Restoring functionally diverse communities enhances invasion resistance in a freshwater wetland

1. Understanding how plant communities reassemble is particularly relevant when invaders are part of the species pool and can compromise restoration or conservation goals. If initial species composition has a lasting impact on community dynamics, it may be possible to enhance biotic resistance to invasion, particularly through diversity effects, by manipulating early community composition. Diversity effects have been investigated much less in wetlands than in uplands.2. We conducted a large plant community restoration experiment in a freshwater wetland to investigate species and functional diversity effects on biotic resistance to invasion during early community assembly. The wetland was under propagule pressure mostly from <i>Phragmites australis</i> (common reed), but all species not sown in this experiment that established in the plots were treated as ‘invaders’, whether exotic or native. We used a two-step model selection approach and a diversity-interaction model framework to test hypotheses about the contribution to invasion success of abiotic conditions, species richness, as well as functional group richness, identity and pairwise interactions, providing a rare test of diversity effect against multiple invaders in wetlands.3. Twenty species invaded the plots. They were grouped for analysis to evaluate the overall resistance of resident communities. The three main invaders, <i>P. australis</i> and two native taxa (<i>Salix spp</i>., <i>Populus deltoides</i>), were also analysed separately. Diversity–interaction models revealed invader–specific responses. Functional group identity determined resistance to <i>P. australis</i>, whereas pairwise interactions between functional groups explained resistance to <i>Salix spp.</i> Both effects were retained when grouping all invaders. Resistance to <i>P. deltoides</i> was determined by abiotic conditions.4. The contribution of specific functional groups varied with invader. Annuals and clonal perennials explained resistance to <i>P. australis</i>, suggesting priority and limiting similarity effects. Interactions, when significant, mostly included fast-growing annuals. When considering all invaders, identity effects were detected for all functional groups.5. Synthesis. Diversity effects on invasion resistance vary with invaders in early assembly of restored wetland communities. Restoring functionally diverse communities provides resistance against multiple invaders. Consistent responses between this field experiment and previous pot experiments with the same species suggest that results are robust enough to inform management.<br>

1. 当入侵物种存在于物种库中并可能损害恢复或保护目标时，理解植物群落的重建过程尤为关键。若初始物种组成对群落动态具有持久影响，则可通过调控早期群落组成，尤其是借助多样性效应（diversity effects）来增强群落对入侵的生物入侵抗性（biotic resistance）。相较于旱地生境（uplands），学界对湿地（wetlands）中多样性效应的研究相对匮乏。
2. 本研究在淡水湿地中开展了大型植物群落恢复实验，旨在探究群落早期组装过程中，物种多样性与功能多样性对生物入侵抗性的影响。该湿地的繁殖体压力（propagule pressure）主要来自芦苇（<i>Phragmites australis</i>，common reed），但所有未在本实验中播种、却在样地中定植的物种，无论外来种还是本地种，均被视为“入侵物种”。我们采用两步模型选择法与多样性-交互模型框架，检验了非生物条件、物种丰富度、功能群丰富度、功能群身份以及两两交互作用对入侵成功的贡献假说，这是湿地环境中针对多入侵物种的多样性效应的罕见研究。
3. 共有20个物种入侵了实验样地。我们将这些入侵物种进行分组分析，以评估本地群落的整体抗入侵能力。针对三大主要入侵物种——芦苇（<i>P. australis</i>）以及两个本地类群：柳属（<i>Salix spp.</i>）和美洲杨（<i>Populus deltoides</i>），我们也开展了单独分析。多样性-交互模型结果显示，不同入侵物种对应的响应模式存在特异性：功能群身份决定了群落对芦苇的抗入侵能力，而功能群间的两两交互作用则解释了群落对柳属的抗入侵能力；当将所有入侵物种合并分析时，这两种效应均依然显著。群落对美洲杨的抗入侵能力则由非生物条件决定。
4. 特定功能群的贡献因入侵物种而异：一年生植物与克隆多年生植物能够解释群落对芦苇的抗入侵能力，这暗示了优先效应（priority effects）与限制性相似性效应（limiting similarity effects）的作用；当交互作用显著时，其组成大多包含速生一年生植物。在综合考虑所有入侵物种的情况下，所有功能群均检测到了功能群身份效应。
5. 综合与结论：在恢复湿地群落的早期组装过程中，多样性效应对抗入侵能力的影响因入侵物种而异。构建功能多样的群落能够提升对多入侵物种的抗性。本野外实验与此前使用相同物种开展的盆栽实验结果一致，表明本研究结果具有足够的稳健性，可为湿地恢复与管理提供参考。