Community Interactions: Competition (-/-) - Online Tutor, Practice Problems & Exam Prep

Competition occurs when organisms vie for limited resources, impacting their fitness negatively. This interaction can be symmetric, with no clear winner, or asymmetric, where one species outcompetes the other. The competitive exclusion principle states that two species with identical niches cannot coexist permanently; one will eventually dominate, leading to local extinction of the weaker competitor. In contrast, resource partitioning allows species with partial niche overlap to coexist by utilizing different resources, which can lead to character displacement, where species evolve distinct traits to minimize competition.

Community Interactions: Competition (-/-)

This video, we're going to talk about the very first community interaction in our lesson, which is competition. And so competition is when two or more organisms compete either directly or indirectly for the same limited vital resource. The overall impact on these competing organisms can either be equal if there's no clear winner or loser of the competition and it's pretty balanced, which we call symmetric competition, or it could be unequal if there is a clear winner and a clear loser of the competition. So it's imbalanced and we call it asymmetric competition. Now moving forward in our next video, we'll talk more about the differences between direct, indirect, symmetric, and asymmetric competition.

But for now, the big takeaway that I'd like you to walk away with from this video is that regardless of the type of competition, competing for a resource always lowers the fitness of the competing organisms. So competition is always a minus-minus interaction compared to getting the resource without competition. And this is because the loser of the competition loses access to the vital resource. But even the winner of the competition must spend precious energy and time in order to do so. And so let's take a quick look at our image down below where notice on the left-hand side, we're showing you that with no competition, it's all easy peasy for this squirrel to get its hands on this acorn right here.

And notice that its energy battery is full. But over here with competition, notice that these losers over here are trying to get some and get hooked up. And so that's negative for their fitness since they lose access to the resource. And this winner over here is like, "Nah, fam. I'm not sharing."

This might be the last acorn in this area. And so notice that although he's winning this competition, his energy battery is depleted, and that's negative for its fitness in comparison to no competition. And so this here concludes our brief introduction, and we'll be able to learn more moving forward. So see you all in our next video.

Community Interactions: Competition (-/-) Example 1

So here we have an example problem that wants us to appropriately match each competitive situation labeled a through d down below to the appropriate part of the table. And so we need to differentiate between symmetric and asymmetric competition and between direct and indirect competition. Symmetric asymmetric competition is all about if there is an obvious winner or loser in the competition. If there's no obvious winner or loser, then the competition is pretty balanced, and we call this symmetric competition. Now if there is an obvious winner or loser, then the competition is imbalanced, and we call this asymmetric competition.

Now direct competition, such as, for example, a face to face physical fight or confrontation over resources, is when a competitor directly and actively interferes with its opponent's chances of success in the competition. Indirect competition, on the other hand, is when a competitor indirectly and passively decreases its opponent's chances of being successful in the competition and is often a result of depleting shared resources. With that in mind, notice a says, a large bear chases away a fox from a deer carcass, then comes back to eat the whole deer for itself. Clearly here, the bear is in a physical confrontation with the fox as it is chasing the fox away from the deer. And because that is the case, this is going to be an example of direct competition.

And what you'll also notice is that the bear comes back and eats the whole deer for itself, so it's clearly winning this competition, and that means that it's a form of asymmetric competition since there's a clear winner and loser. And so what we're saying is that a goes right into this block right here for direct asymmetric competition. So we can cross it off the list and move on to b. B says a nocturnal mouse or a mouse that's active at night depletes a shared food source, leaving little to none available for a diurnal squirrel or a squirrel that's active during the day. So the nocturnal mouse and this diurnal squirrel are active at completely different times of the day, so they're not actually coming into confrontation with each other.

They're not directly competing with each other. Instead, they're indirectly competing with each other for the same food source. And so, we know that it will be categorized as indirect. And also notice that there's a clear winner here in this competition as the nocturnal mouse is getting all of the food and leaving little to none for the squirrel. And so that means that it's asymmetric. So indirect and asymmetric would be b here, and we can cross off b. So now moving on to c, it says 2 lions are injured in a battle over territory, but neither gains an advantage. And so these two lions are battling. This is a physical confrontation, a physical fight, so this is a form of direct competition. And notice that neither of them gains an advantage, so there's no clear winner or loser here.

