A new study by UNSW Sydney researchers suggests that introducing herbivorous insects or animals to control invasive weeds might be triggering a cooperative response among the plants, allowing them to thrive rather than decline.
Lead author Professor Stephen Bonser from the UNSW School of Biological, Earth & Environmental Sciences says that although invasive species are long-studied, the ongoing rise of their impact suggests something is missing from the current approach to controlling their spread.
"I don't think we've been measuring the success of biocontrol agents correctly," Prof. Bonser says.
He says the way biocontrol works is that it takes specialised enemies of the invader species from their home range and adds them to the invaded range.
"They're supposed to knock the invasive species back enough so that we can control their population," he says.
Prickly pear plants are cacti native to the Americas that are now invasive in both natural and rural parts of Australia.
"One of the notable successes of biocontrol so far is a little caterpillar that's quite effective against prickly pear," Prof. Bonser says.
"However, we also have dozens and dozens of cases now where we've applied a biocontrol, we think it should work, and then it doesn't seem to."
He says the bitou bush is a perfect example of this. The plant is a coastal species that grows in dense monocultures, sometimes covering vast areas. The biocontrol agent for this is the caterpillar from the bitou bush tip moth.
Prof. Bonser says the effectiveness of biocontrol agents is currently measured by their impact on individual plants.
"A single caterpillar can knock back the growth of individual plants of the bitou bush quite substantially," he says.
However, he says, "we're trying to control populations. While the moths are now well-established biocontrol agents, populations of bitou bush are still spreading like wildfire."
Cooperating vs competing
Plants compete by racing to grow taller than their neighbours to steal all the sunlight, instead of cooperating by sharing resources and working together, as a team.
Prof. Bonser says the latter is how crops such as corn and wheat took off during the 'Green Revolution'. This period, from the mid- to late-1900s, transformed farming by adopting agricultural technologies to address food production challenges.
"The Green Revolution was all about making plants more cooperative," Prof. Bonser says.
During this period, scientists were able to isolate and grow less competitive plants that focused their energy on making more food. They achieved this by selecting and breeding dwarf varieties, based on their hormone properties.
"Cooperative traits were bred into plants using early genetic engineering techniques," Prof. Bonser says. "This resulted in shorter plants, and this is how farms became much more productive."
He says when plants in crops compete for more light, they get so tall that they are top heavy and fall over.
"That was a real problem up until the 1960s. When wheat or corn act as individuals they fall over and then just rot on the ground."
Plot twist
In forests and other natural areas, it's a different story – the plants that grow fastest and tallest "win". Cooperative plants often get pushed out of an ecosystem by more aggressive, competitive ones.
"Game theory tells us that cooperation is rare in nature," Prof. Bonser says. "If a plant stops fighting for resources, a competitive one will swoop in and take over."
Biocontrol is used to slow down invasive plants such as weeds. However, Prof. Bonser says the very thing meant to stop the invaders might be making them stronger. By damaging individual plants and reducing competition, the group of invasive plants start playing as a team.
"What we think is happening is that the biocontrol agents are turning aggressive invaders into cooperative communities," Prof. Bonser says. "And when they cooperate, they can cover massive areas – even more than before."
"Instead of judging how useful biocontrol is based on how it affects individual plants, we need to look at how whole groups of plants behave. Because what hurts one plant may hurt, or help, the entire team."
He says herbivores eat the parts of plants that help them fight against each other, disrupting vertical growth by eating their shoot tips.
He also says it is well-known that plants growing taller and faster in competition with others often aren't as good at protecting themselves from herbivores. This is because a plant's ability to compete vs its ability to defend are linked by its genes.
"This could work in reverse, where a plant that protects itself with chemicals might not grow as tall to compete with other plants," he says. "And by not spending their energy on competing, they can put that energy into reproducing.
"We think the herbivores trick the plants into cooperating – they stop wasting their energy on fighting and instead start working together."
He says demonstrating that impact of defence responses on competitive responses is a key next step.
Turning over a new leaf
Prof. Bonser says he and his collaborators are now researching how to break up 'invader teams' of plants. This includes experiments that introduce tall, strong native plants that disrupt their teamwork, as well as combining biocontrol with other methods.
"This could change how we fight invasive species in nature but also how we grow crops to further boost food production," Prof. Bonser says.
He says this doesn't mean adding insects to crops.
"But we could perhaps trick plants into thinking they had insects on them – this could possibly help them start growing in a more cooperative fashion," he says.
"This is just the beginning of a new story."
Key Facts:
Trying to curb invasive plants with natural predators such as insects and other herbivores could be helping weeds form unexpected alliances — making them even harder to manage, new research suggests.
