Diversity vs Density

[Bryozoa]

Is there a way of comparing the effects of diversity with those of density?

For example:

You want to know the effects of particular species and its density have on localised currents so you start by using monospecific examples but what happens when you make them heterspecific?

IE;
There is a 10m x 10m beach with 100 oysters on it, all varying in size between 6cm and 20cm shell length. A second 10 x 10m beach has 20 oysters, 30 mussels, 15 snails, 25 worms all of various sizes.

How would you compare the density with the diversity as this could be a fundimental issue of the research. If anyone has any ideas of corrective statistics or experimental methods I would be very grateful!