Alpha and beta diversity and species co‐occurrence patterns in headwaters supporting rare intermittent‐stream specialists

Abstract

Intermittent rivers are the focus of a burgeoning subdiscipline of freshwater science, driven by mounting recognition of their ubiquity, biodiversity, and climate sensitivity. However, our understanding of their ecology is hampered by a reliance on unstandardised diversity measures, an underrepresentation of certain stream types in metacommunity research and a reticence to acknowledge biotic interactions as a potentially fundamental driver of catchment‐scale patterns in assemblage composition. This study analysed both local‐ and catchment‐scale responses of invertebrates to flow intermittence and hydrological isolation in an understudied ecosystem type (low‐gradient, groundwater‐fed headwaters). We compared responses of conventional (taxonomic richness, Shannon diversity) and more robust (coverage‐adjusted Hill–Shannon) α‐diversity metrics to drying, and analysed changes in β‐diversity components as a function of hydrological and spatial distances between samples. We also inferred the influence of biotic interactions on metacommunity structure, using hierarchical species distribution modelling to capture species‐to‐species associations after accounting for hydrological variability and dispersal traits. The conventional and robust α‐diversity metrics exhibited broadly similar shifts, although richness overestimated the decline in α‐diversity with intermittence and the Shannon index failed to detect a significant interaction between flow duration and hydrological isolation. Decomposition of β‐diversity revealed a dominance of species turnover over nestedness along the flow gradient, a finding contrary to prevailing conceptual wisdom in intermittent river ecology and one we attribute to the climatic and geological context of our study. Even after accounting for differences in hydrological niche occupation, we discovered a significant propensity for species to co‐occur with others of similar conservation status, implying that competition/colonisation trade‐offs involving rare specialists and widespread generalists could underpin metacommunity assembly in some drying headwater networks. Our results suggest that enhancing aquatic habitat connectivity in headwaters harbouring intermittent‐stream specialists could be counterproductive, and conservation plans should instead look to safeguard isolated habitats serving as biotic refuges for rare species.

Geomorphological habitat type drives variation in temporal species turnover but not temporal nestedness in Amazonian fish assemblages

“…Between‐habitat variation in turnover is known to arise from differences in connectivity and/or greater seasonal disturbance (Tonkin et al 2017, Perkin et al 2021, Aspin and House 2022). However, higher turnover might also be expected in the river‐floodplain systems simply due to their higher species richness.…”

“…For example, some studies of intermittent streams have found that nestedness contributes more to temporal β-diversity in high-order headwaters subject to extreme stochastic drying, whereas turnover plays a more significant role in lower-order streams with more predictable seasonality (Datry et al 2016, Perkin et al 2021. However, other studies have reported contradictory findings (Aspin and House 2022).…”

Spring forth diversity: Specialist species contribute to the conservation value of headwater springs and streams at the landscape scale

“…We quantified alpha diversity using two metrics, taxa richness (the number of taxa per sample, as a widely used and thus easily interpretable metric) and coverage‐adjusted HS diversity. HS diversity was calculated using the ‘iNext’ package (version 3.0.0, Hsieh et al, 2016), in which rarefaction and extrapolation were used to standardize all samples to 95% coverage, accounting for differences in taxon detection between samples (Aspin & House, 2022; Roswell et al, 2021).…”

“…We attribute this lack of response to the considerable compositional variability both within and among communities at sites with perennial and temporary flow. In turn, this variability was likely driven by the infrequent occurrence of many taxa (Aspin & House, 2022); the geographical isolation of headwaters, which promotes the establishment and maintenance of distinct communities (Finn et al, 2011;Sarremejane et al, 2017); and the recorded habitat heterogeneity.…”

“…Biodiversity is routinely quantified using metrics that describe taxonomic diversity and/or species rarity (Humphries et al, 1995). Alpha diversity is typically measured as taxonomic richness, but metrics that are less influenced by rare species occurrence and sample completeness, such as coverage‐adjusted Hill–Shannon (HS) diversity, provide a more robust measure where many species occur at low abundance (as in some headwaters) or when semi‐quantitative sampling methods (such as kick sampling) are used (Aspin & House, 2022; Roswell et al, 2021). Complementing such richness‐based metrics, indices including the macroinvertebrate‐based Community Conservation Index (CCI) (Chadd & Extence, 2004) combine richness and species rarity information to describe a community's conservation value.…”