Multi‐faceted impacts of native and invasive alien decapod species on freshwater biodiversity and ecosystem functioning

Abstract

Changes to species composition, such as biological invasions and extinctions, have the potential to alter ecosystems. Invaders often replace taxonomically similar species, resulting in potentially redundant impacts. For example, freshwater decapod crustaceans are pervasive invasive alien species but they often extirpate native decapods. This study addresses whether or not these compositional shifts lead to impacts on the structure of the macroinvertebrate community, key ecosystem functions such as decomposition rates and primary productivity, and freshwater properties such as turbidity. In a controlled outdoor mesocosm experiment that ran for 33 days, impacts on biodiversity, ecosystem functioning and properties were compared between a native, endangered crayfish (Austropotamobius pallipes) and two invasive alien decapods: the crayfish Pacifastacus leniusculus and crab Eriocheir sinensis. Equal densities of these decapod species were compared between mesocosms, with a replicated array of decapod free controls. Measurements included macroinvertebrate densities, decomposition of leaf litter, production of biofilms, plankton, macrophytes, gross primary productivity, turbidity, and dissolved nutrients. While taxonomic richness of non‐decapod macroinvertebrates was marginally higher in the invasive alien treatments, differences in Shannon diversity were negligible, and β‐diversity was higher for the invasive alien crab. Gastropod density was reduced in the benthos of invasive alien treatments. This was associated with increased primary productivity of periphyton, particularly in the presence of P. leniusculus. Increased turbidity was, however, inversely correlated with periphyton primary productivity in the E. sinensis treatment. Nitrate concentration was significantly lower in invasive compared to native crayfish mesocosms, but similar to decapod free controls. This reflects the potential for the invasive crayfish to act as a nitrogen sink, mediated through both enhanced periphyton and reduced nitrogen recycling. Other processes, such as decomposition rates, sediment respiration, community respiration, and gross primary productivity, did not differ between treatments. This study demonstrates impacts of both native and invasive alien decapod species on certain aspects of benthic biodiversity and ecosystem processes, but with many of these parameters unaffected. This assumes equal densities of each species in its environment. The enhanced gastropod predation and associated trophic cascade by invasive decapods are likely explained through higher consumption rates, metabolism and activity. These per‐capita impacts are likely to be exacerbated further in‐situ due to typically higher densities of invasive compared to native crayfish.

Non‐native species drive biotic homogenization, but it depends on the realm, beta diversity facet and study design: a meta‐analytic systematic review

“…Non‐native species may also increase instead of decrease beta diversity in a process known as biotic differentiation (Olden and Rooney 2006, Doherty‐Bone et al 2019, Otto et al 2020). This can occur, for example, when different non‐native species establish or when shared species are extinct in a pair of communities as a consequence of the invasion (Rahel 2002).…”

Ecosystem effects of invasive crayfish increase with crayfish density

“…In our study, the abundance of grazers and algal standing stock were negatively affected in enclosures with signal crayfish, indicating a broad spectrum of impacts over multiple components of the ecosystem. Lower algal standing stock in treatment enclosures could also partly be explained by bioturbation due to crayfish activities such as burrowing, inundating biofilms with sediment (Harvey et al., 2014), thereby limiting algal standing stock (Doherty‐Bone et al., 2019). Crayfish in streams with coarse sediment rarely burrow as they use cobbles and boulders for refuges (Galib et al., 2021), and burrowing was not observed in our study.…”

Spatial variation in diatom abundance and composition in Biwase Bay and Hamanaka Bay (Eastern Hokkaido, Japan), with reference to environmental features

“…This affects not only higher trophic levels in the pelagic layers ( Sigman & Hain, 2012 ), but also benthic consumers including important fisheries targets ( e.g . bivalves, decapod crustaceans) ( Caswell & Coe, 2013 ; Doherty-Bone et al, 2019 ). Coastal ecosystems are affected by oceanic dynamics, terrestrial input and geographical (topographical) structures, which make spatial variation in communities and production greater and more complex than offshore pelagic ecosystems ( Burke et al, 2000 ).…”