Oceanographic Conditions Limit the Spread of a Marine Invader along Southern African Shores

Invasive species can affect the function and structure of natural ecological communities, hence understanding and predicting their potential for spreading is a major ecological challenge. Once established in a new region, the spread of invasive species is largely controlled by their dispersal capacity, local environmental conditions and species interactions. The mussel Mytilus galloprovincialis is native to the Mediterranean and is the most successful marine invader in southern Africa. Its distribution there has expanded rapidly and extensively since the 1970s, however, over the last decade its spread has ceased. In this study, we coupled broad scale field surveys, Ecological Niche Modelling (ENM) and Lagrangian Particle Simulations (LPS) to assess the current invaded distribution of M. galloprovincialis in southern Africa and to evaluate what prevents further spread of this species. Results showed that all environmentally suitable habitats in southern Africa have been occupied by the species. This includes rocky shores between Rocky Point in Namibia and East London in South Africa (approx. 2800 km) and these limits coincide with the steep transitions between cool-temperate and subtropical-warmer climates, on both west and southeast African coasts. On the west coast, simulations of drifting larvae almost entirely followed the northward and offshore direction of the Benguela current, creating a clear dispersal barrier by advecting larvae away from the coast. On the southeast coast, nearshore currents give larvae the potential to move eastwards, against the prevalent Agulhas current and beyond the present distributional limit, however environmental conditions prevent the establishment of the species. The transition between the cooler and warmer water regimes is therefore the main factor limiting the northern spread on the southeast coast; however, biotic interactions with native fauna may also play an important role.

外来入侵物种会影响自然生态群落的结构与功能，因此理解并预测其扩散潜力是生态学领域的重大挑战。外来入侵物种一旦在新区域定殖，其扩散过程主要受自身扩散能力、局域环境条件以及物种间相互作用调控。地中海贻贝（Mytilus galloprovincialis）原产于地中海海域，是南非南部海域最成功的海洋入侵物种。自20世纪70年代以来，该物种在当地的分布范围快速且广泛地扩张，但近十年来其扩散已趋于停滞。本研究结合大范围野外调查、生态位模型（Ecological Niche Modelling, ENM）与拉格朗日粒子模拟（Lagrangian Particle Simulations, LPS），旨在评估南非南部海域地中海贻贝当前的入侵分布范围，并解析阻碍其进一步扩散的关键因素。研究结果显示，南非南部海域所有环境适宜的栖息地均已被该物种占据，其分布范围涵盖纳米比亚洛基点至南非东伦敦之间的岩岸带（全长约2800千米），而该分布边界与非洲西海岸及东南海岸的冷温带与亚热带暖温带气候的急剧过渡带恰好重合。在西海岸，漂流幼体的模拟路径几乎完全沿本格拉洋流（Benguela Current）的北向及离岸方向运动，通过将幼体带离近岸海域形成了显著的扩散屏障。在东南海岸，近岸海流使幼体具备向东逆流（即逆主流的阿古拉斯洋流（Agulhas Current））扩散并突破当前分布边界的潜力，但局域环境条件阻碍了该物种的成功定殖。因此，冷暖水团的交界带是限制东南海岸北向扩散的主要因素，但与本土动物群落的生物间相互作用也可能发挥重要作用。