  Understanding factors that shape biodiversity and species coexistence across
scales is of utmost importance in ecology, both theoretically and for
conservation policies. Species-area relationships (SARs), measuring how the
number of observed species increases upon enlarging the sampled area,
constitute a convenient tool for quantifying the spatial structure of
biodiversity. While general features of species-area curves are quite universal
across ecosystems, some quantitative aspects can change significantly. Several
attempts have been made to link these variations to ecological forces. Within
the framework of spatially explicit neutral models, here we scrutinize the
effect of varying the local population size (i.e. the number of individuals per
site) and the level of habitat saturation (allowing for empty sites). We
conclude that species-area curves become shallower when the local population
size increases, while habitat saturation, unless strongly violated, plays a
marginal role. Our findings provide a plausible explanation of why SARs for
microorganisms are flatter than those for larger organisms.
