Exotic species pose one of the world’s biggest environmental problems. But we don’t always know why or how these species can spread so quickly.
“Invasive species are a major factor in the crisis affecting biodiversity now,” says Michael D. Martin, professor of evolutionary genomics at the University Museum at the Norwegian University of Science and Technology (NTNU).
The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) is defined The Five Biggest Global Threats to Species Diversity. Land use change is leading the way, before direct exploitation of natural resources, climate change and pollution.
But the fifth threat may not have occurred to many people: alien species that spread in areas where they do not belong. We know they are a big problem.
What is less clear, however, is how and why exotic species spread so quickly. So an international research group that includes some of the world’s leading genetic researchers studied the case of common ragweed (Ambrosia artemisifolia). Their results have now been published in Science Advances.
Why do weeds spread?
The common ragweed is native to temperate parts of North America, but was introduced to Europe in the 19th century, probably by accident through imported seeds and contaminated horse feed. In recent years, it has spread to large parts of the continent. The main source of introduction today is contaminated bird feed, so if you feed birds outdoors with imported seed, you really should sort out the ragweed seeds first.
Fortunately, many alien species succumb before they can do any harm because they are unable to establish and adapt to new conditions. So why does ragweed grow so well? The answer lies in their genes.
“We examined genetic material in 655 samples of common ragweed, of which 308 were from historical plant groups in the herb. Some were up to 190 years old and are from the time the plant was first introduced to Europe,” says Vanessa C. Baker, an expert in evolutionary genetics at NTNU University Museum.
In this way, the researchers were able to follow how the common ragweed had evolved since the plant’s arrival in Europe. This information provided answers that helped them better understand what led to the massive proliferation today.
But first, a little bit of information about why exotics is something we have to worry about.
causing problems all over the world
Exotic species cause problems in large parts of the world. In Norway, invasive threats include salmon parasite Gyrodactylus salaris, mink, Sitka spruce, garden lupine, American lobster, pondweed Elodea canadensis, red crabs, Canada goose, and giant piggrass.
The impact of man on nature is often at the heart of the problem. The population of Homo sapiens will exceed eight billion this year. Over the past 50,000 years, spreading species to parts of the world they don’t belong has been one of the ways humans have changed the planet.
These exotic species can outgrow those already in an area. Sometimes they simply eat the local species. Other times they eat their own food. They take over the habitats of species that are unable to counter the invaders’ ability to reproduce or make use of the resources in the area.
Common ragweed grows quickly and becomes large and thus can outcompete native species.
rabbits and cats
A famous example is the rabbits of Australia, where Europeans released a few rabbits on their newly discovered continent to make them more friendly and possessive of something to hunt. But in Australia, rabbits had no natural enemies that could keep the population under control.
Half a billion bunnies and massive destruction of nature later, bunnies aren’t so much fun anymore. Even after widespread disease outbreaks and intense population control efforts, Australia still has a few hundred million rabbits, not to mention over a million wild camels, 200 million frogs, and a few million foxes and wild cats.
Cats are one of the biggest threats to birds and other animals around the world. In the United States they kill up to four billion birds and more than 20 billion mammals annually, while in Norway outdoor cats kill about seven million birds. If you really want to help the environment, you should keep your cat indoors – and neuter her, too.
Tougher plants in Europe
So, back to Europe and the common ragweed. The research group found answers that could explain why this plant was so successful.
“Invasive populations in Europe prefer to develop genes that contribute to their defense, such as genes against pathogens that cause disease,” says Vanessa C. Baker.
In Europe, ragweed may have evolved in such a way as to make the plant more resistant to local threats.
Natural selection means that hardy plants have a significant advantage and multiply more often than less hardy specimens. This extended to the offspring who carried the advantage forward. Today, the tougher plants are completely taken over.
Other species contributed to the spread
The common ragweed also received help from outsiders along the way. The common ragweed reproduces sexually and makes up for the lack of partners on a new continent by coming out of its kind.
“We discovered that the plant crossed in Europe with closely related species that were introduced around the same time,” says Michael Martin.
This behavior meant that the common ragweed did not need the common ragweed plant nearby for the plant to gain a foothold where pollen from close relatives could be used to produce seeds. This is particularly useful in the early stages of introduction when population sizes are small.
Spread all the way to Denmark
The plant may also have escaped its North American enemies by coming here. In its normal range, it was susceptible to bacterial pathogens attacking it.
In Europe, native bacteria did not co-evolve with common ragweed, and therefore did not pose an immediate threat. An invasive plant can use more energy to grow and reproduce than to defend, which in turn gives it an advantage over native plants.
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