Aim Isolation is a key factor in island biology. It is usually defined as the distance to the geographically nearest mainland, but multiple alternatives exist. We explored how testing different isolation indices affects the inference of isolation effects on faunal characteristics. We focused on land-bridge islands and compared the relationships of multiple spatial and temporal (i.e., through time) isolation indices with community-, population-, and individual-level characteristics (species richness, population density, and body size, respectively).

Location Aegean Sea islands, Greece.

Time period: current.

Taxon Multiple animal taxa.

Methods We estimated 21 isolation indices for 205 islands, and recorded species-richness data for 15 taxa (invertebrates and vertebrates). We obtained body size data for seven lizard species and population density data for three. We explored how well indices predict each characteristic, in each taxon, by conducting a series of OLS regressions (controlling for island area when needed), and a meta-analysis.

Results Isolation was significantly (and negatively) associated with species richness in ten of 15 taxa. It was significantly (and positively) associated with body size in only one of seven species, and was not associated with population density. The effect of isolation on species richness was much weaker than island area, regardless of the index tested. Spatial indices generally outperformed temporal indices, and indices directly related to the mainland outperformed those related mainly to neighbouring islands. No index was universally superior to others, including the distance to the geographically nearest mainland.

Main Conclusions The choice of index can alter our perception of the effects of isolation on biological patterns. The nearly automatic, ubiquitous, use of distance to the geographically nearest mainland misrepresents the complexity of isolation effects. We recommend to simultaneously test multiple indices, that represent different aspects of isolation, to produce more constructive and thorough investigations and avoid imprecise inference.