Similar yet distinct distributional patterns characterize native and exotic plant species richness across northern New Zealand islands

Aim A better understanding of plant invasions on islands can be gained from comparing patterns of exotic and native species richness. We asked four questions: (1) Is exotic species richness on islands related to native species richness? (2) If they are related, does this result from similar responses of native and exotic species to specific island characteristics? (3) Is residual variation in native‐exotic richness relationships associated with distinctive island characteristics? (4) Are relationships between species richness and island characteristics scale‐dependent, and do they differ between native and exotic species? Location Northern New Zealand. Taxon Vascular plants. Methods We conducted field surveys and augmented our field data with previously published surveys to quantify the number of native and exotic plant species on 264 islands. We then explored the relationship of species richness and several island characteristics (e.g. area and isolation) using multiple and iterative regression techniques. Results Seventy‐two percent of among‐island variation in exotic species richness was positively related to native species richness. Both native and exotic richness increased with island area and declined with isolation and exposure to ocean‐borne disturbances (a proxy for salt spray, wave action, etc). However, exotic species responded more strongly to these three variables. Exotic richness also decreased with latitude and the distance from the nearest urban area, but native species did not. Island area was a better predictor of species richness on larger islands, whereas isolation and exposure were better predictors on smaller islands. Scale‐dependent relationships between species richness and island characteristics were stronger for exotic species. Main Conclusions Insular distribution patterns of native and exotic plant species richness are governed by similar biogeographic principles. However, in New Zealand, exotic species exhibited subtle, yet distinctive, invasion patterns preferring larger, less isolated, less exposed islands that were located at higher latitudes and closer to urban areas.