Entomology

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Author: search.filters.author.Gruner, DanielSubject: 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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    Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment
    (Oecologia, 2006) Gruner, Daniel; Taylor, Andrew
    A longstanding goal for ecologists is to understand the processes that maintain biological diversity in communities, yet few studies have investigated the combined effects of predators and resources on biodiversity in natural ecosystems. We fertilized nutrient limited plots and excluded insectivorous birds in a randomized block design, and examined the impacts on arthropods associated with the dominant tree in the Hawaiian Islands, Metrosideros polymorpha (Myrtaceae). After 33 mo, the species load (per foliage mass) of herbivores and carnivores increased with fertilization, but rarified richness (standardized to abundance) did not change. Fertilization depressed species richness of arboreal detritivores, and carnivore richness dropped in caged, unfertilized plots, both because of the increased dominance of common, introduced species with treatments. Herbivore species abundance distributions were more equitable than other trophic levels following treatments, and fertilization added specialized native species without changing relativized species richness. Overall, bird removal and nutrient addition treatments on arthropod richness acted largely independently, but with countervailing influences that obscured distinct top-down and bottom-up effects on different trophic levels. This study demonstrates that species composition, biological invasions, and the individuality of species traits may complicate efforts to predict the interactive effects of resources and predation on species diversity in food webs.
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    The effects of foliar pubescence and nutrient enrichment on arthropod communities of Metrosideros polymorpha (Myrtaceae)
    (Ecological Entomology, 2005) Gruner, Daniel; Taylor, Andrew; Forkner, Rebecca
    1. Nutrient resource availability and host-plant foliar pubescence both influence arthropod food webs, but multifactor studies are needed to understand their interdependence and relative importance. Arthropods were sampled by clipping foliage from Metrosideros polymorpha (Myrtaceae) trees of pubescent, glabrous, and intermediate leaf forms on fertilised and unfertilised plots. 2. Fertilisation decreased leaf mass per area (LMA) but did not change the relative mass of pubescence within leaf morphological classes. 3. Fertilisation increased densities of individuals in four taxonomic orders, densities of individuals and species of all trophic levels, and the biomass of Collembola and Homoptera. Herbivore relative diversity (Shannon H0) also increased with fertilisation, but detritivore diversity declined due to increasing dominance of Salina celebensis (Schaeffer) (Collembola). 4. Detritivore density, driven again by S. celebensis, increased with decreasing leaf pubescence, but Heteroptera and Acari were most abundant on the intermediate pubescence class, and Psocoptera density and biomass increased with increasing pubescence. Trophic-level species density did not change with leaf morphological class, but relative diversity of all arthropods and of detritivores increased with increasing pubescence. 5. Both resource availability and leaf pubescence affected Metrosideros arthropod communities. However, the pervasive positive influence of fertilisation did not translate to compositional shifts, and there were no interactions with leaf morphological class. In contrast, the effects of leaf pubescence on arthropod density, biomass, and diversity were more restricted taxonomically, and nonparametric MANOVA and redundancy analyses demonstrated significant differentiation in community composition on the pubescent morphology.
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    Attenuation of top-down and bottom-up forces in a complex terrestrial community
    (Ecology, 2004) Gruner, Daniel
    Carnivore (top-down) and resource (bottom-up) influences in food webs are strong and pervasive, but few studies have investigated their interactive effects in species-rich terrestrial ecosystems. This study focused on arthropods associated with the dominant tree species, Metrosideros polymorpha (Myrtaceae), in Hawaiian forests. Severe soil nutrient limitation on a 120-yr-old lava flow was removed by fertilization and combined with bird predator exclusion cages in a randomized block design. Arthropod densities were measured from clipped foliage at the outset and conclusion of a 33-mo experiment, and their biomass was estimated using regression equations. Metrosideros foliar nitrogen, tree growth, and biomass increased directly in response to fertilization but did not change with bird exclusion. Fertilization increased detritivore densities but not biomass, and both density and biomass of herbivores, while bird exclusion increased both density and biomass of carnivores. Fertilization also increased spider density and biomass, but bird exclusion increased spider numbers (15 species) only in high resource plots. Overall, trophic level biomass responses were less pronounced than density because smaller bodied individuals responded more to enrichment. Bottom-up factors controlled basal trophic levels, and detritivores comprised the largest fraction of arthropod density and biomass. Conversely, top-down impacts were apparent but variable, limited to higher order consumers, and did not cascade to the level of primary producers. These experimental results were consistent with the view that complex forest ecosystems are structured on a bottom-up template.
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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.