Cross-system synthesis of consumer and nutrient resource control on producer biomass

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 1) bottom-up control is pervasive but top-down
control is more influential in aquatic habitats relative to
terrestrial systems, and 2) 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 versus 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 its 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 behavior 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.