last time we talked about the trophic levels and how those represent energy flow in an ecosystem and we talked about how most traffic levels start with producers and then the rest are consumers but they are in different levels starting with the primary consumers that eat the producers and ultimately ending up with what we call the higher level consumers like tertiary or quaternary and above that feed on the lower consumer levels so i wanted to start out today with a question that i want you to think about while i'm talking and presenting this lecture and then we'll revisit it at the end and that question is why are upper-level consumers basically tertiary and above often endangered there was a study done in 2015 a paper published in the journal science that looked at 31 of the top predators in the world and 77 percent of those were critically endangered so there's a lot of top predators or upper-level consumers in the world that are on the endangered species list and the question is why is that so i want you to think about that while we're talking today so here's a picture of a food web we talked about this last time remember that the food web represents a flow of energy through an ecosystem the arrows represent that flow so it goes from the thing being eaten the energy goes from the thing being eaten to the consumer right so it starts with sun and ultimately ends up at in the bodies of the higher level consumers and then ultimately as heat to the atmosphere so this is a picture of you know typical picture of a food web but i i wanted to show you a picture of a real food web from a from an actual paper that was done studying a food web in the north atlantic and this is what an actual food web looks like i just thought it would be interesting interesting to show you this it's a lot more complicated than the kind of simplified graphics portray right look at all those arrows so if you look at that kind of what jumps out at you about the food web of the north atlantic if you take a look at that well one thing you might not have realized how many different species there are in those cold waters of the north atlantic seems like a lot of species are found in that ecosystem can you see one that's really really important cod so that's a fish in the north atlantic it's actually um being overfished it's the type of fish that you find in kind of fast food fish like um you know if you get like a mcfish sandwich or something like that it's that kind of very mild white flaky fish that's popular with people because it doesn't really have a lot of flavor but it is being overfished and you know this study showed that it's it is a critical species that's very important to this particular food web a lot of connections go through cod so it is a very important species so just as a review what's the ultimate source of energy to this system in the north atlantic remember with every ecosystem except for the hydrothermal vents under the sea those that come from cracks in the earth's crust every ecosystem the ultimate source of energy is the sun where does the energy enter the ecosystem and in what form it enters with the producers right they have to they then convert that sunlight into food i mean they're basically not converting the sunlight but they're using it as a source of energy to build food molecules that's photosynthesis right so that's basically the first energy transformation in an ecosystem from sunlight to food energy where does the energy end up and in what form what ultimately ends up as heat that's released to the atmosphere because with every energy transformation or every time the energy is used a little bit of the loss is heat so it's a one-way path right it starts with the sun and ends up as heat to the atmosphere so that kind of i just i'll say this in a more formal way there are some laws that describe how energy flows in biological systems how energy is transferred and those are called the laws of thermodynamics so we're going to talk about those laws in the context of biological systems how do they dictate what happens with energy transfers and biological systems so there's two laws that basically are relevant to that the first one is energy cannot be created nor destroyed all the energy in the universe is constant we're not making new energy we're not destroying any energy it's just merely being transformed or transferred okay so that's that's a really interesting thing to think about it kind of sounds simple but it's it's an interesting thing to think about energy cannot be created or destroyed and furthermore every time energy is transformed or transferred some of it is basically lost to the ecosystem as heat that's just released back to the universe and we call that being lost because we can't re we don't recapture that and use it you know to make more energy or use it to do anything so um you know another way to say this is no reaction is 100 efficient okay there's always some energy lost as heat when there's a transformation so basically what life is all about is it starts with the sun starts with the energy from the sun luckily there are producers on earth that can use that sunlight to make food in the form of photosynthesis the rest of life depends on consuming that food as as their source of energy and um that's that food is a source of energy on which life is built in in organisms that are consumers right we have to eat food to get the energy we need all biological organisms need atp is the model the energy molecule that is coupled to chemical reactions requiring energy so that's actually the form of energy that we all need to actually do the things that we do that our bodies do and our cells do and um you know when that energy is used it's some is releases heat so that basically sums up life's