Drift of stream insects is one of the most ubiquitous forms of downstream dispersal and thought to be a key factor influencing the persistence of local populations. Identifying the factors that limit drift dispersal between habitat patches is needed to understand the connectivity of insect populations along river channels. We determined whether insects drifting between riffle habitats (i.e. patches of suitable habitat) were impeded by natural, slow‐moving pools (i.e. unsuitable habitat), limiting dispersal to the next downstream riffle, by estimating drift rates entering and exiting pools. We also investigated whether the frequency of drift dispersal between riffle habitats decreased with increasing pool size (length, width and depth), resulting from increasing areas of low or zero current velocity and/or distance between habitat patches. We found that for the majority of study taxa (7 of the 8 taxa), drift dispersal between riffles was significantly hindered by the intervening pool habitat, supporting our prediction that natural, large slow‐moving pools impede the number of invertebrates drifting between riffle habitats. There were three taxa whose drift rates were significantly reduced by increasing pool depth or width—Offadens hickmani, Austrophlebioides spp. and Austrosimulium spp., and weak relationships (p < .10) were also found for Coloburiscoides munionga and Asmicridea edwardsii. Drift was not associated with pool length for any species, suggesting that hydraulic conditions within a pool limit drift dispersal and not the total distance between riffle habitat patches. Overall, our results suggest drift dispersal seems unlikely to be a major mechanism of long distance dispersal, for at least some common insects. These findings indicate that drift may play a more limited role in stream insect dispersal than is commonly considered and that flying adults may therefore have much greater influence on connectivity among populations.

No abstract

Dams and weirs degrade river ecosystems, reducing diversity and altering the assemblage composition of aquatic biota. These structures may damage rivers by disrupting longitudinal connectivity, fragmenting rivers, and isolating populations. We tested whether a weir could impede the downstream dispersal of stream insects by comparing drift rates through natural pools and through a weir pool. For three of four of our study taxa (Offadens spp., Austrosimulium spp., and Simsonia spp.), we found the numbers of drifters were consistently reduced by the weir across multiple occasions (reduction ranging from 68 to 98%) to a higher degree than natural pools (reduction ranging from 24 to 41%). Drift of Cheumatopsyche spp. through the weir was greatly reduced in December (−95%) compared with natural pools, but the weir had little effect during April (−9%). There were size‐related patterns in drift through the weir pool for some taxa. In the weir pool, emigrating individuals of Austrosimulium spp. were significantly smaller than immigrating individuals, which was not observed in natural pools. In contrast, significant reductions of Cheumatopsyche spp. drifters through the weir only occurred when individuals were small (December). Within the weir pool, the combined effects of lower average water velocity, multiple large low velocity areas, and the weir wall were likely to be the cause of the reduction in stream insect drift through the weir. The impediment of drift of some taxa by weirs may affect dispersal and colonisation processes. This obstruction of downstream movement could undermine the outcomes of river restoration projects.

No abstract

Human-forced climate change significantly threatens the world’s freshwater ecosystems, through projected changes to rainfall, temperature and sea level. We examined the threats and adaptation opportunities to climate change in a diverse selection of rivers and wetlands from Oceania (Australia, New Zealand and Pacific Islands). We found common themes, but also important regional differences. In regulated floodplain rivers in dry regions (i.e. Australia), reduced flooding projected with climate change is a veneer on current losses, but impacts ramp up by 2070. Increasing drought threatens biota as the time between floods extends. Current measures addressing water allocations and dam management can be extended to adapt to climate change, with water buy-back and environmental flows critical. Freshwater wetlands along coastal Oceania are threatened by elevated salinity as sea level rises, potentially mitigated by levee banks. In mountainous regions of New Zealand, the biodiversity of largely pristine glacial and snow melt rivers is threatened by temperature increases, particularly endemic species. Australian snow melt rivers face similar problems, compounding impacts of hydro-electric schemes. Translocation of species and control of invasive species are the main adaptations. Changes to flow regime and rising water temperatures and sea levels are the main threats of climate change on freshwater ecosystems. Besides lowering emissions, reducing impacts of water consumption and protecting or restoring connectivity and refugia are key adaptations for conservation of freshwater ecosystems. Despite these clear imperatives, policy and management has been slow to respond, even in developed regions with significant resources to tackle such complex issues.