I was watching Life of Mammals: Life in the Trees earlier today and it struck me how many lemurs there were on Madagascar. Compared to the rest of Africa, the island is pretty small, yet is has a TON of biodiversity, especially among primates. How could this be? If there are limited resources, wouldn't "the fittest" push all the others out, leading to lower biodiversity and hard times? Nope, and it actually makes sense to have more biodiversity in an environment where there's a lot of competition for resources. It would seem to be counterintuitive, but the island's population speaks for itself: animals in heavy competition in a small area will readily speciate and cause more biodiversity. How does this happen? Well...
Think about it this way. In Africa there is only one species of elephant, the African elephant. There are subspecies with different habits, yes (more on this later), but they are all similar except for minor behavioral and morphological differences. They have certain needs as far as food or water goes, but if they can't find what they need in one area they can walk some distance to another area and get some more. There's not a lot of pressure for them to change or adapt because it's fairly easy to get what they need, so why drastically change something that works? There are reproductively isolated groups (mostly because of geography) and we may see some of the elephants have more variations over time based upon their lifestyle in grasslands, deserts, and wetlands, but there isn't intense pressure for them to eek out a living. This is in stark contrast to ecosystems like rainforests, reefs, and islands, where the organisms in the ecosystems usually have limited mobility and resources, and if they can't make it in that one ecosystem there isn't anywhere to go. Let's take an island as a hypothetical environment.
Now, let's assume that the first group of animals on the island are generalists or omnivores, so fare well because they are not overly specialized. Over time their numbers increase and start to put more of a strain on the environment. There isn't enough of everything for everyone. Where the organisms once ate fruit, insects, and some green plants, now there isn't enough of everything to go around. Some of these animals need to specialize on eating fruit in one area or getting to a hard-to-reach flowers nectar to survive, and that pressure to get resources changes them. They become more specialized, and this trend of competing for resources continues to mold the animals until you get one that only drinks from a particular flower or eats a certain fruit. Perhaps it will take some other sort of food if it can, but the competition for resources is so fierce the only way it can secure a niche for itself is to specialize. This is the case with many animals in biodiverse environments such as tropical fish and the lemurs of Madagascar. In order to survive, they need to specialize, and while this may work, ultimately it can lead to extinction. Environments change, and if you become too specialized and the ecology changes quickly, you may not be able to keep up.
Polar bears in certain areas are a good example. Although they eat various kinds of meat, polar bears are primarily carnivores, and now that global warming is melting certain ice floes the polar bears don't have the access to seals they once did. They were essentially depending on being able to find and catch seals, and with little to no other availible prey, some are starving or resorting to cannibalism. Other polar bears aren't up to the same pressure and seem to be able to cope because they still have access to seals or can vary their diet, and they are more likely to survive, but the amount of resources and environmental change can kick a population or species out pretty quickly.
In short, contrary to what may seem like common sense, competitive environments often have the biggest biodiversity because it allows for a lot of specialists to develop. There are certain flowers that only attract certain insects and so on, and while this is truly amazing, it is not a strategy that pays off in the long run, and puts the animals in a very precarious position. Generalists are more likely to survive, and especially in areas where people are encroaching, it is the animals that can adapt or aren't too picky about where they get their meals like black bear, white tailed deer, cougars, cowbirds, etc. that are thriving while more specialized species are being driven out.
