Catchment Productivity Controls Local Species Richness of Hyporheic Invertebrate Communities in Tropical New Caledonia Streams

At continental to global scales, energy-related predictors relate positively to regional species richness, often explaining much of its variation in different ecosystems. However, the influence of energy on local species richness (LSR) has received much less attention. In aquatic ecosystems, LSR is predicted to increase with increasing catchment terrestrial productivity. Greater food resources resulting from increased terrestrial subsidies of nutrients and organic matter to the stream can support more individuals, thereby enabling more species to attain viable local populations, a hypothesis known as the more-individuals hypothesis (MIH). We used generalized linear models and variance partitioning to quantify the unique and shared effects of nine local predictors and nine catchment predictors on LSR of hyporheic communities sampled at 228 sites in tropical New Caledonia streams. We found that productivity proxies, including the proportions of peridotite and bare soil in the catchment, and the normalized difference vegetation index (NDVI), correlated strongly with proxies of sediment metabolism (dissolved oxygen and pH), which in turn accounted for much of the variation in LSR. However, the positive relationship between LSR and catchment productivity, and the resulting dominance of shared effects of local and catchment predictors, were largely driven by fast-growing taxa. Their richness and abundance increased monotonically with increasing catchment productivity. The LSR of slow-growing taxa was largely explained by local predictors, and it showed a weak, hump-shaped relationship to catchment productivity. The additional food resources from more productive catchments could either not be assimilated by slow-growing taxa or were monopolized by fast-growing taxa. Hence, other processes than those underlying the MIH, including the competitive ability of taxa to acquire available resources, likely play a role in controlling the species-energy relationship in hyporheic invertebrate communities.