Search

Understanding and predicting the geographic distribution of taxa in hierarchical stream landscapes is a cornerstone of river ecology. A central issue is to tease apart the unique and shared effects of local and catchment predictors over species distributions. Here, we tested Hynes's influential hypothesis (1975, Baldi Memorial Lecture) that 'In every respect, the valley rules the stream'. We predicted that if catchment features exert a major control on in‐stream local conditions, the shared effect of local and catchment predictors should largely surpass their unique effects. To test this prediction, we used logistic regression models and variation partitioning to quantify the unique and shared effects of local and catchment predictors on the distribution of two hyporheic crustacean taxa (Bogidiellidae, Amphipoda and Anthuridae, Isopoda) in streams of New Caledonia. We sampled the two taxa at 228 sites. At each site, we quantified nine local predictors related to habitat area and stability, sediment metabolism and water origin, and eight catchment predictors related to geology, area, primary productivity, land use and specific discharge. When analyzed separately, the two predictor types explained the same amount of model variation in occurrence in both taxa. When analyzed jointly, the shared effects of the two predictor types explained twice as much model variation as the unique effect of each. The overriding contribution of shared effects was notably due to controls exerted by catchment area and geology on local habitat size and sediment metabolism, respectively. For both taxa, a model with only these two catchment predictors provided occurrence distribution as reliable as models containing only local predictors or both predictor types. Our findings pave the way for predicting reliably from catchment predictors alone the geographic distribution in local occurrence of taxa in difficult‐to‐access habitats and landscapes, such as here the hyporheic zone of tropical streams. [ABSTRACT FROM AUTHOR]

The recognition of multi-causality and spatial non-stationarity in the determinants of large-scale biodiversity patterns requires to consider the role of multiple mechanisms, their interactions, and how these mechanisms vary in strength relative to each other across geographical space. Here, we challenge the view that historical climate stability primarily drives European patterns of groundwater crustacean diversity by testing also the role of spatial heterogeneity and productive energy. First, we predicted that the three mechanisms would be equally important at continental scale when analyzed separately, but that the importance of historical climate variability would weaken in joint analyses due to co-variation with the two other mechanisms. Second, we predicted that the three mechanisms would exhibit predictable latitudinal changes in their relative strength. To test these predictions, we selected predictors representing each mechanism and analyzed separately and jointly their effects and interactions using global regression models. We further mapped the independent and overlapping effects of mechanisms across Europe using partial geographically weighted regressions. When analyzed separately, the three mechanisms explained the same amount of variation in species richness, but in the joint analysis, the influence of historical climate stability became hidden in the variation shared with the other mechanisms. Topographic heterogeneity interacted synergistically with actual evapotranspiration and habitat heterogeneity on species richness. Spatial non-stationarity in the independent and overlapping effects of the three mechanisms was the most plausible explanation for the hump-shaped latitudinal pattern of crustacean species richness. Productive energy and spatial heterogeneity were important predictors at mid and southern latitudes, whereas historical climate stability overlapped with the two other mechanisms in northern Europe and productive energy in southern Europe. Multi-causality and spatial non-stationarity provide a broader perspective of groundwater biodiversity determinants that revives the importance of spatial heterogeneity and the strong dependence of subterranean communities on food supply from the surface. [ABSTRACT FROM AUTHOR]