Tutorial Chapter 16

7/31/2019 Tutorial Chapter 16

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Community ecology_interaction between

speciesThe ecosystem without the biogeochemistry in a sense.So really what we're looking at here is the interactionbetween species in a localized area with respect to howthey influence each other's fitness, in a sense.

And these interrelationships between these speciesgovern all of the things that we talked about in the firsthalf of my lectures set. It's these interactions that shapethe biogeochemistry of those systems.

So, this is the structure of the system and thebiogeochemistry is the function of the system. And it'salso these interactions. So, they affect the flow of energy,which we talked about.


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What Interactions do? They affect the cycling of elements. They affect the evolution, the

very evolution of species within the community. And thesecommunities self assemble. In other words, if you start with an emptyplot of bare land, we talked about this. Remember the example of theglacier retreating and showing the succession of species as theglacier retreated? We started with a bare rock, and you'll start gettinglichen growing on the rock. And then that lichen will create a little bitof soil, allowing plants to come in, and that the plants increaseproductivity.

Some of them bring nitrogen and, allowing shrubs and trees andeverything. And that's the self-assembly of the community. Once youhave plant there, you can have insects there. Once you have insects

there, you have birds there. It self assembles. And one of thequestions that ecologists ask is, how deterministic is that? I mean, weknow there are some random components to it, and we know thatthere are some things that will happen because we see it happenover, and over, and over.


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Assembly rules ofcommunity

So the big question is what musthappen? What could happen? Andwhat might not ever happen? That's

really one of the challenges ofecologists is to understand thoseassembly rules of community if thereare any.


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Darwinian fitness

Species interactions: so, when we talk aboutspecies interactions and how they structurecommunities, we have to first define some terms.

One is Darwinian fitness. And the fitness of anindividual is the relative ability of an individual in apopulation to survive and reproduce. So, it's allrelative, OK, within a population. And we're goingto define also, we'll be talking about adaptations,and I know you know what this is, but let's just

make sure that we are all operating with the sameassumption. And an adaptation, which issomething that affects the fitness, is a heritabletrait that increases the fitness of an individual with

respect to other individuals in that population.


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Operating assumptions So, these are just some operating

assumptions so that when we talkabout species interactions, there areseveral possibilities. If we have

organism one and organism two,we can have, and we ask how doesthe presence of organism two and

the presence of organism one affectthe fitness of the two organisms?And if the fitness of both organismsis increased by being in the presenceof the other, that's called


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Types of interaction If being together decreases the fitness of both, that's

called competition. And if you have the situation,what we recall that? It could be parasitism, yeah,parasitism. What else? Was the ultimate form ofreduced fitness? Predation, yeah, being dead is the

ultimate reduced fitness. So, parasitism andpredation. And then there are some other sort ofrather vague interactions where when you put twoindividuals together, the fitness of one is notinfluenced but the fitness of the other is either

influenced positively or negatively.


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Intra specific competition

There is intraspecific competition, which meanswithin a species, OK? And that's not what we'regoing to be talking about today, but we've alreadytalked about this without explicitly, remember our

logistic equation and the density dependentfeedback mechanisms in that population thatcaused the population to deviate from exponentialgrowth was due to intraspecific competition:

individuals within a species competing with eachother for resources.


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Inter specific competition

We are going to talking about now is moreinterspecific -- -- which is competition betweenspecies, OK? So, I want to show you some slidesthat are just from your textbook, but just to getyou in the mood for competition. So, it comes in alldifferent forms, and I don't care whether you knowthe names of these. It doesn't matter. This is justto give you an idea of the different types ofcompetition that we see in nature. This is what's

called consumptive competition, and this is justshowing the roots of the trees competing fornutrients in the soil.

