Community Ecology: Feel the Love – Crash Course Ecology #4


I wouldn’t be much of a teacher if I didn’t tell you that life is tough and that everyone’s looking out for themselves in this world. That’s just the way it is, people. You know how I always say that biology is
ultimately about sex and not dying? Well both of those things are more difficult
than we’d like them to be, because of competition. There’s a finite amount of resources on this
planet, so evolution drives us to compete for them so that we can survive long enough to spread
our genes all over the place And naturally, competition is a really important part of how different species interact when their habitats overlap. These interactions between species are what
define ecological communities. So it makes sense that community ecology studies
these interactions anywhere they take place, from a tide pool to the whole ocean,
from a rotting log to an entire forest. But just because inter-species interaction
is mostly competitive doesn’t necessarily mean that community ecology is all about big, bloody, tooth-and-claw scenes like from cable-TV nature shows. Actually, a lot of it is, but we’re not going
to get there until next week. For now, let’s just note that competition,
while prevalent and important, is also pretty dangerous, kind of a hassle,
and can, like, really hurt. So a lot of inter-species interaction is actually
about sidestepping direct competition and instead finding ways to divvy up resources,
or otherwise let species just get along. Can you feel the love? [Theme Music] Careful guys! Because right now we’re surrounded potentially lethal interspecific competition going on all over the place. Since we’re animals, we usually think of competition
as going on between animals, but really it happens between almost all members
of the four kingdoms of life. Whenever species compete, they’re going after
the same resources that they need for their survival and continued population growth. In this garden, the weeds are competing with the sunflower, the corn and the dill for nutrients and water in the soil. So these resources, because they’re finite in this area, are the limiting factors that we’ve talked about. The population can only get as big as these
factors will allow. Now, a particularly nasty weed could, over
time, eliminate the veggies entirely. Such elimination is known as competitive exclusion, and it’s one of the most fundamental properties
in community ecology, and also, like, life. Because the fact is, when two species are
competing for the same resources, one of them is eventually going to be more
successful and eliminate the other. This bitter truth is known as the Competitive
Exclusion Principle, and it was first identified in 1934 by Russian
ecologist G. F. Gause in a study of two closely-related species
of microscopic protists. When he was only 22 years old, Gause made
a name for himself by conducting experiments that pitted one species of protist, Paramecium
aurelia, against another, Paramecium caudatum. First, Gause grew each species separately
with the exact same resources, and found that each developed rapidly and
established stable populations. But, when he grew them in the same container, P. caudatum was soon driven to extinction by P. aurelia. Paramecium aurelia gained a competitive advantage because its population grew slightly faster than P. caudatum’s. So Gause’s experiment showed that, in the
absence of another disturbance, two species that require the same resources
cannot live indefinitely in the same habitat. The inferior competitor will be eliminated. Makes sense, but if competitive exclusion
is the natural law of the land, then why isn’t all of earth just a crazy crap-circus
of constant competition, predation, and ultimately, extinction of all those losers? Well, for a couple of reasons:
first, not all resources are limiting. Two species of sharks may compete for water
in the ocean, but the ocean is, you know, gigantic. So that’s not what limits their population
growth. Rather, the amount of food, like a specific
fish that they both eat, could be limiting, while other resources are plentiful. Second, as the overwhelming diversity of life in almost any community shows us, most species — even ones that are almost identical to each
other — are adaptable enough to find a way to survive in the face of competition. They do this by finding an ecological niche,
the sum of all resources, both biotic and abiotic, that a species uses in its environment. You can think of an organism’s niche as its job in the community that provides it with a certain lifestyle. We tend to keep jobs that we can do better
than anyone else in our community, and if we’re desperate, we do a job that nobody
else wants to do. But no matter what job we have, what it pays
in terms of resources dictates our lifestyle. So finding a nice, comfy niche that you have
pretty much to yourself not only provides a steady income of food and other stuff, it also allows a species to avoid competitive
exclusion, and this, in turn, helps create a more stable ecological community. It’s and elegant and peaceful solution, I wish that we humans could figure out something as good, but as with anything in life, this relative
security and stability comes at a price. The bummer is that it prevents some species
from living the lifestyle that they could have if nobody else competed with them at
all. This ideal situation is called a fundamental
niche, and it’s just that, an ideal. Few, if any species ever get to live that
way. Instead, because of the need to avoid competitive
exclusion in order to survive, many species end up with a different job,
and hence lifestyle. It’s not necessarily the job that they studied
for in college, but it makes a decent living, and that’s called a realized niche. This, my friends, is how nature does conflict
management. But it sounds kind of unnatural, doesn’t it? I mean, Gause taught us that competition, and winning the competition, was the natural order of things. So how could it be that part of the natural order actually involves letting everyone compete and win just a little bit? And how did we ever come to discover that
things actually worked this way? Well, it took a special kind of person, and to to tell you about him, I’m going to need a special kind of chair. Canadian born ecologist Robert MacArthur was
in his late 20s when he made a discovery that made him one of the most influential
ecologists of the 20th century. While researching his doctoral thesis at Yale
University in 1958, he was studying five species of warblers that live in coniferous forests in the northeastern United States. At the time, because there were so many different species of warblers that lived, fed, and mated in such close quarters, many ornithologists thought that the birds
occupied the exact same niche and thus were an exception to Gause’s competitive
exclusion principle. But MacArthur was not convinced. A mathematician by training, he set out to
measure exactly how and where each kind of warbler did its foraging, nesting, and mating. In order to do this, he studied each tree the birds lived in, dividing them into zones, 16 zones to be exact, from bare lichen at the base of the trunk, to new needles and buds at the tips of the branches. After many seasons of observing many birds
in many trees, he found that each species of warbler divided its time differently among the various parts of the tree. One warbler, called the Cape May, for example, spent most of its time toward the outside of the tree at the top. Meanwhile, the Bay Breasted fed mostly around
the middle interior. MacArthur also found that each of the warblers
had different hunting and foraging habits and even bred at slightly different times of the year, so that their highest food requirements didn’t overlap. These differences illustrated how the warblers
partitioned their limiting resources, each finding its realized niche that allowed
it to escape the fate of competitive exclusion. The phenomenon he observed is now known as
resource partitioning, when similar species settle into separate
niches that let them coexist. Thanks in part to this discovery, MacArthur
became known as a pioneer of modern ecology, encouraging curiosity and hypothesis driven research, championing the use of genetics in ecological study, and collaborating with biologists like E.
