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The Role of Riparian Forests for Aquatic-Terrestrial Linkages, Ecosystem Functions, and Biodiversity in Stream Food Webs

Oester, Rebecca. The Role of Riparian Forests for Aquatic-Terrestrial Linkages, Ecosystem Functions, and Biodiversity in Stream Food Webs. 2024, University of Zurich, Faculty of Science.

Abstract

Headwater streams and their surrounding riparian zones are not only fascinating habitats but also biodiversity hotspots. Through their extraordinary biodiversity, streams can provide a variety of important ecosystem functions. Yet, biodiversity and ecosystem functions can extend beyond the boundaries of freshwater and terrestrial ecosystems via flows of energy, nutrients, and organisms. A key resource connecting both ecosystems is terrestrial detritus. Subsidies from the riparian vegetation, such as plant detritus, are the resource of a specialised community of stream microbes and detritivores. The freshwater detrital food web decomposes this dead organic matter and integrates it into biomass of aquatic organisms. These organisms and processes contribute to ecosystem services such as nutrient cycling and water purification. However, these ecosystems, their biodiversity and ecosystem functions, as well as their cross-ecosystem linkages are threatened by land-use changes in the riparian zones. To better protect these dynamic systems and functions therein, we urgently need a more detailed and holistic understanding of the connections between freshwater and riparian ecosystems and of the consequences of changing biodiversity in them. The goal of this thesis is to better understand the ecological connections of small streams and their surrounding landscape and delve into the multitrophic interactions and functions in freshwater detrital food webs. To assess the role of riparian forests on aquatic-terrestrial linkages, ecosystem functions, and biodiversity in stream food webs, I used a combination of field experiments and observations in forested and non-forested sites, as well as literature-based methods. All outcomes of these approaches highlight the ecologically significant role of riparian forests for headwater stream communities, aquatic-terrestrial linkages, and overall ecosystem functioning. First, I found that sensitive macroinvertebrate taxa and shredders were strongly associated with forested stream sites with overall higher values of abundance, diversity, and biomass in forested compared to non-forested sites. Shredder-mediated leaf litter fragmentation rates were on average three times higher in forested compared to non-forested sites. These results demonstrate that not only the composition of the aquatic fauna but also an essential ecosystem function, i.e., organic matter decomposition, depend on the vegetation type in the local riparian zone. Second, I observed that riparian forests shaped the stoichiometric and isotopic properties of stream detrital food webs. Where local riparian forests were absent, leaf litter showed higher nutrient contents, which only some shredders mirrored, whereas others showed differences in their isotopic signatures. Consequently, these findings demonstrate that riparian forests are essential for aquatic food webs by not only influencing organisms themselves but also their trophic interactions and energy flow paths. Third, I identified biodiversity and riparian forest as mutual biological drivers of not only leaf litter decomposition but also multiple aquatic-terrestrial ecosystem functions in temperate and tropical freshwater detrital food webs. Despite major environmental differences between temperate and tropical stream ecosystems, these insights show that biodiversity at multiple trophic level as well as riparian forests are essential for stream ecosystems to fulfil their various functions in very different ecological contexts. Fourth, I could determine that the insights from the previous chapters were not confined to the temperate and tropical study locations from CHAPTERS I–III but expanded on a global scale. In all major biomes, I found that land cover changes in the riparian vegetation, especially through harvest and land-use conversion, had negative effects on adjacent aquatic biodiversity and ecosystem functions. These vegetation changes were correlated with lower diversity, abundance, and biomass across multiple trophic levels in freshwater detrital food webs. Studies observing multiple organism groups, most often observed negative effect sizes across multiple trophic levels, highlighting that the land-cover changes negatively affected multiple detrital food web components simultaneously. Together, the results of my four chapters emphasise the pressing need to consider cross-ecosystem consequences of land-cover changes in conservation management and to holistically protect the riverine zone including the terrestrial ecosystems. From local to cross-continental to global scales, I consistently found that riparian forests are critical for freshwater detrital biodiversity, ecosystem functions and aquatic-terrestrial linkages. Around the world, natural riparian forests not only promote diverse communities and numerous detritivores, but also habitats in which organisms can occupy their preferred dietary niche, which ultimately results in enhanced ecosystem functioning and biodiversity.

Additional indexing

Item Type:Dissertation (monographical)
Referees:Altermatt Florian, Bruder Andreas, Moretti Marcelo, Conti Elena
Communities & Collections:07 Faculty of Science > Institute of Evolutionary Biology and Environmental Studies
UZH Dissertations
Dewey Decimal Classification:570 Life sciences; biology
590 Animals (Zoology)
Language:English
Place of Publication:Zürich
Date:27 December 2024
Deposited On:27 Dec 2024 11:18
Last Modified:27 Dec 2024 11:18
Number of Pages:194
OA Status:Green
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