“Complex interaction of dendritic connectivity and hierarchical patch size on biodiversity in river-like landscapes”

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Francesco Carrara, Andrea Rinaldo, Andrea Giometto, and Florian Altermatt

Connectivity and habitat size rule community composition in riverine landscapes. Included are some of the protist species used in the experiment (from the top, counterclockwise): Euglena gracilis (green), Colpidium sp. (white), Blepharisma sp. (pink), Spirostomum sp. (black and white), Blepharisma (again in pink), Cephalodella sp. (black and white); and in the center, Paramecium bursaria (green).
(Credit: Francesco Carrara; protist images from Regula Illi)

Habitat fragmentation and land-use changes are causing major biodiversity losses. To modify natural waterways is to take a serious risk of endangering species living within such ecosystems, as suggested by the new research by Francesco Carrara et al. in The American Naturalist. Rivers and riverbanks have been constantly altered by introduction of dams, canals, drainage, or massive land use close to the river bed. Riverine ecosystems are hierarchical structures, where the size of the habitat scales locally with the drainage area. Thus, dendritic connectivity and local environmental conditions are interconnected. Such natural links contribute to shaping the unique patterns of diversity observed in natural river basins, and to maintaining a high biodiversity. The research, conducted by scientists from the École Polytechnique Fédérale de Lausanne and Eawag, tests how alteration of the natural hierarchical configuration of riverine habitat may affect biodiversity patterns at the regional scale. The authors use aquatic microcosms arranged in river-like landscapes, where the configuration of patch sizes within river-like networks is suitably varied. They find that riverine biodiversity depends on patch sizes being scaled to their draining tributaries, as opposed to patch sizes that are homogenous or uncorrelated with space. Only in landscapes preserving such geomorphological scaling do the rarer species sustain regionally more abundant populations. There, they are also better able to track their own niche requirements. Findings from such laboratory experiments, even if not directly comparable to natural systems, may cast light on important underlying mechanisms that steer community dynamics in river systems. The experimental results, tested in theoretical community models, demonstrate a principle that can eventually be projected onto real riverine ecosystems. Read the Article