Database compilation
We searched the ISI Web of Science in March 2017 for comparative studies
that included data on phenotypic traits and/or neutral genetic diversity
of populations on marine islands and on mainland sites in any taxonomic
group. Search terms were ‘island’ and (‘mainland’ or ‘continental’) and
‘population*’ and (‘demograph*’ or ‘fitness’ or ‘survival’ or ‘growth’
or ‘reproduc*’ or ‘density’ or ‘abundance’ or ‘size’ or ‘genetic
diversity’ or ‘genetic structure’ or ‘population genetics’) and
(‘plant*’ or ‘tree*’ or ‘shrub*or ‘animal*’ or ‘bird*’ or ‘amphibian*’
or ‘mammal*’ or ‘reptile*’ or ‘lizard*’ or ‘snake*’ or ‘fish’),
subsequently refined to the Web of Science categories ‘Ecology’ or
‘Evolutionary Biology’ or ‘Zoology’ or ‘Genetics Heredity’ or
‘Biodiversity Conservation’ or ‘Marine Freshwater Biology’ or ‘Plant
Sciences’ or ‘Geography Physical’ or ‘Ornithology’ or ‘Biochemistry
Molecular Biology’ or ‘Multidisciplinary Sciences’ or ‘Environmental
Sciences’ or ‘Fisheries’ or ‘Oceanography’ or ‘Biology’ or ‘Forestry’ or
‘Reproductive Biology’ or ‘Behavioral Sciences’. The search included the
whole text including abstract and title, but only abstracts and titles
were searchable for older papers depending on the journal. The search
returned 1237 papers which were distributed among co-authors for further
analyses.
We chose papers for inclusion in the dataset if the same species was
studied on a minimum of two islands and two sites on the mainland (Fig.
1). While we accepted the authors’ judgement about island versus
mainland status, we made our own judgement based on the relative size of
the island or position relative to the mainland i.e., some islands were
reinterpreted as mainland if they were at least four times larger than
smaller islands, with the median size difference for islands
reclassified as mainland being 126 times larger than other islands in
the study (19 papers), or if the distance of the island from the
continent very low compared to the rest of the islands (4 km vs. 1700 km
in one paper, and 300 m vs. 11 km in another paper; Table S1.1). We
eliminated studies comparing populations on several islands where there
were no clear island versus mainland comparisons even after
reinterpreting the island status, studies referring to migratory
species, recent invasions (<50 years), marine species
(including coastline organisms) and ex situ populations. The
complete selection criteria are presented in Supporting material S1. The
initial filter resulted in 235 papers which were then redistributed
among co-authors for a second round of filtering. In the second filter,
we excluded papers that did not provide both population geographic
coordinates and population-level quantitative data, unless data were
provided upon contacting the authors or could be obtained from figures
using DataThief (Tummers 2006). We visually inspected maps plotted for
each study separately and we made minor adjustments to the GPS
coordinates when the coordinates placed the focal population off the
island or mainland. For this study, we included only responses measured
at individual level, therefore we removed papers referring to
demographic performance, and we also excluded traits such as immunity,
behaviour and diet that are heavily reliant on ecosystem context. We
extracted data on population level mean for two broad categories of
response: i ) broad phenotypic measures, which included traits
such as size and weight of entire body or body parts, morphology (e.g.,
mandible shape, number of stamens, wood density), metabolism products
(e.g., colour of skin), physiology (e.g., digestive efficiency), vital
rates (growth, survival, reproduction) and mean age of sampled mature
individuals; and ii ) genetic diversity, which included
heterozygosity, allelic richness, number of alleles per locus (Fig.
S2.2, Table S2.1). The final dataset included 115 studies of 112 species
(74 animals and 38 plants) in 1019 island populations (corresponding to
569 unique islands) and 813 mainland populations, with population-level
taxonomic and biogeographic information, totalling 7736 records (Fig. 2,
Fig. S2.1-S2.2, Table S2.1, Fig. S4.1). The dataset and the
corresponding bibliography are available in the Dryad data repository
(https://doi.org/10.5061/dryad.h18931zqg) .