Sign In | Subscribe
Start learning today, and be successful in your academic & professional career. Start Today!
Loading video...
This is a quick preview of the lesson. For full access, please Log In or Sign up.
For more information, please see full course syllabus of AP Biology
  • Discussion

  • Study Guides

  • Download Lecture Slides

  • Table of Contents

  • Transcription

  • Related Books & Services

Bookmark and Share

Start Learning Now

Our free lessons will get you started (Adobe Flash® required).
Get immediate access to our entire library.

Sign up for

Membership Overview

  • Unlimited access to our entire library of courses.
  • Search and jump to exactly what you want to learn.
  • *Ask questions and get answers from the community and our teachers!
  • Practice questions with step-by-step solutions.
  • Download lesson files for programming and software training practice.
  • Track your course viewing progress.
  • Download lecture slides for taking notes.
  • Learn at your own pace... anytime, anywhere!


  • A community consists of all of the organisms in an area that interact with one another, and their environment. Interactions that occur within a community include competition, predation and symbiotic relationships.
  • Competition is an interaction involving two species competing for the same limited resource.
  • Predation is one species eating another and may include true predation as well as grazing.
  • A symbiotic relationship is one in which organisms of two or more species live in direct contact. Parasitism is a symbiotic relationship between different species in which one organism benefits at the expense of the other.
  • Mutualism is a symbiotic relationship in which both species benefit.
  • Commensalism is a symbiotic relationship in which one species benefits, and there is no effect on the other.
  • The trophic structure of an ecosystem describes the energy-obtaining sequence within a community. A linear sequence of a single species at each trophic level is also known as a food chain. In an ecosystem there are numerous food chains each interlaced with each other into a food web.
  • Re-establishment of a community after a destructive event occurs through the process of ecological succession.


Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Community 0:07
    • Ecosystem
    • Interspecific Interactions
  • Competition 2:45
    • Competition Overview
    • Competitive Exclusion
    • Resource Partitioning
    • Character Displacement
  • Predation 7:46
    • Predation
    • True Predation
    • Grazing/ Herbivory
  • Predator Adaptation 10:13
    • Predator Strategies
    • Physical Features
  • Prey Adaptation 12:14
    • Prey Adaptation
    • Aposematic Coloration
    • Batesian Mimicry
    • Size
  • Parasitism 16:48
    • Symbiotic Relationship
    • Ectoparasites
    • Endoparasites
    • Hyperparisitism
    • Vector
    • Parasitoids
  • Mutualism 21:23
    • Resource - Resource mutualism
    • Service - Resource Mutualism
    • Service - Service Mutualism: Obligate & Facultative
  • Commensalism 26:01
    • Commensalism
    • Symbiosis
  • Trophic Structure 28:35
    • Producers & Consumers: Autotrophs & Heterotrophs
  • Food Chain 33:26
    • Producer & Consumers
  • Food Web 39:01
    • Food Web
  • Significant Species within Communities 41:42
    • Dominant Species
    • Keystone Species
    • Foundation Species
  • Community Dynamics and Disturbances 44:31
    • Disturbances
    • Duration
    • Areal Coverage
    • Frequency
    • Intensity
    • Intermediate Level of Disturbance
  • Ecological Succession 50:29
    • Primary and Secondary Ecological Succession
  • Example 1: Competition Situation & Outcome 57:18
  • Example 2: Food Chains 1:00:08
  • Example 3: Ecological Units 1:02:44
  • Example 4: Disturbances & Returning to the Original Climax Community 1:04:30

Transcription: Communities

Welcome to

We are continuing our discussion of ecology with the focus today on communities.0002

A community consists of all of the organisms in an area that interact with each other as well as their environment.0009

Now, to remind you, we talked about populations.0017

And a population means all of the organisms of the same species that interact and breed and are located in one physical area.0021

So, with the population, we are just talking about one species.0031

Now, with the community, we are talking about multiple different species interacting with each other.0034

And we will be talking about ecosystems as well later in the course.0040

To let you know that what the definition of ecosystem is, an ecosystem consists of all the0045

organisms and the non-living components in one area, so both biotic and abiotic factors.0052

Now, the interaction between individuals of different species - this is called interspecific interactions - are key element that0075

determines the characteristics of a community and how the community changes over time or in response to disturbances.0090

So, these interactions can be positive. They can have a neutral effect, or they can have a negative effect on one or both species.0096

One way to categorize this is to designate these effects with a plus, a minus or a zero.0106

So, this is indicating that there is a positive effect on one species, a negative effect on one species or a net neutral effect on a particular species.0114

To divide these up into some general categories, when we talk about interactions between species in the community,0128

there may be competition, predation, so there could be a predator-prey relationship, parasitism, mutualism, or commensalism.0136

And we are going to talk about each of these and what the effect is on the species,0156

whether it is positive for one, negative for the other, positive for both and so on.0160

