Simple Simulation Model: Successful Water Borne Dispersal in Terrestrial Spiders to Reach Lake Islands

Abstract

Organisms and materials newly arriving on islands impact insular ecosystems. Therefore, the processes around how they arrive on islands are important to understand, along with the ecological dynamics among islands and the mainland. Dispersal materials are thought to move with wind, water or animals, with some ultimately landing on islands. However, most of these passive movements are difficult to observe and mysteries remain as to how and why those organisms/materials arrive at small terrestrial areas surrounded by a vast aquatic space. This is because islands are often far out to sea, and dispersal patterns can be influenced by various factors. These factors include geographical and wind conditions that influence dispersal routes and their outcome for organisms/materials. In this study, I used artificial islands created in the middle of lakes to estimate dispersal rates of waterborne spiders as the models of organisms/materials, which have the possibilities of both wind- and water-borne dispersal. Dispersal possibilities to these islands are expected to be more likely than for typical oceanic islands. Spiders are known to be one of the first animal groups to invade remote islands at the initial stage when new insular ecosystems are forming. Aeronautic spiders have previously been recorded on the young islands created in the Attenborough Nature Reserve, UK, meaning that those spiders arrived on the islands via air or water. A recent study revealed that spiders that can become airborne can also sail on water using wind power. In the present study, I estimated how spiders landing on the water surface of these lakes dispersed to the islands under different geographical and wind conditions. I showed that the distribution of islands in aquatic space, and wind direction influenced on the rate of spider dispersal to islands. In addition, I also showed that water borne dispersal under even the smallest assumptions of survival rate on water surface effectively contributed airborne spiders to reach islands in the artificial lakes created within a 2 km2. Hence, wind borne organisms/materials may arrive at islands via water surface more than they have ever been thought, if they can survive/float on water. A model used in this study is exclusively applicable to estimate the movements of air- and water-borne hydrophobic materials especially in the case that they are highly influenced by wind even on water surface. The simple gridiron model could be a base model to build precise simulation programs taken into account of rheological and current dynamics, to understand dispersal events driven both by wind and water that happens relatively in lightweight materials such as plastic fragments and aeronautic spiders in the future.