For centuries, plants and insects have lived in a delicate dance, with pollinators giving life to flowering plants and plants feeding the masses of insects. But a new study shows that insects are feasting on plants more now than in the last 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 look at plant-insect interactions over time, Azevedo-Schmidt and colleagues compared modern-day plant leaves taken from three forests with fossil leaf prints from the Late Cretaceous period, nearly 67 million years ago.
By quantifying the type and frequency of damage caused by insects, they found a sharp increase in insect herbivory in recent years, with insects burrowing, sucking, puncturing and skeletonizing plant leaves.
“We find that despite the decline of insects, plant damage by insects is elevated in the modern era compared to other time periods represented in the fossil record,” Azevedo-Schmidt and colleagues write.
Plants have evolved to quietly dominate life on Earth, with land plants now accounting for a hefty 80 percent of the world’s biomass. Adolescent insects, although small in size, are unparalleled in species richness. Both have apparently found ways to adapt to environmental change over millennia, despite being extremely sensitive to temperature.
But there is a limit to what they can endure. Some research has shown that insect numbers are declining, at least in some parts of the world. Climate change is also pushing plants to bloom earlier and grow faster, lengthening the pollen season. Not to mention the maddening rates of human-caused habitat and biodiversity loss.
The study by Azevedo-Schmidt and colleagues found that carbon-dated leaves from 1955 to the present had twice the amount of insect damage than any of the 64 fossils dating back tens of millions of years.
Leaves were collected from two forests in the northeastern United States (a cool, wet forest and a second, warm, coastal forest) and a third forest in tropical Costa Rica—a biodiversity hotspot dripping with life.
The fossil data were compiled from published datasets spanning latitudes and climates and spanning from 66.8 million years before the Pleistocene to about 2 million years ago, before the first humans migrated out of Africa.
“We suggest that the comparatively rapid warming trends of [modern] The season may be responsible for its higher herbivory frequencies, so that rapid warming benefits the 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, researchers warn.
Of course, the fossil record records only a slice of life and a snapshot in time, although researchers took steps to figure out how the leaves are preserved. They sampled modern leaves from sediments to mimic fossil estuaries, comparing insect damage to these buried leaves with that of leaf litter and leaf compression fossils.
“A long-term perspective is needed to understand these ancient organisms and their long-term ecological associations, as well as to identify where future collection efforts should be focused,” the researchers write.
What is already clear is that something has changed in the seven decades since 1955, the shortest window compared to the geological epochs that unfolded before we humans began reshaping the biosphere.
Previous research, also from the US, for example, has found significantly more insect damage in herb samples from the early 2000s compared to those collected a decade earlier, a pattern linked to warming.
Insect feeding may be intensifying or locally increasing insect populations in the forests studied – which, in the present study, were located within research station lands, surrounded by roads, housing developments and agriculture.
“Perhaps urbanization has created hot spots of insect biodiversity within research forests,” Azevedo-Schmidt and colleagues write.
A rapidly warming climate – which affects the life cycles and feeding habits of insects and pushes their habitat range towards the poles – and the introduction of invasive species are other big 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 each other to attract pollinators. The situation is dire and human fingerprints are on top of the problem.
“This research suggests that the strength of human influence on plant-insect interactions is not only controlled by climate change but rather by how humans interact with the terrestrial landscape,” they conclude.
The study was published in PNAS.