Herbivore dung stoichiometry drives competition between savanna trees and grasses

The balance between trees and grasses is a key aspect of savanna ecosystem functioning, and so far, believed to be regulated by resource availability, fire frequency, and consumption by mammalian herbivores. Herbivores, however, also impact plant communities through the deposition of growth-limiting nutrients in their dung and urine. Little attention has been paid to the fact that savanna herbivores produce dung containing different concentrations of nutrients and it remains unknown what the effect of this variation is on tree-grass interactions. Here, we investigate if stoichiometric differences in dung from browsers and grazers from an African savanna are large enough to influence competitive interactions between N2-fixing trees and grasses. We performed a competition experiment with seedlings of three common N2-fixing tree species and three common C4-grass species from a Kenyan savanna. Plants were grown in mesocosms as monocultures or mixtures with dung from either zebra (grazer) or giraffe (browser). We show that variation in dung nitrogen (N) to phosphorus (P) ratio between these herbivores was large enough to drive the competitive outcome between tree seedlings and grasses. Under zebra dung with a low N:P ratio (3.5), tree seedlings had an advantage due to increased nodulation and N2-fixation. Under giraffe dung with a high N:P ratio (6.4), grasses suppressed nodulation and tree seedling growth. Synthesis. We identify another potentially important mechanism by which mammalian herbivores can stimulate ecosystem stability, namely through the N:P ratio of their dung. This illustrates how connected browsing and grazing herbivores are in African savannas, through the forage quality they consume, the N:P stoichiometry of their dung, and subsequent effects on plant competition.木本植物与草本植物的相对平衡是稀树草原生态系统功能的核心特征之一，此前学界普遍认为其调控机制包括资源可获得性、火灾频率以及哺乳动物植食动物的取食压力。然而，植食动物还可通过粪便与尿液中沉积的生长限制性养分，对植物群落施加影响。但学界尚未充分关注到，稀树草原植食动物产生的粪便养分浓度存在差异，目前仍不清楚这种养分变异对木本-草本植物互作的具体效应。 在此研究中，我们探讨了非洲稀树草原中，取食型植食动物（browsers）与啃食型植食动物（grazers）的粪便化学计量差异，是否足以影响固氮木本植物（N₂-fixing trees）与草本植物之间的竞争互作。我们以肯尼亚稀树草原中的3种常见固氮木本植物幼苗与3种常见C₄草本植物为材料开展竞争实验，将植株以单种或混种方式种植于中型实验生态系统（mesocosms）中，施加的粪便分别来自作为啃食型植食动物（grazers）的斑马与作为取食型植食动物（browsers）的长颈鹿。 研究结果显示，不同植食动物粪便的氮（N）磷（P）比差异显著，足以决定木本幼苗与草本植物的竞争结局。在施加N:P比为3.5的斑马粪便时，木本幼苗因结瘤率提升与固氮作用增强而获得竞争优势；而在施加N:P比为6.4的长颈鹿粪便时，草本植物会抑制木本幼苗的结瘤与生长。 【研究总结】我们发现了哺乳动物植食动物可通过粪便的N:P比调控生态系统稳定性的另一潜在重要机制。该研究揭示了非洲稀树草原中取食型与啃食型植食动物之间的紧密关联——二者通过各自取食的饲草品质、粪便的N:P化学计量特征，以及由此对植物竞争产生的后续效应，共同塑造稀树草原的生态格局。