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Modeling Species Invasions in Ecopath with Ecosim: an Evaluation Using Ecological Modelling 247 (2012) 251–261 Contents lists available at SciVerse ScienceDirect Ecological Modelling jo urnal homepage: www.elsevier.com/locate/ecolmodel Modeling species invasions in Ecopath with Ecosim: An evaluation using Laurentian Great Lakes models a,∗ b c Brian J. Langseth , Mark Rogers , Hongyan Zhang a Quantitative Fisheries Center, 153 Giltner Hall, Michigan State University, East Lansing, MI 48824, USA b U.S. Geological Survey, Great Lakes Science Center, Lake Erie Biological Station, 6100 Columbus Avenue, Sandusky, OH 44870, USA c Cooperative Institute for Limnology and Ecosystems Research, University of Michigan, Ann Arbor, MI 48108, USA a r t i c l e i n f o a b s t r a c t Article history: Invasive species affect the structure and processes of ecosystems they invade. Invasive species have been Received 7 May 2012 particularly relevant to the Laurentian Great Lakes, where they have played a part in both historical and Received in revised form 20 August 2012 recent changes to Great Lakes food webs and the fisheries supported therein. There is increased interest Accepted 21 August 2012 in understanding the effects of ecosystem changes on fisheries within the Great Lakes, and ecosystem models provide an essential tool from which this understanding can take place. A commonly used model Keywords: for exploring fisheries management questions within an ecosystem context is the Ecopath with Ecosim Invasive species (EwE) modeling software. Incorporating invasive species into EwE models is a challenging process, and Food web models Ecopath descriptions and comparisons of methods for modeling species invasions are lacking. We compared four Ecosim methods for incorporating invasive species into EwE models for both Lake Huron and Lake Michigan based Great Lakes on the ability of each to reproduce patterns in observed data time series. The methods differed in whether invasive species biomass was forced in the model, the initial level of invasive species biomass at the beginning of time dynamic simulations, and the approach to cause invasive species biomass to increase at the time of invasion. The overall process of species invasion could be reproduced by all methods, but fits to observed time series varied among the methods and models considered. We recommend forcing invasive species biomass when model objectives are to understand ecosystem impacts in the past and when time series of invasive species biomass are available. Among methods where invasive species time series were not forced, mediating the strength of predator–prey interactions performed best for the Lake Huron model, but worse for the Lake Michigan model. Starting invasive species biomass at high values and then artificially removing biomass until the time of invasion performed well for both models, but was more complex than starting invasive species biomass at low values. In general, for understanding the effect of invasive species on future fisheries management actions, we recommend initiating invasive species biomass at low levels based on the greater simplicity and realism of the method compared to others. © 2012 Elsevier B.V. All rights reserved. 1. Introduction quagga (D. bugensis) mussel (hereafter referred to as dreissenids), and round goby (Neogobius melanostomus) have all received exten- Non-native species are a continual threat to the maintenance sive attention for their perceived ability to alter Great Lakes food of Great Lakes ecosystems. Approximately 185 non-native species webs. Bythotrephes invaded the Great Lakes during the 1980s have invaded the Laurentian Great Lakes (Ricciardi, 2006) with (Vanderploeg et al., 2002), and preys heavily on mesozooplankton effects ranging from mild to severe. Non-native species that become (Bunnell et al., 2011). Dreissenids were first observed in Lake St. established, increase in abundance, and alter system processes are Clair in 1988 (Vanderploeg et al., 2002), and have been suggested termed “invasive” (Williamson and Fitter, 1996) and are the focus to contribute to shifts in the zooplankton community (Barbiero of policy, research, and restoration efforts within the Great Lakes. et al., 2009; Bunnell et al., 2011), declines in prey fish abundances Several non-native species from the Ponto-Caspian region of (Riley et al., 2008), reductions in the spring phytoplankton bloom Eurasia have recently invaded the Great Lakes. The spiny-water (Barbiero and Tuchman, 2004), and declines in abundance of the flea (Bythotrephes longimanus), zebra (Dreissena polymorpha) and native benthic amphipod (Diporeia spp.; Nalepa et al., 2007). Round goby were first observed in the St. Clair River in 1990 (Vanderploeg et al., 2002) and compete with other benthic fish, predominantly ∗ sculpins. Corresponding author. Tel.: +1 517 355 0126; fax: +1 517 355 0138. Developing ecosystem models facilitate with assessment of E-mail addresses: [email protected] (B.J. Langseth), [email protected] recent changes to Great Lakes food webs, including the effect (M. Rogers), [email protected] (H. Zhang). 