Ecosystems - Online Tutor, Practice Problems & Exam Prep

Ecosystems consist of communities of organisms and their environments, where energy flows and matter cycles. Energy, primarily from the sun, is transferred through trophic levels, with only about 10% efficiency at each level, leading to a pyramid of net production. Matter, however, is recycled, as seen in biogeochemical cycles like the carbon and nitrogen cycles. Limiting factors such as nutrient availability and temperature affect productivity. Decomposers play a crucial role in returning nutrients to the ecosystem, while biomagnification highlights the concentration of toxins at higher trophic levels.

Ecosystems, Energy, and Matter

Hi. In this lesson, we'll be talking about ecosystems, which are made up of a community of organisms and their environment. Now, the biosphere is the sum of all ecosystems on Earth, and when we look at ecosystems, we want to be thinking about energy and matter. Now, the law of conservation of energy states that energy cannot be created or destroyed; it can only be transferred and transformed. So, energy will be transferred from the sun to photosynthetic organisms. And energy is very inefficiently transferred between organisms in an ecosystem. Most of the energy that moves through the ecosystem is going to be lost as heat. And the main thing to bear in mind about energy in ecosystems is that energy is always moving through ecosystems. It doesn't stay in ecosystems. They need a constant influx of energy to keep them powered. Now, matter, on the other hand, behaves differently. The law of conservation of matter states that matter cannot be created or destroyed and will remain constant in a closed system. Meaning, assuming, for example, that no matter can escape the Earth, which, you know, obviously isn't true, but let's just pretend that Earth is a closed system. That basically means that all of the matter that is taking part in this closed system will stay in this closed system, and none of it will be destroyed. It's just going to be moved around, basically. So again, the main point is that energy flows through ecosystems and is continuously lost, whereas matter is recycled through ecosystems, like you see here in the carbon cycle, where carbon will be moved between various sources. And we'll take a closer look at this in a little bit, but for now, let's turn the page.

Producers and Consumers

Trophic levels describe the position an organism occupies on the food chain, and it's determined by its feeding habits. Primary producers are the foundation of these food chains, because they can generate biomass from inorganic matter to support all other trophic levels. Primary producers are autotrophs, or self-feeders. These are mainly photosynthetic organisms, but can also be chemoautotrophs, like the archaea and bacteria that form the foundations of ecosystems around deep-sea hydrothermal vents. Primary consumers feed on primary producers, and these are usually things like herbivores that feed on plants. These and other consumers are considered heterotrophs because they're organisms that cannot fix carbon from inorganic sources, like what happens in the Calvin cycle of photosynthesis. They need to use carbon for growth, so they must consume organic matter, in many cases consuming other organisms. Secondary consumers are generally carnivores that feed on primary consumers, and tertiary consumers are carnivores that feed on other carnivores. As you can see, we are working our way up the food chain, and here in our little food chain model, we have our first level, this is our primary producer. Our primary consumer here at the second level, our secondary consumer here at this third level, and our tertiary consumer here at this fourth level. What is missing from this food chain that is very important, as you can see over here, are the decomposers. The autotrophs, the producers, get energy from the sun, and they use that to create biomass that will feed herbivores, and those herbivores will feed carnivores. However, the decomposers return all of that organic matter back into the nutrient pool. They serve an incredibly important role, as they return matter back to be used by other living organisms. They are essentially recyclers. We call the decomposers detritivores because they consume detritus. They are heterotrophs that consume detritus, which is non-living organic material, such as dead organisms and organic wastes like feces. The food chain, as we just saw right here, is a linear network of trophic levels. We often look at a grazing food chain, which has primary consumers feeding on plants. However, we can also create a decomposer food chain, where the primary consumers actually feed on dead plant matter, and we call those primary decomposers. Food chains are linked together, both grazing and decomposing food chains, into a food web, which is a much better representation of the interactions between the different trophic levels in an ecosystem. Here, I have a picture of some nice primary decomposers, these fungi that are going to eat dead plant matter. With that, let's turn the

Primary Production

Top-down control cascades are seen in ecosystems where a predator high on the food web is going to control prey populations. Now, what we can see is a trophic cascade where the predator is going to lower the abundance of prey, and then reduce the next trophic level from predation, or in this case that we're going to take a look at herbivory. You see, the wolf population in Yellowstone used to keep the elk population in check. But due to overhunting of wolves for very terrible political reasons, the wolf population of Yellowstone was diminished to nothing. They were hunted to extinction, and then the elk population exploded. And this actually led to a lot of herbivory and it greatly reduced the vegetation in the area. So this was an example of a trophic cascade because those wolves were actually protecting the vegetation indirectly by controlling the elk population.

