Nutrient and herbivore alterations cause uncoupled changes in producer diversity, biomass and ecosystem function, but not in overall multifunctionality

Abstract

Altered nutrient cycles and consumer populations are among the top anthropogenic influences on ecosystems. However, studies on the simultaneous impacts of human-driven environmental alterations on ecosystem functions, and the overall change in system multifunctionality are scarce. We used estuarine tidal flats to study the effects of changes in herbivore density and nutrient availability on benthic microalgae (diversity, abundance and biomass) and ecosystem functions (N2-fixation, denitrification, extracellular polymeric substances -EPS- as a proxy for sediment cohesiveness, sediment water content as a proxy of water retention capacity and sediment organic matter). We found consistent strong impacts of modified herbivory and weak effects of increased nutrient availability on the abundance, biomass and diversity of benthic microalgae. However, the effects on specific ecosystem functions were disparate. Some functions were independently affected by nutrient addition (N2-fixation), modified herbivory (sediment organic matter and water content), or their interaction (denitrification), while others were not affected (EPS). Overall system multifunction remained invariant despite changes in specific functions. This study reveals that anthropogenic pressures can induce decoupled effects between community structure and specific ecosystem functions. Our results highlight the need to address several ecosystem functions simultaneously for better ecosystem characterization and management.

Niche dimensionality and herbivory control stream algal biomass via shifts in guild composition, richness, and evenness

“…Species richness tends to decrease less with grazing (Liess and Hillebrand , Alberti et al. ) because tolerant understory species are grazer resistant, while sensitive overstory species can still be present, even if they cannot accumulate considerable biomass, e.g., by maintaining basal growth (Lamberti et al. , Steinman ).…”

“…, Alberti et al. ), shown to cause a logarithmic decline in algal standing crop (Lamberti et al. ) and prevent nuisance levels of filamentous algae (high profile guild) under eutrophication (Anderson et al.…”

“…Species richness tends to decrease less with grazing Hillebrand 2004, Alberti et al 2017) because tolerant understory species are grazer resistant, while sensitive overstory species can still be present, even if they cannot accumulate considerable biomass, e.g., by maintaining basal growth (Lamberti et al 1995, Steinman 1996. Biomass, on the other hand, can be severely reduced by herbivory (Feminella and Hawkins 1995, Gruner et al 2008, Alberti et al 2017, shown to cause a logarithmic decline in algal standing crop (Lamberti et al 1995) and prevent nuisance levels of filamentous algae (high profile guild) under eutrophication (Anderson et al 1999, Sturt et al 2011.…”

Environmental context shapes the long‐term role of nutrients in driving producer community trajectories in a top–down dominated marine ecosystem

“…This follows because changes in community composition may alter trait‐based community properties (Cardinale et al., 2012), with consequences for ecosystem functions, even in the absence of changes in summary measures (diversity, cover; Lefcheck & Duffy, 2015; O’Connor & Donohue, 2013). For example, studies have shown that nutrient enrichment can alter ecosystem functions such as productivity or nitrogen fixation through altered relative species abundances at rates disproportional to those of changes in diversity (tidal flats, no effects on diversity: Alberti et al., 2017; grasslands: Isbell et al., 2013). Our results highlight the need to incorporate temporal turnover in investigations of anthropogenic impacts on assemblage structure and function; they suggest that systems strongly driven by either top–down or bottom–up forces may experience a decoupling of diversity and ecosystem function measures with anthropogenic stress (Souza et al., 2016).…”

“…Increasingly, studies have examined the relative roles of these two factors on plant biomass or diversity (e.g. Alberti et al., 2017; Hillebrand et al., 2007; Souza, Zelikova, & Sanders, 2016). Yet diversity, a measure of the number and relative abundance of species within a defined scale or study unit (Magurran, 2004; Schleuter, Daufresne, Massol, & Argillier, 2010), is underpinned by a complex array of temporally variable processes that define the structure and dynamics of the community (Magurran & Henderson, 2010).…”