In Northern Yellowstone National Park, restoration of terrestrial apex predators had cascading consequences for riparian plant communities, transitioning them from grassland to willow-dominated. However, the extent of these changes was partially mediated by changing prey vulnerability and geomorphic context, resulting in a mosaic. The restored willow communities have been used to suggest that wolves ‘saved’ Yellowstone’s rivers. Though this narrative invokes reciprocal linkages, little is known about how these processes vary across a mosaic. Using a riverscape approach, we investigated how diverse aquatic and terrestrial communities interact to mediate reciprocal (land-water-land) linkages across a mosaic of stream-riparian ecosystems.
From 2018-2021, we intensively sampled eight headwater streams to characterize riparian plant communities, terrestrial organic matter and invertebrate inputs, the diversity and productivity of aquatic primary producers, invertebrates, and fishes, aquatic food web linkages and trophic transfer efficiencies, aquatic insect emergence, and riparian insectivore composition and abundance.
Riparian vegetation state determined dominant basal organic matter, which influenced benthic invertebrate community composition and productivity. The resulting phenology and vulnerability of benthic invertebrates, paired with the fish assemblages’ consumption of benthic invertebrate production and terrestrial invertebrate input, mediated the timing and magnitude of aquatic insect emergence. In turn, the timing and magnitude of emergence, coupled with riparian vegetation, mediated responses by riparian insectivores. At each link of these reciprocal processes, traits of organisms and their interactions mediated subsequent linkages.
Traits and interactions of organisms were critical to understanding localized stream-riparian ecosystems which, rather than occurring as homogenous or categorical states, contribute to heterogeneity in the larger-scale riverscape. Such heterogeneity may diversify a portfolio of ecological patterns and processes that, as models show, can promote foodweb and ecosystem stability. Therefore, the restoration of apex predators did not single-handedly ‘save’ the ecosystem, but contributed important complexity and heterogeneity to reciprocally-linked aquatic and terrestrial communities of Yellowstone.