- Research Overview
- Research Initiatives
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Global change poses
a strong challenge to ecologists, environmental scientists, and conservation
biologists: even as our natural and managed ecosystems become more
stressed by the forces of global change, humans require that they
produce both a greater quantity and variety of ecosystem services.
For instance, we may expect a forested ecosystem to produce timber,
provide clean water, sequester carbon, support wildlife, and provide
recreational opportunities, yet at the same time the forest community
is being buffeted by climate change, invasive species, and land-use
change. In order to ensure that our ecosystems provide the services
society demands, we must be able to predict how ecological communities
will respond to these global forces, and in turn how changes in community
composition will affect ecosystem services. To develop this predictive
framework, I employ a mix of observation, experimentation, modeling
and synthesis, within a diverse array of biological communities. |
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TraitNet,
a research coordination network and biological database designed to
foster the systematic collection and dissemination of species trait
data Ideally, ecologists
eventually will be able to predict many aspects of community interactions
and ecosystem functioning from species traits. However, we have found
that species trait data are at best widely scattered throughout the
literature and at worse simply nonexistent. Many other types of data
are currently online, including community composition data (e.g., VegBank,
CTFS), the phylogenetic relationships of species (e.g., Tree of Life,
Phylocom, TreeBASE), and the genetic sequences of species (e.g., GenBank).
Yet, in spite of the fact that trait measurements are critical to much
ecological and evolutionary research, we know relatively little about
species traits. In response, Shahid Naeem and I are implementing an NSF Research Coordination Network and online database designed to bring together researchers who utilize species traits as a tool for myriad and diverse research efforts. The goal of TraitNet is to build an ecoinformatics backbane that will enable seamless data contribiutions to a fully searchable, geo-referenced database. This databale will allow species trait data to be discovered and shared while protecting the intellectual property rights of data contributors. The proposed network includes more than 40 founding participants, including ecologists and evolutionary biologists as well as ecoinformatics specialists and computer scientists. We believe that species traits are a critical compliment to community, phylogenetic and genetic data, and that TraitNet will foster substantial research that would be otherwise unattainable. |
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In collaboration with Shahid Naeem, Fabrice DeClerk and several other BioMERGE participants, I have been working to quantify the effects of tree species extinctions on ecosystem function. Prior efforts to quantify the effect of biodiversity on ecosystem function have used small-scale experiments in relatively simple systems and have also assumed random extinctions. However, most biological systems are far more complex and less amenable to wholesale manipulation, and species' responses will differ among extinction drivers. To evaluate the effects of tree species loss on carbon storage, we simulated several extinction scenarios within a diverse, well-studied tropical forest, and quantified the effects on above-ground carbon storage. We demonstrated that both the magnitude and variability of carbon storage differed greatly between global drivers of extinction such as fragmentation or climate change. We currently are elaborating on this approach by developing more mechanistic models of both extinction risk and ecosystem function. Because this approach requires substantial species trait data, I have recently begun working with the Species Traits Working Group at the Center for Tropical Forest Science, led by Joe Wright of the Smithsonian Tropical Research Institute, to collect species trait data from the 18 large forest dynamics plots maintained by CTFS. |
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In collaboration with
Elizabeth Nichols, Sacha Spector of the American Museum of Natural History,
as well as numerous data contributors, I am working to quantify the
effects of land use change on dung beetle communities and the ensuing
effects on ecosystem function. Dung beetles perform several important
ecosystem functions - in addition to dung removal, dung beetles recycle
nutrients, disperse seeds, and suppress both flies and pathogens. Dung
beetles are also highly diverse and respond differentially to land use
change. Utilizing samples of dung beetle community composition collected
across seven levels of land use intensity, including intact forest,
secondary forests, clear cuts and pasture , we are identifying the traits
that predispose dung beetle species to population declines. We also
are independently building models of species' contributions to functioning
via their functional traits. When combined, we will be able to assess
broadly the effects of land use change on a suite of important ecosystem
functions. |
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In collaboration with
Shahid Naeem, Andy Hector, Charles Perrings, and Michel Loreau, we are currently preparing
an edited volume intended to assess the current state of biodiversity
and ecosystem functioning research. The volume is being produced in association
