Projects:

1. Native Earthworms Reduce Impacts of Exotic Earthworm Invasions on Soil Processes in Wildland Ecosystems.
   
   National Science Foundation Grant to Paul Hendrix, Institute of Ecology, UGA.
   
   Research over the past century has shown that where earthworms are abundant, they significantly influence soil processes critical to the functioning of terrestrial ecosystems. Recent interest has focused on impacts of invasive, exotic earthworms on soil processes, particularly in temperate regions. Dramatic effects of introduced earthworms on soil organic matter dynamics have been reported in a number of cases. The focus of nearly all of this work has been on a few species of exotic earthworms which have achieved wide distributions and now dominate the soil fauna in many ecosystems. Moreover, the most dramatic effects of exotic species on soil organic matter dynamics have been observed in areas previously uninhabited by earthworms (e.g., north of Pleistocene glacial margins in North America). Such effects have rarely been reported from invasions of ecosystems inhabited by native earthworm assemblages, particularly where soils are undisturbed. These observations lead to the central hypothesis of this proposal--endemic earthworm fauna and/or characteristics of their native habitats are resistant to invasion by exotic earthworm species and, consequently, will reduce the impact of exotic species on soil processes. Objectives of this proposed work are to address the following questions:
   
   
   1. Do native earthworm species maintain exclusivity or dominance over exotic earthworm species in undisturbed, native ecosystems? Three hypotheses will be tested concerning mechanisms by which exotic earthworms may succeed or fail to invade a range of habitats occupied exclusively by native earthworms: 1) propagule pressure hypothesis; 2) habitat matching hypothesis; and 3) biotic resistance hypothesis
      
      
   2. Are the impacts of exotic earthworm species on soil processes reduced in the presence of native earthworm species in their native ecosystems? Effects will be determined of native and exotic earthworms and their interactions on transformations of soil organic matter in O- and A-horizons by measuring: 1) physical and chemical nature of organic and mineral soil horizons; 2) movements of ¹³C from labeled litter through O- and A-horizons and into organic matter fractions in the mineral soil.
      
      
   Field experiments will be conducted in which exotic earthworms will be introduced into plots with and without native earthworms in wildland ecosystems across a spectrum of climates, soils and vegetation at four sites in the Pacific Northwest and Southeastern US where Nearctic earthworms occur in high diversity in undisturbed habitats. Main effects of both exotic and native species as well as their interactive effects on soil structure and organic matter dynamics will be measured over four years. The central hypothesis predicts that interactive effects of native and exotic species will be significantly less than effects of exotic species alone.
   
   Results from these experiments will 1) quantify for the first time under field conditions the effects of native earthworm species and their interactions with exotic invaders on soil processes critical to terrestrial ecosystem function; 2) increase our understanding of mechanisms of biological invasions and their impacts at the ecosystem level; 3) contribute to knowledge of biodiversity and biogeography of Nearctic earthworms, including the likely description of new species; and 4) contribute to a little-studied, but potentially important component of two Long Term Ecological Research sites.
   
   
2. Response of Tropical Stream Ecosystem Structure and Function to Amphibian Extinctions.
   
   National Science Foundation Grant to Catherine Pringle, Institute of Ecology, UGA, as part of a three-institution proposal including Southern Illinois University and Drexel University.
   
   Amphibians represent an important energetic link between aquatic and terrestrial habitats, particularly in the tropics where diversity and abundance are very high. Tadpoles feed on algae and organic materials and transfer energy and nutrients from aquatic to terrestrial habitats when they emerge as frogs, and frogs transfer energy and materials back when they return to water to breed. However, amphibian populations have declined dramatically around the world, including massive losses in 13 Latin American countries in the last 20 years. Most declines involve disappearances of over 75% of amphibians at a given site. This research will examine the consequences of amphibian declines on tropical streams by investigating how losses alter stream biodiversity, energy flow in streams, in-stream and streamside food webs, and energy exchange between streams and terrestrial habitats. This will be accomplished through intensive comparisons of stream communities, food webs, and energy flow patterns in Panamanian highland streams with natural amphibian populations and nearby streams where amphibians have recently disappeared. Preliminary studies suggest that amphibian declines will have large-scale effects on streams, including changes in community structure, shifts to increased importance of algal energy sources, reduced downstream transport of organic materials, and reduced energy transfer from streams to terrestrial habitats.
   
