How do freshwater organisms cross the 'dry ocean'? A review on passive dispersal and colonization processes with a special focus on temporary ponds

Lakes and ponds are scattered on Earth’s surface as islands in the ocean. The organisms inhabiting these ecosystems have thus developed strategies to pass the barrier represented by the surrounding land, to disperse and to colonize new environments. The evidences of a high potential for passive long-range dispersal of organisms producing resting stages inspired the idea that there were no real barriers to their actual dispersal, and that their distribution was only limited by the ecological characteristics of the available habitats. The development of genetic techniques allowed to criticize this view and revealed the existence of a more complex and diverse biological scenario governed by an assortment of historical and ecological factors. In this paper, we review the literature related to the passive dispersal of organisms producing resting stages among inland lentic ecosystems, with special emphasis to temporary ponds, which represent “isolated” ecosystems both in space and in time, and are characterized by high levels of biological diversity. The existence of a sharp decoupling between “dispersal potential” and “actual establishment rates” is stressed, thus urging a definitive overcome of the so-called “Everything is Everywhere” hypothesis in order to gain a proper understanding of the biogeography and ecology of inland water organisms.