The influence of exposed riverine sediment (ERS) physical habitat dynamics and river flow in determining the distributions of specialist ERS invertebrates

Abstract

Exposed Riverine Sediments (ERS) are highly dynamic habitats that support large numbers of rare and scarce invertebrates. ERS are characterised by a high diversity of microhabitats and microclimates. The thesis aimed to assess ERS habitats at a micro, patch and reach scale addressing three broad aims (1) to characterise the nature and dynamics of ERS microhabitat through investigating the thermal characteristics of ERS with respect to sediment calibre, depth and distance from the water‟s edge, and so identify the key factors that control variations in surface and subsurface physical habitat spatially and temporally. (2) Establish the physical drivers of invertebrate microspatial distribution (a) investigate the interaction of microhabitat (moisture and temperature) and food availability in determining invertebrate habitat selection, achieved through experimental habitat manipulation and the use of marked beetles. (b) Determining the microspatial distribution of specialist ERS beetles and spiders, and whether different species were significantly spatially aggregated/ separated from each other. Significant positive or negative association with physical ERS habitat characteristics. Identifying if species show a (3) Examine the spatial dynamics of beetle assemblages in relation to patch environmental variables and, assess population stability, persistence, occupancy and synchrony over time in relation to river flow. ERS microhabitats were found to be subject to strong lateral gradients in sediment, temperature and humidity that are amplified in hot dry weather. Specialist ERS beetles and spiders were found to occupy distinct habitats and showed clear spatial zonation in regard to sediment calibre, vegetation and elevation. Distribution of some species was highly variable and responsive to elevated levels humidity and temperature. Microclimate was found to have a greater influence over invertebrate distributions than food availability. The analysis of the long term data set provided evidence that larger and diverse ERS within sample reaches is associated with larger numbers of beetles of conservation value. Cattle trampling and stabilisation through vegetation were identified as having significant deleterious effects on ERS habitat quality. River flow variability, specifically the timing of high flow events was found to potentially have a negative influence on beetle abundance, population stability and persistence. Population synchrony revealed that the UKBAP beetle Perileptus areolatus exhibits little mobility and is highly resilient to inundation. However this makes it more susceptible to reach scale disturbance and a reduction in either habitat availability or quality