Sign, strength and shape of stream fish-based metric responses to geo-climatic and human pressure gradients

Abstract

Efficient community indices and indicators are crucial for the adequate management and design of measures ensuring the ecosystem integrity. In this study we analyse the shape, sign and strength of the response of some biotic integrity indices and indicators of structure and function of fish communities along geo-climatic and human pressures gradients at catchment scale. To that purpose, >300 sites all over the Iberian Peninsula were characterized at the catchment scale by means of two anthropogenic drivers (agricultural and urban land proportion) and seven natural environmental descriptors covering geographical and climatic aspects. Regarding to fish-based metrics, a set of the most frequently used in stream health assessment studies have been selected, including taxonomic classic indicators, size related indicators and also recent multimetric indices created in the European context (WFD). We applied boosted regression trees that allow estimating the sign and strength of the response as well as considering non-linearity and impact thresholds. Our results show that the jointly contribution of anthropic drivers was lower than geo-climatic drivers. For most of indices and indicators, one single land cover contributed more markedly to the total deviance explained than the other, and they responded rather consistently to land-use variables, i.e., most of them responded negatively to the increase of anthropic use in the catchment. Size diversity, Fish Region Index (FRI) and maximum weight were those more sensible to agriculture land, while EFI+, mean weight, distinctness and FRI were those more sensitive to urban land. Regarding the shape of the response, urban land proportion affects normally at extremely low values, while agriculture land proportion induces smoother changes on a wider range. Our results may have practical implications, such as the selection of an efficient array of fish-based metrics to be included in ecological assessment and monitoring programs.