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\n  \n 2024\n \n \n (1)\n \n \n
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\n \n\n \n \n Díaz‐Yáñez, O.; Käber, Y.; Anders, T.; Bohn, F.; Braziunas, K. H.; Brůna, J.; Fischer, R.; Fischer, S. M.; Hetzer, J.; Hickler, T.; Hochauer, C.; Lexer, M. J.; Lischke, H.; Mairota, P.; Merganič, J.; Merganičová, K.; Mette, T.; Mina, M.; Morin, X.; Nieberg, M.; Rammer, W.; Reyer, C. P. O.; Scheiter, S.; Scherrer, D.; and Bugmann, H.\n\n\n \n \n \n \n \n Tree regeneration in models of forest dynamics: A key priority for further research.\n \n \n \n \n\n\n \n\n\n\n Ecosphere, 15(3): e4807. March 2024.\n \n\n\n\n
\n\n\n\n \n \n \"TreePaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
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@article{diazyanez_tree_2024,\n\ttitle = {Tree regeneration in models of forest dynamics: {A} key priority for further research},\n\tvolume = {15},\n\tcopyright = {All rights reserved},\n\tissn = {2150-8925, 2150-8925},\n\tshorttitle = {Tree regeneration in models of forest dynamics},\n\turl = {https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.4807},\n\tdoi = {10.1002/ecs2.4807},\n\tabstract = {Tree regeneration is a key process in forest dynamics, particularly in the context of forest resilience and climate change. Models are pivotal for assessing long‐term forest dynamics, and they have been in use for more than 50 years. However, there is a need to evaluate their capacity to accurately represent tree regeneration. We assess how well current models capture the overall abundance, species composition, and mortality of tree regeneration. Using 15 models built to capture long‐term forest dynamics at the stand, landscape, and global levels, we simulate tree regeneration at 200 sites representing large environmental gradients across Central Europe. The results are evaluated against extensive data from unmanaged forests. Most of the models overestimate recruitment levels, which is compensated only in some models by high simulated mortality rates in the early stages of individual‐tree dynamics. Simulated species diversity of recruitment generally matches observed ranges. Models simulating higher stand‐level species diversity do not feature higher species diversity in the recruitment layer. The effect of light availability on recruitment levels is captured better than the effects of temperature and soil moisture, but patterns are not consistent across models. Increasing complexity in the tree regeneration modules is not related to higher accuracy of simulated tree recruitment. Furthermore, individual model design is more important than scale (stand, landscape, and global) and approach (empirical and process‐based) for accurately capturing tree regeneration. Despite the mismatches between simulation results and data, it is remarkable that most models capture the essential features of the highly complex process of tree regeneration, while not having been parameterized with such data. We conclude that much can be gained by evaluating and refining the modeling of tree regeneration processes. This has the potential to render long‐term projections of forest dynamics under changing environmental conditions much more robust.},\n\tlanguage = {en},\n\tnumber = {3},\n\turldate = {2024-04-02},\n\tjournal = {Ecosphere},\n\tauthor = {Díaz‐Yáñez, Olalla and Käber, Yannek and Anders, Tim and Bohn, Friedrich and Braziunas, Kristin H. and Brůna, Josef and Fischer, Rico and Fischer, Samuel M. and Hetzer, Jessica and Hickler, Thomas and Hochauer, Christian and Lexer, Manfred J. and Lischke, Heike and Mairota, Paola and Merganič, Ján and Merganičová, Katarina and Mette, Tobias and Mina, Marco and Morin, Xavier and Nieberg, Mats and Rammer, Werner and Reyer, Christopher P. O. and Scheiter, Simon and Scherrer, Daniel and Bugmann, Harald},\n\tmonth = mar,\n\tyear = {2024},\n\tpages = {e4807},\n}\n\n
\n
\n\n\n
\n Tree regeneration is a key process in forest dynamics, particularly in the context of forest resilience and climate change. Models are pivotal for assessing long‐term forest dynamics, and they have been in use for more than 50 years. However, there is a need to evaluate their capacity to accurately represent tree regeneration. We assess how well current models capture the overall abundance, species composition, and mortality of tree regeneration. Using 15 models built to capture long‐term forest dynamics at the stand, landscape, and global levels, we simulate tree regeneration at 200 sites representing large environmental gradients across Central Europe. The results are evaluated against extensive data from unmanaged forests. Most of the models overestimate recruitment levels, which is compensated only in some models by high simulated mortality rates in the early stages of individual‐tree dynamics. Simulated species diversity of recruitment generally matches observed ranges. Models simulating higher stand‐level species diversity do not feature higher species diversity in the recruitment