LINDELL BEACH, B.C. – Plants apparently can’t have it all.
Scientists at the University of California, Irvine, discovered that trade offs are often made between the growth of plants (fast or slow), how they protect themselves against plant eaters (thorns or toxins) and how they attract predators (ladybugs eating plant pests).
“We can breed plants for fast growth but, if we do that, it appears we’re weakening the plant’s immunity against herbivores, making them more dependent upon protection from potentially unreliable predators,” said Kailen Mooney, assistant professor of ecology and evolutionary biology.
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Even though plants have evolved three strategies to survive and thrive, they don’t necessarily have the resources to develop all three. There are trade offs, with some defence strategies developed in lieu of others.
Mooney found that these trade offs influence how ecosystems are structured. In food webs, predators control prey populations by eating them, which allows species lower in the chain such as plants to flourish.
The dynamic is called a trophic cascade.
Mooney and his colleagues looked at the trophic cascades in 16 species of milkweed, famed for their relationship with monarch butterflies whose larvae feed exclusively and copiously on milkweed leaves.
This group of flowering plants is found naturally throughout the Western Hemisphere.
To find out if the evolutionary trade offs among plant traits influenced patterns of predator and prey, the scientists manipulated insect eaters and soil fertility to measure the impact of a major herbivore, the aphid Aphis nerii, on the milkweed plants.
They found that plants that favoured fast growth put less energy into resisting herbivores and depended more on predators for protection.
It is a trade off between growth and insect defence because slow-growing plants don’t appear to need bodyguards while tasty fast-growing ones need that outside protection.
Predators are attracted to a compound called sesquiterpenes, and the trait to produce this compound is also genetically linked to faster growth, or the need to replace leaves faster than they can be eaten.
While natural selection might favour faster growing plants and those that easily fight off insects, it appears that nature has not found a way to combine both traits. This may have implications for efforts to develop fast growing, herbivore resistant crops.
“Plant breeders do in fact breed in resistance with minimal reductions in growth,” he said. “This suggests that it is possible to join these two trains but that, for wild plants, there must be some costs that make this a poor strategy. It’s hard to say what this cost might be but the fact that this trade off appears to occur among wild plants suggests it is in fact enforced under natural conditions.”
Mooney’s research found that fertilizer played a role in this dependency.
“We found that species varied in their responsiveness to fertilization and that the more responsive a species was to fertilization, the more its performance (rate of growth) became dependent on suppressing herbivore populations,” he said.
“In our experiments this was done through biocontrol (predators), but this would presumably apply to herbivore suppression through insecticide application. In other words, as plants evolve, or are selected to better take advantage of soil fertility, they become more vulnerable to herbivores. Whether this trade off translates to crops is unclear.”
However, Mooney said that if plant breeders can put together traits that do not occur together in nature, then it is possible the same could occur in nature.
However, for some reason plants select against it.
“Perhaps this is feasible in crops because the ecological conditions under intensive agriculture are sufficiently different that they actually can co-occur adaptively in that setting,” Mooney said.
