A Comparison of Structural and Functional Approaches to Determine Land-use Effects on Grassland Stream Communities

Effective catchment management in the face of land-use alteration depends on our ability to quantify ecologically significant changes and to discriminate between varying levels of impact. We compare the efficacy of traditional structural indices of change (species composition) with functional measures based on the representation of species traits (including life-history, trophic and morphological features) in an analysis of grassland streams along a gradient of agricultural development (ungrazed native tussock, grazed tussock, extensively grazed pasture, intensive dairy and deer farming). Only 8 of 60 invertebrate taxa demonstrated a significant separation across the land-use gradient wherease 23 of 53 trait modalities did so. Traits associated with population resilience (small size, short generation time, single individual reproduction) became more prevalent with increasing development while gill bearing and the tendency to lay unattached eggs became less prevalent. Multivariate analyses involving species or trait composition were all able to discriminate land uses, but more of the overall between-land use variance was accounted for by trait composition. Rather than simply recording a loss or reduction of species, our functional approach was able to identify sensitive life-history characteristics and provide direct evidence of causative mechanisms, facilitating the development of targeted management actions.