Alien plant invasions: helping British rivers to fight back

First paragraph: From lochs and lakes to rivers, ponds and canals, there is a diverse range of freshwater habitats in the UK, which is good news not just for biodiversity but also the economy, where they are collectively valued at £39.5 billion. Rivers in particular are highly biologically diverse environments, home to a wide variety of plants, invertebrates and fish. But linked together within a river catchment, they are prone to invasion by alien species that can spread quickly between these interconnected habitats. Invasive alien plant species are of one of the biggest concerns to river environments. These contribute to the loss of native plants and invertebrates, as well as altering soil chemistry and impeding river flow. It costs the UK government around £1.7 billion to control invasive alien species and an estimated £6m alone to control the well-known troublesome Japanese knotweed (Fallopia japonica).https://theconversation.com/alien-plant-invasions-helping-british-rivers-to-fight-back-10381

What is the 'hairy panic' tumbleweed that has buried a small Australian city?

First paragraph: We’re used to stories of towns and cities waking up to floodwaters invading their homes. In complete contrast, the Australian city of Wangaratta, 230km north of Melbourne, is dealing with a hairy weed invasion that looks like a scene from the horror edit of an American Western film classic

Effects of invasive alien plants on riparian vegetation and their response to environmental factors

Biological invasions are reportedly one of the major contributory factors to biodiversity loss worldwide. The impacts of invasive alien plant (IAP) species on native communities are widely documented in the scientific literature, however, there is still a lack of detailed information on their impacts within the most vulnerable habitats. Riparian habitats are highly dynamic systems and naturally disturbed, making them particularly vulnerable to invasion. Climate change, directly or indirectly, is also predicted to adversely impact river systems, which may subsequently alter invasion rates and the impacts of IAPs. However, the interactions between climate and IAPs and their combined effects on vegetation have rarely been examined. To address these knowledge gaps, this thesis investigates: (1) the role of environmental variables, such as sediment loading or climate-related changes to river flow regime, on the abundance of IAPs within riparian zones; (2) how variation in IAP abundance impacts native vegetation, relative to the effects of native dominant plant species and (3) some of the mechanisms underlying the effects of IAPs in riparian habitats. Historic and recent field survey data were used to investigate changes in riparian vegetation on British rivers during the last 20 years. Analyses indicate that IAPs had a negative but small effect on native plant diversity. Overall, changes in land use and differences in flow regime between recording periods were the most important predictors of plant community change. Specifically, IAPs had a greater probability of being present along lowland rivers that experienced increased frequency of high flow events. On a local scale across rivers in Scotland, the abundance of IAPs was constrained by greater soil moisture in summer, whilst greater abundance was associated with tree-lined banks. Both native dominant species and IAPs negatively affected subordinate species abundance to a greater extent than species richness, although this effect varied spatially with bank elevation. Artificial turf mats were used to quantify viable propagules within riverine sediment deposited over-winter along invaded riverbanks. The data indicate that there is a legacy effect of IAP abundance, with the most invaded sites being associated with higher sediment loading the following year, though, contrary to the general pattern, 12 sediment associated propagules were scarcer at invaded sites. Moreover, lower above-ground native diversity was associated with sites which had been previously invaded. Plant species composition in the propagule bank and above-ground vegetation were highly dissimilar, particularly closest to the water’s edge at highly invaded sites. This suggests that mono-specific stands of IAPs proliferate best under less disturbed environmental conditions, although fluvial disturbance events may be required to create opportunities for initial establishment. The propagule bank contributed very little to the above-ground vegetation, nor did it limit invasion, suggesting that above-ground plant composition is largely dictated by competitive interactions. The findings presented in this thesis suggest that invasion by IAPs is an additional stressor for native vegetation within riparian habitats, modifying above-ground plant communities via competition and suppressing recruitment from the propagule bank. However, native dominant species common in riparian habitats also negatively impact, subordinate species via competition, in some cases equalling the effect of IAPs. Native dominant and IAP species are differently affected by environmental factors operating in the riparian zone, which may provide future opportunities for reducing and managing invasions

Invasion legacy effects versus sediment deposition as drivers of riparian vegetation