So, let's go ahead and start with competition. Competition is an interaction involving two species competing for the same limited resource.0165

So, this is an interaction involving two species competing for a limited resource such as food or use of a particular area.0177

This is a negative interaction for both species, so this would be categorized as (-)(-) under the scheme that I just described.0201

To give you an example, there could be two insects species with the same food source.0209

That would be competition, or multiple bird species that desire the same type of seed.0214

Predators could have competition.0222

They can both be competing for the same prey like if a bobcat and coyote lived in a community, and they were both competing for mice.0223

Now, there are several possible outcomes for competition among species.0231

The first one is competitive exclusion. In this outcome, one species will eventually dominate the other.0238

If you were looking at a closed controlled system, well, no two species are exactly equal.0259

So, in a close controlled system, there is going to be a slight advantage for one species, and eventually, the other species will become extinct.0266

This is competitive exclusion. You can easily demonstrate this in a laboratory by placing two bacteria species in the same culture.0274

One will always dominate, and the other will end up eliminated, so that is the first possible outcome of competition.0282

A second outcome is called resource partitioning.0288

In resource partitioning, the two species that are competing in a community for resource will eventually develop their own ecological niches.0295

In other words, each one of these species will end up occupying a slightly different space or find the resource in a different way.0304

So, one will not necessarily get wiped out. Instead, they will each find their own way of surviving.0312

An example would be, let's say that there are two species of animals that both prey on a particular insect.0318

And one is a flying animal, and the other does not fly. It is a ground animal.0324

Well, eventually what might happen is that the flying animal will still seek that type of insect, but it will stick to looking for it in the air or up in the trees.0330

Whereas, the ground-based predator would prey on that insect but closer to the ground.0341

So, example would be a flying animal, some type of bird and a ground predator, and they are both prey on the same insect prey.0346

Eventually, the flying animal is going to look for that insect in the air and trees.0364

That is going to be its hunting ground, and the other animal, the ground-based animal will hunt on the ground.0371

So, they each occupy a slight different ecological niche in the same community.0379

The third outcome is called character displacement, and we talked about this when we talked about natural selection in evolution.0383

And what happens in character displacement is the evolution of certain characteristics of a species in order to obtain the ability0392

to take advantage of a resource in a particular location or even a slightly different resource than the species it is competing with.0401

This is best understood through example, and remember when we talked about finches on the Galapagos Islands that Darwin studied.0410

And over time, finch species on these islands evolved different beak shapes.0418

So, example, finch species with different beak shapes, and one might end up with a long narrow beak.0425

And this will allow the finch to find food or seeds in very difficult to access places.0434

Whereas, another finch might evolve to have a large short beak that could be stronger to crack open thicker heavier larger seeds.0443

So, eventually, these two, instead of directly competing are going after slightly different food resources.0453

So, competition, as I sad is (-)(-) over all. It affects both species involved in a negative manner.0461

Now, predation is slightly different. In predation, one species kills and eats the other.0467

This is a (+)(-) because, of course, the predator is benefiting from gaining food, and the prey is losing out by being eaten.0474

What people think when they think of predation is what is actually called true predation.0488

And this is typical predation in which one animal kills and eats another.0493

It could be as simple as a predator swallowing its prey whole or like a whale that is eating krill, just swallowing the prey whole.0506

Or predation can require a very complex hunting and killing process.0515

Now, the other type of predation is grazing.0520

Now, often, when you think of predation, you are probably thinking of true predation.0525

But grazing or herbivore is the eating of a plant by an animal, so the prey is actually a plant.0529

Now, you should note that this does not always kill the plant.0546

A deer could go and eat some leaves off a shrub, and it is not going to kill the shrub, so that is different than true predation.0549

Or if you look at an example of an insect like a mosquito that uses the blood of an animal for food, this can also be considered a type of grazing.0557

Some aspects of predation to consider: many animals are both predator and prey, and when we look at food chains and food webs you will see this.0569

And what we are going to talk about is trophic levels later on.0586

And species are classified by how far they reside up the food pyramid with herbivores being down on a lower level.0591

Animals above them are carnivores. They eat the herbivores, and there are carnivores that eat those carnivores that are higher up.0601

Again, we will go on to this in detail later on.0608

Both predator and prey have adapted to their roles in the community and their situations to survive.0615

Some adaptations by predators can be classified into two broad areas.0623

Well, as a predator, the two strategies can be pursuing prey and waiting for prey.0632

So, some animals, some predators have a strategy where they will just hide and wait for a prey to come along and grab it.0646

Whereas, others will actively pursue and capture a prey. So, these are predator strategies.0653

There are also physical features that predators have developed, and here, in this picture is a picture of various predators.0662

And you can see that some of these adaptations could be claws, the claws on the cat, venom on a snake, for example, sharp teeth.0671

