This project is no longer current
Impact of Exotic Invertebrate Invaders on Food Web Structure and Function in the Great Lakes: a Network Analysis Approach
Doran Mason - NOAA GLERL
The Great Lakes have recently undergone a second wave of species invasions dominated by exotic invertebrates- Zebra mussels (Dreissena polymorpha), Quagga mussels (D . bugensis), Bythotrephes cederstroemi and Cercopagis pengoi. Unlike previous fish invasions (e.g., sea lamprey and alewife), these invertebrates inserted themselves in the lower trophic levels and thus disruption percolates up through the food web with potential serious consequences to fish communities. This bottom-up effect on the food web eliminates the potential application and modification of traditional fisheries models to quantify and predict direction and magnitude of disruption. What is required is an ecosystem level approach to quantify the disruption and magnitude of disruption on food webs, and the differential effect on various regions of a lake and between lakes.
Early in the 20th century, the fish communities of the Laurentian Great Lakes were profoundly disrupted, and permanently changed, by a wave of vertebrate invaders: sea lamprey, alewife, and rainbow smelt (Eshenroder and Burnham-Curtis 1999). These disruptions were largely focused at the upper end of food webs, and, hence, traditional fishery models could be adapted to quantify the associated impacts. During the last 15 years, the Great Lakes have suffered a second wave of invasions featuring invertebrates: two species of dreissenids-Zebra mussels (Dreissena polymorpha) and Quagga mussels (D. bugensis) -and two predatory cladocerans - Bythotrephes cederstroemi and Cercopagis pengoi (MacIsaac 1999; MacIsaac et al. 1999). These four invertebrates are expected to disrupt, or have already disrupted, the fish communities in the Great Lakes in ways quite different from the earlier wave of vertebrate invaders. In contrast to the vertebrate invasion, invertebrate disruptions start at lower trophic levels and percolate up through the food web with potentially serious consequences for fisheries (Dermott et al. 1999; Ryan et al. 1999; Johannsson et al. 2000; Vanderploeg et al., 2002). The bottom-up effect on the food web eliminates the potential application and modification of traditional fisheries models to quantify and predict direction and magnitude of disruption.
Shuter and Mason (2000) argue that an ecosystem level approach, which incorporates field studies and modeling, is necessary to quantify and eventually predict the impacts of these recent invertebrate invaders. Moreover, factors such as lake morphology, trophic status and the temporal sequencing of invasions, all of which differ among lakes, likely modify the magnitude of the invertebrate impact and the current state of food web change. There are many areas of research currently underway that emphasize field and dynamic modeling approaches. However, many of these studies focus only on a subset of the system and none of them evaluates and quantifies the current state of the entire food web before and after invertebrate invasion. What is required are techniques that synoptically evaluate and quantify the structure and flows in food webs across temporal (seasons and years) and spatial (trophic status, nearshore-offshore) gradients, and how changes in the food web, from invasive invertebrates, disrupt and change these structures and flows.
Network analysis is a technique that allows one to quantify the structure and function of ecosystems by evaluating biomasses and energy flow in a food web. Efficiency with which energy and material is transferred, assimilated, and dissipated conveys significant information about the structure and function of food webs (Ulanowicz and Platt 1985; Baird and Ulanowicz 1989 and 1993; Baird et al. 1991; Ulanowicz and Wulff 1991). Network analysis evaluates these components within a food web context using input/output analysis, trophic and cycle analysis, and information theory to calculate ecosystem properties (see below for details). Thus, changes in fish communities can be linked directly to changes occurring within an ecosystem. Network analysis has been used to compare ecosystems of different size, geographical location, hydrological characteristics, and trophic status (Baird et al. 1991; Ulanowicz and Wulff 1991; Baird and Ulanowicz 1993; Monaco and Ulanowicz 1997). Most recently, arguments have been made for the use of network analysis for quantifying the health and integrity of ecosystems (Ulanowicz 2000) and evaluating the magnitude of stress imposed on an ecosystem (Ulanowicz 1995; Mageau et al. 1998). Related to all of these examples, but absent from the list of applications, is quantifying the ecosystem level impact of exotic invaders and how this impact manifests itself in fish communities.
