- The history of life on earth
- Population ecology: The Texas mosquito mystery
- Human population growth
- Community ecology: Feel the love
- Community ecology II: Predators
- Ecological succession: Change is good
- Ecosystem ecology: Links in the chain
- The hydrologic and carbon cycles: Always recycle!
- Nitrogen and phosphorus cycles: Always recycle!
- 5 human impacts on the environment
- Conservation and restoration ecology
Hank discusses ecosystems, trophic structure, and the flow of energy through food webs. Created by EcoGeek.
Want to join the conversation?
- Plants are all at the bottom of the food chain, right? Then what about insect-eating plants like Venus Flytraps?(14 votes)
- The question you asked points to one of the reasons why people usually refer to it as a "food cycle" or "food web" instead of a "food chain" now. There isn't usually a bottom to the cycle, just a reprocessing of energy. Plants can't live on sunlight alone; they need valuable nutrients from the soil, water, air, or -as you mentioned - other creatures. Many of the nutrients in the soil are the result of other plants and animals that have decomposed. The carbon necessary for their life is exhaled from creatures that may end up eating that plant in the future.(13 votes)
- You suggest it is better to eat the lower species to ingest less toxins because of the accumulation of toxins in animals like tuna. Wouldn't you get the same amount of toxins by ingesting higher quantities of lower species?(3 votes)
- Yes, that is true, but I think that the assumption behind that is that there is an equal quantity of the lower and higher species. For example, it's talking about having 100 grams of lettuce vs. 100 grams of tuna. Obviously there are more toxins in 1,000,000 grams of lettuce than in 100 grams of tuna.
Do you see what I mean?(8 votes)
- I have a question: In my biology class, we have discussed both detritivores and decomposers, but what we learned about the two seemed the same. What is the difference between detritivores and decomposers?(2 votes)
- Detritivores are animals, they break things down mechanically, think of a vulture. Decomposers are things like bacteria, fungi, etc. Their role is to recycle nutrients back into the soil.
Hope this helps.(5 votes)
- How are the rests of living beings considered? I mean, what level of the throphic pyramid do we include them in and why?(3 votes)
- Depending on the food chains and food web.
What we know for sure is that we humans are on top - because we eat them all.
Decomposers are on the bottom. But deep bottom is for producers - photosynthetic organisms - plants. Why? Because they are primary producers on which secondary consumers attach. Also, they contribute to the biomass.(2 votes)
- In what section of the trophic pyramid do we come into?(3 votes)
- Humans are omnivores, so we can eat primary producers as well as consumers, though most of us don't tend to eat above primary consumers because herbivores are much easier to farm. Omnivores aren't usually drawn in trophic pyramids because, like detritivores, they occupy more than one level.(1 vote)
- Wait, if each level up in the food chain retains only 10% of the calories, how does it retain all of the toxins?(2 votes)
- Calories can either be used up or stored as biomass. The lost 90% of calories are lost because they aren't converted into biomass, and biomass is the only part that gets passed on to the next level. Toxins don't get used up, so they all end up as biomass, so they all get passed on.(2 votes)
- you said that the toxic stuff gets accumulated, then why does it not just get out in the organisms poop?(2 votes)
- Digestive systems are very good at absorbing and usually need specific ways to not absorb unwanted things. Many toxins just act like another small, potentially useful molecule to an intestine. A tuna's digestive system won't be adapted to reject mercury because tuna haven't had the chance to evolve with mercury around.(2 votes)
- Why are nutrients continually recycled in ecosystems while energy is not?(2 votes)
- To add: the description of why energy is always lost is the Second Law of Thermodynamics, which applies to all systems not just living ones :) Note that the ecosystem of Earth is constantly getting an influx of new energy in the form of sunlight, and this can be used to power biochemical reactions putting atoms in useful configurations - nutrients - which can be used by organisms and re-set into the useful configuration later, recycling them. Examples: Water and carbon dioxide being reacted to sugars/carbohydrates by photosynthetic organisms (which are eaten and split back into water and carbon dioxide by organisms like us); and nitrogen fixing bacteria which react nitrogen gas from the air with other atoms into more useful molecules useful for life, which may eventually be broken down and perhaps return to atmospheric nitrogen gas and recycled (see https://www.khanacademy.org/science/biology/crash-course-bio-ecology/crash-course-ecology-2/v/crash-course-ecology-09).(1 vote)
- What is the difference between food webs and food chains? I know Hank touched base on this slightly at6:37but just wanted to know a little more.(2 votes)
- A food web consists of many food chains. A food chain only follows just one path as animals find food. eg: A hawk eats a snake, which has eaten a frog, which has eaten a grasshopper, which has eaten grass.(1 vote)
- At6:37, shouldn't the arrows represent the flow of energy instead of pointing to what the organism consumes?(2 votes)
- 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 but if you were to corner me at a party and say, Hank what is pop music? I'd be like, it's ah, it's like ah the music that plays on the pop station. Just because we're familiar with a concept does not mean that we actually understand it. Ecology's kinda the same way, even thous it's a common everyday concept and ecosystem is a word that 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, et cetera. I've found it helps to think of an ecosystem, a collection of living and non-living things interacting in a specific place, as one of those magic eye posters for those of 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 sense together until you stare at em long enough, from far enough away, and suddenly a picture emerges. And just like with magic eye posters, it helps if you're listening to gumeric why while you're doin it. So the discipline of ecosystem ecology, just like other types of ecology we've been exploring lately, looks at a particular level of biological interaction on earth. But, unlike population ecology, which looks at interactions between individuals of one species, or community ecology, which looks at how bunches of living things interact with each other, ecosystem ecology looks at how energy and materials come into an ecosystem, move around in it and then get spat back out. In the end ecosystem ecology is mostly about eating, 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. (upbeat music) So ecosystems may be a lot like magic eye posters 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 