So, here, we have an alligator, a cat, an owl, various types of predator that all have adaptations in order to capture, kill and consume their prey.0688

Depending in their strategy, predators typically have keen eye sight, and you will notice that their eyes usually face forward.0698

If you look at a lot of prey animals, the eyes are located on the side of the head so that they can see the predator coming.0704

Predators are usually fast and agile, so eyes facing forward with good eyesight. They are usually fast and quite agile.0711

Alternatively, they may have the ability to hide or camouflage themselves like you see with this owl so that the prey does not see them coming.0725

Now, preys also have their own set of adaptations to survive and evade or sometimes even fight the predator.0735

Adaptations of prey include camouflage-type coloring. The name for this is cryptic coloration.0744

So, you can see here this cryptic coloration on this butterfly where it would blend in with0767

a tree that it was on or the deer blending in with the brush and the tress around it.0772

There can also be a camouflage by virtue of the shape, for example a walking stick. If you had seen a walking stick, their shape helps them to blend in.0778

Also, some other adaptations besides camouflage are toxins. Some prey have toxins that are either poisonous or make them unpalatable to predators.0787

There are physical defenses like shells. A classic one also are the spines on a porcupine.0802

There is another type of coloration that has evolved, which is aposematic.0815

Aposematic coloration is coloration that alerts predators that an animal might be poisonous or dangerous.0823

It alerts other animals that the animal is poisonous or in some other way, undesirable.0835

We can see an example of that here. These black and yellow stripes on the bee or on a wasp are examples of aposematic coloration,0850

bright patterns on a poisonous snake or the stripe on the skunk.0860

Now, Batesian mimicry is coloration or behavior by a harmless animal, but it mimics the coloration or behavior of an animal that is poisonous or unpalatable.0874

So, it is coloration or behavior of a harmless animal to mimic a poisonous or undesirable one.0891

Now, an example would be some other insects like certain types of flies that have colorations similar to that of a bee.0917

So, if an animal sees them, they see those black and yellow stripes.0926

It warns them to stay away because they think there is a stinger there when there is actually is not.0929

Another example are snakes that are actually not venomous, but they have colorations similar to another snake species that is venomous.0933

There also tends to be a correlation between predator and prey size.0944

Predators generally will not attempt to hunt prey that is two large because they will be unlikely to be successful.0951

At the same time, they do not want to waste time going after very small prey because it is still requires energy to capture that prey.0958

And then, the payoff is going to be small, not very much calories, not very much energy for the predators.0964

So, there is usually a fairly good correlation between the size of a predator and the size of a prey. Therefore, size can be a defense for prey.0969

An example would be an elephant. Just by its sheer size, it is going to be more difficult to hunt and limit the number of predators that will be after it.0979

Now, in conclusion talking about predator-prey relationships, all of these elements that we0990

talked about with predation help to determine the structure of each ecological community.0995

And they also determine important aspects of animal behavior, and recall in an earlier lecture, we talked about animal behavior.1001

The next type of relationship between organisms that we are going to discuss is parasitism.1008

And before we do, I want to review the definition of a symbiotic relationship.1014

We have talked about different examples of symbiosis in various sections of this course such as during plants, we talked about it.1017

So, a symbiotic relationship is one in which organisms of two or more species live in direct contact or in close association.1028

We are going to talk about several subtypes of symbiotic relationships.1037

The first of which is parasitism, and this is a relationship between different species in which one organism benefit.1041

And it does so at the expense of another organism. Therefore, this is also a (+)(-) relationship.1051

The organism that benefits is called the parasite. The organism that is being harmed is called the host.1059

Now, parasites normally do not kill their host, although, the parasite might reduce the biological fitness of the host1071

by introducing another disease, by modifying the behavior of the host or by causing physical harm to the host.1080

However, they do not usually kill them. Sometimes, they do though.1088

Parasites are typically highly specialized for a particular host species.1092

As we talked about earlier on when we talked about protist for example, parasites may have two or more host species.1099

So, in different parts of the life cycle, they may have a different host.1107

Parasites can be categorized in some different ways. One way is ectoparasites versus endoparasites.1113

Ectoparasites reside on the surface of an animal. An example or a couple examples would be fleas and ticks.1121

By contrast, endoparasites live inside the animal. They live inside their host.1134

And we have talked about parasitic worms earlier on like tapeworms and flukes. These are examples of endoparasites.1146

Just a few other facts and examples before we go on, there is something called hyperparasitism.1157

Explaining this through an example, there is a type of protozoa that is a parasite of fleas.1170

So, it lives within the fleas, and the flea is a parasite of mammals.1184

So, what this really is, is multilevel parasitism, and it is called hyperparasitism sometimes.1194

Frequently, parasites require a vector, so they would require a vector in order to be transmitted to the host. This is the case with malaria.1209

