Placeholder Image

字幕列表 影片播放

  • I wouldn't be much of a teacher if I didn't tell you that life is

  • tough and 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 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?

  • Careful guys! Because right here 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?

  • 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 a 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 an elegant and peaceful solution,

  • I wish 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 can 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 tell you

  • about him, I'm going to need a special kind of chair.

  • Canadian born ecologist Robert MacArthur was in his late 20's

  • 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 each 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 cancer 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 interspecific interaction in which

  • species actually join forces in the fight 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've heard of both of these cases: First, mutualism,

  • where both species benefit, and commensalism, where one species

  • benefits and other is kind of like, "Whatever."

  • Mutualism abounds in nature, and for those who've been paying

  • attention to CrashCourse,

  • you've heard me talk about it many, many times before.

  • A prime example are micorrhizae, the fungal roots that we talked

  • about a few weeks ago where fungi and plant roots get entangled

  • 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 breakdown the cellulose in the wood they eat

  • without the enzymes produced by 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 grey whales, getting a free

  • ride through swathes 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 Twitter or of course,

  • down in the comments below.

I wouldn't be much of a teacher if I didn't tell you that life is

字幕與單字

單字即點即查 點擊單字可以查詢單字解釋

B1 中級 美國腔

社區生態學。感受愛--生態學速成班#4 (Community Ecology: Feel the Love - Crash Course Ecology #4)

  • 200 20
    羅紹桀 發佈於 2021 年 01 月 14 日
影片單字