The Genetics of Invasion

So far in this series we've looked at the impact of different invaders. But can we predict invasion using genes?

If you’ve read some of our previous posts on invasive species, you’ll have learned that they can be a real danger to local ecosystems and economies, and even have impacts on our health. But as Vanessa Bieker writes, there is a silver lining, and that comes in the form of what invasive species are teaching us about evolution.

Why are invasive species successful?

There is no easy answer as to why some introduced species establish themselves in new environments and others cannot. You would think that local species should be better adapted since they’ve had, in some cases, millions of years to do so, so why can new species outcompete them?

There are different theories out there, one of the most popular being the Enemy Release Hypothesis (ERH). Local species might have had considerably longer to adapt to the environment and to their predators, but their predators (and competitors) have been able to adapt to them too. So a native predator may have a greater impact on native species than on an introduced one, who they haven’t been able to adapt to. For instance, Australian mesopredators such as quolls and snakes haven’t been able to adapt to the introduction of the poisonous Cane Toad, and toad populations have subsequently skyrocketed. At the same time, an introduced predator may be able to easily prey on a native species which hasn’t been able to adapt or evolve any defenses against it. This reduced resistance to introduced species is thought to help them to grow and spread.

Another theory is that successful invasive species are able to rapidly adapt to the new environment. Common ragweed is thought to be one of the species that rapidly adapted to the conditions in the introduced range. Compared to plants from the native range, the introduced plants show improved growth and reproduction.

What can we learn from invasive species?

Each introduction to a new environment can be seen as a new natural experiment. Some of these ‘experiments’ have been going on for several hundred years already, for example, when species from the Americas came to Europe and vice versa after America’s discovery in the 15th century. If species are introduced to several similar environments, you can study whether they adapt in a similar way. You can gauge whether the same traits and genes (e.g. tendency for increased growth) are affected and thus if evolution happens a similar way every time. Some gene variations will probably become common in the new range. The question is if those variants were already present in the native population (standing variation) or if they’re the product of a new mutation (de novo). It is more likely that selection acts on standing variation as it takes some time for new mutations to occur. If that is true, species that have more variation in their native range are more likely to become invasive, as they’ll be able to adapt quicker to a new environment. The understanding of the mechanisms of adaptation to the new environment and the invasive success can not only help in the management of the invaders but also in predicting whether species are likely to become invasive. In addition to that, it can also help us understand how plants or animals react to changes in the environment, which is really helpful in the face of climate change.

Vanessa works with the common ragweed a species we've covered previously

Vanessa works with the common ragweed, a notorious European invader (Image Credit: Vanessa Bieker, NTNU University Museum, CC BY-SA 2.0)

How do you actually do that?

When we want to do genetic studies on invasive species, the first thing we do is actually collect samples (or let someone else do it for us). Since we will likely need data from both the native and invasive ranges, this may involve some travel. So when we choose our species it’s always a bonus if it’s present in some interesting places we always wanted to visit. It is also good to have some samples from the time when it was first introduced in order to see how they have evolved over time. For plants, Herbaria are a good resource for that. Some collections go back to the 16th century. For instance, I study common ragweed and am able to include hundreds of historic samples in my project, some more than 150 years old. Once we have the samples, we need to spend some time in the lab extracting the DNA and building it into so called DNA libraries, which can then be sequenced. For the old specimens, the DNA concentration is often quite low and they are prone to modern contamination. Therefore, we have a special lab that is separated from the modern DNA lab and employs special precautions to prevent contamination. Once you have the sequences, they are ready for analysis!

How do we use this knowledge?

As mentioned before, the data that geneticists like myself gather will hopefully one day contribute to an index of which species are likely to become successful invaders. As we’ve seen so far in this series, it is incredibly hard to eradicate an invasive species once it has become established. So prioritising certain species and ensuring we prevent their establishment will be a key step in protecting our ecosystems.

To read more on Vanessa’s project and to stay up to date with her work, click here.


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