Transformation of detritus by a European native and two invasive alien freshwater decapods

Abstract

Invasive alien species have the potential to alter biodiversity and ecosystem processes. In freshwaters, detritus decomposition is a major ecosystem service but it remains uncertain whether invasive alien decapods process detritus differently to natives. This study examined leaf litter processing, and cascading effects on biofilms, by the European native white clawed crayfish (Austropotamobius pallipes) compared to two invasive alien decapod species: the American signal crayfish (Pacifastacus leniusculus) and the Chinese mitten crab (Eriocheir sinensis). Invasive alien decapods were responsible for higher leaf litter decomposition than the native. In comparison with native crayfish, invasive alien crab and crayfish showed higher rates of litter consumption, increased production of smaller leaf fragments, fine particulate organic matter (FPOM) and dissolved organic carbon. Nutrients (ammonia and soluble reactive phosphorous) derived from excretion (measured separately in the absence of biofilms) varied among decapod species, being lower for P. leniusculus. However, nutrient concentrations did not vary among species in the detritivory experiments with biofilm, implying nutrients were utilised for biofilm production and respiration as no differences in biomass were evident among decapod treatments. These results show invasive alien decapods have the potential to increase the magnitude of detrital processing to FPOM in rivers, but indirect impacts on primary producers due to nutrient release are uncertain based on this experimental context.

Invasive alien shredders clear up invasive alien leaf litter

“…Twelve treatments (10 replicates/treatment, 120 microcosms total) were established containing the conditioned leaf litter from one of the three plant species, and a decapod treatment (native crayfish, invasive crayfish, invasive crab, control without decapod). Subadults of similar sizes were used because they are the most common age class encountered in the field (Doherty‐Bone et al., ). At the start of the experiment, decapods were weighed following being dabbed dry with a paper towel: A. pallipes – 12.87 ± 2.92 g; P. leniusculus – 10.33 ± 2.98 g; E. sinensis – 11.02 ± 4.28 g. The experiment ran for 7 days.…”

“…Decomposition rate was calculated as the change in AFDM of leaf litter per day, final AFDM estimated based on the baseline AFDM estimated from a separate batch of conditioned leaf litter of each species (Benfield, ). As mass of a decapod can influence consumption rates of leaf litter, as well as to enable estimation of impact by mass of animal, rates of decomposition and production of residual materials were divided by decapod mass to show mass‐specific performance (Doherty‐Bone et al., ). This enabled measurement of both the actual impact (decomposition rate, etc.)…”

“…It was hypothesized that (H 1 ) invasive alien decapods would have a higher rate of detrital processing, than native invertebrate shredders as observed by Doherty‐Bone et al. (); (H 2 ) decomposition rates would be lower for the IAS R. ponticum and A. pseudoplatanus due to higher tannin and cellulose content than the dominant native riparian species Alnus glutinosa (Hladyz et al., ). As a consequence, (H 3 ) the assimilation (measured by gain in mass) of invasive alien leaf litter will be higher for invasive alien decapods compared to the native crayfish.…”

“…Invasive alien riparian plants affect freshwater ecosystem processes through reducing light levels and by introducing allochthonous leaf litter that could be novel to the ecosystem either in quantity or in quality (Hladyz, Åbjörnsson, Giller, & Woodward, ; Hladyz, Gessner, Giller, Pozo, & Woodward, ). Invasive alien omnivorous animals (e.g., amphipods, decapods) affect freshwater ecosystems through the direct consumption (shredding) of detritus, as well as trophic cascades following consumption of detritivores and ecosystem engineering (Doherty‐Bone, Dunn, Liddell, & Brown, ; Gallardo et al., ; Harvey et al., ). These two guilds of IAS (riparian plants, shredders) when considered in isolation have the potential to impact detrital processing in freshwater ecosystems.…”

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

“…The following hypotheses were tested: (H 1 ) decapod presence would result in increased consumption of resources, especially by invasive species (Haddaway et al., ; Rosewarne et al., ), particularly slow‐moving species such as gastropods and other invertebrates, leading to (H 2 ) reduced grazing pressure and an increase in primary and gross primary production. It was also expected that (H 3 ) decomposition rates would be maintained or even enhanced in invasive species treatments, despite depletion of shredding invertebrates, due to high omnivory amongst decapods (Doherty‐Bone et al., ; Dunoyer et al., ; Usio, ); and (H 4 ) invasive alien decapods would alter water quality through changes to dissolved nutrients from excretal products (Doherty‐Bone et al., ; Evans‐White & Lamberti, ; Fritschie & Olden, ; Usio, Suzuku, Konishi, & Nakano, ), increased turbidity from bioturbation (Harvey et al., ) and particulate carbon from detritivory (Doherty‐Bone et al., ).…”