Ellenberg numbers revisited
Thompson, K.; Hodgson, J.G.; Grime, J. P.; Rorison, I. H.; Band, S. R.; Spencer, R. E.
Heinz Ellenberg's contributions to ecological thought are many, but this paper concentrates on one which is currently receiving growing attention: namely the quantification of the field response of some 2000 species to a range of climatic and edaphic factors. Ellenberg (1974) numbers are one manifestation of the recognition that the ability to make useful generalisations about vegetation depends on the existence of autecological data for relatively large numbers of species. In the past, such generalisations have been based on easily measured attributes such as growth form (Raunkiaer 1934) and seed size (Salisbury 1942, Baker 1972), but the view that further advances will require large-scale screening for less tractable attributes is gaining ground (e.g. Keddy 1992). Ellenberg numbers can be seen therefore as a short cut to measurement of some of these more difficult variables. Their great value is that although they are based on the experience of many workers, the numbers themselves are ultimately the work of one man, which significantly reduces the objection that they are inevitably rather subjective. Surprisingly, however, they have been relatively little used as aids to the interpretation of spatial and temporal vegetation pattern (cf. Vevle & Aase 1980, Persson 1981, Major & Rejmanek 1992). The reasons for this are numerous: in some quarters there are philosophical objections to generalisation about vegetation per se (Grubb 1985); the intuitive nature of Ellenberg numbers may cause them to be treated with scepticism by those ecologists who value precision above generality; publication in German may have inhibited the use of Ellenberg numbers by the English-speaking world. But in addition there remain unanswered scientific questions about the numbers themselves. To what extent can the numbers be used outside Central Europe? Do the numbers represent the fundamental niches of the species, or are they influenced by the identity and density of potential competitors? What are the mechanisms underlying Ellenberg numbers? Can they be used to help understand the pressing modern problems of changing land-use and climate? How easily can the system be expanded to new species, especially in the absence of Ellenberg himself? And perhaps most fundamentally of all, are there important axes of variation, both in plant attributes and environmental variables, which are not amenable to the Ellenberg number approach? This paper will attempt to answer some, but not all, of these questions. Mostly, we will compare Ellenberg numbers with quantitative data derived from both the field and from laboratory screening experiments. We will then attempt very briefly to use Ellenberg numbers to interpret patterns of commonness and rarity in the British flora, and finally we will compare Ellenberg numbers with other approaches to comparative plant ecology.