Data from: Facultative grazing and bioturbation by macrodetritivores alter saltmarsh plant-plant interactions under stress

The importance of positive plant-plant interactions is generally suggested to increase towards more stressful conditions, due to mutual stress amelioration between plants. Bioturbating macrodetritivores can also ameliorate stress through bioturbation, but can also become selective herbivores under food-limited conditions, making the outcome of plant-plant interactions under stress in the presence of macrodetritivores unclear. We studied how combining environmental stress (waterlogging) with the presence of the soil macrodetritivore Orchestia gammarellus affected the outcome of the interaction between two salt marsh plants: the tall, late successional Elytrigia atherica and the shorter, early successional Festuca rubra. In a replacement design competition experiment under controlled conditions, we found that soil redox potential was negatively affected by waterlogging and positively affected by the presence of O. gammarellus. The survival and shoot biomass of E. atherica was not significantly affected by waterlogging or by the presence of bioturbators. The survival and shoot biomass of F. rubra was especially decreased when waterlogging was combined with the presence of O. gammarellus, as this macrodetritivore turned into a selective grazer on F. rubra under these conditions. We found that E. atherica produced double shoot biomass and F. rubra produced much less shoot biomass in their mixed cultures than was expected from the monocultures of same waterlogging/Orchestia treatments. Hence, the presence of the bioturbator strongly promoted the competitive advantage of E. atherica over F. rubra due to the combination of stress amelioration for the first species and selective herbivory on the second species. Synthesis: This study shows that the inclusion of bioturbating macrodetritivores complicates the standard prediction that plant-plant interactions become more positive towards more stressful conditions. A novel insight is that macrodetritivores can affect the structure and diversity of plant communities through multiple mechanisms. Under benign conditions bioturbating macrodetritivores ameliorated soil conditions, permitting co-occurrence of competing plant species. At high environmental stress (waterlogging) macrodetritivores selectively graze higher quality plant species and emerging seedlings, thus promoting dominance of the lower quality species. Hence the facultative feeding behavior of macrodetritivores deserves further attention.植物间正相互作用的重要性通常被认为会随环境胁迫程度的升高而增强，这源于植物之间能够相互缓解胁迫压力。生物扰动型大型碎屑食性动物（bioturbating macrodetritivores）可通过生物扰动作用缓解环境胁迫，但在食物匮乏条件下也可能转变为选择性植食者，这使得存在该类动物时，胁迫环境下的植物间相互作用结果难以预判。本研究探讨了将环境胁迫（涝渍）与土壤大型碎屑食性动物钩虾（Orchestia gammarellus）的存在相结合，会如何影响两种盐沼植物的相互作用结果：分别为高大的演替后期植物偃麦草（Elytrigia atherica）与较矮小的演替早期植物紫羊茅（Festuca rubra）。在受控条件下开展的替换设计竞争实验中，我们发现：土壤氧化还原电位会因涝渍处理而降低，却会因钩虾的存在而升高。偃麦草的存活率与地上生物量并未受到涝渍处理或生物扰动型大型碎屑食性动物存在的显著影响。而紫羊茅的存活率与地上生物量则在涝渍与钩虾共存的条件下显著下降，因为此时该类大型碎屑食性动物会转变为针对紫羊茅的选择性取食者。我们发现，与相同涝渍/钩虾处理下的单种栽培预期结果相比，两种植物混合栽培时，偃麦草的地上生物量提升了一倍，而紫羊茅的地上生物量则大幅降低。因此，生物扰动型大型碎屑食性动物的存在通过双重机制显著强化了偃麦草相对于紫羊茅的竞争优势：一方面为前者缓解了胁迫压力，另一方面对后者产生了选择性植食作用。综合分析：本研究表明，将生物扰动型大型碎屑食性动物纳入考量后，“植物间正相互作用随胁迫加剧而增强”这一经典预测会变得不再成立。本研究的新发现在于，大型碎屑食性动物可通过多种机制影响植物群落的结构与多样性。在环境条件适宜时，生物扰动型大型碎屑食性动物能够改善土壤状况，使得竞争植物物种得以共存；而在高环境胁迫（涝渍）条件下，这类动物会选择性取食品质更高的植物物种与新生幼苗，从而推动低品质植物物种占据优势地位。因此，大型碎屑食性动物的兼性取食行为值得进一步深入研究。