Gimme Shelter: differential utilisation and propagule creation of invasive macrophytes by native caddisfly larvae

In aquatic systems, invasive submerged macrophytes considerably alter the structure and functioning of communities, thus potentially compromising ecosystem services. The prolific spread of invasive macrophytes is often aided by vegetative fragment propagation, yet the contributions of various commonly occurring invertebrates to such fragmentation are often unquantified. In the present study, we examine fragmentary spread of invasive macrophytes by a group of shredder-herbivores, larval caddisflies. Through novel application of the comparative functional response (FR; resource use as a function of density) approach to the native case-building speciesLimnephilus lunatus, we compared utilisation of non-native waterweedsElodea canadensisandE. nuttalliiin mono- and polycultures. Furthermore, we quantified de-cased and cased caddisfly-induced fragment production and length changes among non-nativeE. canadensis,E. nuttallii,Crassula helmsiiandLagarosiphon majorunder two different plant orientations: horizontal (floating) versus vertical (upright) growth forms. Larval caddisflies exhibited Type II (hyperbolic) FRs towards bothElodeaspecies, and utilised each plant at similar rates when plants were provided separately. When plant species were presented in combination horizontally,E. canadensiswas significantly less utilised compared toE. nuttallii, corroborating observations in the field. De-cased larvae produced new plant fragments for all four aquatic macrophytes, whereas cased larvae fragmented plants significantly less.Elodea nuttaliiandC. helmsiiwere fragmented the most overall.Crassula helmsiiwas utilised to the greatest extent when plants were horizontally orientated, andElodeaspecies when vertically orientated. This study identifies and quantifies a mechanism from a novel species group that may contribute to the spread of invasive macrophytes in aquatic systems. Whilst exploititative interactions are thought to impede invasion success, here we demonstrate how resource utilisation by a resident species may exacerbate propagule pressure from an invasive species.

Automated summary

The study explored the impact of larval caddisflies on fragment spread of invasive aquatic plants and found that these larvae utilize different plants differently under different plant orientations, thus contributing to the spread of invasive species by generating plant fragments.