Data from: Modeling spatial expansion of invasive alien species: relative contributions of environmental and anthropogenic factors to the spreading of the harlequin ladybird in France

Species distribution models (SDM) have often been used to predict the potential ranges of introduced species and prioritize management strategies. However, this approach assumes equilibrium between occurrences and environmental gradients, an assumption which is violated during the invasion process, where many suitable sites are empty because the species has not yet reached them. Here we considered the invasive ladybird Harmonia axyridis as a case study to show the benefits of using a dynamic colonization–extinction model that does not assume equilibrium. We used a multi-year occupancy model incorporating environmental, anthropogenic and neighborhood effects, to identify factors that explained spreading variation of this species in France from 2004, when only a few occupied sites were detected, to 2011. We found that anthropogenic factors (urbanization, agriculture, vineyards, and presence/absence of highways) explained more variation in the diffusion process than environmental factors (winter and summer temperatures, wind-speed, and rainfall). The surface of urbanization was the major anthropogenic factor increasing the probability of colonization. The average summer temperature was the main environmental factor affecting colonization, with a negative effect when high or low. The neighborhood effect revealed that colonization was mostly influenced by contributions coming from a radius of 24 km around the focal cell. The contribution of neighborhood decreases over time, suggesting that H. axyridis is reaching its equilibrium in France. This is confirmed by the small discrepancy observed between the performance of our approach and a SDM approach when predicting a single year occupancy pattern at the end of the study period. Our approach has the advantage of explicitly modelling the state of the biological system during the spatial expansion and identifying colonization constraints. This allows managers to explore the effect of different actions on the system at key moments of the invasion process, hence providing a powerful approach to prioritize management strategies.

物种分布模型（SDM）常被用于预测外来引入物种的潜在分布范围，并制定优先管控策略。然而该方法默认物种出现记录与环境梯度处于平衡状态，但在入侵进程中这一假设并不成立——此时诸多适宜生境尚未被该物种抵达，因此处于空置状态。本研究以入侵物种异色瓢虫（Harmonia axyridis）为案例对象，展示了无需平衡假设的动态定殖-灭绝模型的应用优势。我们采用整合了环境、人为活动及邻域效应的多年占据模型，解析了2004年至2011年间该物种在法国的扩散差异：2004年仅检测到少量占据生境，至2011年已积累该阶段完整的扩散观测数据。研究发现，人为活动因子（城市化水平、农业用地、葡萄园分布及高速公路存在与否）对扩散过程变异的解释度高于环境因子（冬夏气温、风速与降雨量）。其中，城市化用地面积是提升定殖概率的首要人为因子；平均夏季气温则是影响定殖的核心环境因子，无论气温过高或过低均会对定殖产生负向影响。邻域效应分析显示，定殖过程主要受目标单元格周边24公里半径范围内的源种群贡献影响。邻域贡献度随时间推移逐渐降低，这表明异色瓢虫在法国境内正逐步达到种群平衡状态。在研究末期预测单年度占据格局时，本研究方法与传统SDM方法的预测结果差异极小，这进一步验证了上述结论。本研究方法的优势在于，可在空间扩张过程中显式模拟生物系统的状态，并识别定殖限制因子。这一特性可帮助管理者在入侵进程的关键节点评估不同管控措施对种群系统的影响，进而为制定优先管控策略提供强有力的分析工具。