Increasing accuracy of dispersal kernels in grid-based population models

Dispersal kernels in grid-based population models specify the proportion, distance and direction of movements within the model landscape. Spatial errors in dispersal kernels can have large compounding effects on model accuracy. Circular Gaussian and Laplacian dispersal kernels at a range of spatial resolutions were investigated, and methods for minimizing errors caused by the discretizing process were explored. Kernels of progressively smaller sizes relative to the landscape grid size were calculated using cell-integration and cell-center methods. These kernels were convolved repeatedly, and the final distribution was compared with a reference analytical solution. For large Gaussian kernels (σ > 10 cells), the total kernel error was <10⁻¹¹ compared to analytical results. Using an invasion model that tracked the time a population took to reach a defined goal, the discrete model results were comparable to the analytical reference. With Gaussian kernels that had σ ≤ 0.12 using the cell integration method, or σ ≤ 0.22 using the cell center method, the kernel error was greater than 10%, which resulted in invasion times that were orders of magnitude different than theoretical results. A goal-seeking routine was developed to adjust the kernels to minimize overall error. With this, corrections for small kernels were found that decreased overall kernel error to <10⁻¹¹ and invasion time error to <5%.     

Spatially mixed crops to control the stratified dispersal of airborne fungal diseases

Intraspecific crop diversification is thought to be a possible solution to the disease susceptibility of monocultured crops. We modelled the stratified dispersal of an airborne pathogen population in order to identify the spatial patterns of cultivar mixtures that could slow epidemic spread driven by dual dispersal mechanisms acting over both short and long distances. We developed a model to simulate the propagation of a fungal disease in a 2D field, including a reaction-diffusion model for short-distance disease dispersal, and a stochastic model for long-distance dispersal. The model was fitted to data for the spatio-temporal spread of faba bean rust (caused by Uromyces viciae-fabae) through a discontinuous field. The model was used to compare the effectiveness of eight different planting patterns of cultivar mixtures against a disease spread by short-distance and stratified dispersal. Our combined modelling approach provides a reasonably good fit with the observed data for the spread of faba bean rust. Similar predictive power could be expected for the management of resource-mediated invasions by other airborne fungi. If a disease spreads by short-distance dispersal, random mixtures can be used to slow the epidemic spread, since their spatial irregularity creates a natural barrier to the progression of a smooth epidemic wave. In the context of stratified dispersal, heterogeneous patterns should be used that include a minimum distance between susceptible units, which decreases the probability of infection by long-distance spore dispersal. We provide a simple framework for modelling the stratified dispersal of disease in a diversified crop. The model suggests that the spatial arrangement of components in cultivar mixtures has to accord with the dispersal characteristics of the pathogen in order to increase the efficiency of diversification strategies in agro-ecosystems and forestry. It can be applied in low input agriculture to manage pathogen invasion by intercropping and cultivar mixtures, and to design sustainable systems of land use.     

Introduced Species and Management of a <Emphasis Type="Italic">Nothofagus/Austrocedrus</Emphasis> Forest

Isla Victoria (Nahuel Huapi National Park, Argentina), a large island dominated by native Nothofagus and Austrocedrus forest, has old plantations of many introduced tree species, some of which are famed invaders of native ecosystems elsewhere. There are also large populations of introduced deer and shrubs that may interact in a complex way with the introduced trees, as well as a recently arrived population of wild boar. Long-standing concern that the introduced trees will invade and transform native forest may be unwarranted, as there is little evidence of progressive invasion, even close to the plantations, despite over 50 years of opportunity. Introduced and native shrubs allow scattered introduced trees to achieve substantial size in abandoned pastures, but in almost all areas neither the trees nor the shrubs appear to be spreading beyond these sites. These shrub communities may be stable rather than successional, but the technology for restoring them to native forest is uncertain and probably currently impractical. Any attempt to remove the exotic tree seedlings and saplings from native forest would probably create the very conditions that would favor colonization by exotic plants rather than native trees, while simply clear-cutting the plantations would be unlikely to lead to regeneration of Nothofagus or Austrocedrus. The key to maintaining native forest is preventing catastrophic fire, as several introduced trees and shrubs would be favored over native dominant trees in recolonization. Deer undoubtedly interact with both native and introduced trees and shrubs, but their net effect on native forest is not yet clear, and specific management of deer beyond the current hunting by staff is unwarranted, at least if preventing tree invasion is the goal. The steep terrain and shallow soil make the recently arrived boar a grave threat to the native forest. Eradication is probably feasible and should be attempted quickly.     

Behavioral responses to a novel predator and competitor of invasive mosquitofish and their non-invasive relatives (Gambusia sp.)

