Biodiversity is the amount of variety of organisms in a particular area and is typically used to measure the "health" of an ecosystem. Biodiversity usually encompasses genetic diversity (variations within individual populations), ecosystem diversity (within ecosystems) and landscape diversity (range of differing ecosystems within an area).
When studying the environment scientist a lot of times will talk about a term called Biodiversity and it's one of those things where if you know what you are talking about you know what you are talking about but for other people sometimes it's a little bit squishy and that's because in many respect this is not something that you going to just nail down as this the actual definition specifically. So in general biodiversity is the amount of variety of organisms in a particular area but people can talk about it at multiple different levels. Now why should we care? Well it's a way to measure the quote health of an ecosystem the greater the diversity that you see in an ecosystem the much more likely that ecosystem can adapt to changes in the environment and why should we care well we live in that environment, so I don't like the environment changing I like things the way they are. I live in the San Francisco Bay area in wonderful climate I don't want it to change, I don't want to have to deal with more mosquitoes or less mosquitoes, more bees or less bees I like things the way they are, so the greater biodiversity we can have the less likely that on be inconvenience by changes and that is one of the things that some people are concerned about cause with agriculture supposedly we are hugely dependent on maybe 20 or less crops for almost all of our food where there is food that we are consuming or the food that our food is consuming. So how do you figure out biodiversity well you can talk about genetic diversity that's variation within a particular population, individual populations so you could have a population of say Cheetahs that's actually quite large you can have lots of cheetahs but if all those cheetahs were all basically descendants of a few cheetahs that survived in the zoo you don't have great biodiversity in there and we've seen the importance of genetic variety or genetic diversity when back in the 80's and early 90's I believe it was there was a lot of naval oranges here in California it was a great crop but unfortunately because there wasn't a lot of genetic diversity within the naval orange population they got wiped out when a particular disease started attacking the naval orange trees. Now you can also talk about ecosystem diversity within a particular ecosystem is there a wide variety of different organisms or is it an ecosystem made up of just a few organisms. So if A particular predator species is depleted or even wiped out does that throw the entire ecosystem out of lack or are there other predators that can step up and take on the role that the one that went extinct was doing. Then you can also talk about landscaping diversity and that's having a range of differing ecosystems and land types in a particular area to talk about a particular region has great biodiversity and landscape diversity take a look at the State of Hawaii within that really small area you can have snowy mountains, you can have volcanoes, you can have beaches, you can have rain forest it's hugely diverse and that's why you see such a broad variety of organisms there and that is why Hawaii has up to this point at least been able to with stand so many new ecological changes as people have migrated there, different animals have come in from other areas but because of their biodiversity they are much more adaptable and able to change with the change in the environment then others have less biodiversity.