Catch me if you can, or how paradigms of tardigrade biogeography evolved from cosmopolitism to ‘localism’

Abstract

Abstract This review synthesizes recent developments in the field of tardigrade biogeography, tracing back to the origins of Baas Becking’s hypothesis that ‘everything is everywhere but the environment selects’ to the latest empirical studies that favour biogeographic structuring of these microinvertebrates. I recapitulate evidence for diverse means of meiofauna dispersal, focusing on the two principal mechanisms suggested for tardigrades: wind (aeolian dispersal/anemochory) and animals (zoochory). I argue that tardigrade species with relatively well-documented geographic distributions display climatic regionalization. The literature treating purportedly cosmopolitan tardigrades is critically scrutinized and evidence for strict cosmopolitanism is deemed insufficient. Due to the presence of numerous endemics restricted to mountains, elevational diversity gradients can be recognized at a local scale (for example, within a single biogeographic region). Finally, the tardigrade faunas of a given region should be likely regarded as part of a regionalized macrometazoan fauna containing a small fraction of widespread, ubiquitous species (called ‘biogeographic noise’ herein).