https://dl.dropboxusercontent.com/u/67743541/StreamEcosystem.mp3

Hello Students. This is plants and animals of southern California, and we're at Switzer's, ah, in the San Gabriel Mountains about, I don't know, 4000 feet or something, um, and there's this beautiful stream flowing next to us, ah, and then surrounding the stream, we see a lot of alders. The alders have a special relationship with some bacteria in their roots, symbiotic relationship with bacteria in their roots, and the bacteria are able to fix atmospheric nitrogen and make it into nitrogen-ah-like nitrate or ammonium, something useful to the plants. So the plants provide the bacterium with a house and cultivate it, and then they harvest the nitrogen. So the, these, the leaves of these alders are very rich in nitrogen, and the alders are descended from plants that, ah, come from the North where it sort of makes sense to drop your leaves in the winter time when it's snowy, and so even though it doesn't make sense here, they still drop their leaves, and they might drop their leaves into the stream sometimes. So, what I want to talk about today is the flow of two things through this ecosystem, through the stream ecosystem. First, the flow of carbon, so, um, carbon, of course, is in the leaves of these, uh, alders that are dropping into the stream, and that carbon is an energy source that can be use by animals, um, to fuel their metabolism. The second thing I want to talk about is the flow of nutrients through the ecosystem, and that would include things like fixed nitrogen, ah, so we can just use nitrate as our sort-of stand-in nutrient for all the rest. So what happens here is, a leaf falls in the stream, and then it's in a wet place, and microbes, like bacteria and fungi (maybe some algae, but let's say bacteria and fungi), they start digesting that, um, leaf, and so they'll start making it into a mouldy leaf, a soft mouldy leaf. They'll soften it up.  And some of those microbes, like bacteria and fungi, are able to digest cellulose. Cellulose is what the leaf is made out of more or less. There's some other stuff in there too. Most animals can't digest cellulose. Now, of course, you might know that cows eat stuff, cellulose, but then the cows they have microbes in their gut that are really breaking down the cellulose, right? Like the cow by itself couldn't eatÑif you can imagine a cow by itself without its microbes (which is hard to imagine, butÑthe cow by itself could not digest cellulose. And likewise these stream organisms also, by themselves they can't digest the cellulose, but the microbes start digesting the cellulose. And then animals, um, will come along and start, you might say, eating the leaf, or you know, they're using their mouth parts to grind up little the parts of the leaf. And you might see sometimes a leaf that is just the skeleton, just the veins of a leaf, where all of the, the blade of the leaf has been eaten away by these little insects. Now these little insects are stream invertebrates, um, that are eating leaves like this, they are called, said to be, shredders. And shredders is not like a taxonomic group. It's not a family or order or something like that. Shredders include representatives of several different orders of insects, um, so you might have mayflies that are shredders, you might also have stoneflies that are shredders, and so on. Instead it's, instead of being a taxonomic group, it's a trophic role. (Um, just like we might have trophic roles of being, um, grazers ah, out-on-the-land, like you might think of deer as being browsers or grazers, and then mountain lions as being carnivores that eat the deer, and stuff like that.) So, similarly in the stream what we might have is we have different trophic roles, and it's a little more interesting and more complex than just the trophic roles that you might have learned about deers and mountain lions. So the one I'm emphasizing first is the shredders, and they shred the leaf. And they get their carbon and their nutrients to some extent, um, from the microbes that have broken the leaf down. So in other words, they're eating the stuff of the leaf, which includes not just leaf material but also microbes that have started to break it down, and they would get their energy from the microbes that have eaten the cellulose which they can't digest. Now of course there's lots of fiber that they continue, that they're still not able to digest, cellulose that they're not able to digest. Cellulose is a kind of fiber, Um and so, um, they pass it on. They poop out, they poop out some little turds that include some cellulose, and there's still other, there's still microbes working on that too, and those, that little poop, that goes into the stream as fine particulate matterÑit's fine particulate mater that's washing down the stream here. Now, um, what would happen after that is, of course the microbes are still acting on it, and so then there are other trophic roles, trophic groups, that are in the stream that would then collect that fine particulate matter. Some of 'um are filter-gatherers. So they have like nets or they might have, um, strainers, parts of their body bodies again. Some how or other they filter particles out of the water, and then they eat that, and then it would pass through their bodies again, and then it might be ultrafine particulate matter. So a second trophic role that we would find would be the filter-feeders, the filter-gathers. And then a third would be the filter collectors. So this stream has some riffles and it has some pools, and a riffle and then a pool, and a riffle and pool, a riffle and a pool, the water is stiller, it's more still, and in the bottom of the pool there are, um, and