This is a guest post by Emily De Stigter.
Gazing across a north-eastern Victorian landscape, you might notice some unusually fluorescent shades of green mixed into the otherwise earthy tones of Eucalypt forests. Generally, the brightness in hue indicates a species that did not originate in Australia, and if the lime colour appears alongside a riverbank, what you are likely looking at is some form of invasive willow. Since the first species was introduced to Australia in the 1930s, dozens of different willow species now cover hundreds of kilometres of river bed in Victoria, visibly altering the look (and function) of the landscape. So how did these non-native trees end up here?
We put them there, of course! Willows hail from Europe and were brought in at the suggestion of natural resource managers to provide river bank stability in paddocks and shading for cattle. Certainly, at the time this was a reasonable recommendation based on what managers knew of willows: they have thick, matted root systems that excel at holding soil in place. Unfortunately, the managers responsible for introducing the willows did not anticipate the massive problem the shrubs would cause in years to come.
At first, after planting the willows all seemed well. Seedlings grew to trees and the willows were successfully doing their job in stabilising riverbanks. Unfortunately, as time went by river managers and farmers began to slowly recognise the turmoil that the willows were causing. Their fine, dense roots were so successful at holding soil together that instead of stopping erosion, they began causing it. As willows grow larger, their finger-like root systems extend toward the centre of the river, grab onto the soil and pull it upwards, eventually forcing the river bed to shallow.
Then, since the river has less depth, the displaced water is diverted around the willows, widening the river. Over time, this results in islands of willows in the middle of the altered rivers and a patchwork of willows within and along entire river catchments. This pattern is detrimental to the surrounding ecosystems (and avid Victorian fishermen) because shallow rivers cannot house large populations of native fish. Additionally, many willow species form dense, single species stands with shady underbellies where no native species are able to grow due to the lack of light.
With such dire consequences, it may seem like a massive oversight of the managers to introduce these trees, but it was seemingly done with good managerial intentions and minimal public opposition. In fact, this sort of accidental, human-mediated invasion happens all the time and in many different forms across Australia and the world. Within Australia, the most famous accidental invasion story is that of the cane toads that were introduced in 1935 in the hopes that they would eat up the cane beetles that were destroying sugar crops in Queensland.
The fairly significant misstep in the toad story is that cane toads did not actually eat the cane beetles, one of the reasons being that the toads are generalists, happy to gobble down many different types of insects that happen to cross their paths. So, unfortunately, the cane beetles weren’t eaten, the sugar cane was still being eaten, and the cane toads became a new problem, spreading from coastal Queensland all across the northern coast of Australia to the west coast, eating every insect in sight (except the cane beetles).
The cane toad and the willows are examples of intentional human-mediated introduction, leading to accidental invasions. However, very often species will be introduced without humans even realising that they themselves were the cause until much later. For example, it is thought that many non-native plant species are introduced by seeds becoming stuck to boots of unconcerned passengers travelling abroad. The plants can then seed in a new, pest-free location.
Another way that species can become invasive is through human interference in gardens. Green thumbs relish in their unique spread of trans-continental herbs while accidentally introducing them to a new, potentially idyllic habitat. Animals are capable of utilising humans to inhabit faraway locations as well: certain lizards have attached themselves to the undersides of planes and jumped off in a newly discovered home. One of the most successful invaders of all time, the black rat is famous for hitching a ride on ships travelling from Asia to Europe as long ago as the First Century A.D, and now inhabits nearly every continent.
So when you’re glancing around Victorian landscapes and notice some species that don’t quite appear to belong… you’re probably right! Victoria is home to hundreds of species of non-native flora and fauna, most of which have arrived in this country as a result of our recent and ancient human relatives. Perhaps with this information you’ll now feel a bit more patient with airport security when they ask to see the fruits you’re trying to carry across international borders!
Emily De Stigter
Emily moved from the redwoods of northern California where her passion for botany bloomed. Here in Melbourne she is working on her PhD in plant ecology at Monash University, studying the flowering patterns of invasive willows in northeastern Victoria.
Banner image of developing female willow catkins courtesy of Emily De Stigter.