Genomic life‐history traits mediate the impacts of habitat loss and nutrient availability on fragmented grassland communities

•1. Habitat loss and nutrient enrichment are major drivers of plant species declines. However, local extinction risks vary widely and are shaped by life‐history trait syndromes. Genomic life‐history traits, which are simple, stable and widely available, offer a promising yet underexplored approach to improving extinction risk assessments and informing conservation efforts. •2. Using a spatial comparison approach, we analysed 256 semi‐natural grassland plant communities in four Swedish agricultural landscapes to examine whether genomic life‐history traits such as ploidy, genome size and chromosome number mediate differences in plant community diversity and composition along independent gradients of grassland fragment size and phosphorus availability. •3. Our findings show that plant diversity and community composition are increasingly dominated by larger genome species both as grassland fragments become smaller and as more phosphorus becomes available. While all genomic life‐history traits proved important, ploidy plays a particularly significant role in explaining differences between observed plant communities. •4. Ploidy mediated deterministic plant community changes along both environmental gradients. Diploid species are especially vulnerable to local extinction, whereas polyploids demonstrate resilience to habitat loss and benefit from increased phosphorus availability, likely leading to the competitive exclusion of diploids. •5. Synthesis. By revealing how genomic traits shape plant community responses to land‐use change and nutrient pollution—two key drivers of extinction—our study introduces a predictive, genomic trait‐based framework for assessing local extinction risks in fragmented landscapes.