Light competition explains diversity decline better than niche dimensionality

1. One of the most widely documented patterns in plant ecology is the decrease in species diversity following nutrient enrichment. A long-standing explanation for this diversity decline is an increase in the relative importance of size-asymmetric light competition which accelerates the rate of competitive exclusion ( he 'light asymmetry hypothesis'). 2. Recently, an alternative hypothesis has been proposed which attributes the negative effect of nutrient enrichment on species diversity to a reduction in the number of limiting resources (i.e. a reduced niche 'dimensionality'). A recent global-scale experiment demonstrating that increasing the number of added resources leads to a decrease in species diversity was interpreted as a support for this 'niche dimension hypothesis'. 3. Here we highlight a number of theoretical considerations that question this interpretation and demonstrate that a deeper analysis of the new global-scale dataset provides a stronger support for the light asymmetry hypothesis.