The way humans introduced Australian acacias led to the rise of the worst invasive trees

Invasive species are cunning creatures. A species is considered to be invasive if it spreads in a new environment and can outcompete local species. Some of the most well-known invasive plants are the Japanese knotweed in Europe, Norway Maple in the USA, Water Hyacinth outside South America and Giant Hogweed in Canada. 

Some of the world’s most notorious invasive trees are the Australian acacias. The genus Acacia consists of over 1,350 bush and tree species, and over 1,000 of them are native to Australia. Many Australian acacias have been grown around the world in the last 250 years. 

Once an invasive plant species arrives in a new environment, a founder population establishes. Usually, these populations undergo rapid adaptation, become experts at “overtaking” other species and continue to spread. However, the invasive population is thought to be genetically constrained in the new area. This is termed the “genetic paradox” of invasive species. 

PhD student, Sara Vicente and colleagues from the University of Lisbon, Macquarie University, Stellenbosch University and South African National Biodiversity Institute overviewed all previous research on Australian acacias and asked if invasive species are indeed genetically less diverse compared to native acacias. The researchers found that 22 invasive Australian acacias maintained high genetic diversity and did not conform to the “genetic paradox”. The reason? Humans. 

Vicente and colleagues found 51 articles that have previously investigated the genetic diversity of acacias in their invasive and native ranges. The researchers compiled the introduction of all the studied acacias and characterised why and how those populations were introduced (e.g., forestry, ornamental). Three genetic diversity indices were calculated from all studies.

The research team compiled the genetic diversity of 37 acacia species, and of these 14 of them were known to be invasive somewhere in the world. The invasive populations overall gained genetic diversity since their first introductions (sometime between 1827 to 1971).  

“By synthesising available genetic data, we add to a growing body of evidence that shows that some invasive species do not experience genetic bottlenecks or extensive inbreeding,” Vicente and colleagues wrote. 

This has raised many questions about how Australian acacias have been introduced in the past. The scientists overviewed historical data and found that often, millions of seeds have been mixed together from multiple locations when a particular species was introduced. 

For example, “[o]ver 280 million seeds of the Port Jackson willow, A. saligna, have been imported to South Africa and were broadcast into the environment for dune stabilisation,” the authors explain..  

“These seeds were not only sourced from Australia, but also from secondary ranges such as France. For the black wattle, A. mearnsii, millions of seeds were introduced to start forestry plantations for tannin production in South Africa.”

So, the way humans have been introducing new plant species has led to the successful establishment of invasive Australian acacias in other countries. The founder populations did not experience genetic bottlenecks nor lose their adaptive capabilities like other, “more naturally” introduced species. Instead, the invasive acacias have reshaped the native ecosystems and outcompeted local species.

Hopefully, people can learn from this study and be more cautious in the future when thinking about introducing new species to a novel environment.