layer. The effect of light availability on recruitment levels is captured better than the effects of temperature and soil moisture, but patterns are not consistent across models. Increasing complexity in the tree regeneration modules is not related to higher accuracy of simulated tree recruitment. Furthermore, individual model design is more important than scale (stand, landscape, and global) and approach (empirical and process‐based) for accurately capturing tree regeneration. Despite the mismatches between simulation results and data, it is remarkable that most models capture the essential features of the highly complex process of tree regeneration, while not having been parameterized with such data. We conclude that much can be gained by evaluating and refining the modeling of tree regeneration processes. This has the potential to render long‐term projections of forest dynamics under changing environmental conditions much more robust.\n
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\n  \n 2023\n \n \n (1)\n \n \n
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\n \n\n \n \n Fischer, S. M.; Ramazi, P.; Simmons, S.; Poesch, M. S.; and Lewis, M. A.\n\n\n \n \n \n \n \n Boosting propagule transport models with individual‐specific data from mobile apps.\n \n \n \n \n\n\n \n\n\n\n Journal of Applied Ecology,1365–2664.14356. January 2023.\n \n\n\n\n
\n\n\n\n \n \n \"BoostingPaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
\n
@article{fischer_boosting_2023,\n\ttitle = {Boosting propagule transport models with individual‐specific data from mobile apps},\n\tcopyright = {All rights reserved},\n\tissn = {0021-8901, 1365-2664},\n\turl = {https://onlinelibrary.wiley.com/doi/10.1111/1365-2664.14356},\n\tdoi = {10.1111/1365-2664.14356},\n\tabstract = {1. Management of invasive species and pathogens requires information about the traffic of potential vectors. Such information is often taken from vector traffic models fitted to survey data. Here, user-specific data collected via mobile apps offer new opportunities to obtain more accurate estimates and to analyse how vectors' individual preferences affect propagule flows. However, data voluntarily reported via apps may lack some trip records, adding a significant layer of uncertainty. We show how the benefits of app-based data can be exploited despite this drawback.\n\n2. Based on data collected via an angler app, we built a stochastic model for angler traffic in the Canadian province Alberta. There, anglers facilitate the spread of whirling disease, a parasite-induced fish disease. The model is temporally and spatially explicit and accounts for individual preferences and repeating behaviour of anglers, helping to address the problem of missing trip records.\n\n3. We obtained estimates of angler traffic between all subbasins in Alberta. The model's accuracy exceeds that of direct empirical estimates even when fewer data were used to fit the model. The results indicate that anglers' local preferences and their tendency to revisit previous destinations reduce the number of long interwaterbody trips potentially dispersing whirling disease. According to our model, anglers revisit their previous destination in 64\\% of their trips, making these trips irrelevant for the spread of whirling disease. Furthermore, 54\\% of fishing trips end in individual-specific spatially contained areas with mean radius of 54.7 km. Finally, although the fraction of trips that anglers report was unknown, we were able to estimate the total yearly number of fishing trips in Alberta, matching an independent empirical estimate.\n\n4. Policy implications. We make two major contributions: (1) we provide a model that uses mobile app data to boost the mechanistic accuracy of classic propagule transport models, and (2) we demonstrate the importance of individual-specific behaviour of vectors for propagule transport. Ignoring vectors' local preferences and their tendency to revisit previous destinations can lead to significant overestimates of vector traffic and biased estimates of propagule flows. This has clear implications for the management of invasive species and animal diseases.},\n\tlanguage = {en},\n\turldate = {2023-04-25},\n\tjournal = {Journal of Applied Ecology},\n\tauthor = {Fischer, Samuel M. and Ramazi, Pouria and Simmons, Sean and Poesch, Mark S. and Lewis, Mark A.},\n\tmonth = jan,\n\tyear = {2023},\n\tpages = {1365--2664.14356},\n}\n\n
\n
\n\n\n
