The Importance of the Human Footprint in Shaping the Global Distribution of Terrestrial, Freshwater and Marine Invaders

Human activities such as transport, trade and tourism are likely to influence the spatial distribution of non-native species and yet, Species Distribution Models (SDMs) that aim to predict the future broad scale distribution of invaders often rely on environmental (e.g. climatic) information only. This study investigates if and to what extent do human activities that directly or indirectly influence nature (hereafter the human footprint) affect the global distribution of invasive species in terrestrial, freshwater and marine ecosystems. We selected 72 species including terrestrial plants, terrestrial animals, freshwater and marine invasive species of concern in a focus area located in NW Europe (encompassing Great Britain, France, The Netherlands and Belgium). Species Distribution Models were calibrated with the global occurrence of species and a set of high-resolution (9×9 km) environmental (e.g. topography, climate, geology) layers and human footprint proxies (e.g. the human influence index, population density, road proximity). Our analyses suggest that the global occurrence of a wide range of invaders is primarily limited by climate. Temperature tolerance was the most important factor and explained on average 42% of species distribution. Nevertheless, factors related to the human footprint explained a substantial amount (23% on average) of species distributions. When global models were projected into the focus area, spatial predictions integrating the human footprint featured the highest cumulative risk scores close to transport networks (proxy for invasion pathways) and in habitats with a high human influence index (proxy for propagule pressure). We conclude that human related information–currently available in the form of easily accessible maps and databases—should be routinely implemented into predictive frameworks to inform upon policies to prevent and manage invasions. Otherwise we might be seriously underestimating the species and areas under highest risk of future invasions.交通、贸易与旅游等人类活动可能会对外来物种的空间分布产生影响，然而，旨在预测入侵物种未来大范围分布的物种分布模型（Species Distribution Models, SDMs），往往仅依赖环境（如气候）信息。本研究旨在探讨直接或间接影响自然的人类活动（下称人类足迹（human footprint）），会在多大程度上影响陆地、淡水与海洋生态系统中入侵物种的全球分布。本研究选取了72种受关注的入侵物种，涵盖陆地植物、陆地动物、淡水与海洋入侵物种，研究区域位于西北欧（包括大不列颠、法国、荷兰与比利时）。我们基于物种的全球出现记录，以及一系列高分辨率（9×9千米）的环境图层（如地形、气候、地质）和人类足迹代理变量（如人类影响指数、人口密度、道路临近度），对物种分布模型进行校准。分析结果表明，各类入侵物种的全球出现情况主要受气候限制，其中温度耐受度是最重要的影响因子，平均可解释42%的物种分布差异。尽管如此，与人类足迹相关的因子仍可解释相当比例（平均23%）的物种分布格局。当将全球模型外推至本研究区域时，整合了人类足迹信息的空间预测结果显示，在靠近交通网络（入侵路径的代理变量）以及人类影响指数较高的生境（繁殖体压力的代理变量）中，累积风险评分最高。本研究结论认为，当前可通过便捷获取的地图与数据库获得的人类活动相关信息，应被常规纳入预测框架，为预防和管理生物入侵的政策制定提供参考；否则，我们可能会严重低估未来面临最高入侵风险的物种与区域。