Lecture 09 Competition Ecology

8/12/2019 Lecture 09 Competition Ecology

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Lecture 9:

Competition


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What is competition?! An interaction that occurs when a number of

organisms of the same ( intraspecific ) or different(interspecific ) species utilize common resources

! General types of competition:!

Exploitation competition - organisms compete for resourcesthat exist in a limited supply

! Interference competition - organisms compete aggressivelyfor a resource, with the possibility of competitors being harmed orkilled. In this case, the resource need not necessarily be in shortsupply.

! Pre-emptive competition - organisms compete for space as alimiting resource.


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Competition in Hypothetical Space

K for two different species may be different, such that a single areamay accommodate more individuals of one species than another

In resource competition, each species may fill a particular area at adifferent rate, regardless of species density


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What does this model predict?


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What about species 2?!

For species 2 we can take the same approach. First,find a conversion factor:

" N 1 = equivalent # of species 2 individuals! And include this expression into our equation for

species 2:dN 2 /dt = r 2 N 2 ((K 2-N 2- " N 1 )/K 2 )

! We can then represent this graphically with a similarequilibrium isocline


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Species 2 equilibrium


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So if we put these two species together, what happens?

! One of three things:! Both species coexist! Species 1 goes extinct! Species 2 goes extinct

! How do we predict which of these outcomes is mostlikely?

! Solve the following simultaneous equations atequilibrium:

dN 1 /dt =0=dN 2 /dt ! We can do this by vector additions with our isocline

graphs..


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So if we put these two species together,

what happens?


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what happens?


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what happens?


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Outcome of Lotka-Volterra models

! No equilibrium unless the isoclines cross

! If the isoclines do cross, the equilibrium pointrepresented by the crossing can be stable orunstable

! We can make predictions of outcomes based on thevalues of ! , " , K 1 and K 2


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Competition in Tilman

s Model

! We can repeat this analysis for a second species andsuperimpose the two zero growth isoclines

! We can then determine how the two species willinteract when competing for the consideredresources

! Much like the L-V models, extinction or equilibrium(stable or unstable) are possible


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Mathematical models of competition! From the Lotka-Volterra and Tilman models three

important ideas about the competitive interactions oftwo species have arisen! Competition can lead to one species winning and one

going extinct!

Some competitive interactions can lead to coexistence! We can understand competitive interactions only byknowing the resources involved and the mechanisms bywhich species compete

dN 1 /dt = r 1 N 1 ((K 1-N 1-! N 2 )/K 1 )

dN 2 /dt = r 2 N 2 ((K 2-N 2- " N 1 )/K 2 )

dN 1 /dt =0=dN 2 /dt


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Competition in Natural Populations! Does competitive exclusion prevent the simultaneousoccupancy of a niche by two species?! Hutchison (1958) suggested some conditions under which

competitive exclusion might not lead to extinction! Unstable environments that never reach equilibrium and are

occupied by colonizing species! Environments in which species do not compete for resources! Fluctuating environments that reverse the direction of

competition before extinction is possible

! How can we explain the apparent coexistence of largenumbers of species in field communities?! Perhaps competition in nature is actually somewhat rare! Perhaps competition is common, and has resulted in similar

species having adaptations that serve to minimizecompetition


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How can we determine whether

competitive exclusion will occur?! A species will not be excluded from a habitat if it can still

invade under non-ideal circumstances! When abundance is close to 0 and competitor abundance

is close to K! If per capita growth rate is still positive under these

conditions then the species can invade.dN 1 /dtN 1 = r 1((K 1-N 1-! N 2 )/K 1 ) !!!! if K 1 /K 2 > ! , then Sp. 1 caninvade


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The

Ghost of Competition Past

! The apparent coexistence of a number of species ledConnell (1980) to suggest that the currentdistribution of species resulted from a past in whichcompetitive exclusion was a significant selectiveforce

! Competition between species in the evolutionary pasthas led to a current state where species with verysimilar niches have either:! 1) experienced extinctions or! 2) evolved differences in behavior, diet, phenology ordistribution that now minimize competition


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Character Displacement & Speciation! Character displacement takes place when competingspecies evolve to be more distinct from one another in

some attribute so as to minimize competition! Intraspecific competition can select for differences within

a population that can ultimately bring about speciation


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Character Displacement & Speciation

! Resource partitioning in terns: acase of character displacement?

! Species differ in their overall size,and the size of fish they tend to

consume! Two species (Sooty and Brown

Noddy) have similar prey sizedistributions, but search for preyin very different habitats


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Character displacement infinch beak size

Comparison of 5 islandswhere Geospiz fortis and G.fuliginosa occur


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Testing competition using replacement Series Experiments

! Pioneered in the 1960

s,these experiments allow forthe manipulation of thedistribution of plant speciesin a common garden.! Similar to common garden

experiments! Density is held constant

across experimental plots, butthe distribution of the studyspecies is altered.


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Competition and Facilitation

! Interactions between species need not always be a lose-lose situation! For some species, the presence of another species

improves growth, leading to a phenomenon know asfacilitation

! Sometime both species in an interaction arefacilitated; in other cases one species benefits whilethe other experiences a net loss


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Competition, Facilitation & Effect Size! Gurevitch and colleagues (1992) tabulated the results of

218 competition experiments and arrived at the followingresults:

! Thus, for different groups, the mean effect of competitionis quite different

! This is calculated through mean effect size:

(X e-X c )/s! Xc= mean biomass of the control group (w/ competition)! Xe= mean biomass of the experimental group (w/o

competition)! s = standard deviation of both groups pooled


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The

Paradox of Plankton

! Plant communities in many ways compete moredirectly than animal species, as they generally havemore similar needs:! nutrient, water and light--necessities for survival

! Phytoplankton communities are often quite diverse,despite the similarity in niches of the species

! Hutchinson suggested that the phytoplanktoncommunity might serve as an exception tocompetitive exclusion, due to climatic and seasonalvariation in aquatic (freshwater and marine) habitats

! This variability prevents the system from attainingequilibrium and allows a large number of species tocoexist


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Intraguild predation When neighborsattack!!

! Types of IGP! Symmetrical both species prey upon each other.! Asymmetrical one species preys upon the other! Age-structured Predator and prey roles are usually

determined by an age or size!

Juveniles and small individuals become the prey.! Additional term added to the equations for

population growth.

Predator - dN 1 /dt = r 1 N 1 ((K 1-N 1-! N 2 )/K 1 )+ # N 1 N 2

Prey dN 2 /dt = r 2 N 2 ((K 2-N 2-! N 1 )/K 1 )+ $ N 1 N 2


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! How will IGP influence influence our zero-growthisoclines?

Intraguild predation

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Lecture 09 Competition Ecology

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