Horror stories of an alien species taking over and annihilating diversity in a native ecosystem have become pervasive in ecological news. This happens as our world continues to become more and more interconnected: species are moved around by humans- be it intentional or not. There are the rabbit, cane toad and camel population sizes exploding in Australia. There are the well established parrot populations of Europe. Countless “weeds” that have pervaded native ecosystems and are thought to cause trouble.
But it could be that alien species aren’t all the bad guys we make them out to be.
This was recently shown a long term experiment on succession after landslides in New Zealand. Ecological succession is what happens when an ecosystem is deeply disturbed and recolonized by a series of plants species, one after the other. Usually mosses, grasses and sedges,”weedy” species that grow fast in poor conditions, are the first to arrive. They are followed by shrubs, then trees until a full, diverse, forest is recovered. Australian opossums introduced to New Zealand in the 1800’s were thought to hurt this process by eating native plants.
Since the possums were introduced by humans to New Zealand, they have grazed on plants that weren’t grazed on before. To see if this was disruptive, a team of researchers set out on an 11-year long experiment, taking advantage of large landslides that destroyed all vegetation in an area of the Western South island in 2002.
The scientists cleverly created plots and introduced the same vegetation in all of them, the only difference being that some of them were caged off: possum proof. They expected to the possum proof areas to re-vegetate faster, since there would be no inappropriate grazing. However, they were surprised to see the opposite result.
Areas munched on by possums contain more of a “high quality” shrubby plant species: one that can harness nitrogen in the air to use it, also making it available to fellow plants. More plants growing also means more carbon in the ground: more biomass created and stored. Bacteria species are also more abundant and diverse in plots visited by possums. But why? It seems that the possums are grazing on grassy species, freeing up ecological space for the later species in succession. This speeds up the process, aiding in recovery and overall benefitting diversity in the system.
Careful analysis only will allow us to determine whether specific alien species are harmful to ecosystems or not. In a world where we have pervasively modified which species are present where, we should tread carefully in trying to control them. They might actually be reinforcing our efforts to restore the quality of our environment.
Let me know what you think in the comments!
For more on this topic I recommend a great book: The New Wild by Fred Pearce
Article: Bellingham et al. 2016 Journal of Ecology.