Cross-species coprophagy in small stream detritivores counteracts low-quality litter: native versus invasive plant litter

Understanding how stream detritivores can cope with riparian plant invasions is relevant to predict future impacts on detritivore assemblages and the functioning of small stream ecosystems, where litter decomposition mainly fuels food webs. In a microcosm feeding trial, we examined survival, consumption, growth, and energetic status of two detritivores coexisting in nature: the amphipod Echinogammarus obtusidens, and the snail Melanopsis praemorsa. Detritivores were fed two types of leaf litter—the native tree Populus alba, and the invasive reed Arundo donax—and the corresponding con- and heterospecifics faeces produced from the ingestion of the litter from these species. We aimed to assess whether lowland stream detritivores can use coprophagy as a suitable feeding strategy alternatively to Arundo litter; and how coprophagy affects their fitness compared to litter from native and non-native plants. Echinogammarus presented the highest performance when fed Melanopsis’ faeces but the lowest when fed leaf litter, especially the invasive one, exhibiting a high mortality rate. Melanopsis exhibited good and similar performance when fed on both leaf litter species, as well as on conspecific faeces. Echinogammarus performed best as cross-species coprophagous, enabling the amphipods to thrive on the low-quality litter species offered when Melanopsis was present. These results suggest that Echinogammarus obtusidens may act as a coprophagous when no high-quality litter is available. Thus, the snail seems to play a key role facilitating the access to nutrients of recalcitrant leaf litter to sympatric detritivore species via coprophagy.