Seabirds are chemical and physical engineers capable of transforming vegetation by altering edaphic conditions, generating physical disturbance, and affecting seed dispersal.  Although dramatic effects of seabirds on island vegetation have long been noted, the lack of comparative studies across a variety of systems makes it very difficult to make predictions about the role of seabirds on unstudied seabird islands.  We compared the impact of seabird nutrients and disturbance on plant communities across six different island systems: northern New Zealand, Gulf of California (Mexico), Gulf of Maine, French Mediterranean, Aleutians, and Stockholm Archipelago (Sweden).  We used plant species lists to compare floristic composition on islands with different densities of seabirds within and across systems.

Results/Conclusions

Certain plant families showed a strong association with seabirds in multiple systems.  The family Caryophyllaceae was associated with seabird islands in the Aleutians, Gulf of Maine, and New Zealand; species of Brassicaceae frequently occurred on seabird islands in the Aleutians, the Stockholm Archipelago, and Gulf of Maine.  Fabaceae was associated with non-seabird islands in the Gulf of Maine, Mediterranean, and New Zealand; Asteraceae were common on non-seabird islands in the Aleutians, Gulf of Maine, Mediterranean, and New Zealand.  The Gulf of California showed the least floristic overlap with the other systems, and some trends were opposite to those in the other systems.  In New Zealand, the percentage of plant species that was non-native was not a function of island size, but was strongly influenced by island status; the percentage of non-native species was greater on islands without seabirds.  In contrast, the percentage of non-native plant species was greater on seabird islands than on islands without birds in the Gulf of Maine.  There were very few non-native species in the Aleutians and they were associated with some of the seabird islands.  We conclude that floristic composition on seabird islands, particularly at the taxonomic level of family, are highly predictable especially across similar climatic regions.  However, at a whole-island, whole-flora scale the probability that non-native plant species will establish on seabird islands varies across systems.