Now, primary production is something we look at in ecosystems, and this is the synthesis of organic compounds from inorganic carbon dioxide. Usually, we're going to see this in the form of photosynthesis, and you can see the reaction CO2 + H2O → C6H12O6 + O2 here, in the Calvin cycle that will perform this carbon fixation. And we're going to be looking at different measures of production. There is gross primary productivity, which is the amount of energy generated by primary producers in an area over time. So, here, we can see our gross primary productivity, it's just all the energy generated.

Now, a useful measure to look at is the net primary productivity. So, some of that energy that is generated by the plants is going to be used by the plants' own respiration. So they're going to be returning that carbon to CO2. Now what you're left with after you take the gross primary productivity and remove what the plants use for respiration is the net primary productivity. So that's the gross primary production minus the energy consumed by the primary producer's own respiration. And you can think of this as the total amount of new biomass added by primary producers.

Now, net ecosystem productivity looks at the total accumulation of new biomass after the respiration of all organisms. So here you can see the net ecosystem productivity takes that net primary productivity, minus everything that gets diverted to the heterotroph's respiration. So, everything we're left with, all that biomass left over, is the net ecosystem productivity.

Now with that, let's go ahead and flip the page.

Limiting Factors to Primary Production

There are limiting factors to the productivity of ecosystems. In aquatic ecosystems, light penetration can limit primary production. However, nutrient availability is going to severely limit primary production in aquatic ecosystems and is going to really be the bottleneck. And this will, in part, be due to limiting nutrients, which are elements that limit production, and if added, production will increase. Usually, these are going to be nitrogen and phosphorus, but not always. Now in terrestrial ecosystems, temperature and water availability tend to be the main limiting factors. And soils also have limiting nutrients, and like aquatic ecosystems, these usually tend to be nitrogen and phosphorus.

And here, you can see a chart that shows the relative amounts of photosynthesis based on light intensity. And as you can see, when light is very low, down in this area, photosynthesis just plummets. You might also notice that photosynthesis starts to peter off when light intensity gets too high, which can be due to the damaging effects of all that light. Of course, that's not really going to be much of an issue in most marine ecosystems where these organisms are just going to be happy to get some light.

Now coral reefs are the most productive biomes per area, and they're also one of the most threatened. They actually only take up a very small portion of the Earth, and they only account for a small portion of the total productivity of the Earth, but they are still the most productive biomes per area. Now, the most productive marine ecosystems generally are near coasts due to the high nutrient availability there. Terrestrial ecosystems are actually far more productive than marine ecosystems, and this is probably due to the availability of light.

Here in this chart, you can see the most productive marine ecosystems, and you can see that there are these areas in red concentrated in coastal waters. Now here you can see primary production in terrestrial environments, and hopefully, you'll notice that it's the regions known as the tropics that are going to have that very high primary productivity.

Now tropical rainforests actually contain the most productive terrestrial ecosystems in the world, and this is going to be due to the year-round warmth and rain. With that, let's go ahead and flip the page.

Secondary Production and Trophic Efficiency

Secondary production is the amount of energy converted to new biomass by heterotrophs. And we're going to be looking at the production efficiency of heterotrophs, which is how much energy is stored in an organism from the food that it eats. Now, energy is stored after excreting a bunch of it as waste and using some for respiration. So this is usually very inefficient. As you can see here, we have an example of secondary production where this frog is going to be ingesting food, it will simulate it, extract the nutrients, and whatnot. Some of that energy is going to be lost due to respiration and some will be wasted as feces. Of course, many of these frogs are just going to die, so there goes that. But a small amount will be added as new biomass, and that is going to represent the production efficiency.

