A weed is a “plant in the wrong place” but in agriculture, weeds can lead to 10-34% yield loss for major crops. Some of the “weedy” traits include early emergence and groundcover and rapid root, shoot and leaf area growth. Weeds can also out-compete crops by taking up nutrients quicker or more efficiently or tolerate environmental stresses.
Dr Lucie Mahaut and colleagues from France and the UK reviewed how weeds could become model organisms for understanding ecology and evolutionary biology in the Anthropocene. The scientists argue in the Trends in Plant Science journal that weed herbicide resistance and crop mimicry syndromes are prime examples of rapid evolution and adaptation.
A plant species can become a weed through crop domestication, crop-wild hybridisation or after a wild species invades a cultivated field. Mahaut and colleagues argue that weeds might be able to overcome some of the major theories in functional ecology (e.g. constraints in the phenotypic space, tradeoffs between resource-acquisition and resource-conservation strategies) and could be “functional outliers”.
Plant species can be placed within the CSR (Competitive, Stress-tolerant and Ruderal) triangle which represents a three-way trade-off between adaptation to high competition habitats, highly stressed and highly disturbed environments. Ecological strategies of weeds itself vary between competing or avoiding the crop and resisting or avoiding disturbances.
Mahaut and colleagues divided this model into the availability of belowground and aboveground resources and argue that weeds are outliers as they can have high belowground resources but limited aboveground resources (i.e. light availability due to crop canopy) whilst living in a highly disturbed environment.
From a community ecology point of view, the fundamental rules do not fully capture the dynamics of weed communities. Mahaut and colleagues said that weeds represent a combination of transient species which repeatedly colonise fields from the edges and adopt to environmental filtering (stress-gradient hypothesis). There are many fundamental theories (e.g. spatial storage effect, temporal storage effect, source-sink dynamics) which assume stationary environmental variation which is not true for weed communities. Crop fields are frequently managed by humans, using different agricultural practices (e.g. crop rotation), so new theories are needed to better understand weed community dynamics.
In the final section of Mahaut and colleague’s review, the scientists highlight the many questions about that the rapid evolution of herbicide resistance, allelopathy (production of detrimental biochemicals to other organisms) and crop mimicry of weeds. The aim of researching herbicide resistance has been to optimise weed control strategies in the last 60 years but it could be a good model to understand eco-evolutionary feedbacks. Ancient DNA sequencing, resurrection ecology and museum specimen analysis could investigate the genetic basis of the rapid evolution of weeds.
“Because weed evolutionary history and ecological dynamics are linked intrinsically to human activities, these species have great potential to become a valuable model in ecology and evolution,” Mahaut and colleagues said. “We urge (numerical) ecologists not to discard the amazing source of information emerging from weed species and their associated habitats.”
“Let ecologists and evolutionists seize the weeds!”