We propose to construct network models of food webs for the Bay of Quinte Lake Ontario, Oneida Lake, and Lake Michigan across seasons (when data are available), years (pre-invertebrate invasion and post invasion), and trophic status (e.g., Bay of Quinte - western, central, eastern basins; Lake Michigan- nearshore, offshore) to quantify how these invertebrate invasions disrupted lake wide and regional food webs. In additional, our analysis and comparisons within and among seasons, years, regions and lakes should also allow us: (1) to explore various operating hypotheses, acknowledged throughout the Great Lakes Basin, concerning how invaders have changed the ecosystem (e.g., Zebra mussels changing systems to benthic dominated systems), (2) to potential tease apart the effects of nutrient reduction (indirectly determined through changes in primary productivity) and invertebrate invasions on ecosystem structure and function, and (3) to provide a synthesis that may easily be extended to other lakes to understand changes in the ecosystem or to predict potential future changes in other Great Lakes (e.g., Lake Superior).
To achieve our goals and objectives we must upgrade the old DOS code used for network analysis. NETWRK was developed by R.E. Ulanowicz (Ulanowicz and Kay 1991) in the late 1980s (some algorithms where updated in 1999) and is a DOS-based package written in ANSI Standard FORTRAN IV. NETWRK has non-intuitive input structures, data are tedious to enter, programs are command line driven, and results are output as a large report text file. Much of the analysis then comes from extracting the information from the output file, manipulating and finally summarizing the information. The complicated procedures for using the program have restricted its use by others. Heymans and Baird (2000) comment that upgrading NETWRK would “…...be of enormous value to ecosystem analysis′”. Thus, as part of this project, we will upgrade and enhance the NETWRK software to include a graphical user interface, easy and intuitive data input, graphics and analysis, and formatted output. In addition, we will add new features to the program to extend it capabilities and applications. These capabilities may include: uncertainty analysis, improved food web balancing algorithms, system dynamics for evolving systems and potential future states, etc. The product will be a software package available to researchers and resource managers for quantifying the flows and structures in food webs.
Our overall objective is to quantify the change in food web structures and flows on fish communities in Great Lakes because of invasions by invertebrate species (e.g., Zebra mussels, Quagga mussels, Bythotrephes, and Cercopagis). Specifically, our objectives are:
Lake Michigan Food Web Diagram
We participated in the fall 2002 workshop of the Bay of Quinte and Oneida Lake group (also funded by the GLFC) held at the Cornell Biological Field Station to determine the availability of data for constructing the food webs. A second workshop was held this past spring (2003) at the Canadian Center for Inland Waters (Burlington, ONT), but we were unable to attend due to the SARS outbreak in Toronto and the corresponding government travel restrictions. A full-scale effort on this objective will likely begin in the fall of 2003 when the graduate student on this project starts at Michigan State University.
The development and parameterization of the two preliminary food webs for Lake Michigan, pre and post-Zebra mussel invasion, have been constructed using the tools available through ECOPATH (www.ecopath.org). We presented our preliminary results at two scientific conferences last summer (2002). The first presentation was entitled, “Disruption of an Ecosystem: Changes in ecosystem properties following the establishment of an exotic mussel in Lake Michigan,” and was presented at the American Society of Limnology and Oceanography, Victoria, BC. The second one entitled, “Disruption of Lake Michigan’s ecosystem processes by the invertebrate community: implications for the fish community,” was presented at the American Fisheries Society Conference, Baltimore, MD. In addition, DMM was invited to give a seminar on invasive species and food web disruption at the University of Florida. In preparing the material for these presentations, we identified inherent biases in the food web balancing algorithm that estimates respiration rates for species. (Note that in ECOPATH, respiration can only be estimated from the balancing algorithm and not manually entered). For example, the ratio of respiration to consumption (R/C) cannot exceed the value of 1.0 based on the principles of thermodynamics (i.e., you cannot respire more energy than you can consume in a balanced food web). However, on several occasions the balancing algorithm estimated the R/C > 1.0. Because estimates of respiration are critical to the analysis and that the estimates derived from ECOPATH are suspect, we consider the two Lake Michigan food webs preliminary. The problem associated with ECOPATH is rectified in the new software that we are developing to perform the network analysis.
