Effects of kiore (Rattus exulans Peale) on recruitment of indigenous coastal trees on northern offshore islands of New Zealand

Possible effects of kiore (Rattus exulans) on selected indigenous tree species in coastal forests of northern New Zealand are surveyed from recent field sampling and a literature review. Recruitment rates are compared on islands with and without kiore: (i) on the same island before or at the time of rat eradication compared with recruitment some years later, and (ii) on geographically separated islands with and without kiore. In addition, kiore-proof exclosures enabled some comparisons to be made of seed germination and survival in the presence and absence of kiore. There is evidence that kiore have substantially reduced recruitment of Pittosporum crassifolium, Pouteria costata, Streblus banksii, and Nestegis apetala, by eating the seed. Seed consumption and/or depressed recruitment is demonstrated for Rhopalostylis sapida, Vitex lucens and Pisonia brunoniana, but the extent of recruitment reduction is not yet clear. No depressive effect by kiore on the recruitment of some species, including Dysoxylum spectabile, Beilschmiedia tawa, B. terairi, Corynocarpus laevigatus, Melicytus ramiflorus, Pseudopanax arboreus, P.lessonii, and Coprosma macrocarpa, has yet been demonstrated: juveniles remain abundant in the presence of kiore. Some tree species most affected by kiore are now rare in coastal forest of the northern islands and mainland. Evidence from recruitment reduction in these species suggests that the composition of northern coastal forest before kiore arrived was significantly different from that of the present. It also suggests that, if rats are present, current successional pathways following burning or other disturbance of coastal forest will not restore the forest to its pre-human composition.