Versatile Invaders: Exploring the movement and interactions of nonnative pine trees, fungi, and hoofed mammals in Argentina

Featured Image Caption: Pine trees in Illinois. While species like white and red pines are native to North America, they have become a problematic invasive species in the Southern Hemisphere. (Credit: IvoShandor, licensed by CC BY-SA 3.0, via Wikimedia Commons.)

Source Article: Policelli, N., Horton, T. R., Kitzberger, T., & Nuñez, M. A. (2022). Invasive ectomycorrhizal fungi can disperse in the absence of their known vectors. Fungal Ecology, 55. https://doi.org/10.1016/j.funeco.2021.101124

What are Invasive Species?

Early March marks the annual National Invasive Species Awareness Week (NISAW), an event to raise awareness of invasive species for both the public and elected officials. Invasive species are organisms which enter an area they are not originally from, where they may displace native species and cause major habitat disturbance. Often, these organisms are aggressive and grow rapidly. Well-known invasive species include insects such as the emerald ash borer, plants such as garlic mustard and purple loosestrife, aquatic life such as Asian carp and zebra mussels, and even large vertebrates like Burmese pythons. Initial movement of these species outside of their native habitats is usually facilitated by humans, such as through the release of exotic pets, deliberate import for use in gardens, or by ‘hitching a ride’ in cargo or on vehicles traveling from one area to another.

When two or more invasive species exist in the same area, they may interact in a way that amplifies the harm done to the surrounding habitat. For instance, non-native mammals might help spread seeds of non-native plants or selectively eat native plants, damaging native populations and helping invasive plants to survive and spread. Understanding these interactions is essential to helping us predict what areas might be at the greatest risk. 

Pines, Deer, and Fungi: An Unexpected Partnership

One example of such an interaction is the three-way synergy between pine trees, invasive ungulates (hoofed mammals like wild boar and deer), and invasive ectomycorrhizal fungi (EMF). In the Southern Hemisphere, pines in the Pinaceae family were introduced for decoration and timber farming, but became problematic invaders. EMF are fungi which live belowground, attached to tree roots. They provide the tree with nutrients and water, and in exchange receive carbon which the tree produces through photosynthesis. The fungi use this carbon for energy. EMF are incredibly important to many plants, many of which grow and spread significantly slower when EMF are not present. However, invasive pines often partner with invasive EMF, which allows the trees to thrive. Ungulates spread spores from these fungi through their feces, which helps invasive pine trees establish themselves in new areas.

The interactions between these groups clearly help invasive pines spread, but how essential are they? Scientists wondered whether pine invasion would be significantly hindered in places without invasive ungulates, because they wouldn’t be present to disperse EMF spores. Answering this question would help us determine potential ways to limit spread of pines, and understand whether certain areas are naturally more protected from invasion due to a historic lack of ungulates.

This study was conducted in Argentina, around two separated lakes. Three categories of sites were chosen: 1) sites near pine plantations which had been invaded by non-native pines, 2) native forest sites without invasive pines, and 3) islets with very few invasive pines and no history of invasive ungulates. The scientists noted that the sites without ungulates had a very different plant composition, likely because deer, wild boar, and various farm animals were not around to feed on their preferred species. For example, the islets had more ferns, vines and native plants. The other sites had more bare soil and nonnative shrubs that ungulates help to disperse.

The research team collected soil from each site. They grew plants in this soil for a year, after which they inspected the plants’ roots for EMF colonization and identified the specific fungi. The scientists hypothesized that plants grown in soil from the third site would not have the invasive EMF fungi associated with invasive pines, due to the absence of ungulates.

The Mystery of Mycorrhizal Movement

Contrary to expectations, invasive EMF fungi were found on plants grown in soil taken from the islets, despite the fact that the islets lacked the ungulates known for spreading EMF spores. This is an important discovery: it means that areas such as these islets are less protected from EMF and pine invasion than previously thought. How might the fungi have reached these ungulate-free sites? Airborne travel of EMF spores is generally thought to only be short-range, but when fungi release large numbers of spores it increases the chance that a few may reach more distant areas. Soil particles containing spores might travel by air or water. Birds, native mammals, or even humans could also be important carriers!

Further research is required to pinpoint the most common ways these fungi spread, but this experiment has provided insight into invasive species’ interactions and potential dispersal capabilities of EMF. These interactions are likely more complex than initially assumed, and it is important to fully understand them in order to effectively protect natural areas.

Resources for learning more about invasive species:

• The USDA National Invasive Species Information Center has an invasive species overview, a searchable species database, and a list of resources by state or region. 

• The US Fish and Wildlife Service has a page for invasive species Frequently Asked Questions.

• The University of Georgia created an extensive map database for US invasive species ranges.

• The European Union published a brochure describing invasive species of concern in the EU.

• Visit these links to learn more about how you can stop invasive species while spending time outside and taking part in aquatic recreation. Many areas also have “citizen science” programs, where you can help track the spread of local invasive species!

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