So this is a form of symmetric competition. So, c will be direct and symmetric, and we can cross off c and move on to option d. Now d says 2 species of birds feed on an insect population and both experience a decline in food availability. Once again, this is a form of indirect competition as they are depleting a shared resource, and there's no indication that they are directly interfering with each other in any other physical, confrontational way. And so, it's indirect, and it is also symmetric because they are both experiencing a decline in food availability.

So there's no clear winner or loser here in this competition. So, d here is going to be indirect and symmetric. So that here concludes this example problem, and I'll see you on our next video.

Which of the following scenarios is an example of direct, asymmetric competition?

• A. Cheetahs struggling to get enough food because a pride of lions are eating all the zebra in the territory.
• B. Two gorillas fighting for dominance, but both ending up injured and neither emerging as a clear winner.
• C. Sea turtles & crabs both eat algae in a coral reef, leaving less food available for both species.
• D. A plant secretes poison into the soil, inhibiting the growth of other plants in the same area.

Which of the following is always true regarding competition between organisms?

• A. There's a clear "winner" and a clear "loser".
• B. It decreases the fitness of all organisms involved.
• C. It can increase the fitness of one species if they "win" the competition.
• D. None of the above.

Competitive Exclusion Principle (Complete Niche Overlap)

This video, we're going to talk about the competitive exclusion principle, which explains what happens when there is complete niche overlap between competing organisms. First, recall from our previous lesson videos that the term "niche" is a broad, multidimensional term that refers to a species' ecological role or job. It includes a variety of aspects including the interactions and resource utilization of the species and the conditions that the species can tolerate. Now here in this video, we're going to answer the question, what happens to a weaker competitor when its niche completely overlaps with a stronger competitor's niche? Meaning that the weaker and stronger competitors are using the same exact set of resources. When this happens, competitive exclusion will occur.

The competitive exclusion principle tells us that two competing species with completely overlapping niches cannot permanently coexist in the same exact area or in the same exact community. This is because, eventually, one species will always outcompete the other species over time, eventually leading to the local exclusion or the local extinction of the losing species from that particular area or community. This is what we mean by the term "local": that the exclusion or extinction is only happening for the losing species in that particular area or in that particular community where these two species are interacting and competing. Local exclusion or extinction is not to be confused with global exclusion or extinction, which would mean that the losing species would be wiped off of the planet. And again, that's not the case here.

We're looking at local exclusion or local extinction. So, that being said, let's take a quick look at our image down below and notice on the left hand side, we have this graph that shows the niche or the range of resources used by these species on the x-axis, and the amount of resource used by those species on the y-axis. We've got two different species of paramecium: a stronger competitor, Paramecium aurelia in the red color, and a weaker competitor, Paramecium caudatum, in this bluish color. What's really important to note here is that the stronger and weaker competitor have completely overlapping niches.

They're using the same exact set of resources, and we know this will lead to the competitive exclusion of the weaker competitor. We can see the effects of competitive exclusion happening over here in this right graph where we have time on the x-axis and the number of cells on the y-axis. We have the stronger competitor, Paramecium aurelia, growing alone and growing mixed with Paramecium caudatum. Then we have the weaker competitor, Paramecium caudatum, growing alone and growing mixed with Paramecium aurelia. Here, when both of these species are growing alone, they form this classic sigmoidal logistic growth curve that we expect to see. However, when these two species are growing mixed together, notice that they're going to compete with each other.

Competition is a minus-minus interaction where both species are negatively impacted. We can see the stronger competitor is negatively impacted because its growth curve is somewhat diminished and going down. Because these two species have completely overlapping niches, this is going to lead to the competitive exclusion of the weaker competitor. Notice that Paramecium caudatum can temporarily coexist. But eventually, over the long run, it will be driven into local extinction, driven into local exclusion, and it is not allowed to permanently coexist with a stronger competitor if its niche completely overlaps.

This is what we mean by the competitive exclusion principle and complete niche overlap. Moving forward, we'll get to talk about what happens to the weaker competitor when there's only partial niche overlap. So, I'll see you all in that video.

