Taxon and trait-based sampling curves can be used as a tool for assessing impairment in salinized headwater streams

Abstract

Many ecosystems are losing biodiversity, raising concern for the services they provide. However, the extent of loss is uncertain, especially for diverse insects, because of incomplete sampling. Modeling techniques for estimating upper bounds on diversity are needed to assess benthic freshwater insect abundances, taxa richness, and diversity because some species are declining at alarming rates while others are increasing. In central Appalachian streams of the U.S.A., aquatic insect communities have lost diversity following salinization caused by mining activities. However, the number of taxa observed are dependent upon sampling effort. Incomplete sampling could misrepresent biodiversity and functional losses. Our goal was to use taxon sampling curves to estimate sampling effort required to maximize the probability of accurate benthic macroinvertebrate characterization in dominant riffle habitats of headwater streams. We collected 5 to 10 quantitative benthic macroinvertebrate samples in each of six, first-order streams in the central Appalachian region. For our single-habitat, mesoscale approach, we predicted: (1) macroinvertebrate taxa richness would be the most robust indicator of salinization response compared to diversity, evenness, and density, (2) less sampling effort would be needed to capture taxonomic richness in salinized streams compared to reference streams, and (3) response diversity would also be lower in salinized streams because select trait states would be represented by fewer taxa. Results suggested nominally lower taxa richness, evenness, and diversity in some salinized streams but not in all despite greater or more variable within-stream densities. Nonetheless, sampling effort required to characterize macroinvertebrate communities did not differ between reference and salinized streams, though uneven within-stream taxa distributions corresponded with greater sampling effort requirements for complete characterization. Benthic macroinvertebrate community characteristics were different where higher densities and richness of small-bodied and fast-developing taxa were more common in salinized streams. Response diversity depended on trait states. For example, only about five shredder taxa were represented in salinized streams compared to eight taxa in reference streams. Despite some indication of lower response diversity across some functional feeding groups, more than five samples were needed for robust comparisons. Taxon and trait-based sampling curves suggest that greater overall sampling effort is needed and equal samples per stream was critical for complete diversity assessments regardless of the level of mining-induced salinization. Model-based sampling curves can serve as a tool to assess upper bounds on diversity metrics and sample-effort rigor.