Attributes of the recipient community may affect the invasion success of arriving non-indigenous organisms. In particular, biotic interactions may enhance the resistance of communities to invasion. Invading organisms typically encounter a novel suite of competitors and predators, and thus their invasiveness may be affected by how they cope with these interactions. Behavioral plasticity may help invaders to respond appropriately to novelty. We examined the behavioral responses of highly invasive mosquitofish to representative novel competitors and predators they might encounter as they spread through North America. We compared the behavior of invasive Gambusia holbrooki and G. affinis to that of two close relatives of lower invasive potential (G. geiseri and G. hispaniolae) in order to elucidate whether responses to novelty related to invasiveness. In short-term assays, female Gambusia were paired with a novel competitor, Pimephales promelas, and a novel predator, Micropterus dolomieu. Behavioral responses were measured in terms of foraging success and efficiency, activity, refuge use, predator inspections, and interspecific aggression. Contrary to a priori predictions, invasive and non-invasive responses to novel interactions did not differ consistently. In response to novel competition, both invasive species increased foraging efficiency, but so did G. geiseri. In response to novel predation, only G. holbrooki decreased consumption and activity and increased refuge use. No antipredator response was observed in G. affinis. We found consistent differences, however, between invasives and non-invasives in foraging behavior. Both in the presence and absence of the competitor and the predator, invasives foraged more efficiently and consumed more prey than non-invasives.Communicated by P. Bednekoff     

Epidemiological risk assessment using linked network and grid based modelling: Phytophthora ramorum and Phytophthora kernoviae in the UK

We developed a stochastic simulation model incorporating most processes likely to be important in the spread of Phytophthora ramorum and similar diseases across the British landscape (covering Rhododendron ponticum in woodland and nurseries, and Vaccinium myrtillus in heathland). The simulation allows for movements of diseased plants within a realistically modelled trade network and long-distance natural dispersal. A series of simulation experiments were run with the model, representing an experiment varying the epidemic pressure and linkage between natural vegetation and horticultural trade, with or without disease spread in commercial trade, and with or without inspections-with-eradication, to give a 2 × 2 × 2 × 2 factorial started at 10 arbitrary locations spread across England. Fifty replicate simulations were made at each set of parameter values. Individual epidemics varied dramatically in size due to stochastic effects throughout the model. Across a range of epidemic pressures, the size of the epidemic was 5–13 times larger when commercial movement of plants was included. A key unknown factor in the system is the area of susceptible habitat outside the nursery system. Inspections, with a probability of detection and efficiency of infected-plant removal of 80% and made at 90-day intervals, reduced the size of epidemics by about 60% across the three sectors with a density of 1% susceptible plants in broadleaf woodland and heathland. Reducing this density to 0.1% largely isolated the trade network, so that inspections reduced the final epidemic size by over 90%, and most epidemics ended without escape into nature. Even in this case, however, major wild epidemics developed in a few percent of cases. Provided the number of new introductions remains low, the current inspection policy will control most epidemics. However, as the rate of introduction increases, it can overwhelm any reasonable inspection regime, largely due to spread prior to detection.     

The lichen-rich coastal heath vegetation on the isle of Anholt, Denmark — conservation and management

The high conservational value of the lichen-rich vegetation and landscape of the marine foreland Ørkenen on the isle of Anholt is treated from a Danish as well as a European perspective. The sensitivity of the lichen-rich vegetation to physical disturbance is emphasized. The impact of invasive species such asPinus mugo as well as the effect of atmospheric deposition of nutrients on the heaths at Anholt is described. Considerations related to the development of a management plan for Ørkenen are presented.     

预防外来物种入侵的立法对策

对我国目前预防外来物种入侵问题的现状进行了分析,指出了我国在预防外来物种入侵立法上没有专门的针对性法律,立法目的有偏差、法律涉及范围有限的弊病,提出了明确立法目的、制订专项立法与中长期规划、适当扩大相关法律覆盖范围、完善法律法规和顺应国际形势的立法对策.     

What Do the Occurrences of Migratory Fishes in the Upper Mekong Mean?

This paper examines the recent appearance of four migratory species in the Upper Mekong: Dasyatis laosensis, Anguilla bicolor, Anguilla marmorata, and Chitala ornata. Since 2006 these species have been continually sampled in an area below Jinghong Dam in Xishuangbanna, China. This region is far beyond their historical upstream distribution, and the reasons for these appearances and their subsequent impact on river ecosystem are considered.     

生物入侵预测模型研究进展

生物入侵随着全球化进程有进一步加剧的趋势,外来物种已经被视为对本地生物多样性和生态系统功能产生全球威胁的因素之一。生物入侵的影响巨大,开展预测工作以便在一开始就发现并阻止外来种侵入,为生物入侵提供最优化的监测和早期控制手段。为此,本文从环境的可侵入性和外来种的入侵性两个方面综述了目前生物入侵预测模型的研究进展,以便为外来种引入和管理提供依据。     

浅析青藏高原铁路建设的外来物种入侵风险

交通干线的建设和生物入侵的发生紧密相关,由于青藏高原环境的脆弱性,在该区进行交通设施建设更须警惕外来物种入侵的风险。围绕青藏高原这一特殊自然地理区域的铁路建设,初步分析了铁路建设和运行带来的环境变化,同时结合青藏高原生态系统的特点,分析了可能造成的生物入侵风险,并针对生物入侵的防范提出了相关建议。