The importance of windows of opportunity for long-distance dispersal to oceanic islands: examples from the Macaronesian archipelago
José María Fernández-Palacios
Summary
The current species composition of entire oceanic islands or specific insular ecosystems is the result of a complex scenario where several historical and ecological processes, including long-distance dispersal (LDD), subsequent colonization, speciation (within or among islands) and extinction may have played an important rule.
A window of opportunity for a long-distance dispersal (LDD) event could be defined as the time interval throughout which the several different items controlling LDD became simultaneously functional or available. Usually the occurrence of LDD windows of opportunity is analyzed in retrospect, for instance, when palaeobiogeographers try to reconstruct the past events that may have enabled a present bizarre (fossil or extant) species distribution. Among the items that have to be checked for hypothesizing the existence of such window(s) we should consider: i) the existence in the proper historical moment at the continental margins of the ecosystems where the focal species or taxa thrived, ecosystems that may have later persisted or disappeared in situ (as was the case for the Tethyan Palaeotropical Geoflora in Iberian and North Africa in respect to Macaronesia), ii) the existence of islands available to be reached and subsequently colonized (which may as well have later persisted or vanished under the sea, as was the case for several Palaeomacaronesia islands), or iii) the historical occurrence of the proper agents for facilitating the dispersal event, that may be still present today or not (for instance, palaeo-sea-currents or palaeo-wind systems).
Also crucial for the understanding of the progression-rule colonization that may have shaped the archipelago species composition are the iv) temporal availability of stepping stones, for instance, due to eustatic sea-level transgressions that permitted the emersion of non-subsided flat-topped seamounts (guyots) facilitating the dispersal, or v) the existence of peak periods, i.e. the simultaneous occurrence of high islands that may have facilitated, through island hopping, the persistence of endemic species before the disappearance due to erosion and subsidence processes of the mountain and summit ecosystems of the island where they were once present.
These windows of opportunity for long-distance dispersal can be hypothesized as well for explaining retro-colonization events, i.e. the colonization of the mainland by insular species derived from mainland ancestors, when climate changes have transformed in the past the sea-currents and wind systems nowadays prevailing in the focal zone. This is actually the case for several Iberian and North African species with a clear Macaronesian origin, whose insular ancestors have been able to disperse from Macaronesia to the mainland during the prevalence of the Westerlies winds at the mid latitudes (20-30 degrees), what happen several times during the different Pleistocene glaciations maxima events.