Why are species important to maintain biodiversity in an ecosystem?

Answer 1

• Wow! There are so many reasons for maintaining biodiversity. Let's take a
  simple example, cows. Let us say all the farmers in the world that were
  raising cows for milk decided to raise only Holsteins, those are the pretty
  black and white ones we see around here. We raise Holsteins because they
  are champion milk producers. This makes economic sense, raise only the best
  producers. Now let us say there was some sort of virus that could seriously
  weaken the cows and effect milk production. The Holsteins as a group were
  very susceptible to this virus, however, Guernseys were not. Although in
  our simple sample we no longer have Guernseys, so now all our cows are sick
  and failing. This is a really simple example of a problem of greater scope.
  We need to have more than one kind of cow, or chicken, or pig for food
  production to maintain a healthy food source. By the same token, we need
  all the animals and plants on this planet to maintain a healthy world. A
  lot of the plants and insects out there we haven't even classified! Could
  there be a plant out there that might be a key in a cure for Heart disease,
  or cancer? We do not know yet. Although if we keep killing things off, we
  might never get a chance to find out.
  
  A lot of times when we think of conservation, we think of the "pretty"
  animals, whales, dolphins, pandas for example. We need to remember that we
  can not be so bold as to decide that this bug, or that plant is not worthy of
  saving simply because we do not like it or think it is of value. We are all
  closely interlinked in the web of life on this planet. Extinction of
  species effects this web somewhere. Maintaining biodiversity is healthy for
  the planet.
  
  This is really a huge topic. You might find a few more opinions in books at
  your local library. Good luck in your quest. It is something to think
  about.