Remember we talked about Plasmodium, and this is a parasite, which is transmitted via mosquito to its human host; and it causes malaria.1219

So, the vector is a mosquito, and through the mosquito, it is transmitted to the human host.1231

And this is the causative agent of the disease malaria, which we also talked about earlier in the course.1239

Bacterial diseases, viral diseases that humans and other animals can get are parasitic relationships.1247

Parasitoids are parasitic organisms that kill the host. Some even classify this type of relationship as a type of predation.1255

Alright, so now, we have talked about interaction that harms one member of the interaction.1275

We are going to now talk about a different type of relationship, another type of symbiotic relationship, which is mutualism.1280

And this is a symbiotic relationship in which both species benefit. So, this is can be classified as (+)(+).1288

There are three categories of mutualism.1295

One is resource mutualism, and this is when one type of resource is traded by one species for another resource from the other species.1298

So, one type of resource is traded with another species for a different resource.1316

Classic example that we have talked about, we talked about this in the plant section as well as in the fungi section, is mycorrhiza.1338

So, recall, this is when a particular type of fungus lives on the roots of plants, fungus and plant root association.1350

And recall that the fungus provides nutrients. It helps to absorb certain nutrients like nitrogen and phosphate.1366

And so, the plant is getting that resource of nitrogen and phosphate absorption, whereas, the plant is providing the fungus with carbohydrates.1375

So, fungus helps with nutrient absorption by the plant roots, so the plant is benefiting by getting these nutrients.1382

Whereas, the fungus is getting carbohydrates as its benefit, so resource-resource mutualism. This is the first type.1401

The second type is called service-resource mutualism. An example would be pollination.1413

In this case, the bird or the insect, the pollinator is getting the nectar. It is getting the nutrients, the calories which is a resource.1426

And it is doing a service for the plant, which is bringing the pollen to another plant, to another flower.1434

Another example would be sometimes large mammals will... actually birds that, kind of, hang out on their backs or fly over and eat fleas off their backs.1441

So, the service is the mammal is getting these fleas that could cause disease possibly removed.1452

And the bird is getting a resource, so service-resource mutualism.1460

Finally, the third one is a little bit more rare. It is quite rare, but it does exist, service-service mutualism.1464

There is one well-known example, and that is the relationship between a sea anemone and clown fish. They each do a service for the other.1472

The anemone's stinging tentacles protect the clown fish, and the clown fish defends the anemone from the butterfly fish, which can eat the anemone.1485

So, they essentially protect and defend each other.1496

Now, this is one way to classify mutualism. There is another way to classify it, which is looking at is a mutualism obligate or facultative.1501

In obligate mutualism, one or both of the species cannot survive without the other.1516

In facultative mutualism, both species can survive on their own, but they benefit from the relationship.1536

The final type of relationship that we are going to talk about is commensalism.1559

This is a symbiotic relationship in which one species benefits, and there is no effect or a net neutral effect on the other species, so this would be (+)(0).1565

In reality, there are very few relationships that have a true absolute net neutral1579

effect because really, any relationship is going to have some minute effects.1585

However, if the effect on one of the species is nearly insignificant or very difficult to determine, some scientists classify it as a commensal relationship.1590

Other scientists argue that there is actually no commensalism, and that all symbiotic relationships are either mutualism or parasitism.1599

But, to give you some examples of what could be considered commensalism, cattle egrets, so a type of bird that feed on insects stirred up in the1608

Pasteur by grazing livestock, this would benefit the egret, and it does not really have an effect that is known or significant on the livestock.1622

Another example could be moss growing on the side of the tree.1639

It does not really hurt or help the tree, but it gives the moss a place to live, so benefit to the moss, net neutral effect on the tree.1644

Now, I do want to note on the classification system that I am using here,1652

I said that symbiosis is when there are two or more species living in close relationship with one another, a close association with each other.1657

And then, I defined subtypes of this as parasitism, mutualism and commensalism, and this is the way you will see a lot of sources do it.1666

But, I do want to note that in some sources, they define only mutualism as symbiosis and not these other two.1680

And they, sort of, just use these terms as one and the same- mutualism and symbiosis.1691

I am using the classification scheme where symbiosis is the broader term, and then, these are three types of symbiosis.1695

Now, we have talked about some relationships between members of the community.1703

And I introduced very briefly the idea of the trophic structure and food chains when we talked about predator-prey relationships.1707

And now, I am going to go into that in more detail.1714

Key to survival of organisms and species in a community is how they obtain the energy that they need to live and reproduce.1717

And the base word here "troph" means nourish or nutrition.1726

Recall earlier in the course, we talked about autotrophs versus heterotrophs.1735

And autotrophs, for example, if you look at photosynthetic autotrophs like plants, they use energy from light,1740

so photosynthesis to generate complex organic compounds, to make organic compounds.1754

The heterotrophs use compounds that are produced by the autotrophs for energy.1765