\n 1. Management of invasive species and pathogens requires information about the traffic of potential vectors. Such information is often taken from vector traffic models fitted to survey data. Here, user-specific data collected via mobile apps offer new opportunities to obtain more accurate estimates and to analyse how vectors' individual preferences affect propagule flows. However, data voluntarily reported via apps may lack some trip records, adding a significant layer of uncertainty. We show how the benefits of app-based data can be exploited despite this drawback. 2. Based on data collected via an angler app, we built a stochastic model for angler traffic in the Canadian province Alberta. There, anglers facilitate the spread of whirling disease, a parasite-induced fish disease. The model is temporally and spatially explicit and accounts for individual preferences and repeating behaviour of anglers, helping to address the problem of missing trip records. 3. We obtained estimates of angler traffic between all subbasins in Alberta. The model's accuracy exceeds that of direct empirical estimates even when fewer data were used to fit the model. The results indicate that anglers' local preferences and their tendency to revisit previous destinations reduce the number of long interwaterbody trips potentially dispersing whirling disease. According to our model, anglers revisit their previous destination in 64% of their trips, making these trips irrelevant for the spread of whirling disease. Furthermore, 54% of fishing trips end in individual-specific spatially contained areas with mean radius of 54.7 km. Finally, although the fraction of trips that anglers report was unknown, we were able to estimate the total yearly number of fishing trips in Alberta, matching an independent empirical estimate. 4. Policy implications. We make two major contributions: (1) we provide a model that uses mobile app data to boost the mechanistic accuracy of classic propagule transport models, and (2) we demonstrate the importance of individual-specific behaviour of vectors for propagule transport. Ignoring vectors' local preferences and their tendency to revisit previous destinations can lead to significant overestimates of vector traffic and biased estimates of propagule flows. This has clear implications for the management of invasive species and animal diseases.\n
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\n  \n 2021\n \n \n (3)\n \n \n
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\n \n\n \n \n Ramazi, P.; Fischer, S. M.; Alexander, J.; James, C.; Paul, A. J.; Greiner, R.; and Lewis, M. A\n\n\n \n \n \n \n \n M. cerebralis establishment and spread: A graphical synthesis.\n \n \n \n \n\n\n \n\n\n\n Canadian Journal of Fisheries and Aquatic Sciences,cjfas–2020–0352. September 2021.\n \n\n\n\n
\n\n\n\n \n \n \"M.Paper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
\n
@article{ramazi_m._2021,\n\ttitle = {M. cerebralis establishment and spread: {A} graphical synthesis},\n\tcopyright = {All rights reserved},\n\tissn = {0706-652X, 1205-7533},\n\tshorttitle = {M. cerebralis establishment and spread},\n\turl = {https://cdnsciencepub.com/doi/10.1139/cjfas-2020-0352},\n\tdoi = {10.1139/cjfas-2020-0352},\n\tabstract = {M. cerebralis is the parasite causing whirling disease, which has dramatic ecological impacts due to its potential to cause high mortality in salmonids. The large-scale efforts, necessary to underpin an effective surveillance program, have practical and economic constraints. There is, hence, a clear need for models that can predict the parasite spread. Model development, however, often heavily depends on knowing influential variables and governing mechanisms. We have developed a graphical model for the establishment and spread of M. cerebralis by synthesizing experts’ opinion and empirical studies. First, we conducted a series of workshops with experts to identify variables believed to impact the establishment and spread of the parasite M. cerebralis and visualized their interactions via a directed acyclic graph. Then we refined the graph by incorporating empirical findings from the literature. The final graph’s nodes correspond to variables whose considerable impact on M. cerebralis establishment and spread is either supported by empirical data or confirmed by experts, and the graph’s directed edges represent direct causality or strong correlation. This graphical model facilitates communication and education of whirling disease and provides an empirically driven framework for constructing future models, especially Bayesian networks.},\n\tlanguage = {en},\n\turldate = {2021-09-27},\n\tjournal = {Canadian Journal of Fisheries and Aquatic Sciences},\n\tauthor = {Ramazi, Pouria and Fischer, Samuel Matthias and Alexander, Julie and James, Clayton and Paul, Andrew J. and Greiner, Russell and Lewis, Mark A},\n\tmonth = sep,\n\tyear = {2021},\n\tpages = {cjfas--2020--0352},\n}\n\n
\n
\n\n\n
