Why do some introduced plant species become invasive?

Let’s start with thinking about why some species become invasive. Many plants can and do spread to new areas, but only 10-20% become invasive. There are quite a few hypotheses about why this could be, one of which is called the novel weapons hypothesis. This hypothesis predicts that species that become invasive are the non-native species which are able to produce specialised metabolites, which are novel in their new range. These metabolites (also called secondary compounds) can be things like allelochemicals, and volatile compounds, which change the soil chemistry, making it easier for the invader to live by stressing out and even killing native species. Some biochemicals, to which the metabolites belong, can increase the fitness of invasive plants by reducing the damage of herbivores or pathogens, while other biochemicals can accumulate in the soil causing damage to native biodiversity. Nevertheless, the production of high-cost specialized metabolites requires the allocation, in other words redistribution, of limited carbon budgets of the plant away from growth and reproduction.                                                                      

Info: “What exactly are metabolites?”,
you may ask and to answer that question we must dive a little into the biochemistry of a plant. Plant metabolites are organic compounds produced by plants serving a distinct role as they are for instance essential for the growth and development of the plant. In scientific spheres people often use the terms “metabolites” and “biochemicals” interchangeably. But technically, metabolites are a subgroup of biochemicals, whereby the latter term refers to any chemical compound found in a living organism. Metabolites, to be exact, are intermediates or end products of metabolism – the chemical reactions happening inside a plant to keep it alive and growing.                                                                  

Plants have budgets, just like us

To help you picture the process of plant resource allocation, think of a plant like an organization with a limited budget. In our case the “currency” is carbon and the plant must “decide” how to spend its limited budget on different needs like growing, reproducing, or making basic, cheap compounds to function. Some plants produce specialized metabolites that give them advantages, for instance in harming competing plants and consequently becoming more dominant. The ability to produce these chemicals increases the chance of a plant spread in a new area becoming invasive as the metabolites give them an advantage in the new range. But these metabolites are costly to make. If invasive plants spend their carbon budget making these expensive specialized compounds to harm native plants, they have less carbon left over for their other needs which results in a trade-off. In the past, these trade-offs held them back from dominating.

Today, however, with the human-caused rise in atmospheric CO₂, plants exist in a carbon-enriched world and scientific evidence suggests that they have increased carbon allocation budgets. Therefore, Turner and the other authors argue that there are reasons to believe that the increase of CO₂ could be a major driver of plant invasion.

The head start of invasive species

In theory, all plants, invasive or not invasive, should benefit from elevated CO₂, because then they have more carbon available and can therefore allocate more resources towards the production of biochemicals. In reality, however, invasive plants have the advantage that the biochemicals they produce can impact native flora and fauna more than those produced by native species. Therefore, the authors suggest that those exotic species which produce expensive biochemicals should hypothetically benefit more from higher CO₂ levels compared to native species and other exotic species that don’t produce these chemicals.

But not just any biochemicals

The superpower increasing the chance of exotic plants becoming invasive in this case is the production of allelopathic chemicals which can disrupt the performance of indigenous plants. Allelopathic plants release metabolites that specifically hinder the growth and reproduction of plant species. Assumably, they carry a higher energy cost than plants that don’t produce these chemicals.

Let’s look at this trade-off under changing CO₂ levels

For that we compare two types of plants – an invasive plant that produces allelopathic chemicals and a native plant that doesn’t.

Historical CO₂ levels (lower): With low atmospheric CO_2, which we had over the past couple million years, producing allelopathic chemicals would have been very expensive for a plant. This meant that invasive plants couldn’t really dominate as the cost was too high, therefore they stayed relatively rare.

Todays’ and future CO₂ levels (rising): With more CO₂  available in the atmosphere, due to human emissions, there will be an increased photosynthesis resulting in plants growing and therefore increasing their carbon budgets. The authors assume that the higher carbon budget enables invasive plants to either grow taller and outcompete native plants or to allocate more resources to secondary chemical production, like allelopathic chemicals, harming native plants. All this leads to a higher damage the invasive species can make on native species and ultimately, a potential dominance of invasive species under higher CO₂.

Welcome to scientific research

A quick recap: Elevated CO₂ could facilitate plant invasions directly through carbon allocation to growth and indirectly through the production of biochemicals, enabling invasive species to compete over native ones. This would lead to even further plant invasions coupled with more damage to native biodiversity. That doesn’t look very promising… And what happens now? Don’t forget that this is still a hypothesis for the future development of plant invasions. Now, for its validity to be determined, researchers need to experimentally test the hypothesis.

In need of a new perspective on future invasions

Biological invasion research often ignores long-term environmental changes, even though increasing CO₂ can impact plant growth and fitness in ways that alter species composition. Future research needs to separate these mechanisms and understand their interactions. Only then can future plant invasions be accurately projected and respective measures be taken to protect native biodiversity.

You want to find out more about how the authors came to their hypothesis and their theoretical reasoning behind it? Then find the full article here: Global Change Biology | Environmental Change Journal | Wiley Online Library

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