In ecosystems, we're going to want to look at the trophic efficiency, which is how much energy is passed between the trophic levels of the food web. Usually, this is only about 10%. This is highly inefficient. In fact, this can range from about 5 to 20%, but 10% is just a good estimate to work with, and it's a nice round number. Assuming approximately 10% of the energy and biomass in one trophic level gets transferred to the next trophic level, we can construct what's called the pyramid of net production. It's going to show the net production of biomass at each level. You can see the energy transfer in this pyramid. Assuming what the primary producers have, we'll just call that 100%. I'm actually going to switch to red just so it shows up. The primary producers represent 100% right there. That means the primary consumers are only going to get about 10% of the total energy that was stored in the biomass of primary producers. These secondary consumers get only 1% of what was at the bottom of the pyramid because they only get 10% of the rung just below them from the primary consumers. So they're only getting 1% of the base of the pyramid. Likewise, the third level is only going to get 0.1%. This is incredibly inefficient.

We can also sometimes be interested in looking at a biomass pyramid, which is going to show the amount of biomass stored in living tissue at each trophic level. Hopefully, from this energy pyramid you guessed that most of the biomass is going to be concentrated at the bottom of these pyramids and as we move up in trophic levels, it is going to get much, much smaller.

Now with that, let's go ahead and flip the page.

Biomagnification and Decomposers

One of the issues that can result from the fact that the transfer of energy and biomass up the pyramid is so inefficient, is that molecules that accumulate in biomass will actually concentrate at higher levels of the food web. These are going to be substances that are not easily digested or excreted and will efficiently accumulate in trophic levels. Often, these are toxins, such as heavy metals, substances that cannot be broken down or digested by living systems. Now, because organisms have to consume almost 10 times as much food as tissue they produce due to that inefficiency, these molecules are going to concentrate at higher levels, and that is why we call this biomagnification. You can see here at the base level of the producers, there are only a couple of these molecules. But then, these primary consumers are going to eat so much of those primary producers to generate their new biomass, that they are going to accumulate more of those molecules than the previous trophic level had. And so, it gets concentrated more and more as we move up in trophic levels.

Fortunately, primary decomposers, like bacteria, archaea, fungi, and roundworms are going to decompose organic matter and return all this stuff to the ecosystem. Soil organic matter is the component of soil that includes the decomposers and the detritus that they are decomposing at its various stages of decomposition. When it is completely decayed, we call it humus. This is formed from that completely decayed detritus and is super rich in nutrients. It is very good stuff for soil. Now, decomposition cycles nutrients through the soil and generates forms of these nutrients that plants can uptake so that they can return them to the ecosystems.

So, let's actually go ahead, flip the page, and look at some of these cycles.

Water Cycle

Okay, everyone. In this lesson, we're going to be talking about the water cycle. Now, I know you probably have all heard of the water cycle, but you may not know exactly what it entails. What type of process is this? Well, it's important to understand that the water cycle and many other cycles are biogeochemical cycles. What these are are pathways in which chemical substances cycle through the abiotic and biotic components of the earth. So, there's a water cycle, there's also a phosphorus cycle, and a carbon cycle, and many other different types of cycles that we'll talk about in later lessons. But these are going to be pathways and cyclic pathways that substances like water will take as they move throughout the atmosphere, oceans, streams, rivers, and organisms. So, it's like how water is recycled throughout our planet, and this is a biogeochemical cycle, and there are others which we will learn more about later. What's important to understand is that the water cycle is pretty much the most important biogeochemical cycle. Why? Because every living thing requires water to function. So, the water cycle is pretty important for life on earth. And basically, the water cycle is going to be the flow of water above, on, and below the Earth's surface. So, the water cycle is going to contain the different areas that water can exist. It can exist in the atmosphere via precipitation. It can exist on the ground in lakes and rivers and streams. It can exist inside the ground as aquifers or groundwater, and then it can also flow into the oceans as well.

So, this is going to be a depiction of the water cycle, as you can see here. The water cycle has many different steps, but the major steps are going to be that the oceans, obviously, hold a ton of water. They are going to have sun rays hitting them, and then this is going to cause evaporation, turning liquid water into water vapor, and then this is going to go into the atmosphere and condense into clouds. So, then the atmosphere has clouds which are made of water vapor. And then when those clouds get very saturated with water, we're going to have precipitation, whether that be in the form of ice, snow, sleet, or rain, or anything like that. That is going to be water falling onto the earth, leaving the atmosphere and then going to the earth. And then a lot of things can happen from here. We can have streams fill with water. We can have water seep into or infiltrate into the ground and become groundwater. We can also have it collect in lakes and rivers and streams. Usually, groundwater, rivers, and lakes are going to lead back into the ocean over time, and then that whole process is going to start again.