in the bottom of the pool there are animals like caddisflies and um all sorts of different types of animalsÑah, not just a taxonomic groupÑthat would be, ah, the trophic role of collector gatherers. They would collect this fine particulate matter, and they would process it again. And so you'd have that energy that's in the cellulose from the, from the alder leaf, that's cycling through two or three groups of animals. Um. Okay, now, let's go back and think about the nitrate. The nitrate or nutrients from the alder, they get released into the environment, into the stream. Now some of it would be taken up in the food of the shredders. And the shredders would use that nitrogen to build their bodies. Like you need nitrogen in order to build muscles and stuff like that to build their bodies. Um, so the shredders would be building their bodies from the nitrogen. Of course shredders die, and some of the nitrate would leech out, just into the water, and on the rocks, in the stream, you would get a thin film of growth, of little algae, tiny algae that are photosynthetic, and those algae would be of various sorts, but a lot would be diatomsÑso there's a lot of diatoms in these streamsÑah, and the diatoms, they photosynthesize, so they fix carbon from the water and from the atmosphereÑCO2, which in the water becomes bicarbonate, they'd fix that, but they do need nutrients in order to grow diatom bodies, and they would get their nutrients from the water. So they're very good at absorbing the nutrients out of the water, and gathering it up. And there's this thin film of, of, diatoms over the rocks. After we get out of, out from under the trees, the diatoms would grow better because there's more sun, and other algae would also grow, filamentous algae, would also grow better, so there's, ah, then another trophic group of organisms, which I'll call grazers, and those are animals that, they, go on the rocks, and they eat the algae off the rocks, they're like "scratch scratch scratch" to the algae, right on the surface of the rocks. So we have shredders, collector-gathers, collector-filters, and then grazersÑand all of those are consumers, like they are all, they are all like vegetatians sort of, I mean not exactly because they're eating the microbes, but as opposed to being predators. The fifth trophic group that we would have in these streams would be predators that are eating all these other trophic groups. So there's predators that are eating, um, probably fairly indiscriminately amongst shredders, collector-filterers, collector-gatherers, and stuff. And of course, the different predators, they do have different microhabitats that they like. So some might like fast flowing waters. And some might like pools. That type of thing would affect exactly what they eat. [sneeze] But anyway, we have these five different trophic groups, um, that are processing what's falling into the stream, and they're also in a sense they are kind of cleaning up the stream, right? Like, for a stream that flows for any distance, the stuff that goes in it is not the what is in it, because there's all these organisms that are processing everything that goes into the stream, and they, and the result is at the other end you get clean water, so if you have a stream that is flowing for long enough, then it, the water gets all cleaned up. Now, um, there's two carbon sources, notice, there's the carbon source that's the leaves falling into the stream, and that's called allochothonous inputs ('allo-' means other) so there's allochothonous inputs, and then the opposite of allochthonous would be autochothonous ('self-' inputs) and that would be the algae that are growing in the stream that are fixing carbon out of bicarbonate in the water which ultimately comes from carbon dioxide in the, in the atmosphere. If you think about it for a minute, this is kind of a cool thing, in the headwaters up here, underneath all the al- alders, then you'd have more allochothonous inputs than autochothonous inputs, whereas down in, ah, Big Tojunga, where there's hardly any vegetation that's overarching the stream, you'd have more autochothonous inputs than allochothnous inputs, because the sun is shining right on the stream, there's hardly any leaves on, in it. Now you might also find some filter-gathers and stuff like that down in Big Tojunga because there's all this stuff from the mountains that's been processed down to ultra-fine particulate matter, and that ultra-fine particulate matter is being collected by the animals that are filtering the water, that are cleaning up the water. Um, so you would find more shredders here, and we would find more grazers down there. That's what we would predict. Now the exact, um, species of shredder or grazer, that would also sort of change throughout the seasons. You know like early in the season, you would have one species, and then a little later that species dies down and you would have another species, and then a little later that species dies down and you would have another species. And this is a super seasonal ecosystem because, um, all the rain comes, you know, in five episodes during December, January, February, and maybe a little bit of March. Um. So you go from a torrential stream down to something, you know, like this, and the different species of organisms are better able to hold on, or better able to hand oxygen deficit or you know all sorts of things like that or something like that. So it's this constantly changing seasonal, um, turnover of exact species. But meanwhile, while that's happening, you're also still having representation of the different trophic groups in the stream as we find them. Alright, so, that's the spiel. Any questions or comments?