Data from: 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.