Species presence/absence sometimes represents a plant community as well as species abundances do, or better
Many broad-scale surveys are made, and local communities described, with time spent recording some measure of the abundance of each species. The results are always somewhat different from those obtained with presence/absence records, but which best represents the underlying structure of the community? Two partial answers to this question are suggested and tested here: which analysis correlates best with the habitat, and which gives more stable ordination scores under subsampling?
Tests were made on ten field data sets, ranging widely in habitat type, spatial extent, spatial grain and measure of abundance. Correlation with the habitat was examined for the four larger-extent data sets with reasonably complete environmental information, using multiple regression of detrended correspondence analysis (DCA) ordination scores on environmental factors. Stability was tested for each data set using random subsets of the quadrats, and measuring stability as correlation between quadrat ordination scores in the subset and those using all quadrats.
Correlation with the habitat for the four data sets, where possible, was closer with presence/absence in most comparisons. Stability was greater with presence/absence in some cases and with abundance in others. Where abundance analyses were more stable, reduction to abundance categories, which are often used in field sampling, resulted in a loss of stability, although in two out of three data sets some advantage of abundance information over presence/absence was retained. Jittering to simulate subjective recording gave no further degradation.
The data sets in which stability was higher in abundance analyses suggest that abundance is of value only in the rather homogeneous vegetation types that tend to occur over short distances, and with high-quality abundance data. From this, and environmental correlations being on the whole better with presence/absence analyses, I conclude that in broader-scale survey work, abundance information is unnecessary and may even be misleading. It seems that the primary assembly rule control on communities is on the presence of species, not their abundance.