with a joint BioMERGE - DIVERSITAS meeting, and will include both basic
science and applied results. |
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Ecosystems perform numerous functions and provide a wide variety of services. Maximizing the range and magnitude of functions and services requires that
communities fill all available niches. We combine ideas about functional diversity with those of habitat filtering to quantify the proportion of trait space occupied by a given set of species. Advancing prior work using convex hulls, we employ a hulls-within-hulls approach that sums intraspecific hulls, and thus trait space, as a proportion of the total available trait space (the total hull volume of intact communities or of the candidate species pool). We find that while a reduced set of species may fill the available functional trait space, this may come at the price of both reduced redundancy and limited ability to respond to environmental change. |
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For much of my dissertation
work, I focused on testing the ability of three competing theories of
plant competition to predict invasions and biological control. While
many researchers have assumed that species successfully invade because
escape from natural enemies has made them better resource competitors
than the natives they displace, this assumption has never been tested
rigorously in the field. Community ecology offers several models of
plant competition that may predict invasions and biocontrol success,
including Tilman's resource competition model, Goldberg's response to
resource availability model, and the plant size model advocated by both
Miller and Keddy. All three offer trait-based measures that, in theory,
predict competitive ability and thus the invasive potential of an exotic
species. In addition, all three models predict that a biocontrol agent
(e.g., an insect herbivore) will succeed, short of killing its host
outright, only by reducing the competitive ability of its host, for
instance by limiting the invader's ability to reduce resource availability
to lower levels than its native competitors. I have been testing these theories experimentally, in collaboration with Walter Carson, with the widespread invasive, Purple Loosestrife, its native competitor, Broad-leaved Cattail, and Galerucella calmariensis, a leaf-feeding beetle widely released in an effort to biologically control purple loosestrife. Our results suggest loosestrife is able to invade cattail through competition for light and possibly soil nitrogen. However, loosestrife also appears to facilitate cattail by allowing cattail to escape its insect herbivores. The mechanism of this facilitation could be either simple density dependent predation or 'apparent facilitation,' whereby cattail's insect herbivores share a predator with loosestrife's herbivores. Although I have completed my dissertation, I am continuing these long-term experiments as we seek definitive competitive outcomes as well as the mechanisms responsible for the facilitation of cattail by loosestrife. |
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Resource competition
theory was originally modeled under the assumption that resources, such
as soil nitrogen, are distributed homogenously. However, many species
compete primarily for light. Because light is directional and declines
in availability with depth in a canopy, no single measure of light availability,
for instance at the soil surface, adequately describes a species' ability
to compete for light. To address this limitation, I have developed,
in collaboration with Scott Stark and Walter Carson, a model of competition
for light that makes no assumptions about the vertical foliage distribution
of competing species, and thus can be applied to real-world species
such as loosestrife and cattail. In addition, our model allows for the
direct inclusion of effects such as herbivory and leaf litter on competitive
dynamics. |
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Bunker, D.E., and S. Naeem. 2006. Species diversity and ecosystem functioning. Letter to the editor in Science. 312: 846-847. PDF
Stark, S.C., D.E. Bunker, and W.P. Carson. 2006. A null model of exotic plant diversity tested with exotic and native species-area relationships. Ecology Letters. PDF
Bunker, D.E., F. DeClerck, J.C. Bradford, R.K. Colwell, I. Perfecto,O. L. Phillips, M. Sankaran and S. Naeem. 2005. Species Loss and Above-ground Carbon Storage in a Tropical Forest. Science 310:1029-1031. PDF and the Supplement
Bunker, D.E., and W.P. Carson. 2005. Drought stress and tropical forest woody seedlings: effect on community structure and composition. Journal of Ecology. 93: 794-806. PDF
Stevens, M.H.H, D.E. Bunker, S. Schnitzer, and W.P. Carson. 2004. Establishment limitation reduces species recruitment and species richness as soil resources rise. Journal of Ecology. 92: 339-347. PDF
Stevens, M., Z.T. Long, S.A. Schnitzer, D.E. Bunker, R. Collins, A. Bledsoe, W.P. Carson. 2003. Testing Ecological Theory - Lab manual for Ecology Laboratory. University of Pittsburgh. Pittsburgh, PA. PDF not available
D'Arrigo, R.D., C.M. Malmstrom, G.C. Jacoby, S.O. Los and D.E. Bunker. 2001. Correlation between maximum latewood density of annual tree rings and NDVI based estimates of forest productivity. International Journal of Remote Sensing. 21: 2329-2336. PDF
Yamaguchi, D.K., B.F. Atwater, D.E. Bunker, B.E. Benson, and M.S. Reid. 1997. Tree-ring dating the 1700 Cascadia earthquake. Nature 389: (6654) 922-923. PDF
Jacoby, G.C., D.E. Bunker, B.E. Benson. 1997. Tree-ring evidence for an AD 1700 Cascadia earthquake in Washington and northern Oregon. Geology 25: (11) 999-1002. PDF