Paul J. McInerney

Implications of riparian willow invasion to instream community structure and function: a synthesis using causal criteria analysis

Invasive riparian plants are a significant threat to riverine environments and are thought to alter the structure and function of stream ecosystems. Salix spp. are a genus of highly invasive northern hemisphere trees and shrubs that have invaded substantial areas of southern hemisphere riparian corridors. We set out to review the existing peer reviewed literature surrounding the impacts of Salix spp. infestation to streams by rigorously testing a suite of cause–effect hypotheses using a causal criteria analysis. Our analysis found evidence in the literature that infestation by exotic Salix spp. can cause a decrease in incidental illumination and benthic periphyton density, increased rates of allochthonous litter leaching and decomposition and changes to secondary consumer assemblage and trophic organisation. The review also highlighted a number of aspects of Salix spp. invasion for which there are significant knowledge gaps in the literature. Our results emphasise the importance site specificity, seasonal variation, physical properties of supplanted vegetation, stream size and magnitude of infestation when predicting putative cause–effect relationships between Salix spp. invasion and stream structure and function. We show that, by possessing incongruent biological and physical characteristics to native plants, invasive terrestrial trees have the capacity to influence adjacent aquatic ecosystems.

More (or less?) bounce for the ounce: a comparison of environmental DNA and classical approaches for bioassessment

Next-generation sequencing (NGS) techniques are revolutionising the bioassessment of ecosystems. Herein we use a case study to compare environmental (e)DNA and classical sampling and laboratory identification approaches to assess biotic communities in streams. Both techniques were successful in detecting changes to biotic communities following invasion by a non-native riparian plant. The cost of the eDNA methods was one-sixth that of the classical approach and provided a coarse qualitative assessment of overall eukaryotic structure. Classical macroinvertebrate techniques, although they assess only a subset of eukaryotes, provided high-resolution quantitative information that could be applied to assess functional aspects of the ecosystem. Selection of one method in preference over the other is highly dependent on the nature of the hypothesis to be tested.

Co-invasion hypothesis explains microbial community structure changes in upland streams affected by riparian invader

Nonnative riparian plants can affect stream microbial dynamics by altering the quality, quantity, and timing of allochthonous inputs, but little consideration has been given to the potential influence of co-invading mutualists associated with nonnative riparian invaders. We used a high-resolution ecogenomic approach to examine the effects of riparian invasion by nonnative willows on microbial composition in associated small temperate streams. Willow infestation led to significant differences in fungal and bacterial communities between willow-infested and reference reaches. Fungal taxon richness was lower and phototrophic bacteria were less common in willow-infested reaches. Aquatic hyphomycetes contributed less to community composition of fungal communities in willow-infested reaches. Nonnative ectomycorrhizal fungi Inocybe spp. and Tormentella spp., known willow mutualists in their home ranges, were primary drivers of microbial community differences among infested and reference reaches. Nonnative obligate-mutualistic plant invaders can avoid symbiont limitation in their new territories by invading with co-evolved symbionts. We showed how co-invading, nonnative ectomycorrhizal fungi associated with invasive willows can modify microbial dynamics within streams. Our results provide new insights to the effects of riparian invaders, and our methods can be applied to test co-invasion hypotheses in streams and riverine habitats globally.

Long‐term Monitoring of Macroinvertebrate Communities Over 2,300 km of the Murray River Reveals Ecological Signs of Salinity Mitigation Against a Backdrop of Climate Variability

We investigated the ecological effects of salinity mitigation strategies in the Murray‐Darling Basin (MDB) using macroinvertebrate data collected over 2,300 km of the Murray River between 1980 and 2012. The MDB covers 1 × 106km2 and includes both temperate and semiarid climate zones. It was extensively developed to support irrigated agriculture in the early to mid‐1900s, and the secondary salinization that followed has become a major concern. During 1975–1985 daily salinity levels, measured as electrical conductivity above the Murray River off‐take points for South Australias major urban water supplies, were above 800 μS/cm for 40% of the time, necessitating mitigation strategies that have reduced the average salinity by about 150 μS/cm since monitoring began. The MDB has also experienced several major floods and droughts during this time, and surface temperatures in the MDB have increased by 0.8 °C since 1910, mostly in the last 50 years. We hypothesized that (1) taxa richness would increase in response to floods; (2) community structure would shift toward tolerant, opportunistic taxa in response to warming; and (3) geographical ranges of species would change in response to shifting stream isotherms and reducing salinity. Our hypotheses were supported, although increases in water temperature appeared to be due principally to the 1997–2009 Millennium drought. Importantly, against a backdrop of significant climate variability, we believe we have distinguished a change in community structure along a salinity gradient and that changes over the 33 years can in part be attributed to mitigation strategies.

Flooding drives a macroinvertebrate biomass boom in ephemeral floodplain wetlands

Despite broad recognition of the importance of the Flood Pulse Concept to large river ecology, few studies have provided empirical evidence to support a number of its arguments. We compared and contrasted the responses of water quality, nutrients, and the macroinvertebrates of ephemeral and permanent wetlands to a 22-wk managed inundation. We hypothesized that inundation of ephemeral wetlands would mobilize higher concentrations of nutrients from floodplain sediments and C from organic matter, which would, in turn, fuel greater abundance and biomass of aquatic macroinvertebrates than in permanent wetlands. The response of ephemeral wetlands to flooding contrasted very strongly with that of permanent wetlands. Freshly inundated ephemeral wetlands had significantly higher dissolved nutrient and organic C concentrations than permanent wetlands and supported different aquatic macroinvertebrate communities. Macroinvertebrate diversity was higher in permanent wetlands, but ephemeral wetlands supported much greater macroinvertebrate abundance and biomass. Differences between macroinvertebrate communities in ephemeral and reference wetlands were driven primarily by Chironomidae. Chironomus tepperi was the dominant taxon among ephemeral wetlands, both numerically and by biomass. Early-colonizing Coleoptera contributed strongly to taxonomic richness among ephemeral wetlands. Analysis of functional feeding groups (FFGs) indicated that ephemeral wetlands contained a higher proportion of detritus-feeding collector–gatherers compared to permanent wetlands, suggesting an increased reliance on heterotrophic energy pathways. We showed that a managed flood pulse can mobilize latent terrestrial energy sources within ephemeral floodplain wetlands and support a boom of aquatic invertebrate biomass, with important implications for both terrestrial and aquatic food webs.