Resource Partitioning (Partial Niche Overlap)

This video, we're going to talk about resource partitioning and partial niche overlap between competing organisms. And so what happens to a weaker competitor when its niche only partially overlaps with a stronger competitor's niche? Well, in that case, resource partitioning can occur. And this is when a species actually avoids competitive exclusion by differentiating or changing its niche and partitioning the resources that are available. And so resource partitioning can cause an organism to shift from its fundamental niche to its realized niche.

Now, the fundamental niche is the full or total theoretical niche that a species could potentially occupy without any competition. Whereas the realized niche, on the other hand, is the smaller portion of the fundamental niche that a species actually occupies due to competition. So let's take a look at this image down below where we've got these two graphs very similar to the ones that we saw in our last lesson video. And so notice that we're showing you the same exact stronger competitor here with Paramecium aurelia here with this reddish curve. But this time, we're showing you a different, weaker competitor in Paramecium bursaria with this green curve.

And notice that this time, there's only partial niche overlap between these two competing species, which is what allows for resource partitioning to occur. And so notice that the weaker competitor is shifting from its fundamental niche, this full curve here, over to its realized niche. And the realized niche is the smaller darker green portion of the curve. And this shift in niche is occurring because of competition with the stronger competitor. Now, the weaker competitor, by changing its niche in this particular way, it reduces competition and thereby helps the weaker competitor to avoid competitive exclusion.

Therefore, it can coexist in the same community with this competitor. And we can see this actually happening over here in this graph on the right, where once again we've got time on the x-axis and the number of cells on the y-axis. And we've got P. Aurelia growing alone and growing mixed together with P. Bursaria.

And then we've got P. Bursaria growing alone and mixed together with P. Aurelia. And so notice that when these two species are growing alone, they exhibit those classic sigmoidal logistic growth curves that we expect to see. But when they're mixed together, these two species are going to compete with one another.

And we know that competition is a minus-minus interaction where both organisms are negatively affected. And so we can see that P. Aurelia is negatively affected when it's growing together because its growth is somewhat diminished, and we can see something similar happening here with P. Bursaria when it's growing together. Its growth is diminished.

Now the key difference here is that because these two species only have partial niche overlap, resource partitioning can allow the weaker species to avoid competitive exclusion and coexist with the stronger competitor. And so notice that this time around, the weaker competitor is not driven into local extinction or local exclusion like what we saw in our last lesson video, and that's because of resource partitioning. So it's allowed to coexist with the stronger competitor because of resource partitioning. So this here concludes our lesson on this, and we'll be able to learn more and apply these concepts and problems moving forward. So I'll see you on our next video.

Character Displacement

This video, we're going to talk about character displacement. And so the long-term outcome or result of resource partitioning is character displacement. And so character displacement refers to the evolution of new traits between competing species in order to reduce the competition between those competing species. And the reduction of competition is favorable for both species since recall that competition is a minus-minus interaction, and so having less of it will be favorable. Now, a classic example of character displacement is the character displacement that occurs with the beak depth of these two species of ground finch on the Galápagos Islands.

And these two species are the small ground finch or Geospiza fuliginosa and the medium ground finch or Geospiza fortis. And the beak depth really just refers to the vertical height of their beaks, which is going to dictate the size of the seeds that these species are able to consume. Now over here on the right, we have these three separate graphs showing us data for these three islands, and all three of these graphs have beak depth on the x-axis and the number of birds on the y-axis. And so what you should notice is that island number 1 only has significant populations of the small ground finch or Geospiza fuliginosa. And so notice that the beak depth data is falling right into this region here, averaging right around 10 millimeters.

Now on island number 2, on the other hand, it only has significant populations of the medium ground finch or Geospiza fortis. And notice that the beak depth data is falling into a similar region, averaging right around 11 millimeters. And so these two species have partially overlapping niches, which can lead to resource partitioning, and we know that the long-term outcome or result of resource partitioning is character displacement, which we can see actively occurring on island number 3. And so island number 3, unlike island number 1 and island number 2, has significant populations of both the small ground finch, Geospiza fuliginosa, and the medium ground finch, Geospiza fortis. And so these two species with partially overlapping niches are going to be competing with each other.