Preemptive competition shows these are

barnacles. We're going to talk a little bit moreabout that later ust totall takin over the


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More forms of competition

Overgrowth competition in plantswhere this plant would be shading, soother plants that require a lot of light

could not grow underneath. Chemical competition also occurs

where one plant will actually excrete

certain chemicals that create thesecorridors of no growth around them soother plants can't get near to competefor the nutrients. The classic form ofcom etition, sa , in birds and a lot of


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Territorial competition

So these are displays so that anindividual can keep a certainterritory, and therefore make that

food available to itself, thereforeincreasing its fitness because it'sable to feed its young. And then this

is sort of the classic, really tooth andclaw competition where encountercompetition where all these species

are competing over this zebra


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Ecological niche

In this particular competition, the fundamentalecological niche comes from G. Evelyn Hutchinson,who is one of the founders of modern ecology. Hedefined as the fundamental niche of an organism

as an N-dimensional hypervolume, every point onwhich a species can survive and reproduceindefinitely in the absence of other species, OK?So, this is an abstract concept, because those

species are rarely in the absence of other species,except maybe in a test tube.


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Ecological niche contd

But it defines the, and also we can'teven think about N-dimensions,right? We're able to think about, we

can envision three dimensions. Butwhat he's talking about here, is everysingle dimension in the environment

that would have any effect on thefitness of an organism


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Niche overlap

It's an abstract concept. So this is the fundamentalniche, and here's another closely related specieswhose niche has some overlap with this one, buthas different ranges for temperature, humidity,

and food size. And when you have overlappingniches is when you have the possibility, thepotential, for competition.

And two things can happen. If they overlap a lot,

than those two species cannot coexist in the sameenvironment.

One will outcompete the other, and it will move onto some other place where it doesn't have a strong

competitor. But if they overlap a little, you canactuall have com etitive coexistence


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Fundamental and realisedniche

So, if the species can makes it realized, so this isits fundamental niche, this one's fundamentalniche, and what happens if it can make its realizedniche small enough so that there's no niche

overlap, then you can have coexistence of thosetwo species, or very little niche overlap in thesame environment.


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Niche continued

So that's the difference between thefundamental and the realized niche.

. This is just one dimension, seedsized, for, say, a bird eating seeds ofthis size range. We'll be talking aboutbirds a lot in this. And here's partial

niche overlap, species to where theyeat some seeds of the same size, butby and large the mode is different.

You can have spec ies coexisting.


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Partial niche overlap

Partial niche overlap can lead tocompetitive coexistence. And here'sthe complete overlap in just this one

dimension, which would lead tocompetitive exclusion. But obviouslyit matters what's happening on all

the dimensions.


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Growth rate of species inrelation to each other

So, the growth rate of species to inthe presence of species one isreduced by some amount that's

proportional to the abundance ofspecies one. And these, the values ofthese, these are called competition

coefficients. You can actually doexperiments and put values onthese, and they are a measure of

how strong a competitor each of


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Competitive exclusion

Competitive exclusion, that is, theexclusion of one species from anenvironment because of strong

competition in another, is verydifficult to study, because if it's notthere, you don't know it was

excluded, right? I mean, you don't goto some place and say I don't see thespecies here. It must have been

eliminated by competitive exclusion.


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Invasions

It might never have been there. So,the way we learn about thisphenomenon is either through

inadvertent experiments, and that isthe introduction of species to newenvironments and then see what

happens, or actual intentionalecological experiments. So, we'regoing to talk about both of those.

And the first one -- -- we'll talk about-- --


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Example of Zebra mussel

And one of the classic examples of this is the zebramussel. So, the zebra mussel is a tiny mussel thatwas introduced to the United States back in, Iguess, 1988. And up here, introduced into the

Great Lakes by ships just being attached to ships,or it's possible it might have been the larvae inships' ballasts. Ships go into port, they take onwater into their ballasts to stabilize, and then they

go to another port and let it out. And they're filledwith larvae and species. So, the entire worldoceans are now filled with introduced species fromships ballasts.


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E l f Z b l


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Example of Zebra musselcontd

So they actually have increased the clarity of thewater in many ecosystems, filtering out plankton,which allows the light to penetrate deeper in thosesystems, allowing aquatic plants to grow from the

bottom. So the introduction of this one species cancompletely change the structure of the entire

ecosystem.


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References

http://ocw.mit.edu/courses/biology/7-014-introductory-biology-spring-2005/i

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