   
3. Long-Term Consequences of Biochemical and Biogeochemical Changes in the Horseshoe Bend Agroecosystem, Athens, GA.
   
   National Science Foundation Grant to David Coleman, Mark Hunter, Paul Hendrix, and D.A. Crossley, Jr., Institute of Ecology, UGA.
   
   Understanding many soil processes, including the accumulation of organic matter and the formation of soil aggregates, requires research that is conducted over decadal time periods. The dynamics of soil organic matter and soil fauna at the Horseshoe Bend (HSB) agroecosystem site in Georgia have been studied in replicated experimental plots since 1978. Analyses indicate that the experimental treatments (no-tillage and conventional-tillage regimes) are continuing to diverge. It is essential to continue the experiments for an additional six years, to enable prediction of what the long-term consequences of our manipulations will be. Consequently, a continuation of ongoing research on soil processes in no-till and conventional till plots at HSB is proposed, with some significant changes. The new research focuses on two related areas:
   
   
   1. long-term measurements of the gradually-increasing base of soil organic matter from C3-pathway plants, in crop rotations that have been in effect since 1997.
      
      
   2. following the production, accumulation, fate and ecological effects of the Bt proteins from the summer planting of Bt (and non-Bt) cotton in subplots within our main plots.
      
      
   The relationship between these focal areas is simple. The variation in the size of soil aggregates may influence the sequestration of Bt toxins, and their breakdown products, within soils. We have already shown that no-tillage management systems at HSB generate an increasing representation of soil macroaggregates in comparison to the markedly reduced macroaggregates in our conventional tillage plots. We hypothesize that no-tillage systems will therefore sequester more Bt-related products than will conventional-tillage plots.
   
   Several short-term studies have failed to detect deleterious changes in non-target soil fauna as a result of Bt in crop residues. There are three possible explanations for this: a) that no deleterious effects exist, b) that short-term experiments do not adequately predict the long-term consequences of repeated exposure to Bt residues, and c) that effects are contingent upon properties of the soil such as aggregate size. It would seem prudent to be able to distinguish among these possibilities and the long-term plantings at HSB will allow that to occur.
   
   
4. Causes, Consequences, and Control of Outbreaks of the Hemlock Woolly Adelgid, an Exotic Pest of Hemlock in Eastern North America.
   
   United States Department of Agriculture Forest Service Grant to Mark Hunter, Institute of Ecology, UGA.
   
   The hemlock woolly adelgid (HWA), Adelges tsugae, is a destructive introduced pest of forest and ornamental trees (Tsuga spp.) in the eastern United States. HWA feeds at the base of needles, resulting in the loss of needles and new apical buds. When hemlocks become infested with HWA, tree mortality almost always follows, with death of the trees occurring between four and ten years. As a result of HWA infestation, hemlock populations in the eastern United States are in significant decline and, without action, may disappear entirely from eastern forests.
   
   Hemlocks serve important ecological functions in eastern forests. Hemlock stands provide important cover for ruffed grouse, turkey, deer, snowshoe hare and rabbit. More than 100 bird species in the eastern US use hemlock trees for food, nest sites, roosting, or winter cover. A diverse assemblage of plant species are associated with hemlock forests, including leatherwood, rattlesnake plantain, bunchberry, goldthread, bluebeard and wood sorrels. These associated plant species in turn provide important sources of food and habitat for many forest animals. As a dominant component of riparian zones in many eastern forests, hemlocks also act to buffer streams from erosion and nutrient flux, and maintain the cool temperatures required by trout in forest streams. The cool and shade provided by hemlocks during summer make them ideal recreational settings while, in winter, their dense canopies buffer forests from extreme cold. Finally, hemlocks are greatly valued for their beauty, both in natural forests used for recreation, and as ornamental trees. There are 274 cultivars of eastern hemlock, making it one of the most cultured and cultivated landscape trees in the US.
   
