diversityStable community = after a disturbance, remains at, or returns to original function.Does species diversity influence stability?Large experimentsof David Tilman andcolleagues in Minnesota, USA

Stability and species diversityFirst result:Observational.The plots with morespecies were moreresistant to drought.

Stability and species diversitySecond result:Experimentally Manipulated speciesDiversity. High species =Overall biomass higher.After a certain level, Reaches a threshold).Species are redundant to each other.

Stability and species richnessIf species differ in their nutrient requirements or niches.Then if disturbance comes and alters some parts of the environment, the more variety of species, the more likely some will survive and do well.Stability is one way of measuring diversitys impact on community properties. We will revisit this idea when talking about food webs.

HomeworkChapters 20, 21 (Productivity and Energy Flow: Introduction to Ecosystems). Jackson et al. 2001. This is another food web article, because we will talk more about food webs next class (Ch 21). It talks about how humans have impacted marine food webs.Questions:What kind of scientists wrote this article? What do they study and what kind of data did they collect.What kind of marine habitats do they review?Whats their conclusion as to what has been the most important human activity that has harmed these ecosystems?

Key conceptsContinental drift has resulted in differences between regions in their species diversity.For many reasons, species diversity is highest in the tropics.Island biogeography is an elegant theory that explains why species vary in relation to the size and isolation of islands.Heterogeneous resource theory predicts that one can pack in more species (and their niches) into a more diverse environment.The intermediate disturbance hypothesis predicts low diversity at high and low disturbance levels.Diversity appears to have a positive influence of community stability.

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