\n M. cerebralis is the parasite causing whirling disease, which has dramatic ecological impacts due to its potential to cause high mortality in salmonids. The large-scale efforts, necessary to underpin an effective surveillance program, have practical and economic constraints. There is, hence, a clear need for models that can predict the parasite spread. Model development, however, often heavily depends on knowing influential variables and governing mechanisms. We have developed a graphical model for the establishment and spread of M. cerebralis by synthesizing experts’ opinion and empirical studies. First, we conducted a series of workshops with experts to identify variables believed to impact the establishment and spread of the parasite M. cerebralis and visualized their interactions via a directed acyclic graph. Then we refined the graph by incorporating empirical findings from the literature. The final graph’s nodes correspond to variables whose considerable impact on M. cerebralis establishment and spread is either supported by empirical data or confirmed by experts, and the graph’s directed edges represent direct causality or strong correlation. This graphical model facilitates communication and education of whirling disease and provides an empirically driven framework for constructing future models, especially Bayesian networks.\n
\n\n\n
\n\n\n
\n \n\n \n \n Fischer, S. M.; and Lewis, M. A.\n\n\n \n \n \n \n \n A robust and efficient algorithm to find profile likelihood confidence intervals.\n \n \n \n \n\n\n \n\n\n\n Statistics and Computing, 31(4): 38. July 2021.\n \n\n\n\n
\n\n\n\n \n \n \"APaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
\n
@article{fischer_robust_2021,\n\ttitle = {A robust and efficient algorithm to find profile likelihood confidence intervals},\n\tvolume = {31},\n\tcopyright = {All rights reserved},\n\tissn = {0960-3174, 1573-1375},\n\turl = {https://link.springer.com/10.1007/s11222-021-10012-y},\n\tdoi = {10.1007/s11222-021-10012-y},\n\tabstract = {Profile likelihood confidence intervals are a robust alternative to Wald’s method if the asymptotic properties of the maximum likelihood estimator are not met. However, the constrained optimization problem defining profile likelihood confidence intervals can be difficult to solve in these situations, because the likelihood function may exhibit unfavorable properties. As a result, existing methods may be inefficient and yield misleading results. In this paper, we address this problem by computing profile likelihood confidence intervals via a trust-region approach, where steps computed based on local approximations are constrained to regions where these approximations are sufficiently precise. As our algorithm also accounts for numerical issues arising if the likelihood function is strongly non-linear or parameters are not estimable, the method is applicable in many scenarios where earlier approaches are shown to be unreliable. To demonstrate its potential in applications, we apply our algorithm to benchmark problems and compare it with 6 existing approaches to compute profile likelihood confidence intervals. Our algorithm consistently achieved higher success rates than any competitor while also being among the quickest methods. As our algorithm can be applied to compute both confidence intervals of parameters and model predictions, it is useful in a wide range of scenarios.},\n\tlanguage = {en},\n\tnumber = {4},\n\turldate = {2021-05-27},\n\tjournal = {Statistics and Computing},\n\tauthor = {Fischer, Samuel M. and Lewis, Mark A.},\n\tmonth = jul,\n\tyear = {2021},\n\tpages = {38},\n}\n\n
\n
\n\n\n
\n Profile likelihood confidence intervals are a robust alternative to Wald’s method if the asymptotic properties of the maximum likelihood estimator are not met. However, the constrained optimization problem defining profile likelihood confidence intervals can be difficult to solve in these situations, because the likelihood function may exhibit unfavorable properties. As a result, existing methods may be inefficient and yield misleading results. In this paper, we address this problem by computing profile likelihood confidence intervals via a trust-region approach, where steps computed based on local approximations are constrained to regions where these approximations are sufficiently precise. As our algorithm also accounts for numerical issues arising if the likelihood function is strongly non-linear or parameters are not estimable, the method is applicable in many scenarios where earlier approaches are shown to be unreliable. To demonstrate its potential in applications, we apply our algorithm to benchmark problems and compare it with 6 existing approaches to compute profile likelihood confidence intervals. Our algorithm consistently achieved higher success rates than any competitor while also being among the quickest methods. As our algorithm can be applied to compute both confidence intervals of parameters and model predictions, it is useful in a wide range of scenarios.\n
\n\n\n
\n\n\n
\n \n\n \n \n Fischer, S. M.; Beck, M.; Herborg, L.; and Lewis, M. A.\n\n\n \n \n \n \n \n Managing aquatic invasions: Optimal locations and operating times for watercraft inspection stations.\n \n \n \n \n\n\n \n\n\n\n Journal of Environmental Management, 283: 111923. April 2021.\n \n\n\n\n
\n\n\n\n \n \n \"ManagingPaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
\n
