Invasive alien species threaten freshwater biodiversity, with uncertain impacts on ecosystem services. Invasive alien decapods are frequent components of invaded ecosystems, and have been found to impact biodiversity, ecosystem properties and processes. Studies however often do not consider comparisons to native analogues. Native analogues could mask impacts of invasive alien species through functional redundancy. The native White Clawed Crayfish (Austropotamobius pallipes) is declining at the advance of the invasive alien American Signal Crayfish (Pacifastacus leniusculus) and the Chinese Mitten Crab (Eriocheir sinensis). Consequences of this changeover of decapod species to the patterns and processes of river basins are not well understood.
Laboratory microcosm experiments found invasive alien decapods processed native leaf litter at a greater rate than native crayfish, increasing smaller fragments, notably fine particulate organic matter and dissolved organic carbon (Chapter 2). However, these differences in transformation of leaf litter into other products did not increase the productivity of algal biofilms. Each decapod species excreted nutrients at different rates: A. pallipes and E. sinensis had similar nitrogen excretion, while P. leniusculus excreted much less nitrogen suggesting this species could be a nutrient sink. These differences in nutrients were apparently absorbed by biofilms growing on leaf and rock surfaces even though they did not grow as a result.
Chapter 3 found the greater detritivory performance of invasive alien decapods was observed for other species of leaf litter, including increased breakdown of particularly troublesome invasive alien riparian plant Rhododendron ponticum. Derived products varied, with more fine particulate organic matter generated for Acer pseudoplatanus and R. ponticum, suggesting less assimilation of these litter types. This revealed a previously unknown feedback between invasive plants and detritivores, whereby the litter accumulations from riparian invasive plants is potentially consumed by invasive alien decapods. This in turn may subsidise the growth and reproduction of invasive alien aquatic decapods.
Outdoor pond mesocosm experiments revealed that invasive alien decapods altered taxonomic richness, diversity, and structure, with snails particularly found to decline in ponds (Chapter 4). Primary productivity of periphyton was elevated in P. leniusculus ponds compared to natives, but not in the E. sinensis ponds, explained by a combination of trophic cascade and turbidity. Leaf litter decomposition rates, community respiration, gross and net primary productivity were otherwise unaffected. Outdoor flume mesocosms were utilised to replicate headwater streams to measure bioturbation of decapods, and associated changes to biodiversity, water quality and ecosystem processes (Chapter 5). Effects on sediment topology and transport were similar for controls, native and alien species, with the presence of refuges likely causing this lack of effect. Suspended organic matter was similar for all species, but sediment respiration was higher for P. leniusculus. Invertebrate community structure was significantly different for invasive alien decapods, with significantly higher densities of collector-gathering invertebrates such as Baetis sp. and Culicoides.
The the lack of consensus between the laboratory (Chapters 2-3) and mesocosm studies (Chapters 4-5) lead to the conclusion that environmental practitioners should consider context and scaling when interpreting experimental results of biological invasions. The overall findings of this study however do show that P. leniusculus and E. sinensis are not functionally redundant for several ecosystem attributes compared to A. pallipes. These differences in species functional-traits are likely to be exacerbated by greater densities of invasives observed in situ. This supports the potential for invasive alien decapods to alter the ecosystem services of entire river basins, even when a native analogue was previously present.
