A cross‐system synthesis of consumer and nutrient resource control on producer biomass

Abstract

Nutrient availability and herbivory control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that (i) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems and (ii) bottom-up and top-down forces are interdependent, with statistical interactions that synergize or dampen relative influences on producer biomass. We used simple dynamic models to review ecological mechanisms that generate independent vs. interactive responses of community-level biomass. We calibrated these mechanistic predictions with the metrics of factorial meta-analysis and tested their prevalence across freshwater, marine and terrestrial ecosystems with a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients. Our analysis showed that producer community biomass increased with fertilization across all systems, although increases were greatest in freshwater habitats. Herbivore removal generally increased producer biomass in both freshwater and marine systems, but effects were inconsistent on land. With the exception of marine temperate rocky reef systems that showed positive synergism of nutrient enrichment and herbivore removal, experimental studies showed limited support for statistical interactions between nutrient and herbivory treatments on producer biomass. Top-down control of herbivores, compensatory behaviour of multiple herbivore guilds, spatial and temporal heterogeneity of interactions, and herbivore-mediated nutrient recycling may lower the probability of consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently multiple producer resources (e.g. nitrogen, phosphorus, light), multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates) and multiple trophic levels; and -in addition to measuring producer biomass -assess the responses of species diversity, community composition and nutrient status.

Biodiversity impacts ecosystem productivity as much as resources, disturbance, or herbivory

“…Bars with the same letter within each panel do not significantly differ at the P < 0.01 level based on Tukey contrasts (corrected for multiple comparisons). food web structure (15,17,24), the effects of which may be as great as those of plant diversity. For example, although algal biomass production increased with algal diversity in a study of a benthic marine community, this effect was masked when herbivores were present because, in this case, the increased production was consumed by herbivores (15).…”

Interactions among ecosystem stressors and their importance in conservation

“…survival, growth, abundance, behaviour e.g. species diversity, biomass, total primary production additive antagonistic Crain et al [15] Darling & Côté [16] Dieleman et al [32] Gruner et al [26] Harvey et al [47] Przelawski et al [22] Wu et al [31] Stephens et al [29] Wahl et al [33] Jackson [27] Figure 4. Links between various categories of responses to multiple stressors and the types of interactions occurring between these stressors.…”

Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

“…At the scale of our experiment, all main predators moved freely between the treatments and showed aggregation in habitat patches with vegetation, clearly demonstrating that predators did not distribute passively. Interestingly, a survey of manipulative experiments revealed that the marine benthos is the only ecosystem in which grazers generally show an increase with nutrient enrichment and where cascading effects on lower trophic levels from predator removal are the strongest (Borer et al 2005(Borer et al , 2006Gruner et al 2008). Thus, the marine benthos seems to be vulnerable to cascading behavior between trophic levels, which may be promoted by the common conditions of highly mobile predators and a dominance of continuously reproducing and well-mixed distributions of producers (Nisbet et al 1997).…”

“…The prevalence of trophic cascades depends on strong consumer control of the prey community, with a cascading indirect positive effect being generated on the next lower trophic level. There are a number of distinct species traits that determine if species are subjected to consumer control (Duffy and Hay 1990;Coley and Barone 1996;Polis 1999;Hopcraft et al 2010), and the response of the community to changes in predators should ultimately depend on the community composition of each trophic level (Gruner et al 2008). Our results support the concept that the lifestage development of the producer trophic level can be essential for responses to predator declines in the marine benthos, by demonstrating that established larger macroalgae were resource controlled regardless of the composition of the higher trophic levels.…”