Island properties dominate species traits in determining plant colonizations in an archipelago system

The extrinsic determinants hypothesis emphasizes the essential role of environmental heterogeneity in species’ colonization. Consequently, high resident species diversity can increase community susceptibility to colonizations because good habitats may support more species that are functionally similar to colonizers. On the other hand, colonization success is also likely to depend on species traits. We tested the relative importance of environmental characteristics and species traits in determining colonization success using census data of 587 vascular plant species collected about 80 years apart from 471 islands in the archipelago of SW Finland. More specifically, we explored potential new colonization as a function of island properties (e.g., location, area, habitat diversity, number of resident species per unit area), species traits (e.g., plant height, life-form, dispersal vector, Ellenberg indicator values, association with human impact), and species’ historical distributions (number of inhabited islands, nearest occurrence). Island properties and species’ historical distributions were more effective than plant traits in explaining colonization outcomes. Contrary to the extrinsic determinants hypothesis, colonization success was neither associated with resident species diversity nor habitat diversity per se, although colonization was lowest on sparsely vegetated islands. Our findings lead us to propose that while plant traits related to dispersal and establishment may enhance colonization, predictions of plant colonizations primarily require understanding of habitat properties and species’ historical distributions.
Methods
Historical species data from Eklund, O. 1958. Die Gefässpflanzenflora beiderseits Skiftet im Schärenarchipel Südwestfinnlands. – Bidrag till Kännedom af Finlands Natur och Folk 101: 1–342. Skult, H. 1960. Om kärlväxtfloran i Korpo, Brunskär, en utskärsarkipelag.– Acta Soc. Fauna Flora Fenn. 76: 1–101. Contemporary species data collected 1996 2017. For more information contact the author MvN.