Or heterotrophs can then get those compounds from other heterotrophs.1776

But, they do not directly make the organic compounds from inorganic constituents.1780

So, heterotrophs use compounds produced by the autotrophs, directly or indirectly use them.1784

We can look at autotrophs as producers, so autotrophs are producers, whereas, heterotrophs are consumers.1795

And what we are going to look at here is trophic structure, and the trophic structure of an ecosystem describes1809

the energy-obtaining sequence within a community, so the energy-obtaining sequence within a community.1816

And if we start here at the base of the structure, we will see that at the bottom are the producers or sometimes called the primary producers.1833

And these are the autotrophs.1844

The structure goes up from there with the herbivores being the primary consumers, so they eat the autotrophs and gain energy that way.1846

Next are secondary consumers. Secondary consumers eat the primary consumers.1857

And since these are animals that are eating other animals, these are carnivores.1864

Now, it can actually go on from there. There could be additional levels of carnivores.1872

So, a tertiary consumer is a carnivore that is eating the secondary consumer.1877

And it can be even go on to quaternary consumers, again, another carnivore, this one eating the tertiary consumer.1881

So, a linear sequence that shows a single species at each trophic level is called a food chain, and we are going to see an example of that in a minute.1889

So, it is a linear sequence with one species at each trophic level.1901

Now, as I said, this shows the movement of energy, and the primary producers start out obtaining energy from sunlight.1916

And then, the primary consumers on up are taking in energy in the form of food.1924

Most energy is used by the organism in the process of growing and reproducing and by its ongoing metabolism.1931

As a result, only about 10% of the energy is actually passed from one level to the next, so this is a relatively insufficient process.1939

The percent of energy that is transferred from one level to the next is known as trophic efficiency.1948

And this is the percent of the energy transmitted from one level to the next.1960

So, if we start out, the plant gets 100% of the energy. This herbivore will only be able to take about 10%, so there is a lot of loss between each level.1973

And then, we go on, and this secondary consumer will only get 10% of that.1985

So, we can see, even at the level of the secondary consumer, that consumer has only received 1%1990

of the energy that the autotroph obtained because of this 90% loss at each level.1997

So, let's look at a specific food chain to get a little bit more insight in trophic levels and on the stability of species at various trophic levels.2007

If we look at this example here, we have the primary producer. In this case, the produce is a phytoplankton.2019

And then here, we have our first consumer, and this is an herbivore.2027

This is the primary consumer - let me write that out, primary consumer or herbivore - eating the phytoplankton.2038

And then, we have our first carnivore level, this is the secondary, which is a carnivore.2046

So, here, we have a small fish that is eating the phytoplankton. A duck is eating that small fish.2059

And then, finally, at the top of the food chain here is the tertiary consumer, which is also a carnivore, and it is an eagle that preys on the duck.2064

So, if you look down here at the bottom of the food chain, the autotroph depends on sunlight for energy.2077

And sunlight is a very stable source. It is a pretty stable source of energy, and so this leads to a relatively stable population.2086

The primary consumers though are dependent on the autotrophs, and autotrophs are also a very reliable source of energy.2095

But, there is going to be some natural fluctuation in the autotroph population.2102

Now, if you continue up the food chain, then, this secondary consumer, this carnivore,2107

is dependent on this fish population and indirectly dependent on the phytoplankton.2112

So, there is less stability as you move up the food chain.2117

Up here at the top, the eagle is the least stable population of all because it is depending on the secondary consumer, the primary consumer2120

and the producer, and if something goes wrong anywhere in this chain, it is going to affect the eagle up at the top of the chain.2128

Therefore, the higher up in the chain, the less stable the population is.2134

So, the eagle population is going to be much less stable than the small fish or phytoplankton population.2149

For these reasons that we talked about, one is the loss of energy at each level at each trophic level, and the second, the instability.2156

As we go higher up in the level, there is a limit to the number of links in the food chain.2169

So, the number of links is usually less than five, rarely, more than seven.2174

Usually, less than five, and it would be very rarely get above seven.2190

And the reasons for this is because of inefficient transfer of energy between trophic levels.2193

And the second reason is instability at higher levels because of this dependence on each of the levels below it.2218

Biological magnification is a related concept to the food chain.2233

And this is the concept that certain minerals and chemicals become more concentrated in the tissues of species higher up on the food chain.2245

So, chemicals may become concentrated higher in the food chain, and this is very problematic if the chemical is a toxin.2252

An absolute classic example is that of DDT. This is a pesticide that became concentrated in bald eagles after heavy use in United States in the 1950s.2273

So, DDT was heavily used around the US.2283

And because the eagle occupies the top level of food chain, with each level of food chain,2286

this DDT, the toxin became concentrated in the tissue of the animal below.2291

This secondary consumer consumed the primary consumer and so on up.2299

And the result was that high levels of DDT cause eagle shells, their egg shells to become very thin.2304

