Changes in numbers of woody plant seedlings on Kapiti Island after rat eradication.
Seedlings of woody plants were counted on Kapiti Island for two years before and two years after rat eradication in September 1996. A total of 840 sample plots (660 m2) were located at 16 sites from shoreline to over 400 m altitude. Approximately 35 000 seedlings of 49 species were recognised and assigned to three height classes. Data from 21 species were analysed for seedlings 0-5 cm tall. Numbers of seedlings of Elaeocarpus dentatus, Nestegis lanceolata, Passiflora tetrandra, Pittosporum crassifolium, Prumnopytis taxifolia, Pseudopanax crassifolius, and Ripogonum scandens were predicted to increase after rat eradication; in fact, seedling numbers increased by 26 to 100% for all species except Pseudopanax crassifolius (53% fewer). During the four years, a spike of 5-10 times previous seedling numbers was recorded for several species. However, a spike in seedling numbers followed by few seedlings in the following year could not be attributed to rat eradication, especially when Pseudopanax crassifolius experienced a spike before eradication. Without the use of a control area containing rats, comparison of seedling numbers before and after rat eradication could not separate rat effects from natural variations in seedling numbers and ongoing recovery of vegetation after possum removal. However, an untreated contiguous area on Kapiti Island was not possible because of re-invasion risk, and no mainland area would have been suitable as a control.