On correlations and causations between productivity and species richness in vegetation: predictions from habitat attributes

The controversial relationship between productivity and species richness in vegetation is re-examined here in the context of several other important habitat attributes. A simple conceptual model for classifying these attributes and their relationships is developed based on established principles of ecology, evolution and agriculture. Two main predictions derived from this model have important theoretical implications: First, the magnitude of loss in productivity that may result when species are lost from a natural habitat because of extinction, is likely to depend almost entirely upon which species are lost, and should depend only marginally, if at all, upon how many species are lost. Hence, any small role that species richness per se might have in affecting habitat productivity will be detectable only if species composition is controlled explicitly, which has not been the case in several recent empirical studies. Second, the most important determinants predicted for variation in species richness between habitats involve several habitat attributes, but not productivity. The commonly reported decline in species richness across habitats of increasing fertility is not explained in this model by increasing competition intensity as predicted by the popular "hump-backed" model. Frequent and intense competitive exclusion of species from highly fertile (and hence, productive) habitats certainly occurs; however, this accounts only for why many non-resident species are not adapted to this habitat type. It does not account for why the species that are adapted to this habitat type, i.e. the resident, successfully recruiting species, are relatively few in number. The present model, however, accounts for this scarcity of resident species in highly fertile habitats based on three possible hypotheses involving three principal habitat attributes: (i) a scarcity of available resource niches, reflecting relatively low environmental heterogeneity; (ii) a scarcity of opportunities for facilitation between species; and/or (iii) a scarcity of available species, reflecting scarce historical opportunities for origination of adapted species in this habitat type.