And as a result, many of the offspring did not end up hatching and therefore, did not survive.2312

And it end up putting this species as a whole as a risk, and the use of DDT has since been discontinued.2317

Now, as you can imagine, life is not this simple where it is just a single linear chain one on top of the other like this.2324

In reality, food consumption and trophic levels in the ecosystem is far more complex.2335

And so what we will often do is describe a food web rather than a food chain.2341

So, when we talk about numerous food chains interlacing with one another, we have a food web.2347

As you can see, if I look at phytoplankton here, it is not just that a small fish would eat the phytoplankton,2362

one kind of bird would eat that fish, and then, the osprey would eat that kind of bird only.2367

You can see that the phytoplankton is actually food for this fish, this small planktivorous fish, a benthic invertebrate, bivalves.2374

And then, those in turn are eaten by multiple types of carnivores.2386

So, it is much more complicated than showing with just separating out a single food chain.2392

Although, sometimes there are reasons to just separate out and look at one linear chain.2397

Something I did not mention with the food chain before that is important as well is decomposers or detritivores.2403

They are not necessarily shown in food webs, but it is important to remember that they play a key role in returning organic matter to the soil.2420

Decomposers are typically fungi or prokaryotes.2431

And the decomposers play a key role in returning organic compounds to the soil where the autotrophs use them.2441

They are not going to be unnecessarily a particular level because they are going to break down either2462

waste or the remains of an animal or plant that has died at various levels of the food chain.2470

So, they are throughout the food chain or food web.2476

In a later section, we are going to talk about chemical cycles like the nitrogen cycle.2480

And every ecosystem depends on the cycling of carbon, nitrogen, phosphorus, water.2484

And these decomposers are key in closing these chemical cycles so that the primary producers can have access to the compounds that they need for life.2490

Now, certain species play particularly important or significant roles within a community.2504

And I am going to talk about a few different ways in which a species can play a key role in the community.2510

One is that there may be a dominant species, so frequently, communities will have one or more dominant species.2516

And a dominant species is a species that has the highest quantity of individual organisms, or an even better measure is the largest biomass.2524

And a dominant species is going to have a large influence on the other species in the community.2546

Any disruptions to the dominant species, so if the dominant species were to get a disease2550

and die off or be affected by a drought, that disruption can affect the community overall.2555

Another type of species that is particularly significant in certain communities is what is called a keystone species, and that is what is shown here.2568

And a keystone species might not necessary have the largest quantity of individuals or the greatest biomass in a community.2576

But, they occupy a critical position in a food chain or in a food web.2584

As a very simple example, the food web can narrow down to a single species at a particular trophic level.2602

So, if something happens to that species, all these other species at higher trophic levels could be very much affected and the same with the levels below.2609

The third type of species that is of particular significance are species that are called foundation species.2622

Now, they might not necessary have a special place in the food web, but they can affect communities in other ways.2630

An example would be let's say there is a species of tree that is in a forest.2638

And that this tree provides shelter, a tree species in a forest that provides shelter for a variety of animals.2648

So, loss of that tree, if a disease wiped out that type of tree,2657

it is going to affect all the animals that live or rely on that type of tree for shelter even though it might not be a key player in the food web.2661

Now, communities are in a constant state of change.2673

At one time, scientists looked at it this way. They thought specific geographic areas had a default state.2677

And each species, when they reach this state, was represented at its carrying capacity, and then, the species would interact in predictable way.2684

So, they pictured it as once the community was matured, there was a certain type, a certain balance of species.2694

They just went along interacting in certain ways, and it was a stable situation. This is no longer considered the prevailing view.2700

It is now thought that communities are constantly changing in response to inevitable disturbances. Now, what do I mean by disturbances?2712

To give you some examples, there could be a particularly cold winter, a very cold winter that is snowy.2720

And if this is occurring in a coniferous forest, the result could be high mortality of deer, rabbit and2736

other herbivores because of their inability to find food well into spring because of the snow cover.2755

So, that is obviously going to change the community when a lot of the herbivores are wiped out.2763

Or another example of a disturbance could be lightning causes a fire, and that impacts the portion of a deciduous forest.2768

Heavy rains could cause extensive flooding, let's say, in a river valley.2781

Humans can also cause disturbances. There could be construction of a highway in a desert biome that affects drainage in the desert.2787

There is also this idea of introduced species.2797

There could be an exotic species of fungus introduced, and plants in this area or ecosystem might not have defenses against this type of fungus.2802

So, it could cause a lot of disturbance, disruption, damage, disease.2816

Now, disturbances have several critical components. One component that is important when assessing a disturbance is the duration.2821

Is this disturbance extended or is it short?2831

So, in extending disturbance like a multiyear drought is going to have a greater impact than a shorter duration disturbance.2836

