there's a lot of ideas that we just assume that we know a lot about because we hear about them all the time. for instance, i know what pop music is, butif you were to corner me at a party and say, "hank, what is pop music?", i'd be like, "it's, uh...it's like, uh...themusic that plays on the pop station?" just because we're familiar with a conceptdoes not mean that we actually understand it.
crash diet 101, ecology's kind of the same way, even thoughit's a common, everyday concept and "ecosystem" is a wordthat we hear a lot, i think most of us would be a little stumped if somebody actually asked us what an ecosystem is, or how one works, or why they're important,etc.
i find it helps to think of an ecosystem -- a collection of living and nonliving things interacting in a specific place -- as one of those magic eye posters, for thoseof you who were sentient back in 1994. an ecosystem is just a jumble of organisms,and weather patterns, and geology, and other stuff that don't make a lot of sensetogether until you stare at 'em long enough, from far enough away and then suddenly a pictureemerges. and just like with magic eye posters, it helps if you're listening to jamiroquai while you're doing it. so the discipline of ecosystem ecology, just like other types of ecology we've been exploring lately, looks at a particular level of biologicalinteraction on earth.
but unlike population ecology, which looks at interactions between individuals of one species, or community ecology, which looks at how bunchesof living things interact with each other, ecosystem ecology looks at how energy andmaterials come into an ecosystem, move around in it, and then get spat backout. in the end, ecosystem ecology is mostly abouteating -- who's eating whom and how energy, nutrients, and other materials are getting shuffled around within the system. so today, we're setting the record straight: no more not understanding how an ecosystem works, starting now! [theme music] so ecosystems may be a lot like magic eyeposters,
but the way that they're not like a magic eye poster is in the way that posters have edges. ecosystems, i'll just come out and say it: no edge, only fuzzy, ill-defined gradientsthat bleed into the ecosystems next-door. so actually defining an ecosystem can be kind of hard; mostly it depends on what you want to study. say you're looking at a stream in the mountains. the stream gets very little sunlight because it's so small that the trees on its banks totally cover it with shade. as a result, very few plants or algae live in it, and if there's one thing that we know about planet earth, it's that plants are king -- without plants,there are no animals. but somehow, there's a whole community of animals living in and around this mountain stream,
even though there are few plants in it. so what are the animals doing there, and howare they making their living? from the land, of course --from the ecosystems around it. because no stream is an island, it isn't there all by itself. all kinds of food, and nutrient,s and othermaterials drop into the stream from the trees, or are washed into it when it rains. leaves and bugs, you name it, flow down fromneighboring terrestrial ecosystems. and that stuff gets eaten by bigger bugs,which get eaten by fish, which in turn are eaten by raccoons and birdsand bears.
so even though this stream's got its own thinggoing on, without the rest of the watershed, the animals there wouldn't survive. and without the stream, plants would be thirsty and terrestrial animals wouldn't have as many fish to eat. so, where does the ecosystem of the streamstart and where does it end? this is a perennial problem for ecologistsbecause the way it works: energy and nutrients are imported in fromsome place, they're absorbed by the residents of an ecosystem and then passed around within it for a little while, and then finally passed out, sometimes intoanother ecosystem. this is most obvious in aquatic systems wherelittle streams eventually join bigger and
bigger waterways until they finally reachthe ocean. this flow is a fundamental property of ecosystems. so at the end of the day, how you define anecosystem just depends on what you want to know. if you want to know how energy and materialscome in, move through, and are pooped out of a knot in a tree that has a very specific community of insects and protists living in it, you can call that an ecosystem. if you want to know how energy and materialsare introduced to, used, and expelled by the north pacific gyre, youcan call that an ecosystem. and if you want to know how energy and materialsmove around a cardboard box
that has a rabbit and a piece of lettuce init, you can call that an ecosystem. i might tell you that your ecosystem is stupid,but go ahead, do whatever you want. the picture you see in an ecosystem's magic eye is actually dictated by the organisms that live there, and how they use what comes into it. an ecosystem can be measured through figuringout things like its biomass, that is, the total weight of living thingsin the ecosystem, and its productivity -- how much stuff is produced and how quickly stuff grows back, how good the ecosystem is at retaining stuff. and of course, all these parameters matterto neighboring ecosystems as well because if one ecosystem is really productive,the ones next-door are going to benefit.
so, first things first, where do the energyand materials come from? and to be clear, when i talk about materials, i'm talking about water or nutrients like phosphorus or nitrogen, or even toxins like mercury or ddt. let's start out by talking about energy becausenothing lives without energy and where organisms get their energy tellsthe story of an ecosystem. you remember physics, right? the laws of conservation state that energyand matter can neither be destroyed or created; they can only get transferred from place toplace to place. the same is true of an ecosystem.
