Plant diversity loss affects stream ecosystem multifunctionality

Abstract

Biodiversity loss is occurring globally at unprecedented rates, altering the functioning of the Earth's ecosystems. Multiple processes are often key components of ecosystem functioning, but it is unclear how biodiversity loss affects ecosystem multifunctionality (i.e., the ability of ecosystems to maintain multiple processes simultaneously). This is particularly true for some ecosystem types such as streams, which have been understudied, despite their key role in global biogeochemical cycles and their serious impairment by the widespread loss of riparian vegetation as a result of global change. Using a microcosm experiment, we tested whether losing riparian plant diversity affected stream multifunctionality, taking into account nine key processes related to litter decomposition, animal biomass production, and nutrient cycling, and simulating plant species loss from four to one in the presence or absence of litterfeeding detritivores. Multifunctionality increased with plant diversity in the presence of detritivores and decreased in their absence, evidencing a key role of detritivores in biodiversity-ecosystem-functioning (BEF) relationships. Moreover, by exploring effects of plant diversity on each process individually we were able to reveal potential mechanisms underlying BEF relationships; for example, effects of plant diversity on nutrient cycling occurred at least partly via indirect nutrient transfer, and were possibly accompanied by changes in microbial stoichiometry. Such mechanisms were unnoticeable when examining multifunctionality metrics, suggesting that individual processes provide crucial information to understand how stream ecosystem functioning is impaired by biodiversity loss.

Effects of two measures of riparian plant biodiversity on litter decomposition and associated processes in stream microcosms

“…In our study, in the absence of detritivores, negative complementarity and positive selection were similar in magnitude (53% and 47% on average, respectively), resulting in a non-significant net diversity effect. This suggests that selection effects were more relevant for microbial than for detritivore-mediated decomposition, and indicates that the lack of net diversity effects on microbial decomposition found here and in other studies 15,30 could be due to different mechanisms operating in opposite ways, rather than to the absence of interactions between litter diversity and microbial decomposers.…”

“…Our experiment revealed a negative effect of plant species richness on litter decomposition: monocultures decomposed, on average, faster than litter mixtures. This result was unexpected when compared with several other microcosm experiments, which have found faster decomposition of litter mixtures than monocultures 15,28,30,31,34 . In most of the above-mentioned microcosm experiments, diversity effects occurred only in the presence of detritivores, suggesting that they were the key drivers of such effects, and the main underlying mechanism was a positive complementarity effect.…”

“…We used leaf litter from 9 tree species belonging to 3 families (Betulaceae, Salicaceae and Fagaceae), which were introduced in microcosms (with and without detritivores) as monocultures (SR = 1) or mixtures (SR = 3) with either low PD (3 species from the same family) or high PD (3 species from 3 different families). The above processes were quantified after 6 weeks, and the following hypotheses were examined: (1) plant SR enhances all studied processes (i.e., they have greater values in mixtures than in monocultures) 15,28 , mostly due to complementarity effects 29 ; (2) the difference between monocultures and mixtures is greater for high-PD than for low-PD mixtures; and (3) all the above patterns are more marked in the presence of detritivores, which often are key drivers of biodiversity-ecosystem process relationships [30][31][32] .…”

A common fungicide impairs stream ecosystem functioning through effects on aquatic hyphomycetes and detritivorous caddisflies

“…We used larvae of the cased caddisfly Sericostoma pyrenaicum Pictet (Trichoptera: Sericostomatidae), which is a common detritivore in the study area, previously used in experiments assessing litter decomposition (Correa-Araneda et al, 2017;Tonin et al, 2017;L� opez-Rojo et al, 2018L� opez-Rojo et al, , 2019. Larvae of similar size (mean case length � SE: 12.78 � 0.16 mm; larval instar between IV and VII based on Basaguren et al, 2002) were collected manually from the benthos of Perea stream in November 2018 and transported to the laboratory in refrigerated containers.…”

“…Even if black alder is often a dominant riparian species across Europe, litter in streams is usually a mixture of species that can differ in terms of fungicide adherence and their subsequent toxicity. Moreover, it is possible that the diversity of resources provided by litter mixtures enhances decomposition (Tonin et al, 2017;L� opez-Rojo et al, 2018L� opez-Rojo et al, , 2019 and microbial decomposer diversity (Kominoski et al, 2010). Thus, litter diversity could compensate for the negative effects of fungicides on decomposition, as observed for other stressors (Duarte et al, 2008).…”