@article{fischer_managing_2021,\n\ttitle = {Managing aquatic invasions: {Optimal} locations and operating times for watercraft inspection stations},\n\tvolume = {283},\n\tcopyright = {All rights reserved},\n\tissn = {03014797},\n\tshorttitle = {Managing aquatic invasions},\n\turl = {https://linkinghub.elsevier.com/retrieve/pii/S030147972031848X},\n\tdoi = {10.1016/j.jenvman.2020.111923},\n\tabstract = {Aquatic invasive species (AIS) cause significant ecological and economic damages around the world. A major spread mechanism for AIS is traffic of boaters transporting their watercraft from invaded to uninvaded waterbodies. To inhibit the spread of AIS, Canadian provinces and American states often set up watercraft inspection stations at roadsides, where potentially infested boats are screened for AIS and, if necessary, decontaminated. However, since budgets for AIS control are limited, watercraft inspection stations can only be operated at specific locations and daytimes. Though theoretical studies provide managers with general guidelines for AIS management, more specific results are needed to determine when and where watercraft inspections would be most effective. This is the subject of this paper. We show how linear integer programming techniques can be used to optimize watercraft inspection policies under budget constraints. We introduce our approach as a general framework and apply it to the prevention of the spread of zebra and quagga mussels (Dreissena spp.) to the Canadian province of British Columbia. We consider multiple scenarios and show how variations in budget constraints, propagule sources, and model uncertainty affect the optimal policy. Based on these results, we identify simple, generally applicable principles for optimal AIS management.},\n\tlanguage = {en},\n\turldate = {2021-06-18},\n\tjournal = {Journal of Environmental Management},\n\tauthor = {Fischer, Samuel M. and Beck, Martina and Herborg, Leif-Matthias and Lewis, Mark A.},\n\tmonth = apr,\n\tyear = {2021},\n\tpages = {111923},\n}\n\n
\n
\n\n\n
\n Aquatic invasive species (AIS) cause significant ecological and economic damages around the world. A major spread mechanism for AIS is traffic of boaters transporting their watercraft from invaded to uninvaded waterbodies. To inhibit the spread of AIS, Canadian provinces and American states often set up watercraft inspection stations at roadsides, where potentially infested boats are screened for AIS and, if necessary, decontaminated. However, since budgets for AIS control are limited, watercraft inspection stations can only be operated at specific locations and daytimes. Though theoretical studies provide managers with general guidelines for AIS management, more specific results are needed to determine when and where watercraft inspections would be most effective. This is the subject of this paper. We show how linear integer programming techniques can be used to optimize watercraft inspection policies under budget constraints. We introduce our approach as a general framework and apply it to the prevention of the spread of zebra and quagga mussels (Dreissena spp.) to the Canadian province of British Columbia. We consider multiple scenarios and show how variations in budget constraints, propagule sources, and model uncertainty affect the optimal policy. Based on these results, we identify simple, generally applicable principles for optimal AIS management.\n
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\n  \n 2020\n \n \n (2)\n \n \n
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\n \n\n \n \n Fischer, S. M.\n\n\n \n \n \n \n \n Locally optimal routes for route choice sets.\n \n \n \n \n\n\n \n\n\n\n Transportation Research Part B: Methodological, 141: 240–266. November 2020.\n \n\n\n\n
\n\n\n\n \n \n \"LocallyPaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
\n
@article{fischer_locally_2020,\n\ttitle = {Locally optimal routes for route choice sets},\n\tvolume = {141},\n\tcopyright = {All rights reserved},\n\tissn = {01912615},\n\turl = {https://linkinghub.elsevier.com/retrieve/pii/S019126152030391X},\n\tdoi = {10.1016/j.trb.2020.09.007},\n\tabstract = {Route choice is often modelled as a two-step procedure in which travellers choose their routes from small sets of promising candidates. Many methods developed to identify such choice sets rely on assumptions about the mechanisms behind the route choice and require corresponding data sets. Furthermore, existing approaches often involve considerable complexity or perform many repeated shortest path queries. This makes it difficult to apply these methods in comprehensive models with numerous origin-destination pairs. In this paper, we address these issues by developing an algorithm that efficiently identifies locally optimal routes. Such paths arise from travellers acting rationally on local scales, whereas unknown factors may affect the routes on larger scales. Though methods identifying locally optimal routes are available already, these algorithms rely on approximations and return only few, heuristically chosen paths for specific origin-destination pairs. This conflicts with the demands of route choice models, where an exhaustive search for many origins and destinations would be necessary. We therefore extend existing algorithms to return (almost) all admissible paths between a large number of origin-destination pairs. We test our algorithm and its applicability in route choice models on the road network of the Canadian province British Columbia and empirical data collected in this province.},\n\tlanguage = {en},\n\turldate = {2020-12-14},\n\tjournal = {Transportation Research Part B: Methodological},\n\tauthor = {Fischer, Samuel M.},\n\tmonth = nov,\n\tyear = {2020},\n\tpages = {240--266},\n}\n\n
