Size-dependent impacts of invasive alien crayfish on a littoral marsh community

The invasive signal crayfish (Pacifastacus leniusculus) may undergo ontogenetic diet shifts with increasing size, but the consequences of crayfish size–structure changes for aquatic food webs are little known. We evaluated size-dependent impacts of signal crayfish on a littoral marsh food web using in-situ enclosures in a northern Japanese marsh. During the 60-day experiment, large crayfish (<30 mm orbital carapace length) rapidly eliminated submerged macrophytes through mechanical destruction while the comparable impacts of small crayfish (>30 mm) appeared after more time had passed. Benthic algal biomass was reduced in the presence of large crayfish but was affected little by small crayfish, suggesting that large crayfish were significant bioturbators. On the other hand, total invertebrate biomass was reduced while total invertebrate density was enhanced in the presence of both crayfish size classes. Potential crayfish impacts on invertebrates are: (1) predation on large vulnerable taxa such as Trichoptera and predatory invertebrates, (2) mechanical destruction of macrophytes, thereby reducing microhabitats for invertebrates, (3) heightened emigration of invertebrates in response to bioturbation and/or (4) predation or competition release on small invertebrates as a result of crayfish reducing their predators/competitors. Invertebrate taxa richness was nearly halved in the presence of large crayfish, probably reflecting reductions of rare taxa. Overall, functional roles of signal crayfish as omnivores and ecosystem engineers remained similar during the ontogenetic stage but the magnitude and rate of crayfish impacts intensified with increasing crayfish size. Thus, removal of large invasive crayfish alone may reduce detrimental impacts on aquatic biodiversity.