Invasive species are able to take over and vastly change the ecosystems where they invade. On par with climate change and habitat destruction, they are one of the top threats to biodiversity. A recent example in the news, Asian carp, threatens to invade the Great Lakes and decimate fish populations – this species alone could cause a $7 billion fishing industry to collapse, so we can see why it is so important to try to predict and prevent invasions before they occur. How can scientists try to stop the next big invasion before it starts?
They can start by using niche theory. Hutchinson (1957) defined the fundamental niche as the environmental area a species could occupy without other interacting species, where the realized niche is the environmental area that a species occupies when interacting species are present (for example, herbivores and disease). We cannot measure the fundamental niche of a species, so we must rely on our knowledge of the realized niche, which we can get by measuring environmental conditions where a species occurs.
To prevent invasions, we could use the realized niche of a species to predict its realized niche in areas where it might be introduced (see figure). These predictions would have to assume niche conservatism, or that an invasive species will grow under the same environmental conditions in its invaded range as it did in its native range (a). However, this might not be true, and we know that species interactions change during the invasion process (Callaway & Aschehoug 2000, Hallett 2006).
Niche shifts could occur for two potential reasons in an invasive species: First, when a species is moved across continents its biotic interactions change. Perhaps a disease that could infect it is left behind, increasing the species performance in its new range and allowing it to increase its fundamental niche and expand into new niche space (b). Second, a species can evolve to use new niches in its introduced range, for example, evolving to consume a new prey item that did not exist in the native range (c). Both of these niche shifts expand the potential range of an invasive species in its introduced range, compared to its native range.
What would it mean if invasive species were able to evolve quickly and change their niche in a new range? It would eliminate one of the major tools scientists use to predict and attempt to prevent the next big invader; scientists could no longer identify areas under threat of invasion by using the native range of a species to predict its potential introduced range.
Should we give up on the idea of niche modeling to help predict invaders? This is the focus of the recent Science paper by Petitpierre and others (Climatic Niche Shifts Are Rare Among Terrestrial Plant Invaders (2012) Science). The researchers in this paper set out to identify if niche shifts were a common occurrence in today’s invasive species. If they are not, the niche modeling may remain a helpful tool.
What they did:
The authors defined a niche shift by looking at all the available niche space in a species range, and comparing it to the niche space occupied in the invasive range. If a species occupied a niche in the introduced range that existed in the native range but was unoccupied by that species, it was said to have undergone a niche shift. Why is this? It makes the assumption that a species in its native range has had time to encounter and exist in all the niches available to it.
What they found:
The researchers concluded that niche shifts in general are rare amongst plant invaders – only 15% of invasive plants had expanded their niche more than 10% during invasion. They identified some interesting outlier species that have shown a large niche shift in their invaded range; for example, Centaurea stoebe (spotted knapweed) has expanded its niche space by > 50%, and is a species known to have altered interactions with above and belowground competitors and natural enemies.
Why I liked it:
- It takes the ideas of enemy release and evolution in invaders a step forward, and asks whether we can model how an invasive species will change based on shifting biotic interactions and selection pressures.
- Their methods could be used to identify species that have undergone large changes in biotic interactions or evolution during the invasion process. These would be good candidates for future study, such as cross continental enemy release experiments.
- As the climate changes, it could make it even more difficult to predict future invasions. Niches may expand even in the native range, making niche modeling even more difficult.
- It sets up a standard framework for identifying niche shifts, previously defined in many different ways in the literature.