Entomology

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Author: search.filters.author.Bracken, MatthewSubject: search.filters.subject.fertilization

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    A cross-system synthesis of herbivore and nutrient resource control on producer biomass
    (2008-07) Gruner, Daniel; Smith, Jennifer; Seabloom, Eric; Sandin, Stuart; Ngai, Jacqueline; Hillebrand, Helmut; Harpole, Stanley; Elser, James; Cleland, Elsa; Bracken, Matthew; Borer, Elizabeth; Bolker, Benjamin
    Nutrient availability and consumption by herbivores 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 often interact to synergize or dampen relative influences on producer biomass. We use a simple set of dynamic models to review mechanistic hypotheses for these questions, and compare model predictions to empirical data from a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients from freshwater, marine and terrestrial ecosystems. Parameterized model equilibria suggest that interactive outcomes should be weak and less common than strict additivity. Producer community biomass responded positively to fertilization across all systems, although effects were most pronounced in freshwater. Herbivores suppressed producer biomass in both freshwater and marine systems, but effects were inconsistent on land. Importantly, we observed a striking absence of either synergistic or dampening interactive effects of nutrients and herbivores across ecosystem types and within most habitats. Marine temperate rocky reef systems, which showed superadditive synergism of nutrient and herbivore controls, represented an exception to this pattern. Experimental studies showed limited support for emergent interactive effects on producer community-level biomass. We suggest that compensation by multiple herbivore guilds, top-down control of herbivores, spatial and temporal heterogeneity, and herbivore-mediated nutrient recycling tend to reduce the expectation for 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 both multiple producer resources (e.g. nitrogen, phosphorus, light) and multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates); and simultaneously assess the effects on not only producer biomass but also species diversity, community composition and structure, and nutrient status.
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    Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure
    (Proceedings of the National Academy of Sciences of the USA, 2007) Hillebrand, Helmut; Gruner, Daniel; Borer, Elizabeth; Bracken, Matthew; Cleland, Elsa; Harpole, Stanley; Ngai, Jacqueline; Seabloom, Eric; Shurin, Jonathan; Smith, Jennifer
    Consumer and resource control of diversity in plant communities have long been treated as alternative hypotheses. However, experimental and theoretical evidence suggests that herbivores and nutrient resources interactively regulate the number and relative abundance of coexisting plant species. Experiments have yielded divergent and often contradictory responses within and among ecosystems, and no effort has to date reconciled this empirical variation within a general framework. Using data from 274 experiments from marine, freshwater, and terrestrial ecosystems, we present a cross-system analysis of producer diversity responses to local manipulations of resource supply and/or herbivory. Effects of herbivory and fertilization on producer richness differed substantially between systems: (i) herbivores reduced species richness in freshwater but tended to increase richness in terrestrial systems; (ii) fertilization increased richness in freshwater systems but reduced richness on land. Fertilization consistently reduced evenness, whereas herbivores increased evenness only in marine and terrestrial ecosystems. Producer community evenness and ecosystem productivity mediated fertilization and herbivore effects on diversity across ecosystems. Herbivores increased producer richness in more productive habitats and in producer assemblages with low evenness. These same assemblages also showed the strongest reduction in richness with fertilization, whereas fertilization increased (and herbivory decreased) richness in producer assemblages with high evenness. Our study indicates that system productivity and producer evenness determine the direction and magnitude of top-down and bottom-up control of diversity and may reconcile divergent empirical results within and among ecosystems.