   Motivation. In 2002, HWA reached the forests of western North Carolina and north Georgia, the southern-most distribution of hemlock in the US. This region of the southern Appalachians is home to the some of the greatest biodiversity in north America. Forests in the southern Appalachians are important habitat for hundreds of species of vertebrate and invertebrate animals and refuges for a wide diversity of plant species. Their recreational value is enormous as these forests host hikers, campers, hunters, and fisherman from all over the world. Streams that arise in these forests provide water both for natural and human use, and hemlock trees are important components of riparian forest. The potential loss of hemlock from the southern Appalachians raises significant concerns about the ecology, economics, and natural beauty of one of the United States biodiversity hot-spots.
   
   Objectives. In 2003, HWA infestations were reported within the Coweeta Hydrologic Laboratory in North Carolina. Coweeta is a Forest Service research site and is home to a Long-Term Ecological Research (LTER) Site. With the long-term ecological records available, Coweeta is an ideal site for studies of HWA in the southern Appalachians. There are two primary goals for our research at Coweeta:
   
   
   1. Survey the current distributions of both hemlock trees and HWA infestations at Coweeta. There are 399 permanent vegetation plots within the Coweeta basin that were surveyed for the abundance of hemlock in 1934 and in 1970. Over this time period, hemlock occurrence increased from 16% to 42% of the plots. We will re-survey these plots and record occurrence of hemlock, basal area under hemlock, and infestation by HWA. Within all plots containing hemlock, we will individually mark 10 mature trees and 10 saplings. These sentinel trees will allow us to monitor over time any apparent resistance to HWA attack or recovery from infestation. Long-term monitoring will continue over the next ten years.
      
      
   2. Examine the long-term consequences of hemlock loss from riparian zones. From our permanent plots, we will choose 20 riparian plots for long-term monitoring. Ten plots will serve as controls (no hemlock present) and 10 (>30% basal area in hemlock) will be used to follow the effects of hemlock loss. Within each of these long-term plots, we will survey biodiversity of key groups (trees, understory plants, amphibians, stream insects), key ecosystem processes in soils and streams (nutrient dynamics, productivity) and the progression of HWA infestation and hemlock loss.
      
      
5. The keystone role of heterotrophic microbes in driving ecosystem-level effects of nutrient enrichment.
   
   National Science Foundation Grant to Amy Rosemond, Bruce Wallace, and Keller Suberkropp. Institute of Ecology, UGA, Department of Entomology, UGA, Biology Department, University of Alabama.
   
   We have been continuously enriching a detritus-based headwater stream for 2.5 years. We have observed remarkable changes in the production of heterotrophic microbes and invertebrates, and consequent effects on ecosystem function in terms of decomposition rates and carbon balance. As a result of our long-term nutrient enrichment, fungal production and activity dramatically increased, decay rates of leaf litter increased, benthic organic matter standing crop has been reduced and export of fine particulate organic matter has increased. Long-term data from these streams indicate that benthic invertebrate production is highly correlated with benthic organic matter. Thus, despite increased invertebrate biomass and production we have observed after one year of enrichment, we predict that over longer time scales (> 3 yr) annual invertebrate production will return to a lower stable mean in the treatment stream, based on carbon limitation. The first objective of this study is to continue the nutrient enrichment of this headwater stream for one year to examine longer-term trends in system changes. The effects we’ve observed in this system have been largely driven by fungal rather than bacterial response, suggesting a keystone role of fungi in driving the response of a detritus-based system to nutrient enrichment. The second objective of the study is to determine the role of fungi in controlling the system-wide response to nutrient addition. We propose to test the keystone role of fungi by excluding or reducing fungal response to enrichment (by continuous addition of a fungicide) and measuring changes in the response of other variables. Critical to our predictions is whether there is a synergistic, facilitative relationship between fungi and bacteria, or whether these groups of microbes compete (e.g., for labile carbon or nutrients). Whether bacteria respond positively or negatively to reductions in fungi will test these alternative hypotheses. To the extent that microbial diversity and production is important to invertebrate production and ecosystem function will also be shown via differences in production and litter processing following reductions in fungi. The proposed study will add to our understanding of mechanisms driving detritus-based system response to nutrient enrichment and the role of microbial diversity in ecosystem functioning.
   
   

For further information about Center activities and opportunities, or to
send news about your own research programs, please contact

Mark D. Hunter, Director
Center of Biodiversity and Ecosystem Processes
Institute of Ecology, University of Georgia
Athens, GA 30602-2202
Phone (706) 542-1801
Email mhunter@sparc.ecology.uga.edu.

 

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