Final answer:
New Zealand has more alien plant species than native, with around 2100 introduced species now established and outnumbering the indigenous plants. This has been enabled by New Zealand's unique and isolated ecosystem, where many introduced species such as nitrogen-fixers have thrived, often to the detriment of native species.
Step-by-step explanation:
New Zealand has more alien plant species than native. This is a result of human activities that have introduced a vast number of plant species to the region. Notably, the invasional meltdown in certain areas, owing to nitrogen-fixing plants like gorse, scotch broom, and tree lupine, has altered the native plant community structure and soil composition, making it difficult for native species to compete. Invasive species have seen immense proliferation partly due to New Zealand's unique ecosystem, which has been isolated for millennia since the break-up of the supercontinent Gondwana, leading to a lack of natural predators for these alien species.
Ecologists point out that island ecosystems, such as those in New Zealand, are particularly vulnerable to the introduction of non-native species, which can have detrimental effects on the native flora and fauna. This has been reflected in New Zealand's biodiversity, where approximately 2100 species of introduced plants are now established in New Zealand, outnumbering native species.