Ancient lake deep drilling allows for fresh look at theory of island biogeography
Friday, 14 December 2018
During the past decades, considerable progress has been made towards understanding biotic and abiotic factors governing diversification, particularly in highly isolated ecosystems. Accordingly, the general dynamic model (GDM) of island biogeography predicts that diversification rates are mainly driven by geoenvironmental and climatic processes (environment dependence), and balanced by species interaction (diversity dependence). While the GDM is broadly accepted, empirical evidence from nature is scarce. Moreover, most studies favor either diversification mode, thus fueling an ongoing debate on the drivers of diversifications. These deficiencies call for evolutionary studies in model systems. A paradigm is the Balkan Lake Ohrid – Europe’s oldest and most biodiverse lake. Recently, it became the subject of the International Continental Scientific Drilling Program (ICDP), resulting in continuous environmental, climate and diatom fossil records, spanning the lake’s entire lacustrine phase of 1.4 Myr. Integrating these data, the main goal of the project was to unravel the interplay of environmental- and diversity-dependent diversification over time. Our Bayesian birth-death-sampling analyses showed decreasing speciation and extinction rates over most of the time, including a significant rate shift, dividing the lake’s history into two evolutionary phases. Diversification rates during the shallow-water phase were mainly driven by geomorphology, environment and species interactions, causing diversity-dependency. During the deep-water phase, diversification rates were primarily determined by species interactions, with the system approaching equilibrium. Our findings demonstrate that diatom diversification in Lake Ohrid was largely governed by the dynamic interplay of geomorphological/environmental parameters and species interactions. There is strong evidence that the impact of environmental forcing has decreased over times. Finally, we show for the first time that an isolated ecosystem may reach a dynamic equilibrium when both speciation and extinction rates slow down. This, in turn, questions the trajectories of speciation and extinction rates proposed by the GDM.