|MadSci Network: General Biology|
You have asked a fairly complicated question. Generally, the idea that everything has a purpose assumes that ecosystems are in balance and that all organisms in the ecosystem play a role in maintaining that balance. I believe that you are asking me what purpose mosquitoes have in maintaining balance in an ecosystem.
In a classic example of a balanced ecosystem, there is a prairie of grass, rabbits that eat the grass, and wolves that eat the rabbits. If there were no wolves, the rabbit population would get too large, eat up all of the grass, and then starve themselves to death. If the rabbits were not there, then the grass would not be trimmed, trees would take over the prairie, and the ecosystem would be lost. In this example of a balanced ecosystem, every animal is necessary for the survival of the ecosystem. If one of the organisms is gone, then the ecosystem will collapse. Each of these organisms have a purpose, as most people would define it.
Now let us add a parasite to this system. Let's say that ticks feed on both the wolves and the rabbits. This takes blood away from both the predator and prey, and makes the animals less energetic. The animals won't be sick enough to stop their important role in balancing the ecosystem, but there might be fewer numbers of them. Is the tick important? The tick really doesn't have a role in balancing the ecosystem. It could be said, therefore, that ticks do not have a purpose.
Let's go one step further and say that a certain bacteria infects the rabbits. In order to get from inside one rabbit to inside another rabbit, the bacteria must be transported by a tick. In that case the tick has a purpose for the bacteria. But neither the tick nor the bacteria have a role in balancing the ecosystem.
Now let's say that a scientist comes and collects some of the ticks. She takes the ticks back to her laboratory and finds a chemical inside them that is useful in treating humans with heart disease. She collects more ticks and uses them to make a new medicine that saves many lives. Now the tick has a purpose to humans.
Let's say that an ecologist goes into the ecosystem and discovers how the bacteria get moved from one rabbit to another through the tick. He studies the ways in which the bacteria lives, and then writes a scientific article on the interesting life of the bacteria. Other people read about this interesting bacteria, and it is very inspiring to them. The bacteria has a purpose to the people who find it interesting.
The mosquitoes that were biting you in Coasta Rica probably do not have a purpose in maintaining the balance of the ecosystem there. Some may be eaten by frogs or fish, but there are plenty of other insects for those animals to eat. If all of the mosquitoes were killed, the ecosystem would probably not suffer, unless the poisons used to kill them also killed organisms that are required for the balance of the rainforests. (This is one very difficult problem with trying to control pest insects!)
No one knows, however, all of the secret chemicals those mosquitoes might have. One day, one of the chemicals inside a mosquito might be used to make a new drug. If all of the mosquitoes were killed, scientists would never have the chance to find out.
Also, mosquitoes have many fascinating life styles. They carry many fascinating parasites between different animals. So for people who love to learn about the strange way that life works, those mosquitoes have a purpose that way too.
Everything I've written so far has been for an ecosystem in balance. Some times, an organism will get into an ecosystem that it's not supposed to be in. The new organism can be very dangerous to the ecosystem, and throw it out of balance. Several mosquito species have spread around the world posing a serious risk to native mosquito species. It is these mosquitoes that most of the mosquito pest control measures are targetted against. I have written another MAD Scientist essay, Why are House Flies important? for someone that had a similar question on introduced insects, which I recommend that you read also.
For a similar discussion with great information and references, I'd also recommend reading The response that MAD Scientist Justin Remais wrote when he was asked, Shoul d we cut down the rainforests?
To understand why some species are very important and some are not so important in maintaining a balanced ecosystem, I recommend reading What happens when a botanical species disappears from the Amazon rainforest? , a well written, easy to understand essay by MAD Scientist Keith. McGuinness.
Thank you for your interesting question!
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