Functional Diversity of Freshwater Crustaceans Along Natural River Gradient Transitions

Crustaceans connect algae, detritus, and fishes in rivers, but their functional diversity is rarely investigated in relation to the natural gradient transitions that guide a river from its headwaters to its floodplains. This paper will consider how the functional diversity of crustaceans may be affected by natural gradient transitions in freshwater ecosystems and human impacts that may override them. Given the paucity of research on crustaceans, we have developed an evidence map based on the findings of twelve recent peer-reviewed papers on fishes, benthic macroinvertebrates, plankton and reef molluscs, and translating their results into a framework for crustaceans. Each source was coded by its main cause, which includes natural gradients, land use impacts, or river regulation/regime changes, as well as organism type. The literature corpus was provided rather than extracted from a formal systematic database search. Five sources were based on land use impacts, four sources on natural gradients, and three sources on river regulation/regime changes. Fishes and benthic macroinvertebrates were predominant in the corpus, representing nine out of twelve sources, while one source was about crustaceans. Nine of the 12 sources cited were written between 2020 and 2022. In light of this trend, a proposed path is one that involves four stages where trait filtering by gradient transition occurs, functional diversity reacts to this transition, and land use and regulation modify both. This model produces four hypotheses, including one that states functional diversity will be most sensitive to gradient transition at zone boundaries of the rivers and that regulation will have more influence on functional diversity than seasonal flow. Crustaceans need to be included in functional biomonitoring, and we provide a checklist and a research plan.