Abstract
Invasive Alien Species (IAS) are a major cause of global ecological change and biodiversity loss and constitute the greatest threat to fragile ecosystems. The overall economic impacts of biological invasions are difficult to quantify, but the estimated cost for controlling IAS in the UK and Ireland in 2013 was £2 billion. While IAS affect both terrestrial and aquatic habitats, in water they pose a more challenging threat to assess as it is extremely difficult to identify their full impact given IAS can spread inconspicuously, often detrimentally, thereby altering the receiving environment.This thesis aimed to investigate a range of aquatic invasive alien species, what is driving their distribution, how they impact the receiving environment and their interactions with other native and non-native species. Better understanding the factors instrumental in their spread allowed for the testing of management options, including in-situ control and biosecurity for secondary spread, and where best to focus limited resources, bearing in mind the balance of ecosystem services. Through an interdisciplinary approach, this was achieved using a combination of laboratory testing, field surveys and novel mesocosms experiments.
Submerged aquatic IAS macrophytes such as Nuttall’s pondweed (Eluded nuttallii) can grow rapidly and displace native plants. Being highly adaptable and extremely competitive, they can cause severe damage to ecosystem services. Other non-native species, such as the earlier invader and closely related species, Canadian pondweed (Elodea canadensis), are less detrimental. In fact, E. canadensis is now considered naturalised and has been displaced by later invaders, including E. nuttallii. These congeneric macrophytes have proved difficult to differentiate visually as they exhibit a wide range of morphological variation owing to phenotypic plasticity. Therefore, to support morphological identification, their genetic differentiation was assessed using DNA barcoding and species-specific sequencing markers, and specimens of each species were cultivated for future experimentation.
To understand what makes a species a successful invader, the environmental correlates of the E. nuttallii invasion were investigated by comparing mined, long-term water chemistry data following a distribution survey carried out in Lough Erne, County Fermanagh. The distribution and abundance of E. nuttallii was highly correlated with biotic (zebra mussels Dreissena polymorphic) and various abiotic (e.g. pH, suspended solids, phosphorus, ammonium) factors. Whether such associations represent habitat selectivity during dispersal and establishment (affects), or are the consequences and impacts of its invasion (effects) are discussed.
To explore the impact of D. polymorphic on macrophyte growth and water chemistry, the competitive and facilitative interactions among various combinations of E. canadensis and E. nutlallii, and D. polymorphic were experimentally tested. Using a mesocosm-based, factorial experimental design, the effect of interspecific competition on macrophyte growth rates in the absence and presence, at varying densities, of D. polymorpha was assessed. The findings show that dense D. polymorphic colonization strongly facilitates E. nnttallii growth but not E. canadensis growth, promoting the dominance of the former over the latter where the two species co-occur, thereby supporting the invasional meltdown hypothesis.
Aquatic non-native invasive species do not just interact with other IAS, they can interact with native species, potentially further facilitating the spread of the IAS. The production of macrophyte fragments by shredder-herbivores, when such consumers are abundant, could have serious implications for invasive macrophyte spread in terms of propagule pressure. Resource-use patterns of native Limnephilid caddisfly larvae towards E. canadensis and E. nuttallii were quantified using “functional responses” (FRs). FRs were applied in a novel way to quantify shredder-plant interactions, which contributes to understanding and predicting invasion success in the context of propagation. E. lunatics larvae shows a preference for invasive E. nuttallii over naturalised E. canadensis, and de-cased caddis can generate fragments in all of the examined IAS macrophytes.
IAS control can be difficult, labour-intensive and expensive. Reductions in light intensity through shading has been shown to control some aquatic plants. Therefore, the use of biodegradable blue dyes to absorb light and reduce photosynthesis was tested experimentally on a range of aquatic IAS macrophytes. The treated macrophytes showed an initial period of etiolation. However, over time it resulted in reduced biomass and therefore the use of dyes should be investigated further.
Biosecurity protocols designed to prevent invader spread have become integral to invasive species management strategies. However, application of many proposed spread-prevention practices is inhibited due to low practicality, high expense, undesirable non-target effects and a lack of known efficacy. The use of direct steam exposure to induce substantial fragment degradation of seven invasive macrophytes was tested. Steam treatments were observed to be highly efficacious, with total degradation being induced by 10 seconds of direct steam exposure. Therefore, this innovative, yet simple technique can be used to improve biosecurity practices to inhibit the spread of invasive macrophytes.
The interdisciplinary outputs of this thesis are a timely contribution to aquatic IAS identification, the factors influencing their distribution, their control and biosecurity.
| Date of Award | Jul 2019 |
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| Original language | English |
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| Supervisor | Jaimie Thomas Allan Dick (Supervisor), Neil Reid (Supervisor), Louise Kregting (Supervisor), Anthony Ricciardi (Supervisor) & Hugh J. MacIsaac (Supervisor) |