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B7 D) Food Webs & Predator-Prey Cycles
B7 D) Food Webs & Predator-Prey Cycles
A food web shows all of the different food chains in an ecosystem. There is an example of a food web below.
The producer for the above food web is the algae. The primary consumers are the tadpoles, snails and water beetles. The secondary consumers are the small fish and frogs. The tertiary consumers are the kingfishers.
We can draw out individual food chains from a food web. Here is an example of an individual food chain for the above food web.
We can draw out individual food chains from a food web. Here is an example of an individual food chain for the above food web.
Here is another individual food chain for the food web.
We are now going to have a look at the effect that a decrease in the population of tadpoles would have on the whole food web (the decrease in tadpoles could have happened because of a new pathogen). If the population of tadpoles decreased, there would be more algae (food) available for the water beetles and snails, so we would expect their populations to increase. If the populations of water beetles and snails increases, there would be more food for the frogs, so the population of frogs would increase.
Let’s now have a look at the small fish who eat the tadpoles. The tadpole population decreased which means that there is less food available for the small fish, which will result in the population of small fish decreasing.
The final organism in the food web that we need to look at is the kingfisher who eat small fish and frogs. The population of small fish has decreased, and the population of frogs has increased. This means that the population of kingfishers could go either way; the kingfishers have fewer small fish to eat but more frogs to eat, so the effect of a decrease in tadpoles on the population of kingfishers is uncertain.
We are now going to have a look at the effect that a decrease in the population of tadpoles would have on the whole food web (the decrease in tadpoles could have happened because of a new pathogen). If the population of tadpoles decreased, there would be more algae (food) available for the water beetles and snails, so we would expect their populations to increase. If the populations of water beetles and snails increases, there would be more food for the frogs, so the population of frogs would increase.
Let’s now have a look at the small fish who eat the tadpoles. The tadpole population decreased which means that there is less food available for the small fish, which will result in the population of small fish decreasing.
The final organism in the food web that we need to look at is the kingfisher who eat small fish and frogs. The population of small fish has decreased, and the population of frogs has increased. This means that the population of kingfishers could go either way; the kingfishers have fewer small fish to eat but more frogs to eat, so the effect of a decrease in tadpoles on the population of kingfishers is uncertain.
Predator-Prey Cycles
The population of organisms in a stable community depends on the quantity of available food. A predator is an animal that hunts, kills and eats other animals for food. Prey are the animals that predators kill for food. In a food chain, if the population of prey increases, there will be more food available for the predator, which results in the population of the predator increasing. But, if the population of the predator increases, the population of prey will decrease as there are more predators eating the prey. A decrease in the population of prey means that there is less food available for the predator, which results in the population of predators decreasing. The process of predator and prey populations changing is known as the predator-prey cycle.
Let’s have a look at the predator-prey cycle for the food chain below.
The population of organisms in a stable community depends on the quantity of available food. A predator is an animal that hunts, kills and eats other animals for food. Prey are the animals that predators kill for food. In a food chain, if the population of prey increases, there will be more food available for the predator, which results in the population of the predator increasing. But, if the population of the predator increases, the population of prey will decrease as there are more predators eating the prey. A decrease in the population of prey means that there is less food available for the predator, which results in the population of predators decreasing. The process of predator and prey populations changing is known as the predator-prey cycle.
Let’s have a look at the predator-prey cycle for the food chain below.
For the above food chain, the grass is the producer, the rabbits are the primary consumers and the foxes are the secondary consumers. The predators in this food chain are the foxes and the prey are the rabbits.
Let’s now suppose that there was a boom in the growth of grass. This will mean that there is more food available for the rabbits, so the rabbit population increases. The rabbits are eaten by foxes, so an increase in the population of rabbits means that there is more food available for the foxes, which results in the population of foxes increasing. An increase in the population of foxes now means that more rabbits are eaten, so the population of rabbits will decrease. A decrease in rabbits means that there is less food available for the foxes, which will result in the population of foxes decreasing. Fewer foxes means fewer predators, so less rabbits get eaten and the population of rabbits increases… this process keeps continuing.
The graph below shows how the populations of rabbits and foxes changes over time; it is known as the predator-prey cycle.
Let’s now suppose that there was a boom in the growth of grass. This will mean that there is more food available for the rabbits, so the rabbit population increases. The rabbits are eaten by foxes, so an increase in the population of rabbits means that there is more food available for the foxes, which results in the population of foxes increasing. An increase in the population of foxes now means that more rabbits are eaten, so the population of rabbits will decrease. A decrease in rabbits means that there is less food available for the foxes, which will result in the population of foxes decreasing. Fewer foxes means fewer predators, so less rabbits get eaten and the population of rabbits increases… this process keeps continuing.
The graph below shows how the populations of rabbits and foxes changes over time; it is known as the predator-prey cycle.
The populations of the predator and prey in the predator-prey cycle are always slightly out of sync with each other – the peaks and troughs for the predator and prey do not occur at the same times. This is because it takes a while for the populations of the predator and prey to change.