WHAT CONSTITUTES EVIDENCE OF COMMUNITY STRUCTURE - A REPLY TO VAN DERMAAREL, NOEST AND PALMER

Van der Maarel et al. (1995) - hereafter VNP - criticize our analyses of Oland limestone grassland. They have four general objections, addre ssed below. 1. VNP attribute our significant results to randomness ('v olatility'); however, significant deviation from randomness cannot be explained by randomness. VNP's conclusion of volatility derives from r esults that are inconsistent with ours. Their ecological interpretatio n assumes spatial and temporal near-constancy in the vegetation; we de monstrate that these assumptions are not correct. 2. We discussed phys ical limitations to plant module packing. VNP give estimated module si zes. We appreciate this information, though data on actual module size s and overlap patterns would be required before the real module packin g effects could be determined. If module packing were really the main cause of deficits of variance in richness, the effect would not fluctu ate between sites and years, as VNP admit it does. 3. The shape of the richness frequency distribution is a potentially interesting addition al form of analysis, albeit one that we had chosen not to include in o ur analysis. However, it is surprising that VNP dismiss analysis of ri chness variance, but then interpret a more subtle aspect of the richne ss distribution - the skewness. VNP's redefinition of the terms 'Niche facilitation' for a deficit of low richness values, and 'Niche limita tion' for a deficit of high values, leads them to misrepresent our arg uments. They are mistaken in suggesting that niche limitation will nec essarily lead to a skewed curve: they do not allow for the fact that t he species frequencies, on which the null model is based already, inco rporate effects of species interactions. 'Niche facilitation', as defi ned by VNP, would lead to a variance excess, not a deficit as they ass ume. 4. VNP's criticism of a priori guild classifications had already been met by our use of the intrinsic guild approach. Guild analyses of fer the best way forward. VNP use principally the methodology of 1987. There have been a number of methodological advances since that time. We used these advances in our original paper specifically to circumven t the kinds of problems that they identify. We agree with VNP that it would be very useful to explore the mechanisms behind assembly rules w ith experiments, but the logical first step is to identify potential a ssembly rules, as we have been attempting to do. We conclude that, whi lst VNP make some interesting observations, none of their criticisms i nvalidate our results or conclusions. Our original approach stands as the best known approach, we believe, for searching for community struc ture in such data, and we reaffirm the validity of our ecological conc lusions.