The ecology of rafting in the marine environment. III. Biogeographical and evolutionary consequences

Abstract

Rafting of marine and terrestrial organisms has important ecological, biogeographical and evolutionary implications. Herein the general principles of rafting are described and how they contribute to population connectivity. Rafting dispersal has particular characteristics, which may differ substantially from those of species with planktonic larval dispersal. Dispersal distances achieved via rafting can vary considerably: journeys may be very short or in some cases extremely long, depending on currents and wind. Accumulation of rafts in convergence zones facilitates cohesion of travelling groups, possibly reducing the risk of founder populations being very small. This becomes particularly important over long distances where singular founder events could provoke strong reduction of the genetic variability in the founded population. The frequency of transport affects the degree of connectivity between local populations. Three important rafting routes are distinguished: frequent, intermittent and episodic. Frequent rafting routes are found in bays, lagoons and estuaries, and they are typically facilitated by substrata of biotic origin (seagrass, saltmarsh vegetation, intermediate-sized algae and mangroves). Intermittent rafting routes are found along temperate continental shores where they are facilitated primarily by giant kelps. In the subtropics and the Arctic intermittent rafting routes facilitated by wood are particularly important. Episodic rafting routes, which often cross vast areas of open ocean (biogeographic barriers), are facilitated by volcanic pumice, floating trees and occasionally by giant kelps when these are pushed beyond intermittent routes by strong winds or currents. Dispersal events occur in a highly sporadic manner in this latter category of rafting route, but when they happen, large amounts of floating substrata and rafters may be dispersed simultaneously. Intervals between events can be decades, centuries or even millennia, and consequently populations resulting from these events may be isolated from each other for long time periods. Population connectivity on frequent, intermittent and episodic rafting routes is high, intermediate and low, respectively. Genetic studies support these predictions, and furthermore underline that rafting may contribute to population connectivity over a wide range of geographic scales, from 5000 km. Rafting also has a strong effect on evolutionary processes of the organisms dispersed by this means. It is suggested that local recruitment (consequence of direct development) contributes to enhanced rates of population divergence among local populations of common rafters, but occasionally high genetic diversity may result from secondary admixture. Isolation of colonisers after singular episodic rafting events facilitates allopatric speciation. Through these processes rafting dispersal may support local species richness and thus have an influence on local biogeography and biodiversity. Human activities affect rafting connections in the oceans either by reducing or enhancing the possibility of transport and landfall. In many cases it cannot be safely decided whether the appearance of a species in a new habitat is due to rafting or to other transport mechanisms, and genetic studies can help to identify the most likely causes. Future field and laboratory studies on the ecology of potential rafters in combination with genetic studies on different spatial and temporal scales will contribute to a better understanding of the mechanisms of rafting dispersal, consideration of which is crucial in developing efficient conservation measures in the marine environment.