Non-natives at the end of the world

The Crozet archipelago. A few tiny specks in a vast ocean, ‘on the road’ from South Africa to Antarctica. A tough climate, inhabitants limited to a bunch of winter-hardy researchers and the occasional seabird. But also: Poa annua, the common street grass you’d find in cracks in the streets in any European city.

The non-native Poa annua weathering the elements on Possession Island on the Crozet archipelago. Picture by Rémi Joly.

A species perhaps a bit out of place on the island, but it’s far from alone: there are already 68 non-native plant species recorded on Possession Island alone. Some of them very local, restricted to the few human settlements and the trails connecting them, while others have managed to spread quite a lot throughout the island.

That brought us to an important question: what is driving the distribution of these non-native species on the island? Is it climate that limits them, or human-related factors? Luckily, those scientists on the island haven’t been idle: they collected highly detailed survey data on non-native plant species distributions on the island yearly since 2010, making the archipelago and its vegetation into a perfect case study for cold-climate plant invasions. We used that dataset and went ahead to make species distribution models for each of the 6 most important non-natives. The results of this modelling exercise are now published here.

Interestingly, we observed two very distinct invasion patterns: species were either predicted to occur over a narrow spatial extent, with their occurrence probability strongly affected by human-related variables; or they occurred over a wide spatial extent, only limited by particularly harsh climatic conditions (see figure).

Graphical summary of the main findings of the paper, distinguishing between the two types of non-natives: left, low-spread species, mostly tall annuals, who are limited to human settlements and trails. Right, high-spread species, typically short-statured perennials, who have spread beyond the limits of human settlements and now are largely restricted by climate conditions-only.

So some species were highly climate-limited, while others were mostly driven by disturbance. Although the sample size was small, our species suggested that it were mostly perennial and low-stature species, historically introduced earlier, who appeared less dependent on human-induced dispersal and disturbance, and thus more widely distributed on the island.

Tall annual non-natives thus seem to lack the necessary toolkit to successfully spread far from introduction sites under the harsh sub-Antarctic climate on the island. Additionally, the coldest inner parts of the island are currently still free even from those widely-spread short perennials, suggesting that at least some parts of the island are still highly resistant against plant invasions.

So what to do next? Our study clearly exemplifies that even those harsh and remote places are not spared from non-native plants, and that with the right traits, non-natives can become highly successful even there. As climate warms further, these last climatic barriers will also lower, tilting the balance even more in their favour. It is thus extremely urgent to identify current – and future – potential non-natives on the sub-Antarctic islands across the region, and see if sufficient regulations are in place to contain them.

Reference:

Bazzichetto et al. (2021). Once upon a time in the far south: Influence of local drivers and functional traits on plant invasion in the harsh sub-Antarctic islands. Journal of Vegetation Science. https://doi.org/10.1111/jvs.13057

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