What are the disadvantages and advantages of low diversity ? What are the advantages and disadvantages of high diversity ?
There are very few benefits to low biodiversity
The problems with low biodiversity are many. Firstly, low biodiversity means that the trophic system is likely less functional. Since there are less species in the various tropic levels, meaning the paths of energy and nutrients are fewer in number. On top of the ecosystem being much less productive, this also means that if one species goes extinct or a natural disaster occurs, the ecosystem is extremely vulnerable to collapse.
Simple example: If there are only three species that are the primary organisms in the ecosystem (we'll say a hawk, mouse, and a type of grass), one of them could collapse the whole thing. If hunters shoot all the hawks, there will be no other predator to control the mice so the ecosystem will domino to a quick end.
The places where there is low biodiversity (such as open oceans, deserts, and tundra) tend to be very vulnerable to disturbances. The only benefit (I suppose you could call it one) is that often there aren't enough nutrients to go around, such as in a low-nutrient place like a desert, low biodiversity is the only way that an ecosystem could exist at all. I might also add that low biodiversity usually means diseases spread much more easily, which is a definite disadvantage.
High biodiversity is like a bed of nails, except the nails are what's keeping the ecosystem alive. These areas tend to be very resilient because while disturbances such as fires or human intrusion might affect one or two species, there are plenty of others, so nutrients and energy can still transfer from one trophic level to the next.
One could say that the disadvantage to this system is that it requires a lot of energy and nutrients to exist due to the sheer number of energy and nutrient-consuming organisms compared to a low-diversity place. I would say, however, that this is more of a reason for the existence of a high versus low diversity biome rather than an advantage or disadvantage to either system.