When weed is good

Weeds are the bane of life for gardeners, farmers and conservationists, but they may have some previously unrecognised benefits in highly degraded landscapes.

Whether it be onion weed taking over gardens, Paterson’s curse spreading across paddocks and poisoning cattle or Japanese knotweed chocking waterways – Its easy to see where the inspiration behind John Wyndam’s The Day of the Triffids, or HG Wells’ red weed in War of the Worlds comes from. Transported from one side of the planet to another, and without natural predators or biological control agents, these ‘alien’ plants run amok in new territories smothering landscapes and killing wildlife.

What’s needed is a transition to a new paradigm […] in which human perceptions about weeds should not be negative by default.

“Negative by default” is how humans perceive weeds. They’re a threat to biodiversity, so we spend millions of dollars on their eradication. But is this paradigm true for all ecosystems and all weeds? And, do we really understand the exact interactions between weeds and native species in our managed and natural ecosystems?

According to an international team of researchers, the answer is ‘no, not necessarily’. These researchers have been working to fill this knowledge gap by examining weed-biodiversity interactions in detail. And what they’re finding is unexpected, potentially controversial and may challenge prevailing notions of best practice weed management among conservation practitioners.

Weeds can have positive effects on biodiversity

The research, lead by Dr Irene Martín-Forés of the Spanish National Research Council (CSIC) at the National Museum of Natural Sciences in Madrid, found that, contrary to most people’s notions,  weeds have a positive or neutral association with biodiversity in Australian grassland ecosystems. That is, the more weeds found at sites often the higher the level of native species diversity. In fact, they found no apparent competition between weeds and natives or other negative effects on overall native diversity.

Irene and her colleagues from The University of Adelaide, came to these conclusions after analysing data on species richness, cover and diversity collected from hundreds of grassland monitoring sites of varying levels of disturbance throughout the greater Mount Lofty Ranges region of South Australia.

At the most disturbed grasslands sites with land-uses such as cropping and modified pastures, rather counterintuitively, the positive association between weed and natives was highest, and is most likely due to the complementary roles of exotic and native species, and the opportunity for rapidly establishing weeds to aid  the occupation of space and use of resources by all species. They work together, facilitating each other to colonise degraded sites, instead of competing.

A new way of managing weeds

As highlighted by research team member, Dr Greg Guerin, these results indicate that although a minority of weed species are having strong negative impacts on native biodiversity, these interactions can’t be generalised across all weeds and ecosystems. This challenges the notion that all weeds need to be removed from remnant Mediterranean-climate grasslands, regardless of identity, or that weed cover or diversity per se is a valid condition measure.

Habitat management that focusses on general weed eradication rather than targeting particularly problematic species in grasslands may well turn out to be ineffectual.

What’s needed is a transition to a new paradigm. A model in which human perceptions about weeds should not be negative by default.  Instead, a more nuanced understanding of conservation is required, one that involves assessing exotic species in native systems on a case-by-case basis needs to be considered.

Multiple sources of open data were utilised in this research, including from TERN and its cognate National Collaborative Research Infrastructure Strategy (NCRIS) projects:

• • Vegetation data from the Government of South Australia’s Biological Survey of South Australia (BSSA) downloaded freely via TERN’s AEKOS data portal;
• • Rainfall data from TERN’s modelling capability;
• • Long-term climatic data and information on the nutrient status, including percent soil organic carbon, nitrogen and phosphorus, from the Atlas of Living Australia (ALA); and
• • The environmental gradient infrastructure of the Transect for Environmental Monitoring and Decision-Making (TREND), which forms part of TERN’s Ecosystem surveillance field observatory capability.

Read more about this research in the recently published paper in the journal PlosOne.

This blog is rewritten with permission from a Terrestrial Ecosystem Research Network newsletter article by Mark Grant and the authors of the article.