0304-3800/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecolmodel.2012.08.015 252 B.J. Langseth et al. / Ecological Modelling 247 (2012) 251–261 of invasive species. Ecosystem model development also supports 2. Methods ecosystem based approaches to fisheries management, which are becoming more prevalent (Pikitch et al., 2004). A popular tool for 2.1. Ecopath with Ecosim models exploring food-web dynamics and assessing multi-species man- agement objectives is the Ecopath with Ecosim (EwE) computer We developed two Ecopath with Ecosim models for lakes Huron software (Halfon and Schito, 1993; Halfon et al., 1996; Kitchell et al., and Michigan, to evaluate alternative methods for modeling inva- 2000; Cox and Kitchell, 2004; Stewart and Sprules, 2011). Ecopath sive species. Both models focused on the offshore fish communities, with Ecosim requires a mass-balance description of a food web at an so groups primarily inhabiting nearshore areas were excluded. initial point in time (Ecopath; Christensen and Pauly, 1992), which Within the offshore fish community, only groups hypothesized as is then used as the basis for forward-projecting time-dynamic sim- most important to food-web function were included in the mod- ulations (Ecosim; Christensen and Walters, 2004). els. The Lake Huron Ecopath model was parameterized using data Simulating species invasions in EwE models is challenging. collected around 1981 for 21 unique species or groups of species, Species included in Ecosim time-dynamic simulations must have while the Lake Michigan Ecopath model was parameterized using positive biomass when the model is initialized and balanced in Eco- data collected around 1987 for 29 unique species or groups of path. This presents a problem for species that invade the system species. Invasive species invading after the initial Ecopath year after the initial Ecopath year. One solution to account for inva- in the Lake Huron model included Bythotrephes, round goby, and sive species effects has been to construct separate pre-invasion dreissenids, whereas invasive species in the Lake Michigan model and post-invasion Ecopath food-web models (e.g. Jaeger, 2006; included round goby and dreissenids. Bythotrephes invaded Lake Stewart and Sprules, 2011). Such an approach only utilizes Eco- Michigan prior to the year we initialized the Lake Michigan model path, and precludes opportunities to explore dynamic simulations (i.e. 1987), and therefore were modeled as if they were non-invasive in Ecosim. A solution that utilizes Ecosim is to run simulations in (i.e. actually already present in the system in the initial Ecopath Ecosim based on an Ecopath model initialized in a year after all year). Age-stanzas were included for some species that underwent invasive species were present, and therefore when biomass val- ontogenetic diet shifts or were targeted by fisheries, increasing the ues were truly positive. Time series of data are used by Ecosim total number of modeled groups to 36 for the Lake Huron model to tune model parameters and evaluate model performance, and and 42 for the Lake Michigan model (Table 1). therefore initializing Ecopath in a later year excludes available time series data from the model-fitting process that could inform species Table 1 interactions prior to the invasion. Species or species groupings in the Lake Huron (H) and Lake Michigan (M) Ecopath Another approach to simulate species invasions in Ecosim mod- with Ecosim models. Age stanzas for multi-stanza groups are provided in years. els has been used, and does not reduce the length of data time Group/species name (age stanzas) Scientific name Model series. With this approach, invasive species are initialized at some a positive biomass value in Ecopath, prior to actual invasion, arti- Sea lamprey Petromyzon marinus H, M Lake whitefish Coregonus clupeaformis M ficially maintained at negligibly low biomasses until the year of (0, 1–3, 4+) H invasion, and then afterwards allowed to proliferate. For exam- Lake trout Salvelinus namaycush ple, Pine et al. (2007) simulated the invasion of flathead catfish (0, 1, 2–4, 5+) H (Pylodictis olivaris) into an inland reservoir by artificially increas- (0, 1–5, 6+) M Stocked Chinook salmon Oncorhynchus tshawytscha ing fishing mortality on catfish, to explore what the system might (0, 0.5, 1–5, 6+) H have looked like prior to invasion, and then reducing fishing mor- (0, 1–5, 6+) M tality to allow catfish to invade. Similarly, Espinosa-Romero et al. Wild Chinook salmon (0–5, 6+) Oncorhynchus tshawytscha M (2011) initially suppressed biomass of sea otters (Enhydra lutris) Steelhead Oncorhynchus mykiss by applying high artificial fishing mortality, and then released fish- (0, 1, 2–5, 6+) H (0, 1–5, 6+) M ing mortality in order to simulate reintroductions across a spatial Brown trout (0, 1–5) Salmo trutta M gradient. Forcing biomass of an invasive species (i.e. specifying a Coho salmon (0, 1–2, 3+) Oncorhynchus kisutch M time series rather than dynamically modeling it) has also been Burbot Lota lota M successfully employed to model lionfish (Pterois volitans) invasion (0–3, 3+) H Alewife (0, 1+) Alosa pseudoharengus H, M in the Caribbean (V.
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