Light competition explains diversity decline better than niche dimensionality

Abstract

Summary 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.

Out of the shadows: multiple nutrient limitations drive relationships among biomass, light and plant diversity

“…). In some cases, we might expect addition of particular combinations of resources to restore a balanced supply of resources that promotes coexistence, which is the mechanism that Hautier, Niklaus & Hector () found when they experimentally added light back to the understorey of fertilised communities, refuting the interpretation of DeMalach & Kadmon (). The consequences of altered resource supply also depend on the relationships between the pool of species present, the multivariate set of traits and trade‐offs they represent, and how these map onto multivariate resource supply clouds (Fig.…”

“…DeMalach & Kadmon (; abbreviated D&K hereafter) reanalysed our experimental data (Harpole et al . ) with the goal to disentangle two hypotheses that can explain loss of plant diversity due to multiple resource addition: the light asymmetry hypothesis and the niche dimension hypothesis (Harpole et al .…”

“…(): Multiple nutrient addition drives a suite of correlated responses, including diversity and light (ultimate causal factors shown by bold solid arrows; response variables and potential proximate causal relationships shown as curved, dotted arrows). (c) DeMalach & Kadmon () tested the relative effects of live and dead biomass, light and nutrients as independent and direct drivers of diversity.…”

“…The model is technically incorrect because live and dead biomass and light are dependent variables and correlated, violating assumptions of linear regression, but we used the model as an ‘added variable’ test that provided just one of a number of lines of evidence that we used to support our conclusions. However, D&K next used the standardised multiple regression coefficients from that model to conclude that light was the ‘main factor explaining diversity loss’ and that the actual experimental treatment factors – multiple added nutrients – were unimportant and that therefore there was ‘no support for the niche dimension hypothesis’ (DeMalach & Kadmon ). They further suggested that the effects of nutrients were exaggerated because they were ‘measured without variance’ – a misunderstanding of the design because they were factor level treatments (Oehlert ).…”

Grazing Exclusion, a Choice between Biomass Growth and Species Diversity Maintenance in Beijing—Tianjin Sand Source Control Project

“…There are several explanations for the phenomenon that the different practices commonly tended to decrease the species diversity index H'. These include the light competition hypothesis [15,34] and litter accumulation hypothesis [35] which have been widely discussed and used to explain diversity decline in nitrogen addition experiments. The restoration practices which reduce the pressure of grazing and trampling by livestock [29] result in those high and palatable forage grasses recovering rapidly in restored areas.…”