processes right there okay but let's go into a little bit further how does that first energy transformation occur starts with the sun and then it becomes food right so that is the process of photosynthesis which i'm not going to go into in great detail but i do want you to understand the concept of what happens with it um and of course you know the basic thing that happens is that producers or photosynthesizers make food using sunlight as an energy source okay now they don't make food from the sun sometimes that's how people describe photosynthesis they say photosynthesis is plants making food from the sun that's that's not really 100 correct and the reason why is here's a picture of the actual food molecule that's made during photosynthesis it's a glucose molecule okay this is basically food um now you know don't get confused because i know that you know that food is made up of lots of different types of molecules right we've got all different types of carbohydrates not just glucose we've got fats we've got proteins and things like that but but ultimately glucose can be used to build all those different molecules this is kind of the simplest form of food this is what food is broken down to before it's made into atp okay so you could think of this glucose molecule as being the foundation of food it's food so this is what plants and other producers are building in the process of photosynthesis this is what they're actually building so if you were going to build this molecule what would you need to build it you would need a source of what you would need a source of those c's right carbon where do you think photosynthesizers get the source of carbon in that molecule they get it in the form of a gas that is available in the atmosphere which is carbon dioxide they just absorb it through their tissues so that's what they use to build this molecule what other raw material would they need to build this molecule well they need a source for the h's and o's right what do you think they use for that well what what else is around them that's readily available that contains h's and nose water they just absorb that through their roots from the soil so these are the two molecules carbon dioxide and water that photosynthesizers actually use to build food those are the raw materials that they need they can't build glucose they can't do photosynthesis without these two molecules available so photosynthesis is processed by which food is built from carbon dioxide and water well where does the sunlight come in then what do they need the sunlight for well it's not free to take a little tiny co2 molecule and a little tiny h2o molecule and bind those together to make this big glucose molecule that's not free that takes energy right it takes energy to build all those bonds see those lines there connecting the carbons to the hydrogens and the oxygens and the hydroxyls those are not free it takes energy to make those bonds and that's where the sun comes in the sun is the energy source that is required to build glucose from carbon dioxide and water so producers make food from carbon dioxide and water and you could think of this food as um as a type of potential energy it's stored energy right and so we take in food but we have to do something with the food our bodies have to do something with the food to actually access the energy in it so that's what we'll talk about next notice too so here's kind of the formula for photosynthesis you need carbon dioxide and water and you need sunlight and then you end up with glucose and then look what else we end up with as a byproduct of photosynthesis o2 which is oxygen gas that's a byproduct that means it's a waste product of the photosynthetic process oxygen gas is produced and it's just released out of the plant or out of the producer it just diffuses out okay it is produced because during the process of photosynthesis the water molecule is split and the water molecule is split in other words the the bonds between the hydrogens and the oxygens are split apart the reason for that is the plant is going to use those electrons that are in those bonds to do stuff okay to do part of the process and so the water molecule is split to harvest the electrons and then so doing if you split a bunch of water molecules you end up with hydrogen ions right and you end up with oxygens and they and they then form oxygen gas and are released so that comes from the splitting of water okay so we're left with food food is stored energy our bodies and the bodies of other consumers have to break down that food in order to access and use energy remember what is the form of energy that all of our cells need to do reactions that require energy do you remember what that was it's atp right so the next question is how do we get from food to atp um and that is the process of cellular respiration so this explains it you know when we are losing a lot of energy and we need energy to do more work we need it in the form of food all consumers do not just humans anything that's not a producer has to consume food in order to get energy right that energy is stored in the chemical bonds of the molecules in the food so um the ultimately what happens is all those different molecules are broken down until we end up with glucose and then glucose is broken down it's burned up basically oxygen is required just like when you burn wood or burn a candle you need oxygen so this process requires oxygen to do and what is happening is this food molecule is going to get broken apart it's going to get broken apart releasing all these electrons that are that are making up these bonds between the carbons and the hydrogens and the hydroxyls and those electrons are going to be used in a chemical process that ends up building this molecule of atp atp