The spread of invasive species can be limited or promoted by the distributions of mutualists, which presents an opportunity for managing biological invasions. However, the ways in which the geographical distributions of mutualists shape the range limits and invasion dynamics of introduced species remains unclear. Legumes include noxious invaders whose success is tied to their mutualism with nitrogen-fixing rhizobium bacteria. We examine whether the availability of compatible rhizobia limits the spread of an invasive legume at both small (50 m) and large (United States) spatial scales across natural invasion fronts. Using greenhouse experiments we simulated the dispersal of Medicago polymorpha into soils at increasing distances from a host patch. We find that rhizobial mutualist availability rapidly declines to almost zero within 20 m of established legume patches and legume fitness and the percentage of leaf nitrogen derived from symbiotic nitrogen fixation show a concomitant decline. The decline of fitness due to the lack of rhizobium mutualists differs among M. polymorpha genotypes. Our findings support the missed mutualist hypothesis whereby mutualist limitation reduces invasiveness. As M. polymorpha colonizes novel areas, seeds following either short- (50 m) or long-range seed dispersal will be mutualist-limited and exhibit low fitness and nitrogen fixation. Without co-introduction via transport of soil containing both mutualists, the patchy distribution of E. medicae may limit M. polymorpha invasion. Plant lineages exhibit genetic variation upon which selection could act to reduce dependence on rhizobia, thus a scarcity of symbionts could cause plant symbiosis traits to evolve during invasion.

互助主义者的分布可以限制或促进入侵物种的扩散，这为管理生物入侵提供了机会。但是，尚不清楚共生者的地理分布如何影响范围限制和引入物种的入侵动态。豆类包括有害入侵者，其成功与他们与固氮根瘤菌的共生联系在一起。我们研究了相容性根瘤菌的可用性是否会限制侵袭性豆科植物在自然入侵前沿的小（50 m）和大（美国）空间尺度上的扩散。使用温室实验，我们模拟了紫花苜蓿的扩散与宿主斑块之间的距离越来越远。我们发现，在已建立的豆科植物斑块和豆科植物适宜性的20 m内，根瘤菌的互惠可用性迅速下降至几乎为零，而源自共生固氮的叶氮百分比也随之下降。多形支原体基因型因缺乏根瘤菌互生体而导致适应性下降。我们的发现支持遗漏的互惠主义者假说，从而使互惠主义者的局限性降低了侵略性。由于多形麦芽孢杆菌在新的区域定居，种子在短距离（50 m）或远距离扩散后将受到相互制约，并且适应性和固氮性低。如果不通过运输包含两个互惠生的土壤而共同引入，则斑块的斑片状分布药用大肠埃希菌可能会限制多形支原体的入侵。植物谱系表现出遗传变异，因此选择可以减少对根瘤菌的依赖性，因此共生菌的缺乏可能导致植物共生性状在入侵过程中演变。