Riparian zones are formed by interactions between fluvio-geomorphological processes, such as sediment deposition, and biota, such as vegetation. Establishment of invasive alien plant (IAP) species along rivers may influence vegetation dynamics, evidenced as higher seasonal or inter-annual fluctuations in native plant diversity when IAP cover is high. This could impact the overall functioning of riparian ecosystems. Conversely, fine sediment deposited in riparian zones after floods may replenish propagule banks, thus supporting recruitment of native species. The interactive effects of invasion and fine sediment deposition have hitherto, however, been ignored. Vegetation surveys across rivers varying in flow regime were carried out over 2 years to assess changes in community composition and diversity. Artificial turf mats were used to quantify over-winter sediment deposition. The viable propagule bank in soil and freshly deposited sediment was then quantified by germination trials. Structural Equation Models were used to assess causal pathways between environmental variables, IAPs and native vegetation. Greater variation in flow increased the cover of IAPs along riverbanks. An increased in high flow events and sediment deposition were positively associated with the diversity of propagules deposited. However, greater diversity of propagules did not result in a more diverse plant community at invaded sites, as greater cover of IAPs in summer reduced native plant diversity. Seasonal turnover in the above-ground vegetation was also accentuated at previously invaded sites, suggesting that a legacy of increased competition in previous years, not recent sediment deposition, drives above-ground vegetation structure at invaded sites. The interaction between fluvial disturbance via sediment deposition and invasion pressure is of growing importance in the management of riparian habitats. Our results suggest that invasion can uncouple the processes that contribute to resilience in dynamic habitats making already invaded habitats vulnerable to further invasions

Riverbanks as battlegrounds: why does the abundance of native and invasive plants vary?

The abundance of invasive alien plants (IAPs) can vary dramatically over small spatial scales for reasons that are often unclear. Understanding these could offer key insights for containing invasions, accepting that eradication is often no longer feasible. This study investigated determinants of IAP cover on riverbanks, a well-known hotspot of invasion, using Impatiens glandulifera, a prolific invader across the Northern hemisphere, as a model species. Within this framework we included the potential for dominant native vegetation cover, mediated by favourable environmental conditions, to resist invasion by I. glandulifera through negative association. Our analyses, using structural equation modelling, showed that I. glandulifera is more sensitive to environmental conditions, than dominant native vegetation. High soil moisture was a key determinant of I. glandulifera cover, having negative effects across the riparian zone. Spatially, I. glandulifera and dominant native vegetation responded differently to environmental conditions. Sites with steeper banks had less dominant native vegetation at the water's edge, potentially favouring I. glandulifera cover through reduced competition. In general, greater abundance of dominant native vegetation presented a more invasion-resistan

Twenty years of change in riverside vegetation: What role have invasive alien plants played?

Question  Which environmental factors influence the occurrence of invasive alien plants (IAPs) in riparian habitats, and how much can IAPs account for change in native vegetation compared with other environmental variables?  Location  Rivers distributed throughout mainland Britain.  Methods  We quantified change in river bank vegetation using survey data collected approximately 20yr apart and assessed the contribution of major IAPs (Impatiens glandulifera, Heracleum mantegazzianum and Fallopia japonica) to these changes. We also determined the importance of abiotic factors such as flow regime and land use in driving these changes.  Results  Comparing data from pre- and post-1990 surveys revealed that IAPs occurred mainly on lowland rivers (<200m a.s.l.), regardless of time period, and their probability of occurrence increased over time and with rising frequency of high flows. Native plant species diversity declined over time with increasing IAP cover, along lowland rivers and along all rivers that experienced extended low flows during the growing season. These conditions particularly favoured native dominant species, whereas native subordinate species responded both positively and negatively to increased flood frequency depending on survey period. Over time, Salix spp. and larger native hydrophilic species, such as Sparganium erectum, increased along lowland rivers, replacing smaller-statured ruderal species and driving a shift towards increased shade tolerance of subcanopy and groundcover species. Smaller compositional changes occurred in the uplands and these changes lacked a clear environmental signature.  Conclusions  National-scale changes in native riparian vegetation are likely driven primarily by environmental changes and land-use effects, rather than invasion by IAPs. However, IAPs, and indeed native species that benefit from abiotic changes, in turn, likely exert secondary effects on native riparian vegetation. The trend towards reduced diversity, increased shade tolerance and increased dominance of some native species and IAPs is likely linked to a set of interacting factors, including drier summers, wetter winters, increased riparian tree cover, reduced livestock access to river banks and increased fine sediment input. Determining combined effects of land use, IAPs and climate-related changes in flow regime over decadal time scales (i.e. ~20yr) is important for predicting ecological responses of vulnerable habitats under future disturbance scenarios

All change at the water's edge: invasion by non-native riparian plants negatively impacts terrestrial invertebrates

