I will try to explain my thoughts on lack of bio diversity with this great article:
http://reefkeeping.com/issues/2002-07/mvd/index.php
Maintain Biodiversity
The more biodiverse an ecosystem is, the more stable it is. As species go extinct, not only is that organism lost forever, but it contributes to the destabilization of the ecosystem it inhabited. All organisms fulfill a role or "niche" in an environment. In a sense, they all work together to make the system work. The same principles apply to an aquarium. One way diversity increases stability is by increasing the complexity of the food webs and nutrient pathways. As aquarists rely more and more on natural forms of filtration such as deep sand beds, live rock, and macroalgae-filled refugiums, they are beginning to understand the importance of biodiversity.
I can remember a time when bristle worms were the bane of every reef keeper's existence. Now, we admire them fondly as "detritivores." A deep sand bed would not function properly without the presence of various detritivores to break down large organic particles in order to make this material available to bacteria and smaller organisms. Other fauna contribute by breaking up bacterial clumps and inorganic material. There are also "critters" that contribute to the health of the tank by grazing on algae or perhaps acting as a food source for corals and fish. Unfortunately, many of these critters eventually dwindle and disappear from the tank over time. Predation from fish and corals, competition from other critters, and random catastrophic events (like a wild temperature swing from a failing air conditioner) all contribute to the disappearance of these beneficial critters from the reef aquarium. There are many steps an aquarist can make to alleviate this loss of diversity.
One popular method to increase biodiversity is to add a refugium. By definition, a refugium is an area where organisms are allowed to thrive without the presence of predators. This could be a separate tank or even a section of the sump. Today, manufacturers are making hang-on-the-back refugium units for aquariums. A refugium may alleviate the predation problem for the organisms found there, but what about the competition and random catastrophic events?
click here for full size picture
Refugiums such as the one above are relatively simple to add on to existing systems, and are an effective way to maintain faunal biodiversity in a reef aquarium.
In natural ecosystems, there is immigration and emigration. Many organisms will move from one area to another. If something catastrophic occurs and destroys a whole population of trees or birds in a given habitat, neighboring communities can help recolonize the area. In a reef tank, this does not occur. Despite having a refugium to mimic some of this recolonization, a reef tank is still a closed system. Consider a tank as a very remote island. It is isolated. If a particular population of organisms is eliminated from the tank and refugium, there is no way for that type of organism to recolonize the tank...unless the reefkeeper steps in.
I personally think it is good practice to play Mother Nature from time to time and restock the tank with micro- and meiofauna. Many online stores carry detritivore "kits," which would allow one to re-establish some of these critters in a reef aquarium. Another way is to replace a couple of pieces of old live rock with fresh (cured) rock. The new live rock can aid in reintroducing some of the lost populations. Unfortunately, there is also a risk of reintroducing nuisance organisms, but I think the benefits far outweigh the risks.
Choosing fish wisely is another good way to maintain diversity. Reducing the amount of predatory fish will certainly help. Feeding more heavily and providing a variety of food types and sizes will contribute to sustaining a diverse reef, by meeting the nutrient demands of more organisms. Support of this suggestion is found in the various comments I hear from aquarists that begin feeding plankton substitutes. They notice an increase in many organisms, from sponges to microfauna.
Perhaps the reader may feel that the suggestion of maintaining biodiversity conflicts with the suggestion of keeping less corals, but many corals tend to fill the same or similar niches. By maintaining diversity, I refer to the goal of filling and maintaining a large variety of niches.
Simulate Storms
Detritus builds up in areas of the tank despite our efforts to maintain large populations of detritivores, strong current, and proper filtration. In a closed reef aquarium, it may even become a problem. Some suggest that long-term accumulation of detritus can lead to mysterious algae problems. Others suggest that detritus can also clog the porous live rock in our system, reducing the surface area on which bacteria, sponges, and other filter feeders settle. I'm not entirely sure if these concerns are legitimate, but I personally think it is beneficial to occasionally simulate the action of storms with large powerheads and other devices to remove detritus build-up. I also try to occasionally manage stagnant areas with a turkey baster when doing water changes. Stirring up the detritus is also a good way to feed your corals. I would, however, recommend only disturbing small pockets of detritus. Disturbing large sinks of detritus, such as stirring a deep sand bed, can have catastrophic effects on the tank. Releasing large amounts of detritus will also release large densities of aerobic bacteria and microbes into the water column. These organisms may use up a lot of the free oxygen in the water column thereby reducing the overall oxygen levels in the tank. The freeing of large amounts of organic material may also cause an algae bloom down the road.