In today's video we're going to see how energy passes through an ecosystem, which we can do using food chains. And then at the end we'll take a look at Predator Prey Cycles. Now all a food chain does is show what gets eaten by what in an ecosystem.
It's basically a simplified version of a food web, because it doesn't show all of the interactions, just one chain of them. All food chains start with a producer. which is some kind of photosynthetic organism, like a green plant or alga.
We're going to use grass here as our example. And if you haven't heard of it before, all photosynthetic means is that the organism can produce glucose by photosynthesis. So they're effectively using the sun's energy to make biological molecules.
And once these molecules have been incorporated into the plant, we call it biomass. The next step in the chain are the primary consumers, which eat the producers. And even though there might be lots of different primary consumers in a particular ecosystem, like rabbits, mice, aphids, and so on, we only show one of these in the food chain.
So let's stick with mice for now. Next up, we have the secondary consumer, which feeds on the primary consumer, and so must be some type of predator. So here it would probably be something like an owl.
Now if you're noticing a pattern here, primary means first, so they're the first consumers. Secondary are the second, and another word for third is tertiary. So next we have the tertiary consumer. Some ecosystems would then have even more levels after this, while others might not even have this many. Something really important to remember with food chains is that as the energy gets passed up the different levels, most of it gets lost.
For example, if there were 1000 joules of energy in the grass, then only around 100 joules or so might be passed on to the mites, and only around 20 joules might be passed on to the owl. Whenever you're drawing out a food chain, the arrows between the different levels actually represent this flow of energy up the chain. So we need to get rid of these. and draw them the other way around, like this.
One of the most important concepts in ecology is the idea that organisms all interact and influence each other. And we can see this if we look at this graph, which shows how the population of field mice, which are a type of prey, and owls, a type of predator, vary together over time. We call this type of graph a predator prey cycle. What we find is that the populations of both species cycle up and down, but the change in the predator population always lags behind that of the prey very slightly. It can be a bit hard to see, but if you draw a line, you can see that the mouse population peaks here, but the owl population doesn't peak until a bit later, just here.
Because of this lag, we say that they're out of phase with each other. Let's now consider what's actually happening at each point in this graph. At the beginning of our graph, just here, we can see that the owl population is low, and the mouse population is increasing, which kind of makes sense, because if there aren't many predators around, then the prey population is free to increase, because they're not really being eaten very much.
However, in this next section, we can see that there are now loads of mice, which means that there is lots of food for the owl. and so the predator population starts to increase again. Once the owl population gets too high though, it starts to cause a decline in the mouse population, because the mice are suddenly all being eaten. Because there are now fewer mice, and so less food for the owls, the owl population starts to decline as well, and this in turn will allow the mouse population to rise again, meaning that we're back to where we started. and the cycle will just repeat again and again.
The main reason that we get this cycle, rather than everything reaching a steady equilibrium, is that it takes a long time for entire populations to increase or decrease. For example, even if there is plenty of food around for the owls, it still takes multiple generations of breeding before there are lots more owls around due to the good source of food. Anyway, that's all we have to say for food chains. So, hope you enjoyed it, and we'll see you next time.