Aquatic-terrestrial linkages and biodiversity spillover in forest streams

Biodiversity is changing globally, with disproportionate losses in freshwater ecosystems. While the impact of biodiversity loss on ecological functions and stability within a single ecosystem is well understood, its influence across ecosystem boundaries remains less clear. Forested streams and their adjacent riparian zones offer a model "meta-ecosystem" to investigate these spill-over effects. Forest stream invertebrates heavily depend on terrestrial leaf litter for energy, and emerging aquatic insects are a crucial food source for riparian consumers like birds, bats, and spiders. To test how donor ecosystem biodiversity might influence recipient ecosystem functioning and create reciprocal feedback loops by facilitating prey diversity, we have begun a field experiment in the North Island of New Zealand manipulating terrestrial litter inputs at the reach-scale in two forested streams with markedly different vegetation. One stream is dominated by pine (Pinus radiata), while the other stream features diverse native forest. Our experiment involves a BACI (Before, After, Control, Impact) design, where terrestrial litter inputs are being manipulated in two 30-m reaches of each stream for one year and compared with before data alongside control reaches. The two litter input treatments involve a labile titter type from a single tree species (monoculture) and a polyculture of litter from three tree species. Our sampling has included benthic and emergent aquatic insects, organic matter processing, and eDNA. Preliminary results show that overall aquatic invertebrate composition is similar between the two stream, but with key differences including significantly diminished diversity of Ephemeroptera in the pine stream. These diversity responses will be complemented with food-web analyses using functional traits, biotracers, and gut-content metabarcoding. By using these innovative techniques in an ecosystem manipulation at the reach scale, we can better understand the importance of biodiversity within a landscape context. Addressing this frontier in ecology will improve our ability to conserve, restore, and manage biodiversity and ecosystems across spatial scales.