Research
My research projects cover a lot of ground in basic and applied entomology and ecology. Some of the broad questions we explore in my lab:
- How do geochemical resources and consumers interactively constrain community diversity, function, and invasion? [i.e., how do bottom-up and top-down processes affect trophic structure and ecosystem processes?]
- How do the dynamical scales of organisms at different trophic positions bear on the persistence and strength of food web interactions? [i.e., do large mobile predators initiate stronger and more persistent trophic cascades than small, immobile counterparts?]
- How do global patterns of food web structure vary across ecosystems and over time, and how resilient are these interactions to perturbation? [i.e., are there real differences in trophic structure across ecosystem types, and how do they respond to change?]
Here are some of the projects in the lab:
1. Soil ecology and entomopathogenic nematodes
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| The nematode-ghost moth-lupine trophic cascade at Bodega Marine Reserve. a) Bush lupine (Lupinus arboreus) in flower in June 2001; b) the same stand in July 2003 after ghost moth outbreak; c) ghost moth caterpillars (Hepialus californicus) in a lupine stem; d) healthy Galleria waxworm used as sentinel bait insect; e) Galleria waxworm infected by nematode Heterorhabditis marelatus, indicated by the red-gold color of symbiotic luminescent bacterium Photorhabdus; f) release of thousands of EPN infective juveniles (all photos: Don Strong). |
In collaboration with my postdoctoral advisor Dr. Donald Strong at the University of California-Davis Bodega Marine Lab, I went gone underground to study the dynamics of a powerful trophic cascade in coastal prairie soils. Entomopathogenic nematodes (EPN) in the soil can confer to shrubs a blanket of protection from fatal herbivory by infecting and killing caterpillars in roots. However, the population dynamics of these nematode predators are highly volatile – with wide variance in local persistence – and mathematically unstable. Combining spatially explicit field studies, statistical models, and Geographic Information Systems, we are testing several hypotheses for the observed persistence of the trophic cascade. For example, a rescue effect from very deep reservoirs (>80 cm) – where favorable conditions and alternative hosts are available during otherwise harsh seasonal periods near the surface – may drive the long-term persistence of this interaction. These EPN have a fascinating intrinsic biology, with two independent evolutionary lineages converging to the same solution: symbioses with a bacterial partner that packs a lethal punch to a variety of soil arthropod hosts.
My lab at the University of Maryland continues to work with entomopathogenic nematodes at a range of local sites. Our initial goals are to characterize the community of soil EPN and host arthropod species as functions of soil abiotic variables and land management. With these data we will develop experiments designed to understand persistence of EPN species. We aim to develop habitat manipulations that will improve the endemic biological control of outbreak insect crop pests in the soil.
Gruner, D.S., A. Kolekar, J.P. McLaughlin, and D.R. Strong. 2009. Host resistance reverses the outcome of competition between microparasites. Ecology 90:1721-1728
Ram, K., E. L. Preisser, D. S. Gruner, and D. R. Strong. 2008. Metapopulation dynamics override local abiotic limits on long-term parasite persistence. Ecology 89:3290-3297
Denno, R.F., D.S. Gruner, and I. Kaplan. 2008. Potential for entomopathogenic nematodes in biological control: insights from trophic cascade theory. Journal of Nematology 40:61-72
Ram, K., D.S. Gruner, J.P. McLaughlin, E.L. Preisser, and D.R. Strong. 2008. Dynamics of a subterranean trophic cascade in space and time. Journal of Nematology 40:85-92
Gruner, D.S., K. Ram and D.R. Strong. 2007. Soil mediates the interaction of coexisting entomopathogenic nematodes with an insect host. Journal of Invertebrate Pathology 94:12-19
2. Trophic structure and restoration in Hawaiian forests
| With replicated islands and evolutionary lineages, simplified biotic communities relative to tropical continental systems, all along well-characterized ecosystem gradients… the Hawaiian Islands are an extraordinary field system for studying basic community and ecosystem ecology. In an integrated program at multiple spatial and temporal scales, I showed that forest birds and arthropods on a model tree species (Metrosideros polymorpha) were most abundant and diverse on fertile, developed volcanic soils in the Hawaiian Islands. Historical legacies determined the regional pools of arthropod species composition across islands, but local community abundance and diversity converged to common limits set by resource availability. In a complementary large-scale factorial experiment using the same model community, I demonstrated that forest birds can suppress spiders and outbreaks of invading arthropod species, although strong bottom-up control of resources dampened these effects from cascading to herbivores or trees. (to right: Anomalochrysa machlachlani) |  |
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In a current collaboration with Tad Fukami (Stanford), Christian Giardina (USDA Forest Service), and David Flaspohler (Michigan Tech), we hope to use the natural landscape of volcanic forest fragmentation (forest islands called ‘kīpuka’) to 1) test the importance of ecosystem size and productivity as determinants of food web complexity and food chain length (via stable isotope methodology), and 2) experimentally eliminate invasive rodents to determine their impact as omnivorous consumers of bird nestlings, arthropods, and plants. (to left: a forested kīpuka on Hawai`i island) |
