Community invasibility along environmental harshness gradients in a subarctic ecosystem

Background/Question/Methods  

In subarctic regions, the climate is warming rapidly, which is resulting in the migration of native species and in an increased establishment of non-native species. However, our understanding of which subarctic communities are most prone to colonization and invasion, and will thus face new species assemblages is very limited. This is due to poor empirical knowledge and mechanistic understanding of community invasibility in subarctic and other cold-climate ecosystems.

We addressed the following questions: (1) Which subarctic communities are most likely to get colonized or invaded? (2) What drives community invasibility in cold-climate ecosystems? (3) What is the relative importance of abiotic stress and biotic resistance for invasibility in subarctic biomes? (4) Is there a general pattern of invasibility along different types of environmental harshness gradients?

We studied the invasibility (seedling emergence and establishment of 22 sown species) of four common subarctic community types (Salix scrub, meadow, rich heath, poor heath) at five elevations along an altitudinal gradient. The community types differed in their position along local topographic gradients, from benign depressions to wind-exposed ridges. We also examined the role of small-scale disturbances for invasibility and tested if the invasibility could be predicted from abiotic and biotic community characteristics.

Results/Conclusions

We found that, next to abiotic stress, biotic resistance plays an important role for invasibility in subarctic communities and that small-scale disturbances significantly improve the invasibility in the shrubless and treeless tundra zone. Our data further indicate that along altitudinal gradients the increase in abiotic stress with altitude is compensated by a similar decrease in biotic resistance, resulting in a similar invasibility along the entire gradient. This suggests that, given a sufficient seed supply, species can equally well colonize or invade at high (alpine) as at low (subalpine) elevations in subarctic ecosystems. Contrary to our expectations, the biotic resistance was higher in the harsher than in the milder end of the topographic gradient due to the presence of a dominant clonal species (Empetrum hermaphroditum). This resulted in a higher invasibility in Salix scrub  than in heath communities. With disturbance, the invasibility was highest in the meadow plots.

Overall, we expect community changes mainly to occur in less common Salix or meadow communities, whereas little changes are expected in dwarfshrub heath. The dominant vegetation type in subarctic regions thus shows a high resistance against colonization and invasion and is very likely to remain unchanged.