stands for adenosine triphosphate it's just a molecule but it is coupled to reactions that require energy okay so that's how it works food is basically burned up by the body broken apart using oxygen and the electrons are harvested and used to make atp and that process of converting food to atp is called cellular respiration and all organisms that are exposed to oxygen do this process do plants do it yes they do they do it just like this so plants actually do this process the only difference between plants and us when it comes to energy use is they can make their own food and then they break it down the same way we do we have to consume our food but in terms of this process it's exactly the same between plants and humans okay okay the next thing is um when we talk about the transfer of energy and an ecosystem well how is it transferred it comes from the tropic levels right we have the producers that make the food in photosynthesis but then the rest of the energy is transferred through consumption so we've already talked about that but now let's focus on the efficiency of that process it's inefficient the transfer of energy from one trophic level to the next from the herbivores eating the plants from the secondary consumers eating the herbivores it's inefficient okay and that's what we're going to talk about now so one way to to kind of illustrate this inefficiency is that the mass of producers the total mass we call it biomass biomass is just the measure of the amount of basically tissue the amount of tissue that makes up a certain trophic level right so if we were going to measure the biomass of the producer level in this prairie ecosystem we would just harvest a sample of the all the plants you'd have to pull them up by their roots we would want to get all the tissue that makes up that level right basically dry it to get rid of the water weight and you would just weigh it and so you would end up with certain weight of producer material that would be the biomass okay so the biomass of producers in an ecosystem is always going to be greater than the biomass of the upper level consumers hmm why is that that's really inefficient and there's a rule called the 10 percent rule which says that in general only 10 percent of the energy that's stored in the biomass of one trophic level is actually transferred and represented in the biomass of the next level above it so for example if we had 100 kilograms of plant matter of plant biomass in an ecosystem that could support only 10 kilograms of primary consumers and that could support only one kilogram of secondary consumers that's how inefficient the energy transfer is that's called the 10 roll only 10 of the energy stored in one trophic level is actually transferred to the level above it so why is that that's the question can you think of any reasons why that would be okay so this graph explains the inefficiency of energy transfer the 10 roll why is only 10 of the energy stored in one trophic level actually passed on to the level above it so let me orient you to this graph if you look over here on the left side you see this white bar and that represents the biomass of the producers in other words the the if you weighed all the bodies of the plants or the producers in an ecosystem that's what that bar represents that's the food that's available to the next level above that's the food that's available to the herbivores right so why if we have 100 kilograms of plant matter why is that only enough to support 10 kilograms of herbivores well the first reason is that do the herbivores eat every single blade of grass and every single plant available to them in an ecosystem no they don't so some of that is not consumed some of that plant matter is not consumed it's left on the ground and therefore it doesn't go into bodybuilding growth of the primary consumers right it's kind of like if you have a hamburger on a plate that you know has a thousand calories in it and you only eat half of it well then you're only going to get 500 calories of energy into your body because you left 500 on the plate okay so only what's consumed in terms of plant biomass goes into bodybuilding growth of the primary consumers right but even what's consumed some of it is undigested and when we're talking about plants that can be quite a bit plants plant plant tissue has a lot of different molecules like cellulose that are difficult for most organisms to digest and what's undigested by the body what happens to it it just goes out of the body as waste so it leaves the body therefore it doesn't go into bodybuilding growth so now of that whole white bar 100 kilograms of plant matter what is going into bodybuilding growth of the primary consumers now is just this which was eaten and digested but it actually is worse than that because of what's digested some of that food some of the energy in that food is going to be used to kind of run around and do daily activities right if the deer eats some grass and then goes and runs around it's going to use some of the calories in that grass use some of the energy to do its thing so that doesn't go into bodybuilding growth if you eat a hamburger that's a thousand calories and then you go run a marathon well that a thousand calories can be used to run that marathon it's not going to go into bodybuilding growth right it's why marathoners tend to be so skinny but yeah so energy used for metabolic activity um so now from that whole big white bar 100 kilograms of plant matter this is only the amount of energy that goes into bodybuilding growth of the herbivores of all that grass that was available to them because of all the reasons we talked about some of it's not consumed some of it's not digested some of it's used for metabolic activity and also this thing here what's