organisms in an ecosystem organize themselvesinto a trophic structure, with each organism situating itself in a certainplace in the food chain. all of the energy in an ecosystem moves aroundwithin this structure, because when i say energy, of course i meanfood. for most ecosystems, the primary source ofenergy is the sun, and the organisms that do most of the conversion of solar energy into chemical energy -- you know this one. who rules the world?the plants rule the world. autotrophs like plants are able to gatherup the sun's energy, and through photosynthesis, make something awesome out of it: little stored packets of chemical energy.
so whether it's plants, bacteria, or protiststhat use photosynthesis, autotrophs are always the lynchpin of everyecosystem -- the foundation upon which all other organismsin the system get their energy and nutrients. for this reason, ecologists refer to plantsas primary producers. now obviously, the way that energy gets transferred from plant to animals is by the animal eating the plant. for this reason, herbivores are known as primaryconsumers, the first heterotrophs to get their grubbypaws on that sweet, sweet energy. after this stage of the trophic structure, the only way to wrestle the solar energy that was in the plants that the herbivore ate is to -- you guessed it --
eat the herbivore, which carnivores, known assecondary consumers, are very happy to do. and assuming that the ecosystem is big enoughand productive enough, there might even be a higher level of carnivore that eats other carnivores, like an owl that eats hawks, and these guys are called tertiary consumers. and then there are the -vores that decomposeall the dead animal and plant matter, as well as the animal poop: detritivores. these include earthworms and sea stars andfiddler crabs and dung beetles and fungi and anything else that eats the stuff that none of the rest of us would touch with a three-meter pole. so that's a nice hierarchical look at who's getting energy from what or whom within an ecosystem,
but of course organisms within an ecosystemdon't usually abide by these rules very closely, which is why these days we usually talk aboutfood webs rather than food chains. a food web takes into consideration that sometimes a fungus is going to be eating nutrients from a dead squirrel, and other times squirrels are going to beeating the fungi. sometimes a bear likes to munch on primaryproducers, blueberry bushes, and other times it's going to be snackingon a secondary consumer, like a salmon. and even at the tippy-tippy top, predatorsget eaten by stuff like bacteria in the end, which might or might not be the same bacteria that eat the top predator's poopies. circle of life! it's also worth noting that the size and scope of the food web in an ecosystem has a lot to do with things
like water and temperature, because waterand temperature are what plants like, right? and without plants, there isn't going to bea whole lot of trophic action going on. take for example the sonoran desert, whichwe've talked about before. there aren't many plants there, compared tosay, the amazon rainforest, so the primary producers are limited by thelack of water, which means that primary consumers are limitedby lack of primary producers. and that leaves precious few secondary consumers:a few snakes and coyotes and hawks. all this adds up to the sonoran not being a terribly productive place, compared to the amazon at least, so you might only get to the level of tertiaryconsumer occasionally.
now all this conversation about productivity leads me to another point, about ecosystem efficiency. when i talk about energy getting passed alongfrom one place to another within an ecosystem, i mean that in a general sense organisms are sustaining each other, but not in a particularly efficient way. in fact, when energy transfers from one place to another, from a plant to a bunny or from a bunny or a snake, the vast majority of that energy is lost alongthe way. so let's take a cricket. that cricket hasabout one calorie of energy in it. and in order to get that one calorie of energy,it had to eat about 10 calories of lettuce. where did the other nine calories go?it is not turned into cricket flesh. most of it is used just to live, like to power its muscles or run the sodium-potassium pumps in its neurons.
it's just used up. so only the one calorie of the original 10 calories of food is left over as actual cricket stuff. and then right after his last meal, the cricket jumps into a spider web and is eaten by a spider, who converts only 10% of the cricket's energyinto actual spider stuff. and don't get me started on the bird that eats the spider; this is not an efficient world that we live in. but do you want to know what's scary efficient?the accumulation of toxins in an ecosystem. elements like mercury, which are puffed outof the smokestacks of coal-fired power plants, end up getting absorbed in the ocean by greenalgae and marine plants. while the tiny animal that eats the algae only stores 10% of the energy it got, it keeps 100% of the mercury.
so as we move up the chain, each trophic levelconsumes ten times more mercury than the last. and that's what we call bioaccumulation. concentrations get much higher at each trophiclevel until a human gets ahold of that giant tunathat's at the top of the marine food chain, and none of that mercury has been lost. it's all right there in that delicious tunaflesh. because organisms only hold on to 10% of theenergy they ingest, each trophic level has to eat about ten timesits biomass to sustain itself. and because 100% of that mercury moves upthe food chain,
that means that it becomes ten times moreconcentrated with each trophic level it enters. that's why we need to take the seafood advisoryseriously. as somebody who could eat anything you wanted, it's probably safest to eat lower on the food chain: primary producers or primary consumers. the older, bigger, higher-in-the-food-chain,the more toxic it's going to be. and that's not just my opinion, that's ecosystemecology. thank you for watching this episode of crashcourse: ecology and thank you, everyone who helped us putthis episode together. if you want to review any of the topics wewent over today, there's a table of contents
over there that you can click on, and if you have any questions or commentsfor us, we're on facebook or twitter or, of course, down in the comments below.we'll see you next time.
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