A mechanistic framework for cross-habitat bottom-up effects of the invasive terrestrial shrub Lonicera maackii on the biology of headwater streams
Ecological invasions provide a unique opportunity to study basic ecology. By definition, invasive species move into ecosystems displacing native species and form high density populations. Invasions result in a shift in the dominant species within a particular habitat and, in some cases, are associated with a shift from a diverse community into a quasi-monoculture. This is the case in some portions of the Eastern Deciduous Forest of North America where Amur honeysuckle (Lonicera maackii) is extremely effective at colonizing and growing at high densities. This invasion is particularly prevalent in the riparian zones of headwater streams in the Ohio Valley where L. maackii forms near monocultures. This invasion results in a shift in dominant traits of the riparian vegetation and has the opportunity to substantially impact the biology of headwater streams, which is tightly linked to allochthonous inputs. Studying the impacts of this invasive shrub on headwater streams has allowed for the development, presented here, of a testable, mechanistic, framework that connects the ecology of terrestrial habitats and headwater streams.
Ecological invasion of the terrestrial habitat, by definition, includes the replacement of native biomass with that of the invading species. In most cases, invasion results in a simplification of the community and a contraction of the total trait space maintained by the system. The terrestrial-aquatic framework we developed assumes this effect is manifested as an alteration of allochthonous materials. In the particular case of our study system, using a litter bag experiment, we discovered a 4× increase in decomposition rate of the invading species (L. maackii) in comparison to native species. This establishes a trait-transition of a basal resource in the aquatic system associated with the terrestrial invasion. Using instream plots, we discovered that the presence and abundance of particular species entering the stream was altered by the terrestrial invasion. This reinforces the idea of strongly altered terrestrial subsidies coming into the aquatic system when the terrestrial habitat is highly invaded. Our data suggest that altered subsidies are reflected in (a) nutrient status of the stream, (b) changes in the microbial biofilm, and (c) alterations in the composition and structure of the macroinvertebrate community.