Although the 40-hour long flight to New Zealand was long and tiring, it took me no longer than an hour to be surprised by how many European plants there were in Christchurch. Some of my favorite plants from home flower were here in abundance. At the same time, it is no secret that few countries fight invasive weeds as hard as New Zealand. The invasion of plants is prominent – it was not until I entered Arthurs Pass, in the Southern Alps, that I came across communities of native plants, such as the fragrant kanuka.

Why are native plants important? As a part of an ecosystem, the indigenous plants play significant roles in feeding native nsects, birds, and lizards. For example, it is not a coincidence that the tui is not found in Christchurch, they need native plants to feed on. Therefore, it is a high priority to protect the native plants and control the invasive weeds.

Since invasive species develop resistance to control methods over time, New Zealand constantly needs to develop new methods of pest control. To do this researchers needs to know as much as possible about their enemies for the best chance of defeating them. With this in mind, a group of researchers, including Philip Hulme from Lincoln University, set out to study native and invasive plants on the Banks Peninsula, south-east of Christchurch, to gain the understanding why many alien plants species have such success in New Zealand. Often it is assumed that foreign species have fundamentally different traits than native species, but the underlying reasons that are needed for a greater understanding, are not addressed.

In New Zealand, more than 30% of the land is protected. After seeing both invasive plants and animals in your national parks, I came to wonder… protected from what? Sure, protection from human activities and industrial impact is great, but does the country really manage to protect many of these areas from invasive species?

Much of the protected land also serves to protect scenic landscapes rather than actually preserving the native biodiversity. Scenic reserves are the most common protected area in New Zealand. They are often small and therefore likely to be inefficient for the protection of native species. Edge effects have a large impact on small reserves causing disturbances and, according to previous studies by Philip Hulme, the possibility for invasive plants to establish in native forests is high.

Controlling pests in New Zealand seems to be a never-ending war, especially on the invasive animals, but there is also a big cost for biodiversity due to invasive plants. On top of that, it’s calculated that clearing weeds and mitigating their success cost New Zealand more than 300 million dollars annually.

The team investigated the dominance of plants on the Banks Peninsula. Land modified by humans, such as sheep farms, seem to hold less native species than land that has been left alone, such as a national park. What the team of researchers wanted to know was if the plant distribution on the Banks Peninsula could be explained by past human impact, or if other traits such as adaptation to pests played a bigger role.

Alien plant species were six times more common than native plant species on the peninsula. This would imply that most of New Zealand’s flora on Banks Peninsula is under threat from the ongoing invasion. Hulme also found that native and alien species largely have different environmental preferences. Native species preferred south facing slopes (related to cooler environment), high elevation and steep slopes while alien plants preferred the opposite.

While all of these environmental variables are parts of the landscape, Philip, and his team argue that environmental preferences are easy to connect to human impact. For example, the dry north facing slopes have likely been more easily cleared by fire than south facing slopes. Due to a longer coevolution with humans, alien plants in New Zealand tend to be better adapted for human disturbances. In non-disturbed areas, alien plants did not seem to have any advantages over native plants.

How are alien plants a threat to biodiversity? If native and alien plants occupy different habitats, the alien plants shouldn’t pose a threat to the native ones. Unfortunately, ecology is rarely that simple. First of all, the “exceptions” of alien plant species that break this rule do cause serious damage to native plant communities. An example is wilding pines (Pinus radiata) which I have seen frequently in protected areas. These colonizing pines permanently change soil composition making it toxic for other plant species.

Alien plants may also support alien invertebrate species and grazers. Deer roam almost everywhere on the South Island and as most of New Zealand’s native plants are not well adapted to browsing this may give exotic species, that evolved with deer-browsing, a competitive advantage. Since deer were introduced by humans, they can also be considered a human impact.

What is a human-modified landscape in New Zealand, and how do we protect New Zealand from it? The latter question probably troubles every ecologist in New Zealand – but there is no certain answer. However, there is no doubt about the importance of protecting New Zealand’s nature both for the survival of native species and for the economy. New Zealand is often portrayed as a country with unique nature both overseas and locally. Tourism is the biggest income export industry of the nation and people come from all over the world to enjoy the country.

Protecting the unique nature that still exists in New Zealand ought to be a key priority. Ending the dominance of invasive plants is a hard task to undertake. To minimize the human disturbance will not solely be enough, but would provide an important base for conserving plants as well as the animals relying on them. However, just as you, the people of New Zealand, don’t enjoy freedom campers on your lawn, your plants are no more tolerant! We better keep our disturbances low.

The author Wilhelm Osterman is a postgraduate student in the Master of International Nature Conservation taught jointly at Lincoln University and University of Göttingen. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.