Gruner, D.S., N.J. Gotelli, J.P. Price and R.H. Cowie. 2008. Does species diversity drive speciation? A reassessment with the Hawaiian biota. Ecography 31:279-285
Gruner, D.S. 2007. Geological age, ecosystem development, and resource constraints on arthropod community structure in the Hawaiian Islands. Biological Journal of the Linnean Society 90:551-570
Gruner, D.S. and A.D. Taylor. 2006. Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment. Oecologia 147:714-724
Gruner, D.S., A.D. Taylor and R.E. Forkner. 2005. The effects of foliar pubescence and nutrient enrichment on arthropod communities of Metrosideros polymorpha (Myrtaceae). Ecological Entomology 30:428-443
Gruner, D.S. 2005. Biotic resistance to an invasive spider conferred by generalist insectivorous birds on Hawai‘i Island. Biological Invasions 7:541-546
Gruner, D.S. 2004. Attenuation of top-down and bottom-up forces in a complex terrestrial community. Ecology 85:3010-3022
Gruner, D.S. 2004. Arthropods from ‘ōhi‘a lehua (Myrtaceae: Metrosideros polymorpha), with new records for the Hawaiian Islands. Bishop Museum Occasional Papers 78:33-52
3. Trophic structure and dynamics across ecosystems
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Over the past three years, I co-led (with Jon Shurin and Helmut Hillebrand) a working group, through the National Center for Ecological Analysis and Synthesis, to compare trophic structure across systems. Our ‘trophic comparisons’ group – comprising graduate students with new ideas alongside leading researchers in community and ecosystems ecology – explores the root causes of variation in trophic structure across global ecosystems. We have created the largest existing databases of herbivore and nutrient manipulation experiments and patterns of biomass partitioning among trophic levels in diverse freshwater, marine and terrestrial ecosystems. Our meta-analyses and theory aim to test candidate hypotheses based on factors that distinguish food webs among these habitat types, such as consumer metabolism and body size ratios, biomass and nutrient turnover rates, productivity and nutrient limitation (resource quantity), and nutrient stoichiometry (resource quality). This work is increasing our mechanistic understanding of how trophic interactions ramify in natural webs as a result of human impacts like eutrophication or extirpation of top consumers. (to left: fig. from Shurin et al. 2006) |
Moreover, an outgrowth of this working group is an exciting global research cooperative project called NutNet. The NutNet project, which involves more than 40 research sites worldwide, investigates the effects of resources and consumption on herbaceous plant communities and ecosystem processes. The combined power of this site replication and a single, shared experimental protocol allows for an unparalleled, piercing analysis of fundamental unresolved questions in community and ecosystem ecology. (to right: experimental setup inlaid on scene from Sagehen Creek Station in the California Sierra Nevada) |
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Borer, E.T. and D.S. Gruner. 2009. Top-down and bottom-up regulation of communities. Pages 296-304 in S. A. Levin, S. R. Carpenter, H. C. J. Godfray, A. P. Kinzig, M. Loreau, J. B. Losos, B. Walker, and D. S. Wilcove, eds. Princeton Guide to Ecology. Princeton University Press
Hillebrand, H., E.T. Borer, M.E.S. Bracken, B.J. Cardinale, J. Cebrian, E.E. Cleland, J.J. Elser, D.S. Gruner, W.S. Harpole, J.T. Ngai, S. Sandin, E.W. Seabloom, J.B. Shurin, J.E. Smith, and M.D. Smith. 2009. Herbivore metabolism and stoichiometry each constrain herbivory at different organizational scales across ecosystems. Ecology Letters 12:516-527
Gruner, D. S., J. E. Smith, E. W. Seabloom, S. A. Sandin, J. T. Ngai, H. Hillebrand, W. S. Harpole, J. J. Elser, E. E. Cleland, M. E. S. Bracken, E. T. Borer, and B. M. Bolker. 2008. A cross-system synthesis of herbivore and nutrient resource control on producer biomass. Ecology Letters 11:740-755 Elser, J.J., M.E.S. Bracken, E.E. Cleland, D.S. Gruner, W.S. Harpole, H. Hillebrand, J.T. Ngai, E.W. Seabloom, J.B. Shurin, and J.E. Smith. 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine, and terrestrial ecosystems. Ecology Letters 10:1135-1142
Hillebrand, H., D.S. Gruner, E.T. Borer, M.E.S. Bracken, E.E. Cleland, W.S. Harpole, J.T. Ngai, E.W. Seabloom, J.B. Shurin, and J.E. Smith. 2007. 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 104:10904-10909
Shurin, J.B., D.S. Gruner and H. Hillebrand. 2006. All wet or dried up? Real differences between aquatic and terrestrial food webs. Proceedings of the Royal Society B: Biological Sciences 273:1-9
4. Other initiatives
Members of the lab are variously involved with other research projects, including meta-analyses of experiments on insectivorous vertebrates, structural equations modeling of kelp forest systems, arthropod surveys and conservation assessments on Pacific Islands, food web ecology of Spartina marsh systems, and so on.
Van Bael, S.A., S.M. Philpott, R. Greenberg, P. Bichier, N.A. Barber, K.A. Mooney, and D.S. Gruner. 2008. Birds as predators in tropical agroforestry systems. Ecology 89:928–934
Handler, A.T., D.S. Gruner, W.P. Haines, M. Lange, and K.Y. Kaneshiro. 2007. Arthropod surveys on Palmyra Atoll, Line Islands, with insights into the decline of the native tree Pisonia grandis (Nyctaginaceae). Pacific Science 61:485-502
