Think of a weed and that annoyingly stubborn plant you can never quite rid from the veggie patch inevitably springs to mind. We are accustomed to viewing weeds as the ultimate pests of the plant word, apparently existing only to give gardeners and conservationists a headache. It may even be tempting to distinguish between native plants and weeds as a simple case of good versus bad.
But is this distinction accurate, or even useful? A look at the complex nature of weeds can shed some light on this issue.
The lowdown on weeds
A weed is typically a plant that inhabits and endures in an ecosystem where it did not occur before. It can be either native or exotic, and generally moves into new habitats when it has an advantage over resident plants.
Weeds commonly produce a large number of seeds and they reproduce and survive well in environments as diverse as cities, deserts, farmland, bushland and gardens. They succeed in a range of temperatures and climates.
A native weed you may be familiar with in Australia is the Cootamundra wattle (Acacia baileyana), which has established itself outside its natural range. This species flowers profusely and as a result has been planted in a variety of gardens.
It is also used as a food source by ants and birds, which then transport the seeds to different areas. Because these seeds remain viable for decades and germinate quickly after fire, this species can take over some habitats and gardens.
Are weeds bad for the environment?
It is important first to recognise that species classified as native do not always have positive effects on the local environment. The native Christmas beetle, for example, is suspected of killing multiple broad-leaf peppermint gums (Eucalyptus dives) in Australia. Similarly, categorising flora according to how long these have occupied an ecosystem does not tell us much about a plant’s role in that habitat.
Nevertheless, the underlying assumption for conservation and restoration remains that non-native species offer little by way of ecological benefits. But this is not necessarily the case.
The devil’s claw plant (Martynia annua) was brought to Australia from Mexico in the 19th century as a garden plant, like many weeds now present here. This plant has been removed by hand for the past 20 years in Gregory National Park in the Northern Territory.
However, there is little evidence to suggest this species has negatively changed this ecosystem by increasing nitrogen or reducing biodiversity. Instead, native birds and insects gather nectar and pollen from the devil’s claw, so its removal could be detrimental to the native fauna that regularly feed on it.
Blackberry is another exotic weed, introduced for its fruit, which has spread over more than 8.8 million hectares in Australia. It has reduced the productivity of primary industries, restricting access to land and water as it forms large thickets and overgrown shrubs.
But despite its negative influence on farmland and bushland, this species also provides benefits for fruit and honey production, fodder for browsing animals and food and habitat for some native fauna species, including bandicoots and blue wrens. So its removal, like many other weeds, could be detrimental to the survival of native fauna.
The case of Lantana
My research has focused on another prominent weed known as Lantana (Lantana camara). It is admired for its mass of pink, yellow and red flowers, but also derided for its negative environmental impacts.
After beginning as a humble garden plant, Lantana unfortunately escaped and has been spread by birds and small mammals that feed on its seeds and fruits. This species has changed soil nutrient characteristics by increasing nitrogen, which can cause stress and even death in forest trees. It has been blamed for the death of thousands of eucalypts in northern New South Wales.
Lantana is so invasive that it is now illegal to sell it in nurseries or even grow it in your garden. Any plant is a potential seed bank for more invasion.
But what if, despite all the negative influences of this species on floral diversity, the removal of Lantana actually harms native fauna? Our research at the University of New England’s Avian Behavioural Ecology Lab shows that this might indeed be the case in forested areas in NSW.
When Lantana in this area began dying after an eradication program, we noticed a fall in the numbers of certain species such as Antechinus and bush rats. It turns out that this weed provides invaluable habitat for these small native mammals.
So it seems that at least some native species are starting to adapt to, and even rely on, weeds as a food source and habitat. What would happen if, in the name of conservation, we completely removed them from these areas and even our own backyards?
As we’ve seen, the answers aren’t always straightforward, and there’s a chance it could end up doing more harm than good.