gradients that bleed into the ecosystems next door. So actually defining an ecosystem can be kinda hard, mostly it depends on what you wanna study. Say your 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 pants in it. So what are the animals doing there and how are 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 nutrients and other materials drop into the stream from the trees or are washed into it when it rains, leaves and bugs, you name it, flow down from neighboring terrestrial ecosystem and that stuff gets eaten by bigger bugs which get eaten by fish which in turn are eaten by raccoons and birds and bears. So even though the stream's got it's own thing going 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 stream start and where does it end? This is a perennial problem for ecologists because the way it works, energy and nutrients are imported in from someplace, they're absorbed by the residents of an ecosystem and passed around within it for a little while and finally passed out, sometimes into another ecosystem. This is most obvious in aquatic systems where little streams eventually join bigger and bigger waterways until they finally reach the ocean. This flow is a fundamental property of ecosystem. So at the end of the day, how you define an ecosystem just depends on what you want to know. If you wanna know how energy and materials come in, move through and are pooped out of a knot in a tree that has a very specific community of insects and protists living it in, you can call that an ecosystem. If you wanna know how energy and materials are introduced to, used and expelled by the North Pacific Gyre, you can call that an ecosystem. And if you wanna know how energy and materials move around a cardboard box that has a rabbit and a piece of lettuce in it, 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 a ecosystems 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 figuring out things like its biomass, that is the total weight of living things in the ecosystem, and its productivity, how much stuff is produced and how quickly stuff goes back how good the ecosystem is at retaining stuff, and of course all these parameters matter to neighboring ecosystems as well because if one ecosystem's really productive, the ones next door are gonna benefit. So, first things first, where do the energy and materials come from? And to be clear, when I talk about materials, I'm talking about water, or nutrients like phosphorous or nitrogen or even toxins like mercury or DDT. Let's start out by talking about energy, because nothing lives without energy and where organisms get their energy tells the story of an ecosystem. You remember physics, right? The laws of conservation state that energy and matter can either be destroyed or created. They can only get transferred from place to place to place, the same is true of an ecosystem. Organisms in an ecosystem organize themselves into a trophic structure when each organism situating itself in a certain place in the food chain. All the energy in an ecosystem moves around within this structure because when I say energy, of course I mean food. For most ecosystems the primary source of energy 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 gather up the suns energy and through photosynthesis make something awesome out of it, little stored packets of chemical energy. So whether it's plants, bacteria or produce that use photosynthesis, autotrophs are always the linchpin of every ecosystem, the foundation upon which all other organisms in this system get their energy and nutrients. For this reason, ecologists refer to plants as primary producers. Now obviously the way that energy gets transferred from plants to animals is by the animal eating the plant. For this reason, herbivores are known as primary consumers, the first heterotrophs to get their grubby paws 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 as secondary consumers are very happy to do. And assuming that the ecosystem is big enough and 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 decompose all of the dead animal and plant matter as well as the animal poop, detritivores, these include earth worms and sea stars and fiddler 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 ecosystem don't usually abide by these rules very closely which is why these days we usually talk about food 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 gonna be eating the fungi. Sometimes a bear likes to much on primary producers, blueberry bushes, and other times it's gonna be snacking on secondary consumer like a salmon. And even at the tippy tippy top, predators get eaten by stuff like bacteria in the end, which might or might not, be the same bacteria that ate the top predators' 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 water and temperature are what plants like, right? And without plants there isn't gonna be a whole lot of trophic action going on. Take for example the Sonoran Desert which we've talked about before, there aren't many plants there compared to say the Amazon Rain Forest. So the primary producers are limited by the lack of water which means that primary consumers are limited by a lack of primary producers and that leaves precious few secondary consumers, a few snakes and coyotes and hawks. All this add to the Sonoran not being a terribly productive place compared to the Amazon at least, so you might only get to the level of tertiary consumer, occasionally. Now all this conversation about productivity leads me to another point about ecosystem efficiency. When I talk about energy getting passed along from 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 to a snake, the vast majority of that energy is lost on the way. So let's take a cricket, and that cricket has about one calorie of energy in it, and in order to get that one calorie of energy in it, 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 into sodium potassium pumps and 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 crickets' energy into 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 wanna know what's scary efficient? The accumulation of toxins in an ecosystem. Elements like mercury, which are puffed out of the smokestacks of coal fired power plants end up getting absorbed in the ocean by green algae 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 level consumes 10 times more mercury than the last and that's what we call bioaccumulation. Concentrations get much higher at each trophic level until a human gets a hold of a giant tuna that'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 tuna flesh because organisms only hold onto 10% of the energy they ingest, each trophic level has to eat about 10 times its biomass to sustain itself and because 100% of that mercury moves up the food chain, that means that it becomes 10 times more concentrated with each trophic level it enters. That's why we need to take the seafood advisory seriously. 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 gonna be. And that's not just my opinion, that's ecosystem ecology.