Nature 244, 310 (03 August 1973); doi:10.1038/244310a0

Competition and Diversity in Herbaceous Vegetation

E. I. NEWMAN

Botany Department, The University, Bristol BS8 1UG

GRIME¹ points out that herbaceous vegetation on highly favourable, "low-stress" sites often contains few species compared with that on intermediate-stress sites. He proposes an explanation for this which rests upon the claim that competition is less intense at intermediate-stress sites than at low-stress sites. I question this claim. The only evidence Grime gives to support it is based on a competitive index which he has devised. But he uses characters which are mainly related to competition for light, and seems to ignore the possibility of root competition. That root competition does occur between grassland plants is shown by experiments in which either shoot or root competition was prevented by partitions between the plants²⁻⁷. (Admittedly the conditions in such experiments are artificial.) Evidence that competition can be intense in species-rich grassland is as follows. (1) If seeds are sown into such vegetation and into control plots from which the vegetation has been removed, seedling establishment and growth are often better in the control plots^8,9. (2) If some species are removed from established vegetation, some (at least) of the remaining species show increased growth^9,10. (3) If organic nitrogenous fertilizer is added to the soil, the bulk of vegetation usually increases¹¹⁻¹⁴. This shows that at such sites nitrogen is usually a factor limiting growth, and as nitrate and ammonium ions are mobile in soil, it is virtually certain that before addition of the fertilizer the plants were competing for nitrogen.

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