When we talk about legumes, most people think of food—lentils, peas and beans are all famous examples of legumes. But what you may not know is that the legume plant family, Fabaceae, is actually much larger than these food crops. It is the third largest plant family in the world, with about 19,000 known species, which can be found on every continent except Antarctica.
One of the things legumes are known for, particularly among agricultural circles, is the ability of many species to introduce nitrogen into soils—a vital ingredient for plant growth. It’s this trait that makes them a very handy crop to have as part of an agricultural rotation cycle, as other non-legume crops can benefit from the nitrogen that legumes have left behind in the soil.
Legumes have this ability, known as nitrogen-fixing, because of a very special relationship they have with a group of bacteria called rhizobia that are found in most soils. It is the bacteria that do all the work in converting nitrogen gas (N2) from the air into a more useable form of nitrogen for the plant (typically ammonia, NH3). The plants use this nitrogen in order to form proteins and amino acids to grow.
The catch is that rhizobia also use legumes to maintain their population and, in turn, convert the nitrogen that the plants need. These rhizobia infect the roots of legumes and form nodules, where they integrate themselves into the cellular structure of the plant roots and can go about their nitrogen-fixing activities. In return, the plant supplies the bacteria with the carbohydrates, proteins, and oxygen they need to grow and reproduce. So, it is a mutually beneficial relationship between legumes and rhizobia, something known in science as a type of symbiosis. They can grow independently of one another but they do much better when they’re working together.
While these relationships have been known about for quite some time, our research published last week in Nature Communications has shed some new light on just how important they are in determining where legumes can successfully grow in ecosystems worldwide. More specifically, we found that these symbiotic relationships hinder the ability of legumes to spread and establish themselves outside of the areas they are native to.
At this point we should make clear that not all species of legumes are dependent on rhizobia. There are some that get by just fine without these symbiotic relationships; they are known as non-symbiotic legumes. So, it was by comparing the establishment patterns of symbiotic legumes with non-symbiotic legumes that we were able to show symbiotic legumes have been far less successful at spreading to areas where they are not native compared with non-symbiotic legumes. It appears this special relationship comes at a cost—the symbiotic legumes aren’t as free to migrate about the globe, as their fortunes are bound to their bacterial partners.
But this isn’t simply a tale of woe for the would-be travellers held back by the demands of their partners, these findings have some important implications for the conservation of native ecosystems. How and why plant species are able to establish and spread outside their native range is a big concern for environmental managers because of the potential for damage to native ecosystems from non-native species.
When a plant is introduced (accidentally or intentionally, but usually by humans) into a new region, many factors can influence the ability for that species to become established. One major factor at play is the different set of species it will interact with in this new environment—will they work with it or against it? We naturally tend to focus on the negatives; like whether there are enemies like pathogens, predators or other competitors that will control it.
What has received less attention is how positive interactions are affecting the spread of non-native species. For example, we know that the availability of pollinators is important for many plant species. So when a species moves, it doesn’t just leave its enemies behind, it also leaves its friends, its beneficial partnerships.
And it appears that for symbiotic legumes, these beneficial partners matter a lot. Their associated rhizobia matter so much that we can see their impacts on legume species spread at a global scale, across multiple continents and islands.
We believe this elevates bacteria and other microbes in the soil to a level of importance in conservation management that they probably haven’t had previously. Right now, we have only scratched the surface in our knowledge of the diversity and distribution of soil microbes. And, generally, we aren’t that interested in them unless they’re causing a problem—like attacking a plant or animal species that is valuable to us. Because microbes involved in mutual symbiosis (mutualists) aren’t doing any harm—like in the case of legumes and rhizobia— we’ve tended to ignore them.
Our study, along with others, suggests that intentional or accidental introductions of mutualist microbe partners could play an important role in the spread of exotic or invasive legumes. To counter this and be able to monitor it, we have some more learning to do about these bacteria and how we may be spreading them.
Access the publication at Nature Communications: Simonsen et al. (2017)