Riparian zones are complex, dynamic habitats that play a critical role in river ecosystem functioning. Terrestrial invertebrates comprise much of the diversity found in riparian habitats and facilitate the transfer of energy between aquatic and terrestrial systems. However, the consequences for terrestrial invertebrates of invasion of riparian zones by invasive non-native plants (INNP) remain poorly understood. Responses of terrestrial macroinvertebrate morphospecies to invasion by two common INNP, Fallopia japonica (Japanese knotweed) and Impatiens glandulifera (Himalayan balsam) were assessed, relative to local environmental factors. Terrestrial invertebrates were collected from 20 sites on low order streams in June and August alongside data on physical attributes and land use. Greater cover of F. japonica and I. glandulifera cover reduced total invertebrate abundance and morphospecies diversity at the individual sample scale, whilst increasing spatial heterogeneity of invertebrates at the site scale. Impatiens glandulifera reduced morphospecies diversity at the site scale with increasing cover, but this was not observed for F. japonica. INNP affected terrestrial invertebrate morphospecies abundance and diversity, to a greater extent than prevailing environmental conditions. Our findings therefore offer support for managing riparian plant invasions to improve habitat heterogeneity, restore terrestrial invertebrate diversity and repair aquatic-terrestrial linkages

Raw water transfers: why a global freshwater invasion pathway has been overlooked

Zhu et al. (Science 380:1230, 2023, https://doi.org/10.1126/science.adi6022) recently reported in Science that ‘water diversions’ carry a range of freshwater invasive non-native species (INNS) in China, and make calls for the Chinese government to take actions to monitor this invasion pathway and prevent future spread. This issue is not confined to China however. Water diversions, more commonly known internationally as ‘water transfers’ or ‘raw water transfers’, exist globally in large numbers. Despite worldwide occurrence, this major invasion pathway has received little attention within the field of freshwater ecology or invasion ecology, beyond a limited number of regional studies which offer little context regarding the global nature of the pathway. We discuss the factors contributing to the widespread lack of awareness of this uniquely complex invasion pathway, and emphasise the importance of future collaboration and knowledge sharing in this emerging field. Transdisciplinary research is needed to develop effective management techniques for this invasion pathway and ensure that the threat to global freshwater biodiversity posed by INNS is dealt with comprehensively

Stream invertebrate diversity reduces with invasion of river banks by non-native plants

1. Invasion of riparian zones by non-native plants is a global issue and commonly perceived as a challenge for river and fishery managers, but the type and extent of ecological changes induced by such invasions remain poorly understood. Established effects on sediment delivery, allochthonous inputs and channel shading could potentially alter aquatic macroinvertebrate assemblages, with implications for in-stream ecological quality. 2. We assessed responses in the diversity, quality and heterogeneity of stream macroinvertebrate communities to riparian invasion by non-native plants. Macroinvertebrates were collected from 24 sites on low order streams in central and southern Scotland during spring and autumn. The effect of invasive non-native plants (INNP) on macroinvertebrates was assessed relative to that of local physical and chemical factors. 3. INNP cover was associated with stronger effects than other factors on local diversity of macroinvertebrates (33% reduction at the highest INNP cover) but also increased macroinvertebrate abundance across sites. Invaded sites were also associated with lower macroinvertebrate biomonitoring scores. Community composition differed between invaded and uninvaded sites in autumn, but not in spring. However, INNP influence on macroinvertebrate composition was generally secondary to that of physicochemical variables (e.g. channel shade, substrate diversity). 4. We demonstrate that the influence of INNP extends beyond well-known impacts on plant communities to reductions mainly in stream macroinvertebrate diversity. Combined with the negative impact on pollution-sensitive macroinvertebrate taxa this raises concerns over the ecological health of streams with heavily invaded riparian zones. Our findings suggest that efforts to improve low order streams by actively managing severe riparian invasions are merited, but the size and uncertainty of the likely ecological gains must also be evaluated against the effort involved

Drivers of moth phenology in England and Wales

Climate change has led to changes in the phenology of Lepidoptera species. While phenological shifts have been previously measured for moth species in England and Wales, the drivers of these shifts are not well known. Here, we use data from the National Moth Recording Scheme and the Garden Moth Scheme to investigate the drivers of phenology in 149 moth species over a 50 year period from 1970 to 2019. We investigate whether there have been phenological shifts in adult emergence using three phenology metrics: First Emergence (FE), Peak Emergence (PE), and Emergence Standard Deviation (ESD) in relation to life history traits and temperature. Overwintering stage had a significant impact on moth phenology, so we analysed species that spend the winter as eggs, larvae or pupae separately. Overall phenological changes were different depending on overwintering stage category and phenological measure, with the rate of phenological shifts increasing with later overwintering life stages in response to both temperature and year. The overwintering stage larva was the only one impacted by diet, with those that feed on woody hostplants emerging ~ 17 days later than species with herbaceous hostplants. These results indicate that species that either overwinter in earlier life stages or have woody hostplants may be less adaptable to climate change, and thus should be the targets of conservation efforts