Nonrandom species loss in phytoplankton communities and its effect on ecosystem functioning

Abstract

The effects of species diversity on ecosystem functioning have been broadly studied, mostly considering random artificial assemblages. However, natural communities are shaped by ecological interactions and environmental conditions often leading to nonrandom species extinctions. Here, we manipulated a natural phytoplankton community by generating a taxonomic diversity gradient based on rare species exclusions and conducted a mesocosm experiment to investigate the diversity effects on ecosystem functioning (resource use efficiency and biomass) under two nutrient levels. We quantified two functional traits (size and photosynthetic pigments) to evaluate the relation of functional diversity and ecosystem functioning. In a second experimental phase we simulated temperature fluctuations to assess the role of diversity on temporal stability of ecosystem functioning. We did not find a significant effect of diversity on ecosystem functioning and the temporal stability of ecosystem functioning, regardless of nutrient level. These results indicated that loss of biomass caused by rare species extinctions was compensated by the species retained in the diversity gradient. Phytoplankton size diversity was positively related to diversity, but this was not transferred into a positive diversity effect on ecosystem functioning. Additionally, the loss of species did not result in a loss of pigment diversity. The lack of a biodiversity-ecosystem functioning (BEF) relationship in our study may be due to the weak coupling of functional and species diversity and a low manifestation of functional diversity under the evaluated conditions. We emphasize that more realistic biodiversity loss scenarios in experiments can yield different results from those in classical BEF research paradigms.

Predators mitigate the destabilising effects of heatwaves on multitrophic stream communities

“…By revealing that predators can mitigate the destabilizing effects of heatwaves on multitrophic stream communities, our study adds to a growing body of literature exploring the contribution of individual species to the stability of communities, ecosystem functioning and service supply (Gerhard et al, 2020;Kordas et al, 2017;Oliver et al, 2015;Post & Pedersen, 2008;Ross, Arnoldi, et al, 2021;Smale et al, 2019;White et al, 2018White et al, , 2020Worm et al, 2006).…”

“…Biodiversity change across the globe is a non-random process (Chen et al, 2020;Estes et al, 2011;Gerhard et al, 2020;Gross & Cardinale, 2005;Purvis et al, 2000). Of particular concern is trophic downgrading, whereby species extinctions follow a pattern of systematic community disassembly starting from the highest trophic levels, as a result of the larger body sizes, metabolic demands and resource requirements of apex predators (Estes et al, 2011;Tanentzap et al, 2020).…”

Phytoplankton diversity affects biomass and energy production differently during community development

“…Coadaptation might thus facilitate coexistence but weaken diversity effects, explaining some variation in the strength and direction of BEF relationships among studies (Aubree et al., 2020; Chen et al., 2019). For instance, artificially assembled communities might contain species with greater trait diversity than natural communities that have been exposed to longer environmental filtering (Gerhard et al., 2021). Since our experiment only covered ~10 generations, evolutionary changes probably played a minor role.…”

Environmental stoichiometry mediates phytoplankton diversity effects on communities' resource use efficiency and biomass

“…The diversity gradient (six levels, increasing in diversity from the lowest diversity at D1 to the highest at D6) was generated by dilution of the natural community (dilution factors ranged from 1:1 × 10 5 to 1:1) and thus removal of rare species (Engel et al, 2017;Hammerstein et al, 2017). Each diversity level was incubated in flasks for 26 days to increase biomass prior to the start of the experiments (for methodological details, see Gerhard et al, 2020). The dilution created a diversity gradient that was maintained during the incubation until the inoculation in the mesocosm and the microcosm experiment, and therefore the diversity manipulation was based on the initial pool of species (Figure 2).…”

“…In a previous mesocosm study, we showed that the loss of rare species does not result in a significant decay in RUE and standing biomass as long as the common or abundant species are maintained, considering two nutrient levels (Gerhard et al, 2020). Here, we investigated the effect of the rare species loss combined with a wide N:P supply ratios) and the diversity of (natural) communities experimentally, which is conceptually understood but poorly tested for phytoplankton.…”