And again, that competition is not favorable for either of them. And so what they can do to reduce that competition is character displacement. And so that's just the displacement of their characteristics. So notice that here in this region of the graph where we used to have data, there's now a gap here in the data. And that's because they've displaced their characteristics, where the small ground finch has displaced its characteristics so that it now has evolved smaller beak depths, averaging right around 7 millimeters.

And the medium ground finch has evolved larger beak depths, averaging right around 14 millimeters. And so this displacement of their characteristics creating this gap here, again, is going to reduce the competition between these two species, which is favorable for both of them. And this is what we refer to as character displacement. So hopefully, this was clear for you. Moving forward, we're going to apply these concepts and problems, and so I'll see you all there.

Community Interactions: Competition (-/-) Example 2

So here we have an example problem that wants us to appropriately match each term on the left-hand side over here to their correct definition over here on the right-hand side. And of course, there are many different ways we can approach this, but the way that I'm going to approach this is by starting with the definitions and matching them to their terms as we go down. So number 1 here says the ability of a species to survive and pass its genetic material onto offspring. Now this matches with fitness, and so we can put 1 here next to fitness. Recall that the term fitness refers to the organism's ability to survive and reproduce, which means they survive and pass on their genetic material to their offspring.

Now we can cross off 1 and move on to 2. 2 says the principle that 2 or more organisms with the same niche cannot permanently coexist. And so if they have the same niche, that means that they are completely overlapping niches, and they cannot permanently coexist in the same community. And so this principle is the competitive exclusion principle. So Roman numeral II goes here, and we can cross that off and move on to 3.

3 says the range of resources used by an organism due to competition. Now this range of resources is going to be the niche. And so the niche that an organism has due to competition is going to be the smaller realized niche. And so 3 here matches with realized niche, and we can cross that off. Now 4 says species evolve new traits in order to avoid competition.

Now the evolution of those new traits is going to be character displacement, which is the end result or the outcome of resource partitioning. And resource partitioning is going to be the last one here, number 5, which says species niches diverge from one another to avoid competition and increase fitness. So that will be 5. So that here concludes this example problem, and I'll see you in our next video.

In which scenario is competitive exclusion most likely to occur?

• A. When there are multiple predators in a community.
• B. When there are 2 species with partially overlapping niches in a community.
• C. When there are 2 species with completely overlapping niches in a community.
• D. Both b & c.

The total range of resources that an organism can use & the conditions it can tolerate is its _____ niche, while the actual range of resources it uses & conditions it lives in is its _____ niche.

• A. Fundamental; realized.
• B. Realized; fundamental.
• C. Competitive; realized.
• D. Fundamental; competitive.

What could an ecologist do to find out if a species is occupying its fundamental or its realized niche?

• A. Remove a species it consumes & measure the effect.
• B. Observe how its use of resources changes upon the introduction of a competitive species.
• C. Take half the population out the community and observe the species' use of resources.
• D. Observe changes in the species' range of resources used after removal of a competitor.

Two barnacle species, Chthalamus stellatus & Balanus balanoides, can be found in communities on rocks along the coast. C. stellatus live higher up on the rocks than B. balanoides, who live closer to the surface of the water. In an experiment, all the B. balanoides are removed from the rock and over time the C. stellatus colonized the entire surface of the rock, including the area where only B. balanoides previously lived. In another area, all the C. stellatus were removed but the habitat of B. balanoides did not change. What does this suggest about the relationship between the two species?

• A. C. stellatus is a stronger competitor and forces B. balanoides into a realized niche.
• B. B. balanoides is a stronger competitor and forces C. stellatus into a realized niche.
• C. Both species have undergone resource partitioning to avoid competition.
• D. Both species have undergone character displacement to avoid competition.

In which of the following scenarios is character displacement most likely to occur?

• A. Two allopatric species that have complete niche overlap.
• B. Two allopatric species that have partial niche overlap.
• C. Two sympatric species that have partial niche overlap.
• D. Two sympatric species that have complete niche overlap.
• E. None of the above.