The area that the disturbance covers or areal coverage, the area affected by the disturbance.2844

If the disturbance is a fire, and it just burns down a few trees,2855

that is going to have a very different effect than a huge wide-spread fire that wipes out acres of trees.2860

Frequency: rare versus frequent, if there is a very rare flooding,2869

that is going to affect the community much differently than occasional flooding or frequent flooding would.2878

Finally, intensity: each storm, flood, fire has a different intensity or the strength of the storm, flood, etc.2884

Studies have shown that intermediate levels of disturbance lead to the greatest amount of biological diversity in a community.2902

So, intermediate level of disturbance leads to the greatest biological diversity. To give you a simple example, let's consider a stream.2909

So, an example here, a stream that has very constant flow rate, temperature etc. It is very stable, minimal disturbances.2942

What can happen, then, is that one species will tend to dominate, a species that is optimized for that flow rate, temperature and the conditions.2953

A stream with stable conditions, what will often happen is one dominant species.2963

Now, look at the other end of the spectrum that there is a strain with very intense frequent2974

disturbances that may be alternates between drying out and having catastrophic flooding.2979

Well, that type of stream is going to also have low diversity because many species cannot survive these swings in the level of the stream.2985

Looking between the two, very stable conditions versus vastly fluctuating conditions, would be an intermediate level of disturbance.2997

What this is going to lead to is constant adjustment by species in the communities.3005

There is going to be an opportunity from minor species to increase the number.3009

And the dominant species will have more difficulty overtaking or dominating the ecosystem for an3014

extended period of time because as things change, there will be opportunity for other species to thrive.3021

Some type of disturbances may lead to a process of re-establishing life.3030

So, there can be a level of destruction of a community or a portion of the community.3039

And the result is that life, one level at a time or one layer at a time, is re-established, and this is called ecological succession.3044

An example would be what is shown here, which is a fire. It could be a landslide, glacier, volcanic eruptions.3054

Also, human activities like clear-cutting a forest or farming can destroy portions or all of the community.3063

Now, the level of destruction can vary.3073

If you are looking at a low-intensity slow burning fire, that might destroy most of the undergrowth in the fire and some of the trees in the forest.3076

But seeds, certain plants, many of the insects and lot of other animals could survive.3089

This situation is very different if it is a high-intensity fire that destroys nearly every living thing,3096

and in various severe cases could even destroy organic compounds in the soil.3103

After a destructive event such as a fire, the area will be colonized by plant in the animal species seeking resources that the area might still offer.3109

These colonizing species presence will subsequently provide opportunities for additional species to migrate into the area,3136

so then, others species colonized after.3148

And this process of reestablishment of a community is known as ecological succession or initial establishment of a community. There are two broad types.3155

In primary ecological succession, this is a situation where a community is being re-established in an area where there is no soil.3176

When would this occur? Well, let's say there was a volcanic eruption where complete ash or lava coverage results in an area.3203

Another example could be receding glaciers because the receding glaciers are going to expose3222

organic free rock, gravel and sand, not soil with a bunch of organic compounds in it.3229

So, what is going to happen in primary ecological succession, first thing is going to be that pioneer organisms will get set up there.3243

And we talked about this earlier on when we talked about plants because mosses and lichens are pioneer organisms.3253

Also, prokaryotes are pioneer organisms.3261

Over time, the decomposition of these pioneer organisms along with the erosion of rocks will lead to the formation of soil, so eventually, soil will form.3267

And in this way, these pioneer organisms facilitate actually the ability of later species to become established in the area.3278

So, once the soil is formed, then, larger species can come and take over.3290

Early larger species that are going to come along are grass, weeds and other small plants.3297

This is going to continue on often with woody shrubs, trees along the way.3304

Insects, birds, mammals and various other vertebrates are going to additionally settle.3314

As each species occupies the area, it can do one of two things.3323

A species settling an area can either facilitate the appearance of additional species or inhibit other species from settling.3329

So, a plant that becomes established can provide food for an herbivore to come and settle an area.3350

On the other hand, the one that got there first of a particular species that is using a certain food source3356

might prevent other species after the same resource form settling just by virtue of being there first.3365

Eventually, the community will become relatively stable.3374

It is going to settle into what could be called a final mix of plants, animals, soil, etc., and this is what is known as the climax community.3377

Like I said, sort of a final community because in reality, as we talked about, there would always be disturbances.3387

There is going to be constant change in the interactions and the mix of species in a community. There is no actual, permanent climax community.3394

So, this is all primary succession that I talked about when you are starting out with a situation where there is no soil.3405

Secondary succession occurs when plant and animal life has effectively been destroyed from an area, but the soil is intact.3413

Match the competition situation with the likely outcome, and the outcomes are competitive exclusion, resource partitioning, and character displacement.3439

Looking at the situations, the first one, two frog species live in wetlands.3449

One tends to favor the mud near the waterline for its habitat. The other lives under rocks and small raised areas.3455

