Can ecosystem functioning be maintained despite climate-driven shifts in species composition? Insights from novel marine forests

1. Climate change is driving a redistribution of species and reconfiguration of ecological communities at a global scale. Persistent warming in many regions has caused species to extend their geographical ranges into new habitats, with thermally-tolerant species often becoming competitively dominant over species with colder affinities. Although these climate-driven changes in species abundance and diversity are well documented, their ecosystem-level implications are poorly understood, and resolving whether reconfigured communities can maintain fundamental ecosystem functions represents a pressing challenge in an increasingly warmer world. 2. Here, we investigated how climate-driven substitutions of foundation species influence processes associated with carbon and nutrient cycling (biomass production, detritus flow, herbivory, decomposition) by comparing two habitat-forming kelp species with contrasting thermal affinities. We examined the wider consequences of the observed (and predicted) emergence of novel marine forests in the NE Atlantic, which are expected to become more dominated by range-expanding, warm-temperate kelps. 3. Warm-temperate kelps both accumulated and released 80% more biomass than the cold-temperate species despite being taxonomically closely-related and morphologically similar. Furthermore, the warm-temperate species accumulated biomass and released detritus year-round, whereas the cold-temperate species did so during short, discrete periods. The warm-temperate kelps supported higher densities of invertebrate grazers and were a preferred food source. Finally, their detritus decomposed 6.5 times faster, despite supporting comparable numbers of detritivores. Overall, our results indicate an important shift in the cycling of organic matter in kelp forests along parts of the NE Atlantic coastline following climate-driven expansion of a warm-affinity kelp, with novel forests supplying large amounts of temporally-continuous—yet highly labile—organic matter. 4. Synthesis. Collectively, our results show that, like species invasions, climate-driven range expansions and consequent shifts in the identity of dominant species can modify a wide range of important ecosystem processes. However, alterations in overall ecosystem functioning may be relatively limited where foundation species share similar ecological and functional traits.

1. 气候变化正在全球尺度上驱动物种分布再分配与生态群落重构。诸多区域的持续升温已促使物种将地理分布范围拓展至新的生境，耐热物种通常相较于耐寒偏好的物种更具竞争优势。尽管这些由气候驱动的物种丰度与多样性变化已有充分研究记录，但其生态系统层面的影响仍不明晰；在全球持续变暖的背景下，厘清重构后的群落能否维持基础生态系统功能，已成为一项紧迫的科学挑战。
2. 本研究通过对比两种热适配性截然相反的造生境海带物种，探究了气候驱动的建群种（foundation species）更替如何影响与碳、养分循环相关的生态过程，包括生物量生产（biomass production）、碎屑流（detritus flow）、植食作用（herbivory）与分解作用（decomposition）。我们还评估了东北大西洋海域已观测到（及预测将出现的）新型海洋森林所带来的更广泛影响——该海域未来有望被分布范围扩张的暖温带海带所主导。
3. 尽管分类学上亲缘关系密切、形态学特征相似，暖温带海带的生物量积累与释放量均较寒温带物种高出80%。此外，暖温带海带全年均可进行生物量积累与碎屑释放，而寒温带海带仅在短暂且间断的时段内开展此类过程。暖温带海带可支撑更高密度的无脊椎草食动物，且是其偏好性食物来源。最终，其碎屑的分解速率是寒温带物种的6.5倍，尽管两者所支撑的食碎屑动物（detritivores）数量相当。总体而言，本研究结果表明，随着气候驱动的暖适配性海带在东北大西洋部分海岸沿线扩张，该海域海带森林中的有机质循环已发生显著改变；新型海洋森林将提供大量时间上连续但极易被降解的有机物质。
4. 综合分析。本研究结果显示，与物种入侵类似，气候驱动的分布范围扩张以及由此引发的优势物种身份转变，可改变诸多关键生态系统过程。不过，若建群种拥有相似的生态与功能性状，则整体生态系统功能的改变可能相对有限。