this remember with every energy transformation sum is lost as heat that's the second law of thermodynamics right we end up with only this many primary consumers can be supported with all that grass and then the same thing happens when you go to the secondary consumers here's the biomass of the herbivores this is the biomass right here of the herbivores right in this blue box that's what's available to the secondary consumers but again not all of them will be eaten some of what is eaten is going to be used for energy some is going to be lost as heat some is going to go out as waste and be undigested so now we're talking about the secondary consumer biomass look look at this tiny bar that can be supported from all that 100 kilograms of plant matter this is how much biomass can be supported in the secondary producer level or the secondary consumer level that's the 10 rule so an important measure of ecosystem health and kind of processes that happen in ecosystems is called primary productivity and primary productivity just measures the rate at which carbon is fixed in a given ecosystem the rate of photosynthesis is what it meant is what it's measuring carbon fixation taking inorganic carbon as carbon dioxide and making it into organic tissues which that's photosynthesis right so primary productivity is a measure of the rate of photosynthesis in an ecosystem now why first of all why would that be an important measure to be able to measure the rate of photosynthesis in an ecosystem why would that be important well obviously however much photosynthesis is going on really tells you a lot about what can happen in terms of the other trophic levels right if there's not a lot of productivity you know you're not going to be able to support a lot of different trophic levels right or a lot of biomass so productivity is really an important measure what are the factors that affect how much productivity is happening how much primary productivity is happening in the ecosystem at a given time can you think of some of the factors well one is the amount of light what affects photosynthesis well there's got to be light if if you have a lot of light you know that that's going to support more productivity what else would support high productivity water you need water for photosynthesis so there needs to be enough water available another thing that affects how much photosynthesis happens is how much how many producers there are just the biomass of producers it doesn't really matter the species diversity it just matters plant biomass or producer biomass if there's a lot of producers that means a lot of photosynthesis because they're the ones that do photosynthesis so we have light we have water we have density of producers and there's another thing which is nutrients nutrients are things like nitrogen phosphorus sulfur these things are typically available in the soil and those are important because those nutrients are used to build the biomass of the producer to build their bodies it's what their bodies are made out of so you can't build a producer if you don't have all of these nutrients so those are the things that really support high productivity so if you look in this ecosystem like the savannah you could see why there's so such high productivity in savannahs you got a lot of light there a fair amount of water they're a little bit arid of ecosystems but it's enough a lot of plant density look at all those grasses and a lot of nutrients where do you think the nutrients are coming from well first of all they come from the grasses that die at the end of every growing season and where else do they come from see all those animals their fecal matter their poop is is actually releasing nutrients into that system too okay juxtapose that to the desert which has low productivity well there's plenty of sun there but it's limited photosynthesis can be limited by the lack of water right and because of the lack of water you don't have a lot of plant biomass look at look at there's just a few plants there and because you don't have a lot of plant biomass you're not going to support a lot of other trophic levels there's not going to be a lot of decomposition of organic matter there's not a lot of nutrients in this system so less productivity in a desert so now i want you to just consider that question that we started with why do you think it is that top predators tend to be endangered what do you think the answer to that is well there are some things like that that come from human impacts like we do tend to prize these top predators and hunt them and things like that that that is a factor but actually the biggest factor that makes these top predators tend to be endangered is the lack of producers right the lack of habitat and oftentimes people think well they just need a lot of room to roam that's why the lack of habitat affects them and that's true but it really comes to energy if you don't have a lot of producers there's not going to be a lot of energy available in that ecosystem because that's where the energy enters the system right through photosynthesis and there's no way you can make up for that that's where it enters so no matter what happens with the other trophic levels if you don't start with that energy base of all that producer biomass you can't support higher level consumers even though those tertiary and quaternary consumers never eat a plant they are not plant eaters they still rely on plants to to be that first energy transformation where energy enters the ecosystem so in order to save higher level producers or higher level consumers we actually have to save the producers and not just for habitat for the predators to have space and things like that it's for the influx of energy that comes into an ecosystem from production