Inferring community assembly processes from mangrove species–area relationships

The increasing species–area relationship (SAR) is a nearly universal ecological law. But recent theory has predicted that in systems with low large‐scale diversity the law should be violated and the SAR should be nearly flat at intermediate scales, with species richness roughly constant at some value typically greater than one. We tested this prediction using a global dataset of mangrove trees – a species‐poor group. We used a published global dataset of mangrove tree distributions to construct an SAR spanning local to global scales. We found that over a large range of scales (≈10 −4 to 10 6 km 2 ) the SAR was close to flat, in stark contrast to a classical power‐law SAR, which would predict roughly a 300‐fold change in species richness over these scales. Importantly, species richness was not simply equal to the minimum value of one or the maximum value of global mangrove richness over these scales, either of which possibilities would be reconcilable with the classical theory, but instead was maintained at an average value of between two and three species. Our theoretical interpretation of the results is that there are two to three stabilising niches (i.e. niches that would allow two to three species to stably coexist without substantial immigration) for mangrove trees in typical coastal settings and that immigrant propagule diversity is in most cases too low (because of low mangrove tree metacommunity diversity) for there to be more species than niches. Only at scales greater than ≈ 10 6 km 2 is the diversity of immigrants typically sufficient to yield more species than niches. We speculate that in other systems, local niche diversity may be similarly low but that the nearly flat SAR phase is hidden by immigration from diverse source pools.