Warming reverses directionality in the richness–abundance relationship in ephemeral Mediterranean plant communities

Recent findings in forests worldwide have demonstrated how directionality in the richness–abundance causality shifts along global climate gradients: The so-called more-species hypothesis (richness determines abundance) prevails in Earth’s most productive climates, whereas the opposite, the so-called
more-individuals hypothesis (abundance determines richness), is more likely to prevail in climatically harsh conditions. Since temporal variability is the norm, a critical question is whether this directionality shift is also a function of temporal climatic fluctuations locally. Here, we analyze whether directionality in the richness–abundance relationship is contingent on temporal variability over 10 annual consecutive realizations in ephemeral plant assemblages. Our results support the idea that the more-species hypothesis prevailed
in the most benign years, whereas the more-individuals hypothesis did so during less productive years, which were significantly linked to the warmest years. These results support the idea that rising temperatures can reverse directionality in the richness–abundance relationship in these annual plant communities, and therefore, climate warming can have a significant effect on the relationship between diversity and ecosystem functions, such as productivity, by altering the prevalence of primary mechanisms involved in species assembly.