Eradication of five invasive alien plants on the Poor Knights Islands (New Zealand)
The Poor Knights Islands are situated 16 km off the coast of Northland, New Zealand. The two main islands are administered by the New Zealand Department of Conservation. The general public is not allowed to access the islands. There are numerous alien plant species on the islands, but only five have been identified as likely to have adverse impacts on the islands if left uncontrolled. These species are:
- Ageratina adenophora (Asteraceae), originating from Central America,
- Ageratina riparia (Asteraceae), originating from Central America,
- Cortaderia selloana (Poaceae), originating from South America,
- Cortaderia jubata (Poaceae), originating from South America,
- Araujia sericifera (Apocynaceae), originating from South America.
These species invade open disturbed areas and outcompete natural regeneration. All five species were introduced to New Zealand as ornamental plants around 1900.
Where unmodified indigenous forest (Metrosideros excelsa) is found, there is a striking absence of invasives. All existing weed sites occur in areas of disturbance associated with exposed coastal faces, shrubland and broadleaf forests.
Every spring, four people spend in total seven days on both islands searching invaded sites. Some coastal faces could only be searched by aerial observations. An aerial search was conducted annually in early summer during the flowering of Araujia serifera and Cortaderia spp., and every second spring for the flowering of Ageratina spp. Information on locations and size of the sites was stored in a database.
On the Poor Knights Islands, 142 weed sites of these 5 species have been recorded since 1995. In 2001, 112 of these sites were weed free. All flowers and seed heads were removed from the plants and placed in secure bags for removal from the island. All plants were hand-pulled, soil was shaken from the roots and the plant was placed so that the roots were freed of soil to prevent any regrowth. Herbicides were also used.
Field trips in 1996 required 96 person/days per year: 40 person/days searching, and 56 person/days completing surveillance for new infestations. In 2002, it took 56 person/days: ;40 person/days completing surveillance for new infestations and 16 person/days searching and controlling the existing sites.
Ageratina adenophora numbers have been reduced from several thousands to fewer than fifty. Ageratina riparia, Cortaderia selloana, and Cortaderia jubata have also drastically been reduced. Araujia sericifera continued to have high seedling germination in the following years, but it is now in decline, probably because of the depletion of the seedbank.
Coulston GJ (2002) Control of invasive plants on the Poor Knights Islands, New Zealand. In Veitch CR, Clout MN (Eds) Turning the tide: the eradiction of invasive species. IUCN SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambrigde, UK. 79-84.