Demetris Koutsoyiannis responds:
I agree that a bimodal distribution is seldom seen. Well, my experience is not from ecological but mainly from hydrological processes but I suspect that the behaviours would be similar.
I have seen claims of bimodality several times but I was never convinced about them as I did not read any argument supporting it except empirical histograms. However, we must be aware that the uncertainty of the histogram peaks is large. A simple Monte Carlo experiment with say a normal distribution suffices to demonstrate that (unless the number of generated values is very high) it is very common to have a histogram with two, three or more peaks. This however is totally a random effect; obviously the normal density is unimodal.
So, I think that one must have theoretical reasons to accept a bimodality hypothesis. As a simple illustration, consider a system described by a random variable X, which switches between two well defined states, 1 and 2 with probabilities p and 1-p. Assume that the conditional density of X given the state is normal in each of states 1 and 2 and denote it f1(x) and f2(x), respectively. Then the unconditional density will be p f1(x) + (1-p) f2(x). It can be easily observed that if the means of the two densities are different, then certain combinations of the standard deviations and the probability p result in a bimodal unconditional density.