\n
\n\n\n
\n Route choice is often modelled as a two-step procedure in which travellers choose their routes from small sets of promising candidates. Many methods developed to identify such choice sets rely on assumptions about the mechanisms behind the route choice and require corresponding data sets. Furthermore, existing approaches often involve considerable complexity or perform many repeated shortest path queries. This makes it difficult to apply these methods in comprehensive models with numerous origin-destination pairs. In this paper, we address these issues by developing an algorithm that efficiently identifies locally optimal routes. Such paths arise from travellers acting rationally on local scales, whereas unknown factors may affect the routes on larger scales. Though methods identifying locally optimal routes are available already, these algorithms rely on approximations and return only few, heuristically chosen paths for specific origin-destination pairs. This conflicts with the demands of route choice models, where an exhaustive search for many origins and destinations would be necessary. We therefore extend existing algorithms to return (almost) all admissible paths between a large number of origin-destination pairs. We test our algorithm and its applicability in route choice models on the road network of the Canadian province British Columbia and empirical data collected in this province.\n
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\n \n\n \n \n Fischer, S. M.; Beck, M.; Herborg, L.; and Lewis, M. A.\n\n\n \n \n \n \n \n A hybrid gravity and route choice model to assess vector traffic in large-scale road networks.\n \n \n \n \n\n\n \n\n\n\n Royal Society Open Science, 7(5): 191858. May 2020.\n \n\n\n\n
\n\n\n\n \n \n \"APaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
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@article{fischer_hybrid_2020,\n\ttitle = {A hybrid gravity and route choice model to assess vector traffic in large-scale road networks},\n\tvolume = {7},\n\tcopyright = {All rights reserved},\n\tissn = {2054-5703, 2054-5703},\n\turl = {https://royalsocietypublishing.org/doi/10.1098/rsos.191858},\n\tdoi = {10.1098/rsos.191858},\n\tabstract = {Human traffic along roads can be a major vector for infectious diseases and invasive species. Though most road traffic is local, a small number of long-distance trips can suffice to move an invasion or disease front forward. Therefore, understanding how many agents travel over long distances and which routes they choose is key to successful management of diseases and invasions. Stochastic gravity models have been used to estimate the distribution of trips between origins and destinations of agents. However, in large-scale systems, it is hard to collect the data required to fit these models, as the number of long-distance travellers is small, and origins and destinations can have multiple access points. Therefore, gravity models often provide only relative measures of the agent flow. Furthermore, gravity models yield no insights into which roads agents use. We resolve these issues by combining a stochastic gravity model with a stochastic route choice model. Our hybrid model can be fitted to survey data collected at roads that are used by many long-distance travellers. This decreases the sampling effort, allows us to obtain absolute predictions of both vector pressure and pathways, and permits rigorous model validation. After introducing our approach in general terms, we demonstrate its benefits by applying it to the potential invasion of zebra and quagga mussels (\n              Dreissena\n              spp.) to the Canadian province British Columbia. The model yields an\n              R\n              2\n              -value of 0.73 for variance-corrected agent counts at survey locations.},\n\tlanguage = {en},\n\tnumber = {5},\n\turldate = {2020-10-03},\n\tjournal = {Royal Society Open Science},\n\tauthor = {Fischer, S. M. and Beck, M. and Herborg, L.-M. and Lewis, M. A.},\n\tmonth = may,\n\tyear = {2020},\n\tpages = {191858},\n}\n