Now, this is competition between two frog species, but you will notice that one has a certain niche where it stays near the waterline.3464

The other has a different niche where it lives under rocks and small raised areas.3476

So, they are not directly competing, and what we call this is resource partitioning, so resource partitioning is no. 1.3481

And this is when two species in a community compete for a resource, and then, they developed their own ecological niches.3494

Two: a none-native fish is introduced to a small stable pond. The existing fish and the introduced fish both rely on the same larva for food.3503

Well, what is probably going to happen when you have an introduced species is competitive exclusion.3515

Here, under A we have no. 2. One species could very well end up dying out, and the other will take over.3525

It could be either species. Sometimes introduced species could be devastating to the native species.3533

On the other hand, the introduced species might just get out competed by the native species and never really take hold.3538

A pond is likely too small for resource partitioning.3546

And character displacement is going to take too long because here, we just have a newly-introduced species so things are going to happen more quickly.3547

So, eventually one species will most likely dominate over the other.3556

Finally, several species of bees inhabit an island with a variety of flowers.3560

So, an island, there is going to be a pretty big area even if it is relatively small island considering the small size of bees.3565

And then, over time, what is probably going to happen is character displacement, so C matches three. This is character displacement.3570

And what is going to happen is over a long period of time, this bee species will evolve certain characteristics in order to obtain the ability to take3581

advantage of a resource in a particular location or perhaps, take advantage of a slightly different resource than the competitor species.3591

So, that is what possibly could happen, although, resource partitioning could happen, as well.3603

OK, example two: explain why food chains usually have less than five to seven links.3609

So, remember that there are couple major reasons why food chains are limited. The first is energy insufficiency.3616

Recall that with each trophic level only approximately 10% of the energy is passed on.3627

So, in order to support a very large food chain, primary producer productivity and biomass needs to be very large.3653

Most ecosystems cannot support enough primary producers with enough productivity to support more than five links.3661

So, a large food chain meaning many links would need very high productivity from the producers, from the base of the food chain.3670

And usually, an ecosystem simply cannot support that large of a producer biomass.3690

OK, so this is one reason why food chains have less than five to seven links typically.3697

The second reason, recall, is instability- the higher trophic levels are dependent on the health and productivity of more species.3701

If there is an animal that is up on the fifth level, it is directly or indirectly dependent on all the species below it.3726

So, if something happens to one of those species, it is wiped out by a disease or a change in3735

temperature or climate that it cannot survive, then, that will affect that higher up species.3740

So, high trophic levels are dependent on the health and productivity of more species.3746

This makes their population size less stable, and that means that more levels are just less viable because as you go higher up, less stability.3754

Example three: list the following ecological units from order of smallest to largest or most broad.3766

And we have community, ecosystem, biosphere, population and organism.3774

The smallest level is the level of the individual organism, so we did that one.3781

The next level that we talked about were all the individuals of one species in an area that interact and breed.3791

So, the next level up would be the level of population, all the individuals of one species in a given area that are interacting and interbreeding.3800

After that, what we just talked about today is when we were talking about multiple species,3811

all of the organisms or species that are living in one area, not just one species but multiple species.3817

And that, as I said today, is a community.3825

The next level is when we talked about all the living organisms in an area and the non-living factors in an area,3832

so both the biotic and abiotic factors in an area, and you will recall that, that is an ecosystem.3839

Sometimes we talk about the entire planet as an ecosystem.3848

So, the global ecosystem taking the entire earth is one ecosystem. It is often referred to as the biosphere, so that is the largest unit,3853

so organism, population, community, ecosystem and finally, biosphere.3862

Example four: which are the following disturbances would require the most time to return to its original climax community.3872

A: 2 square kilometer of coniferous forest is buried in 3 meters of volcanic ash.3882

B: 5 square kilometers of grassland is ploughed by a farmer and then, left alone.3890

C: spring flooding submerges 50 meters on either side of a river in a temperate broadly forest. The flooding last 5 days.3897

D: a hurricane causes wide-spread flooding and tree damage in a tropical wet land system.3907

Well, remember we talked about primary versus secondary ecological succession.3918

And primary succession takes longer because in primary succession, you are starting out with no soil.3924

You have to get those pioneer organisms like lichen and prokaryotes in there and wait for weathering and breakdown of rocks and the creation of soil.3929

So, let's look for a situation that would cause primary succession to have to occur. Which disturbance would result in primary succession?3939

And the only disturbance that would do that is coverage with volcanic ash.3950

This is the only disturbance that results in primary succession, and these other disturbances, the soil would be still be intact.3955

So, reestablishment is quicker, and these would represent secondary succession. This would be primary ecological succession.3963

The others represent disturbances that would cause secondary ecological succession.3973

So, that concludes this lesson on

Thank you for visiting.3986