Adding fungi to soil can introduce invasive species, threatening ecosystems


Invasive alien species are bad for ecosystems. They reduce bidoversity and disrupt food chains, including ours.

History is replete with examples of intentional and unintentional introductions of invasive species. The introduction of cane toads to northern Australia in the 1930s to control cane beetles led to the decline of many native predators. The fungus that causes chestnut blight was introduced to North America via infected nursery stock; four billion trees have died in 40 years.

It’s easy enough to see the devastation caused by invasive plant species, just look out the window: Spotted Knapweed, Eurasian Watermilfoil and Giant Hogweed have completely changed communities across North America.

Soil ecosystems

What about creatures in the ground? Have they been affected by invasive species? Which species have disappeared? Which proliferate? It is important to consider the soil as an invisible ecosystem, as many agricultural practices include the deliberate addition of microbes to the soil, biofertilizers.

Biofertilizers are microbes grown specifically to be applied to soil. Many microbes are used as biofertilizers, including bacteria and fungi, and the most common application is to improve the nutrient status of crops. These products are considered by some to be a more sustainable alternative to synthetic fertilizers.

The use of mycorrhizal fungi – fungi that grow on the roots of plants – as biofertilizers is becoming more common. Applying them as a kind of fertilizer makes sense because these fungi grow in plant roots and help plants get more nutrients from the soil.

Companies are encouraging farmers to use biofertilizers with the promise that biofertilizers will lead to healthier soil. The number of companies making mycorrhizal fungi has increased dramatically over the past decade, but there’s no easy way to track what they’re selling, where they’re being used, and how much is being released into the environment.

The root structure of red daikon radish. Farmers are being sold biofertilizers to increase crop yield.

My lab examines how mycorrhizal biofertilizers move through the environment and how they affect native ecosystems. Because mycorrhizae are an important part of all ecosystems, the introduction of an exotic mycorrhizal fungus can have unintended consequences for native mycorrhizae and ecosystems in general.

Exotic species

The application of biofertilizers and mycorrhizal products involves the introduction of potentially invasive species. These products, which are foreign to the environments in which they are placed, must establish themselves in a new environment under a wide range of conditions. To do this, they must compete with and replace native fungi. This is the definition of an invasive species.

Using biofertilizers may not be a big deal if these products stay where we put them, like in the greenhouse or in a farmer’s field. But if there’s one thing we’ve learned about germs over the past 24 months, it’s that they move, and they move fast. There is evidence that mycorrhizal fungi can travel long distances, through air currents or even as passengers of migrating birds.

In all ecosystems, mycorrhizal fungi connect plants in a community through hyphae – thin fungal strands that transport nutrients to plants. In this way, mycorrhizal fungi and their plant hosts become a superorganism – with plants of different species linked via mycorrhizal hyphae (the filaments that make up a fungus’ web).

This allows plants to detect conditions elsewhere in the web by receiving warning chemicals through hyphae if there is a herbivore somewhere in the web and increasing defense chemicals before a attack does not occur. Mycorrhizal fungi can also alter the flow of canopy sugars when a seedling is shaded and needs more carbon.

Read more: How plants rely on friendly fungal bodyguards

The problem is that while these networks are crucial to ecosystems, science doesn’t understand how they are affected by biofertilizers. There is currently no research on how mycorrhizal networks are affected by the introduction of biofertilizers or what this means for ecosystems. There is also no research beyond my lab on how far these products travel. But the science is clear on one thing: once we release these organisms into the environment, we lose the ability to control them.

white mushroom on the ground
Some soil fungi can be helpful, others can damage the soil ecosystem.

Regulate biofertilizers

This is the crux of the problem: we do not know the magnitude of the threat that biofertilizers represent for ecosystems. Yet these products continue to be marketed and traded around the world, with little or no regulation. In Canada, they are considered soil additives under the Fertilizers Act, which is the federal law for the safety of fertilizers and soil supplements. The regulations focus on the toxicity of biofertilizers to humans and other animals, not their risk as an invasive species.

A better framework might be the Plant Protection Act, which exists to protect plants, agriculture and forestry from the spread of plant pests. Although mycorrhizal fungi are not pests, they are not universally beneficial in all settings. For example, these fungi can act as a carbon sink for plants, suppressing their growth under certain conditions. It is no exaggeration to say that in some cases they could act as plant pests.

While biofertilizers are not universally beneficial to all plants under all conditions, they pose a real threat to soil biodiversity and perhaps even plant diversity. If biofertilizers outcompete local fungi, this could alter the composition and productivity of plant communities. This is a problem for natural systems, but also for agriculture and forestry.

We need to better regulate these products to ensure that they do not pose a threat to ecosystems. The thin skin of our planet’s soil is home to the creatures that keep our ecosystems functioning – we must not forget them in our quest to make agriculture more sustainable.


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