"On a tree", "terrestrial", or "on the rocks"? Habit diversity in the megadiverse genus Peperomia.

Categorical thinking conceptualizes continuous gradients into distinct entities and is problematic because it obscures the natural variation that exists within and between members of a category. A case in point is the common practice of sorting species into different habits, that is, epiphytic, lithophytic, or terrestrial. This work aims to emphasize that habitat use should be treated as a continuous gradient to avoid missing biological variations and diversity. We focused on the genus Peperomia and collected relevant information from >400 sources, including online databases, flora checklists, species descriptions, type specimens, and herbarium vouchers. Literal habitat descriptions were translated into numerical proportions that allowed for a quantitative analysis of the tendency to grow on either substrate, and to depict the distribution of Peperomia species in the epiphyte-lithophyte-terrestrial (ELT) space. Triangular ordinations allowed the visualization of the habits of 1375 species at the genus and subgenus level and portray geographical differences. Species were distributed across the ELT space and not limited to the extremes as specialists, which we defined as species that occur (almost) exclusively in one habitat. Online databases with oversimplified habitat descriptions do not adequately present the existing inter- and intraspecific variations in habitat use. We present the arguably most comprehensive and up-to-date database on Peperomia, which will be an important source for future ecological and phylogenetic studies. The main takeaway message that is also applicable to other taxonomic groups is that our numerical approach showed succinctly that habitat use should not be categorized but instead viewed as continuous to properly capture biological variabilities such as inter- and intraspecific variation.