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\n Human traffic along roads can be a major vector for infectious diseases and invasive species. Though most road traffic is local, a small number of long-distance trips can suffice to move an invasion or disease front forward. Therefore, understanding how many agents travel over long distances and which routes they choose is key to successful management of diseases and invasions. Stochastic gravity models have been used to estimate the distribution of trips between origins and destinations of agents. However, in large-scale systems, it is hard to collect the data required to fit these models, as the number of long-distance travellers is small, and origins and destinations can have multiple access points. Therefore, gravity models often provide only relative measures of the agent flow. Furthermore, gravity models yield no insights into which roads agents use. We resolve these issues by combining a stochastic gravity model with a stochastic route choice model. Our hybrid model can be fitted to survey data collected at roads that are used by many long-distance travellers. This decreases the sampling effort, allows us to obtain absolute predictions of both vector pressure and pathways, and permits rigorous model validation. After introducing our approach in general terms, we demonstrate its benefits by applying it to the potential invasion of zebra and quagga mussels ( Dreissena spp.) to the Canadian province British Columbia. The model yields an R 2 -value of 0.73 for variance-corrected agent counts at survey locations.\n
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\n  \n 2019\n \n \n (1)\n \n \n
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\n \n\n \n \n Fischer, S. M.; and Huth, A.\n\n\n \n \n \n \n \n An approach to study species persistence in unconstrained random networks.\n \n \n \n \n\n\n \n\n\n\n Scientific Reports, 9(1): 14110. December 2019.\n \n\n\n\n
\n\n\n\n \n \n \"AnPaper\n  \n \n\n \n \n doi\n  \n \n\n \n link\n  \n \n\n bibtex\n \n\n \n  \n \n abstract \n \n\n \n\n \n \n \n \n \n \n \n\n  \n \n \n\n\n\n
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@article{fischer_approach_2019,\n\ttitle = {An approach to study species persistence in unconstrained random networks},\n\tvolume = {9},\n\tcopyright = {All rights reserved},\n\tissn = {2045-2322},\n\turl = {http://www.nature.com/articles/s41598-019-50373-z},\n\tdoi = {10.1038/s41598-019-50373-z},\n\tabstract = {The connection between structure and stability of ecological networks has been widely studied in the last fifty years. A challenge that scientists continue to face is that in-depth mathematical model analysis is often difficult, unless the considered systems are specifically constrained. This makes it challenging to generalize results. Therefore, methods are needed that relax the required restrictions. Here, we introduce a novel heuristic approach that provides persistence estimates for random systems without limiting the admissible parameter range and system behaviour. We apply our approach to study persistence of species in random generalized Lotka-Volterra systems and present simulation results, which confirm the accuracy of our predictions. Our results suggest that persistence is mainly driven by the linkage density, whereby additional links can both favour and hinder persistence. In particular, we observed “persistence bistability”, a rarely studied feature of random networks, leading to a dependency of persistence on initial species densities. Networks with this property exhibit tipping points, in which species loss can lead to a cascade of extinctions. The methods developed in this paper may facilitate the study of more general models and thereby provide a step forward towards a unifying framework of network architecture and stability.},\n\tlanguage = {en},\n\tnumber = {1},\n\turldate = {2020-02-13},\n\tjournal = {Scientific Reports},\n\tauthor = {Fischer, Samuel M. and Huth, Andreas},\n\tmonth = dec,\n\tyear = {2019},\n\tpages = {14110},\n}\n\n
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\n The connection between structure and stability of ecological networks has been widely studied in the last fifty years. A challenge that scientists continue to face is that in-depth mathematical model analysis is often difficult, unless the considered systems are specifically constrained. This makes it challenging to generalize results. Therefore, methods are needed that relax the required restrictions. Here, we introduce a novel heuristic approach that provides persistence estimates for random systems without limiting the admissible parameter range and system behaviour. We apply our approach to study persistence of species in random generalized Lotka-Volterra systems and present simulation results, which confirm the accuracy of our predictions. Our results suggest that persistence is mainly driven by the linkage density, whereby additional links can both favour and hinder persistence. In particular, we observed “persistence bistability”, a rarely studied feature of random networks, leading to a dependency of persistence on initial species densities. Networks with this property exhibit tipping points, in which species loss can lead to a cascade of extinctions. The methods developed in this paper may facilitate the study of more general models and thereby provide a step forward towards a unifying framework of network architecture and stability.\n
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