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.

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.

Metadata:

outcome	Colonization outcome (0=failure, 1=success)
Euref_X	EUREF-FIN X-coordinate.
Euref_Y	EUREF-FIN Y-coordinate.
Euref_X_original	EUREF-FIN X-coordinate. Untransformed values.
Euref_Y_original	EUREF-FIN Y-coordinate. Untransformed values.
Residents_per_area_log	Number of resident species per unit area.
Area_log	Island area.
Buffer_2_km_log	Land area within a 2 km buffer around an island.
Buffer_5_km_log	Land area within a 5 km buffer around an island.
Shannon_habitats	Shannon's diversity index calculated from the proportional cover of CORINE level 4 land cover types.
Convolution	Index of island convolution as the perimeter ratio of island and circle of the same area.
Limestone	Presence of limestone on the island according to Eklund (0=absent, 1=present)
Buildings	Proportion of built area on an island based CORINE land cover data.
Meadow_or_pasture	Proportion of meadows and pastures on an island based on CORINE land cover data.
Deciduous_forest	Proportion of deciduous forests on an island based on CORINE land cover data.
Coniferous_forest	Proportion of coniferouse forest on an island based on CORINE land cover data.
Mixed_forest	Proportion of mixed forests on an island based on CORINE land cover data.
Scrub	Proportion of Scrub on an island based on CORINE land cover data.
Sand	Proportion of sand on an island based on CORINE land cover data.
Open_rock_or_bare_ground	Proportion of open roch or and bare ground on an island based on CORINE land cover data.
Marsh	Proportion of marsh on an island based on CORINE land cover data.
Shore_meadow	Proportion of shore meadows on an island based on CORINE land cover data.
Dist_to_historical_log	Distance to the nearest island historically colonized by the focal species.
Gowdis_traits	Grower distance between  life history traits of the focal species and resident species of the focal island.
Historical_total_log	Number of islands historically colonized by the focal species.
North_limit	North limit of distribution of the focal species in Finland
Ellenberg_Light	Ellenberg's score for light for the focal species.
Ellenberg_Temperature	Ellenberg's score for temperature for the focal species.
Ellenberg_Moisture	Ellenberg's score for moisture for the focal species.
Ellenberg_Reaction	Ellenberg's score for soil reaction (pH) for the focal species.
Ellenberg_Nitrogen	Ellenberg's score for nitrogen for the focal species.
Eklund_culture	Eklunds score for focal species affinity to human modified habitats.
Life_cycle	Life cycle length for the focal species.
Life_form	Life form of the focal species.
SLA	Specific leaf area of the focal species.
Plant_height_log	Maximum height of the focal species.
Dispersal_vector	Dispersal vector of the focal species.
Seed_bank	Seed bank longevity of the focal species.
Seed_mass_log	Seed mas of the focal species.
Pollen_vector	Pollen vector of the focal species.
Apomictic	Apomictic reporduction of the focal species.
Veg_repr	Vegetative reproduction of the focal species.
spp.name	Focal species name.
holmkod	Focal island code.
Family	Focal species family.
Genus	Focal species genus.