Now, there are some terms here that you might be interested in. The first one that we have that's a little bit different is sublimation. Sublimation is when a substance goes from a solid form directly into its vapor or gaseous form. So, this would be for water, for whenever it goes from ice into water vapor. It's very difficult to see water do this, and it doesn't do it all that often. You have to have very unique circumstances for this to happen, but this is called sublimation. Desublimation is the exact opposite where water vapor turns directly into ice. But again, this is very difficult to see, and you will probably never witness it. But if you were looking for a great example of sublimation, dry ice turns into carbon dioxide. But again, that is sublimation, but it's not water. Water sublimation is very difficult to visualize. And then there is another word here which is very interesting, evapotranspiration. Evapotranspiration is actually a very specific form of evaporation of water from plants. This is the water vapor that plants release from their bodies. This is commonly called transpiration or evapotranspiration, so a lot of our water in our atmosphere is also coming from plants as well.

So that's what that term means. So now, let's go down and let's talk about some specific types of water that are very important to life on Earth. A very important type of water is groundwater. Any idea why? Because a lot of the fresh water that is utilized for life on our planet is going to be stored as groundwater. Especially for us human beings, a lot of our drinking water and our agricultural water for crops and livestock comes from groundwater. What is groundwater? It is water that sits underground inside soil or rock, and most of it is stored in a specific rocky structure called an aquifer. You probably have heard of an aquifer before, but you might not know exactly what it is. Basically, an aquifer is a layer of rock that is porous, meaning it has holes inside it. So, water-permeable rock. Water can get into those holes in the rock and then be stored there for hundreds or thousands of years. So, aquifers are this porous layer of stone that holds a ton of water. Now in this image up here, this line right here — let me see if I can pick a good color — this line that I'm drawing in red, this is representing groundwater. This is also representing groundwater, and that is water that just exists under the Earth's surface. Now the more specific type of groundwater in aquifers is going to be here. So this, a confined aquifer right here, is going to be that porous rock that is going to hold water inside of those holes in the rock. Now, whenever you are digging into the earth and you hit water, you're going to hit the water table. The water table is the level in the earth or in the ground that is saturated with water. So, basically, once you hit the water table, you've hit the top layer of groundwater, the water being held inside of the ground. And as you can see here, the water table is not very deep. And in many regions of the world, the water table is not very deep, and you can dig maybe 10 feet and hit water. It's a very high water table. In these areas of the world, you probably won't build a basement, but in other areas of the world, you can build a basement because the water table is not so high. So just understand that the water table is that very top layer of groundwater. So, all of this is groundwater here, and then we have a specific aquifer, which is this porous rocky structure that holds water. Now, it's very important to understand that a lot of our water, especially in the United States, around 40 to 50 percent of the water we utilize for drinking and bathing and for our animals and our livestock and our and our crops, comes from groundwater, so it's very important.

Okay, everyone. Let's go on to our next topic.

Carbon, Nitrogen, and Phosphorous Cycle

The carbon cycle is the flow of carbon through the abiotic and biotic components of the biosphere. And what's amazing about the carbon cycle is that photosynthesis removes a ton of CO₂, and that amount is roughly equivalent to the amount of CO₂ that's added by cellular respiration. So in this way, these two processes feed each other. Cellular respiration produces CO₂, and photosynthesis removes CO₂ to generate biomass. Now, the major reservoirs of carbon include biomass, soil, sediment, fossil fuels, and in the atmosphere, there is some carbon dioxide.

Now, the nitrogen cycle is a particularly interesting one, because most nitrogen is actually found in the atmosphere. And nitrogen is only able to enter ecosystems through this special process called nitrogen fixation, which will be carried out by organisms like bacteria. And this is going to be super important, for supporting the growth of many primary producers, which need that nitrogen to live.

Now, the phosphorus cycle tends to recycle locally in ecosystems, unlike, for example, the water cycle or the carbon cycle that has very far-reaching effects. The phosphorus cycle tends to stay pretty local, because most phosphorus is found in rock and soil. So it's not going to get too far; it's not going to travel too far. That's all I have for this one. I'll see you guys next time.