O. Wilson and Jared Diamond. Sadly, he died of renal caner at the age of
42, but his study of northern warblers remains a classic example of community ecology that is still taught today. So, if organisms can do this, if they can
behave in ways that help minimize competition while increasing their odds for survival, it follows that traits associated with this
behavior would start being selected favorably. After all, that’s what natural selection is for. When this happens, it’s known as character displacement. To demonstrate, let’s go back to some other
famous ecologists, our favorite couple of evolutionary biologists
and love birds, Peter and Rosemary Grant. I told you before about how they observed the process of speciation among Darwin’s famous Galapagos finches. Well on the same island, Daphne Major, in 2006, they witnessed character displacement in action. For a long time, a small population of finches
had the island to themselves, where they ate a variety of seeds, including
seeds of the feverplant, which were bigger and more nutritious than
the smaller seeds that were available but were harder for the little finches to open. Then in 1982, a group of much bigger finches
showed up on the island, and they began to commandeer the island’s
abundant supply of feverplant seeds. Within just 20 years, the Grants found that
the small finches’ beaks shrunk to allow them to specialize in eating only the smaller,
less nutritious seeds. But now the little finches had those seeds
all to themselves. The traits of the two populations had actually diverged to help facilitate the partitioning of resources. See? Competition can be hard on us, but it
also can make us better people, or you know, finches or warblers or kangaroo
mice. But there are also kinds of interspecies interaction in which species actually join forces in the fights for survival. This is the ultimate in conflict-avoidance. In these cases species in a community actually
manage to avoid competition altogether by forming downright tight relationships that
benefit one, if not both, of the parties involved. You may have heard of both of these cases:
First, mutualism, where both species benefit, and commensalism, where one species benefits
and the other is kind of like, “Whatever.” Mutualism abounds in nature, and for those
who’ve been paying attention to Crash Course, you’ve heard me talk about it many, many times
before. A prime example [of mutualism] are mychorrhizae, the fungal root that we talked about a few weeks ago, where fungi and plant roots get tangled and essentially rub each other’s backs for nutritious favors. Others you may have heard about include flowering
plants that produce nectars to attract pollinators, and that bear fruit to attract animals to
help spread the seeds inside. Oftentimes these relationships become rather
needy, like in the case of termites — they can’t break down the cellulose in the
wood they eat without the enzymes produced by the microorganisms that live inside their
digestive systems. Without the little critters, the bigger critters
would die. Such a needy relationship is called obligate
mutualism. By contrast, commensalism is where one species definitely benefits and the other isn’t really hurt or helped. Such neutrality, of course, is difficult to prove because even a seemingly benign interaction probably has some effect. Barnacles, for example, hitchhike on gray
whales, getting a free ride through swaths of plankton-rich water for feeding. While clearly a benefit to the barnacles,
the relationship is often considered commensal because the whales probably don’t really care
whether the barnacles are there or not. Or do they? The barnacles might slow down
the whale as it swims through the water, but on the other hand, they might also serve as a type of camouflage from predators like orcas, in which case they confer an advantage. So it probably comes down to “meh” for the
whale. And when you consider all the other possibilities out there when species interact, “meh” isn’t such a bad option. Especially considering that next week, we’re
going for the throat, by which I mean we’ll be investigating the
kill-or-be-killed world of animal predation and all of the fantastic evolutionary changes it can trigger that lead to even greater diversity in ecological communities. There probably is going to be a lot of blood
though, so you might want to bring your poncho. Thank you for watching this episode of Crash
Course: Ecology. If you want to review anything, there’s a
table of contents over here for you to click on any of the parts that you may want to review. Much love and appreciation to all the people
who helped us put this episode together, and if you have any questions or comments or ideas, you can leave them for us on Facebook or on Twitter, or, of course, down in the comments below.