Despite the prerequisites of having the food webs developed from objectives 1 and 2 prior to working on this objective, we have made theoretical progress. We have used a new technique from the social sciences towards understanding community structure in complex food webs. The technique “Cohesion Analysis” identifies compartments (communities) where interactions within compartments are stronger than interactions between compartments. The technique has four properties that are crucial towards us achieving our objectives of quantifying structural changes in the food web. First, taxa are assigned to non-overlapping compartments that maximize the concentration of interactions within all of the compartments of the food web. Second, the algorithm generally requires no a priori specification of subjective parameters (including number of compartments) and thus removing arbitrary decisions of who belongs where. Third, the algorithm has been applied to extensive simulated data with known compartment assignments allowing one to calibrate the performance of algorithm. Fourth, compartment boundaries can be embedded in a graphical representation of the food web, thus facilitating interpretation. We will use this new technique on the various food webs to explore and quantify how invasive invertebrates have restructured community dynamics within complex food webs of the Great Lakes.
Krause, A.E, K.A. Frank, D.M. Mason, R.E. Ulanowicz, and W.W. Taylor. 2003. Compartments revealed in food web structure. Nature 426:282-285.
Mason, D.M. 2003. Quantifying the impact of exotic invertebrate invaders on food web structure and function in the Great Lakes: A network analysis approach. Interim Progress Report to the Great Lakes Fisheries Commission- yr 1. 3pp.
EcoNetwrk is based on the text-based application Netwrk 4.2 by Robert E. Ulanowicz. EcoNetwrk performs all of the analysis of Netwrk 4.2 but in a windows friendly environment. Netwrk 4.2 is copyrighted (1982, 1987, 1998, 1999) and was used with the author’s permission and guidance.
Mason, D.M., A. Jaeger, A.E. Krause. Food web disruption: Comparison of Lakes Michigan and Huron. Salmonid Communities in the Great Lakes: Special session- What does the Future Hold for the Laurentian Great Lakes. 134th Annual Meeting of the American Fisheries Society. Madison WI. August 21-26, 2004.
Krause, A.E., K.A. Frank, D.M. Mason, and W.W. Taylor. Changes in the connectivity pattern of a food-web network after biological invasions. Special Session- Integrating Approaches to Connectivity: Landscapes, Patches & Networks. 89th ESA Annual Meeting of the Ecological Society of America, Portland, Oregon. August 1-6, 2004.
Krause, Ann E., W. W. Taylor, and D. M. Mason. 2004. Assessing the potential impact of fishing on compartments in the food web of southeastern Lake Michigan. Quantitative Ecosystem Indicators for Fisheries Management. International Symposium. March 31-April 3, 2004. Paris France.
Mason, D.M. Structure and function in aquatic ecosystems: from food webs to habitat. Florida Atlantic University. March 1, 2004.
Mason, D.M. Application of network analysis to the Great Lakes. National Science Foundation Long Term Ecological Research (LTER)-All Scientist Meeting. Seattle, WA. September 19-21, 2003. (Invited)
Krause, A.E., and D.M. Mason. Invasive species and food web disruption: Example from the Laurentian Great Lakes. 17th Biannual conference of the Estuarine Research Federation. Seattle, WA. September 14-18, 2003. (Invited)
Mason, D.M. Invasive species and food web disruption. University of Florida. February 29, 2003. (Invited)
Krause, A.E., D.M. Mason, K. Franks, and R.E. Ulanowicz. Intuitive compartments: the other half of trophic structure in food webs. 88th Annual Meeting of the Ecological Society of American. Savannah, GA., August 3-8, 2003.
Mason, D.M., and A.E. Krause. Food web structure and energy flow: implications for lake whitefish production in Lake Michigan. Great Lakes Whitefish-Diporeia Workshop. Ann Arbor, MI February 26-27, 2002.
Mason, D.M. Disruption of Food Webs in the Great Lakes. The Great Lakes: Current Issues and Future Challenges. Annual Great Lakes Conference, Agriculture and Natural Resources Week, Michigan State University. March 7, 2002.
Krause, A.E., and D.M. Mason. Disruption of an ecosystem: Changes in ecosystem properties following the establishment of an exotic mussel in Lake Michigan. 2002 summer meeting of the American Association of Limnology and Oceanography, June 10-14, 2002 · Victoria, British Columbia, Canada.
Krause, A.E. and D.M. Mason. Disruption of Lake Michigan’s ecosystem processes by the invertebrate community: implications for the fish community. 132nd AFS Annual Meeting, Baltimore, MD, August 18-22, 2002.
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