Nav: Home

Picky pathogens help non-native tree species invade

July 24, 2019

 

Walk into a forest made of only native trees, and you probably notice many different tree species around you, with no one species dominating the ecosystem. Such biodiversity - the variety of life and species in the forest - ensures that each species gets a role to play in the ecosystem, boosting forest health and productivity. However, when non-native trees invade, they form dense groups of a single species of tree. This bucks conventional wisdom because, in theory, pathogens - microscopic disease-causing organisms - should prevent this from happening.

Trees have many natural enemies, such as herbivores and insects that nibble on their leaves. But their main foes are invisible to the naked eye. In older forests especially, fungal pathogens evolve to attack the seedlings of certain tree species and, over time, accumulate in the soils around the adults, hindering the growth of their seeds. Seeds that fall far away from their parent typically survive better. The pathogens thus help dictate where native trees can grow and prevent some species from dominating others.

This effect is part of the Janzen-Connell hypothesis, a widely accepted explanation for the promotion of biodiversity in forests. The theory was developed in the 1970s by ecologists Daniel Janzen and Joseph Connell, who said that species-specific herbivores, pathogens, or other natural enemies make the areas near a tree inhospitable for the survival of its seedlings. If one species becomes too abundant, there will be few safe places for its seedlings to survive, thus promoting the growth of other plant species within one area.

Why, then, do introduced tree species often invade, outcompete, and displace native trees, even those of the same genus? The subject is the focus of a new study published in the Ecological Society of America's journal Ecosphere.

Aleksandra Wróbel, a PhD candidate in the Department of Systematic Zoology at Adam Mickiewicz University in Poland, says that the relationship between native trees and their enemy pathogens is a tightly co-evolved one; so tight, in fact, that pathogens may not be able to recognize or attack even closely-related introduced tree species. "Enemy-release" theory states that because introduced species are new to the ecosystem, they do not have enemies yet in the soil, and their seeds can fall densely and thrive, becoming invasive. This relaxation of the Janzen-Connell effect, says Wróbel, "gives non-native species a big competitive advantage over native species."

To test how the intensity of the effect differs between tree species of a same genus, Wróbel and colleagues use greenhouse and field experiments to study two pairs of invasive and native tree species in Central Europe: boxelder (Acer negundo) vs. Norway maple (Acer platanoides), and Northern red oak (Quercus rubra) vs. pedunculate oak (Quercus robur). They also conduct surveys of natural forests containing populations of all four species to investigate their survival in an uncontrolled environment.

Under controlled conditions, the invasive species of both pairs fare better than their respective native cousins, avoiding attack by soil pathogens; they are released from the Janzen-Connell effect, giving them the survival advantage. However, under natural conditions, one genus pair (Acer) followed the expected pattern, with non-native seedlings escaping pathogen attack, while the other pair (Quercus) did not. Wróbel chalks this up to the complexity of interactions with other plants and animals that tree seedlings experience in the wild.

This study indicates that freedom from disease partially explains why non-native tree species dominate in areas where they are introduced. Improving understanding of the role that plant-soil interactions play in the establishment and spread of invasive plants is critical for developing effective management and control strategies.

"Figuring out why invasive species are able to proliferate so readily in new areas is one of the most important issues in ecology and nature protection," concludes Wróbel, "and the results of our research provide additional insight into the factors responsible for the success of certain non-native species."
-end-
Journal article

Wróbel, Aleksandra, et al. 2019. "Differential impacts of soil microbes on native and co?occurring invasive tree species." Ecosphere. DOI: 10.1002/ecs2.2802

Authors

Aleksandra Wróbel, Rafa Zwolak; Department of Systematic Zoology, Faculty of Biology, Adam Mickiewicz University, Pozna?, Poland

Elizabeth E. Crone , Department of Biology, Tufts University, Medford, Massachusetts, USA

 

Author contact:

Aleksandra Wróbel, wrobel_a1@wp.pl

The Ecological Society of America (ESA), founded in 1915, is the world's largest community of professional ecologists and a trusted source of ecological knowledge, committed to advancing the understanding of life on Earth. The 9,000 member Society publishes fivejournalsand a membership bulletin and broadly shares ecological information through policy, media outreach, and education initiatives. The Society's Annual Meeting attracts 3,000-4,000 attendees and features the most recent advances in ecological science. Visit the ESA website at http://www.esa.org.

Contact: Zoe Gentes, 202-833-8773 ext. 211, zgentes@esa.org">zgentes@esa.org

Ecological Society of America

Related Biodiversity Articles:

Biodiversity and wind energy
The location and operation of wind energy plants are often in direct conflict with the legal protection of endangered species.
Mapping global biodiversity change
A new study, published in Science, which focuses on mapping biodiversity change in marine and land ecosystems shows that loss of biodiversity is most prevalent in the tropic, with changes in marine ecosystems outpacing those on land.
What if we paid countries to protect biodiversity?
Researchers from Sweden, Germany, Brazil and the USA have developed a financial mechanism to support the protection of the world's natural heritage.
Grassland biodiversity is blowing in the wind
Temperate grasslands are the most endangered but least protected ecosystems on Earth.
The loss of biodiversity comes at a price
A University of Cordoba research team ran the numbers on the impact of forest fires on emblematic species using the fires in Spain's Doñana National Park and Segura mountains in 2017 as examples
Biodiversity and carbon: perfect together
Biodiversity conservation is often considered to be a co-benefit of protecting carbon sinks such as intact forests to help mitigate climate change.
The last chance for Madagascar's biodiversity
A group of scientists from Madagascar, UK, Australia, USA and Finland have recommended actions the government of Madagascar's recently elected president, Andry Rajoelina should take to turn around the precipitous decline of biodiversity and help put Madagascar on a trajectory towards sustainable growth.
Biodiversity draws the ecotourism crowd
Nature -- if you support it, ecotourists will come. Managed wisely, both can win.
Biodiversity for the birds
Can't a bird get some biodiversity around here? The landscaping choices homeowners make can lead to reduced bird populations, thanks to the elimination of native plants and the accidental creation of food deserts.
Biodiversity can also destabilize ecosystems
According to the prevailing opinion, species-rich ecosystems are more stable against environmental disruptions such as drought, hot spells or pesticides.
More Biodiversity News and Biodiversity Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

Accessing Better Health
Essential health care is a right, not a privilege ... or is it? This hour, TED speakers explore how we can give everyone access to a healthier way of life, despite who you are or where you live. Guests include physician Raj Panjabi, former NYC health commissioner Mary Bassett, researcher Michael Hendryx, and neuroscientist Rachel Wurzman.
Now Playing: Science for the People

#544 Prosperity Without Growth
The societies we live in are organised around growth, objects, and driving forward a constantly expanding economy as benchmarks of success and prosperity. But this growing consumption at all costs is at odds with our understanding of what our planet can support. How do we lower the environmental impact of economic activity? How do we redefine success and prosperity separate from GDP, which politicians and governments have focused on for decades? We speak with ecological economist Tim Jackson, Professor of Sustainable Development at the University of Surrey, Director of the Centre for the Understanding of Sustainable Propserity, and author of...
Now Playing: Radiolab

An Announcement from Radiolab