Describing a curve in order to compare with species abundance


I was wondering if there was an established method / algorithm for describing a curve (in this case, grain-size data) in order to compare resulting values with species abundance, in order to determine if there is a relationship between the shape of a curve and the abundance of species.

To help you answer this, grain-size is traditionally measured semi-quantitatively (i.e. usually the modal grain size is used). However, this very often does not reflect reality, and does not give you any information about the spread of data, or even if it is in fact bimodal (which is very common). With new technology, it is now possible to measure grain-size data quantitatively, but there is now no way of comparing this data to species abundance. The problem for me is that I am just as interested in the shape of the curve (i.e., how skewed it is) as well as the modal value.

Any ideas?

Thanks, Nick