For eons plants and insects have lived in a delicate dance, pollinators giving life to flowering plants and plants feeding the masses of insects. But a new study suggests insects are feasting on plants more now than they did in the past 66.8 million years.
“The difference in insect damage between the modern era and the fossil record is striking,” says University of Maine paleoecologist Lauren Azevedo-Schmidt, who led the study.
To examine plant-insect interactions over time, Azevedo-Schmidt and her colleagues compared the leaves of modern-era plants sampled from three forests to fossil assemblages of leaf prints dating as far back as from the Upper Cretaceous, about 67 million years ago.
Quantifying the type and frequency of insect damage, they found a sharp increase in insect herbivory in recent times, with insects piercing, sucking, puncturing and skeletonizing plant leaves.
“We find that despite the decline of insects, insect damage to plants is high in the modern era compared to other periods represented in the fossil record,” Azevedo-Schmidt and colleagues write.
Plants have evolved to quietly dominate life on Earth, with terrestrial plants now accounting for 80% of global biomass. Tiny insects, although small in size, are unmatched in species richness. Both have obviously found ways to adapt to environmental changes over millennia, although they are very sensitive to temperature.
But there is a limit to what they can endure. Some research has suggested that insect numbers are declining, at least in some parts of the world. Climate change is also causing plants to flower earlier and grow faster, lengthening the pollen season. Not to mention the sickening rates of human-caused habitat and biodiversity loss.
The study by Azevedo-Schmidt and colleagues found carbon-dated leaves from 1955 to the present had twice as much insect damage as any of the 64 fossil assemblages dating back tens of millions. of years.
Leaves were collected from two forests in the northeastern United States (a cool, humid forest and a second warm coastal forest) and a third rainforest in Costa Rica – a biodiversity hotspot dripping with life.
The fossil data was compiled from published datasets spanning latitudes and climates, and spanning from 66.8 million years ago in the Pleistocene to about 2 million years before the first humans migrated out of Africa.
“We propose that the relatively rapid warming trends of the [modern] Era may be responsible for its higher herbivory frequencies, so that rapid warming benefits insects in the arms race against their food source: plants,” Azevedo-Schmidt and colleagues write.
Increased insect herbivory could have unknown consequences for plants and forest communities, the researchers warn.
Of course, the fossil record only captures a slice of life and a snapshot in time, though researchers have taken steps to account for how the leaves are preserved. They sampled modern leaves from sediments, to mimic fossil outcrops, comparing insect damage on these buried leaves to that of leaf litter and leaf compression fossils.
“A long-term perspective is needed to understand these ancient organisms and their long-standing ecological associations, as well as to determine where future collecting efforts should focus,” the researchers write.
What is already clear is that something has changed in the seven decades since 1955, the briefest of windows compared to the geological eras that unfolded before we humans began to reshape the biosphere. .
Previous research, also American, for example, found significantly more insect damage on herbarium specimens from the early 2000s than on those collected a decade earlier, a pattern linked to rising temperatures.
It could be that insect feeding is intensifying or insect populations are increasing locally in the forests studied – which in the present study were located within the confines of the research stations, surrounded by roads, housing estates and agriculture.
“Perhaps urbanization has created insect biodiversity hotspots in research forests,” Azevedo-Schmidt and colleagues write.
A rapidly warming climate – which influences the life cycles and feeding habits of insects, and pushes their range of habitats towards the poles – and the introduction of invasive species are other important factors that could lead to an increase in insect herbivory.
At the same time, agriculture is decimating insects and research suggests that plants may have to start fighting for pollinators. The situation is dire and human fingerprints are everywhere in the problem.
“This research suggests that the strength of human influence on plant-insect interactions is not controlled solely by climate change, but rather by how humans interact with the terrestrial landscape,” they conclude.
The study was published in PNAS.
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