Plant-eating insects are the most diverse group of multicellular organisms on Earth. The most discussed drivers of their inordinate taxonomic and functional diversity are high niche availability associated with the diversity of host plants and dense niche packing due to host partitioning among herbivores. However, the relative contributions of these two factors to dynamics in the diversity of herbivores throughout Earth’s history remained unresolved. Using fossil data on herbivore-induced leaf damage from across the Cenozoic, paleontologists from the Hessian State Museum Darmstadt and the Senckenberg Biodiversity and Climate Research Center Frankfurt now show that the diversity of plant-eating insects evolved over the past 66 million years primarily through the shared use of food plants.
Plant-herbivore interactions from the Cenozoic fossil record: (A) meta-network of plant-herbivore interactions, based on 10,714 damaged leaves from 21 fossil floras; colored bars represent the total interaction frequencies of host plants and herbivores; the ten most abundant plant orders are labeled; gray lines represent pair-wise interactions, with more frequent interactions being thicker and less transparent; (B) examples of damage types: (a and b) mines mainly produced by moths; (c and d) galls mainly produced by gall midges; (e-h) external foliage feeding including hole feeding, margin feeding, surface feeding, and skeletonization; (i) endophytic oviposition; (j) in situ preservation of armored scale insects. Scale bars – 10 mm. Image credit: Albrecht et al., doi: 10.1073/pnas.2300514120.
“Herbivorous insects are the most diverse group of multicellular organisms on Earth,” said Dr. Jörg Albrecht, a researcher at the Senckenberg Biodiversity and Climate Research Center.
“The variety of their mouthparts and feeding modes also testify to a high degree of diversity: for example, there are caterpillars or beetles that chew on leaves with their powerful jaw-like mouthparts, bugs and aphids that pierce plants to get at their sap, or animals that stimulate plants to form galls — an excessive tissue growth — in which they can develop and feed protected from enemies.”
“The feeding traces of such insects are also clearly visible on fossil leaves,” he added.
“They can help us to identify the factors that led to the enormous diversity of herbivorous insects.”
In their research, the authors classified 47,064 fossil leaves of 436 plant species from 16 sites in Central Europe, Iceland, and Norway.
“The fossils we studied cover nearly the entire Cenozoic era, i.e., the period between 66 and two million years before present,”
“Moreover, the fossilized leaves originate from a variety of climates — from subtropical to oceanic to humid continental,” said Hessian State Museum’s Professor Torsten Wappler.
The researchers examined the fossil leaves for traces of insect damage; more than one-fifth of the studied specimens revealed corresponding traces.
“Based on these data, we can show that food plants were already used by a variety of herbivorous insects in early geological history,” Dr. Albrecht said.
“Moreover, the detailed evaluation of the feeding traces shows that the shared use of a plant species by different groups of herbivorous insects contributed twice as much to their functional diversity — in terms of their feeding mode — as the species diversity of the food plants themselves.”
The results provide new insights into the origin of insect diversity and show that the co-occurrence of many specialized insect species on the same plant species is the primary factor driving the functional diversity of herbivorous insects.
“When different insect species share a food plant species, they also have to adapt their diet and feeding mode to avoid direct competition with each other,” Dr. Albrecht said.
“Over millions of years, this led to the emergence of an incredible diversity of mouthparts, and ultimately species.”
The findings are reflected in today’s tropical forests, where most herbivorous insect species specialize on specific plant families, which in turn provide food for a variety of insect species.
“Our study emphasizes that the fossil record can be used to test fundamental theories about the origin of biodiversity,” Professor Wappler said.
“The results of our study are also an important benchmark for identifying the factors that determine the diversity of herbivorous insects in contemporary ecosystems.”
A paper on the findings was published in the Proceedings of the National Academy of Sciences.
_____
Jörg Albrecht et al. 2023. Fossil leaves reveal drivers of herbivore functional diversity during the Cenozoic. PNAS 120 (32): e2300514120; doi: 10.1073/pnas.2300514120
