Relationships between population size and fitness in four common and four rare alpine plant species

Due to habitat fragmentation and climate change, many plant populations become smaller and more isolated and thus more prone to local extinction. Whereas it is well established for lowland species that plants of small populations have lower individual fitness, alpine species have not been sufficiently studied in this respect. It is also not clear whether relationships between population size and fitness vary between naturally rare and common species. We assessed how population size and rarity affect seed set, seed mass, seed number, total seed mass per fruit, germination, time to germination, offspring survival and offspring size in four congeneric alpine plant species pairs (Androsace chamaejasme Wulfen, A. puberula Jord. & Fourr., Primulaceae; Gentiana acaulis L., G. alpina Vill., Gentianaceae; Potentilla crantzii (Crantz) Fritsch, P. nivea L., Rosaceae; Viola calcarata L., V. lutea Huds., Violaceae). Across all eight species, plants from smaller populations produced fewer seeds and had lower total seed mass per fruit than plants from larger populations. This demonstrates that population size also affects fitness in alpine species. Rare species did not have lower individual fitness than common species. Therefore, naturally rare species might be well adapted to their environment. Relationships between population size and fitness were equally pronounced in rare and common species. We conclude that plant fitness is reduced in small populations in alpine species, also in common species.