Marine heatwaves leap tropical, herbivorous fish into temperate ecosystems and threaten future kelp strongholds

As oceans warm and environmental isotherms shift, so does the distribution of species to track regions that encompass their preferred thermal ranges. Whilst gradual warming is a major driver of this phenomenon, marine heatwaves can also play a role by creating sudden shifts that facilitate longer-term ‘tropicalisation’, indirectly affecting the resilience of temperate kelp forests through heightened herbivory. Predicting the future distribution of temperate kelp forests is key to making robust conservation management decisions, however, current studies do not take discrete climatic events or their indirect effects into account. Here, we incorporate climatic extremes and biotic interactions into species distribution models to help understand future kelp forest persistence, by projecting distributions of tropical, herbivorous fish as they respond to future extreme marine heatwaves. Other studies indicate kelp forests are likely to be limited to the southern coast of Western Australia under gradual warming scenarios, however we show this region will also be highly suitable for tropical herbivores who contribute to the erosion of kelp forest resilience, and promote the persistence of canopy free-states. We demonstrate major differences in fish distributions between Representative Concentration Pathway scenarios (2.6 ‘most optimistic’, versus 8.5 ‘business as usual’), with the latter resulting in the majority (~70%) of temperate Western Australia becoming highly suitable for tropical herbivores. Whilst seemingly inevitable that some degree of kelp forests will be lost from the region over the coming decades, our ability to incorporate climate-mediated indirect impacts into targeted intervention could help direct the trajectory of change towards more desirable outcomes.

随着海洋升温与环境等温线（environmental isotherms）迁移，物种分布也随之改变，以追踪包含其适宜温度范围的区域。尽管渐进式升温是这一现象的主要驱动因素，但海洋热浪也可通过引发突发性变化，推动长期的“热带化（tropicalisation）”进程，并通过增强植食作用间接影响温带海藻林的恢复力。预测温带海藻林的未来分布是制定稳健的保护管理决策的核心前提，但当前研究并未将离散气候事件及其间接影响纳入考量范畴。本研究将气候极端事件与生物相互作用纳入物种分布模型（Species Distribution Models），通过预测响应未来极端海洋热浪的热带植食性鱼类分布，以解析温带海藻林的未来存续态势。另有研究表明，在渐进式升温情景下，温带海藻林的分布范围或仅能局限于西澳大利亚州南部沿岸；但本研究显示，该区域也将高度适宜热带植食性鱼类栖息，而这类鱼类会削弱海藻林的恢复力，并促进无冠层状态的持续存在。本研究揭示了典型浓度路径（Representative Concentration Pathway, RCP）不同情景下鱼类分布的显著差异：其中RCP2.6为“最乐观情景”，RCP8.5为“一切如常情景”，后者将导致西澳大利亚州约70%的温带海域高度适宜热带植食性鱼类栖息。尽管未来数十年内该区域的温带海藻林或将出现一定程度的损失看似不可避免，但我们若能将气候介导的间接影响纳入针对性干预措施，则有望引导生态系统的变化轨迹朝向更理想的方向发展。