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Speciation, extinction, and dispersal processes related to fragmentation of riverine networks: a multiscale approach using freshwater fishes

Team 7: Biodiversity & Macroecology


Year of defence: 

Understanding factors driving the variation of diversity across the Earth is the main goal of ecology and biogeography. To reach this goal one needs to study the drivers of three main processes directly causing variation in diversity: speciation, extinction and dispersal. Speciation increases and extinction decreases diversity levels through time, whereas dispersal has a dual effect: it may increase or decrease diversity depending on the circumstances. Fragmentation, which stands by the emergence of natural or human-driven discontinuities on a given environment, is ubiquitous and has multiple ecological and evolutionary implications for the three diversity processes mentioned above (i.e., speciation, extinction and dispersal). Riverine fishes (i.e., strictly freshwater fishes) naturally experience fragmentation, as natural elements may disrupt connectivity of rivers (e.g., waterfalls) and river basins are separated from one another by barriers (oceans or land) which are insurmountable for these organisms. Riverine fishes thus constitute an ideal model for studying fragmentation effects. Here, I evaluated the effect of fragmentation on freshwater fish diversity by testing its effects on speciation, extinction, and dispersal processes at spatial and temporal scales ranging from a single river basin to worldwide rivers and from decades to million of years. In a first study, by analyzing endemism level of tributaries from the Orinoco river basin, I showed that tributaries highly fragmented by waterfalls have higher speciation probability and higher neoendemism levels (i.e., species presumably originated by in-situ cladogenetic speciation) than their less fragmented counterparts. In a second study, I tested whether the historical connectivity between basins left an imprint on the global patterns of freshwater fish biodiversity. After controlling for contemporary and past environmental conditions, I found that palaeo-connected basins (those connected during the Last Glacial Maximum; 18-21 kya) displayed greater species richness but lower levels of endemism than did palaeo-disconnected basins. Palaeo-connected basins exhibited shallower distance decay of compositional similarity, suggesting that palaeo-river connections have favored the exchange of fish species. In a third study, I evaluated to what extent, if any, anthropogenic threats related to fragmentation (e.g., damming of rivers, agricultural practices) have been promoting fish extinctions in river basins. Focusing on Western Europe and North America, two strongly impacted regions, I showed that the percentage of cropland in the river basin and river fragmentation by dams are the main causes of present riverine fish species extinction. These extinctions, even if still reduced, correspond to extinction rates 40 times higher than background, natural rates. Overall, my results point for a prominent role of fragmentation as a driver of fish diversity through speciation processes and highlight the strong role played by history in explaining the global contemporary patterns of biodiversity via colonization processes. Moreover, they support the need of maintaining connectivity within river basins where human-made barriers have been created to avoid a substantial increase in species extinction rates.

Keywords: speciation; extinction; colonization; freshwater fish; river basins; richness, endemism, beta diversity; history effects; natural barriers, anthropogenic disturbances.