Light competition explains diversity decline better than niche dimensionality

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 (the ‘light asymmetry hypothesis’). 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’. 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.