Thesis talk - Denise Ruijsch

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The Lemurs of Madagascar: Determining Eocene Ocean Connectivity between Southeast Africa and Madagascar by using Dynamic Network Theory

Madagascar is an island in the Indian Ocean that hosts one of the most unusual, endemic and diverse concentrations of fauna around the world. However, the fauna found on Madagascar is very unbalanced. This pattern of imbalance, endemism and diversity begs an obvious question: what forces have created the pattern? In this thesis, I will investigate the over-water dispersal hypothesis first proposed by Simpson in 1940 by using model simulations under Eocene conditions and applying Lagrangian particle tracking techniques. While the simulated ocean surface currents are driven by wind patterns, the effect of the wind on the trajectory of floating particles still needs to be included. This was done by first determining and then adding a windage factor to the ocean circulation. With the knowledge of how particles travel between Africa and Madagascar and what windage to choose, the connectivity of these areas could be investigated by using dynamic network theory to calculate the most probable path between Africa and Madagascar. We conclude that all signs point to Simpson's sweepstakes model being correct: ocean currents could have occasionally transported rafts of animals to Madagascar from Africa during the Eocene, with the journey taking at least 10 days.  Specifically, Autumn would be the best season for lemurs to make the crossing in the Eocene, which is in contrast with present-day simulations showing Spring to be the most favourable season. The starting location of the lemurs in the Eocene should have been between 27-30°S, corresponding to a region spanning the present-day east coast of Mozambique.

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Buys Ballot building 6.07