A questioning look at the most colorful of all animal groups: why did the herbivorous insects fan out into so many amazing species over the course of evolution? The main driving force was competition, as evidenced by an evaluation of feeding marks on more than 45,000 fossil leaves. According to the researchers, the results reflect that sharing of food plants was the key factor behind the strong diversification over the past 66 million years.
From nibbling caterpillars and beetles to plant sap-sucking aphids and bugs to gall wasps that produce leaf growth: In many forms and sometimes very different ways, the world’s herbivorous insects attack their plant victims. No other group of animals has produced such a variety of species and diets. Researchers are also still discovering many new species of insects with unique traits in the world’s ecosystems. Looking at the thousands of species, one can ask why herbivorous insects of all things have fanned out so enormously.
Two aspects are considered to be potentially formative: On the one hand, the splitting up of plants into many species over the last 66 million years has led to a corresponding diversification of their parasites. On the other hand, the strong competition among herbivorous insects could have contributed significantly to their specialization. “The relative contributions of these two factors to the dynamics of herbivore diversity over the course of Earth’s history are still unclear,” writes the research team led by Jörg Albrecht from the Senckenberg Biodiversity and Climate Research Center in Frankfurt.
What was the deciding factor?
In order to gain insight into this question, the scientists chose a paleobotanical approach: “The feeding tracks of insects are often clearly visible on fossil leaves. They can help us identify the factors that led to the enormous diversity of herbivorous insects,” explains Albrecht. The researchers devoted an enormous amount of work to the approach: they examined a total of 47,064 fossil leaves from 436 plant species from 16 localities in Central Europe, Iceland and Norway. “The fossils we examined cover almost the entire Cenozoic period, i.e. the period from 66 to two million years ago. The petrified leaf fossils also come from different climatic conditions – from subtropical to oceanic to humid continental,” says co-author Torsten Wappler from the Hessian State Museum in Darmstadt.
As it turned out, a fifth of the examined leaves showed traces of the machinations of herbivorous insects. These were various forms of gnawing marks, remains of parasites or growths attributed to insects such as gall wasps. The researchers assigned these tracks to the different groups of insects and also identified species-specific features. “Using this data, we have now been able to show that food plants were used by a large number of herbivorous insects early in the history of the earth,” says Abrecht.
Competition led to specialization
The scientists then integrated the new information into models of diversity development. Eventually, it became apparent that the sharing of a plant species by different groups of herbivorous insects was the main factor in the evolution of functional diversity in herbivorous insects. According to the results, this aspect influenced evolution twice as much as the development of species diversity in the food plants themselves.
Albrecht explains: “If different insect species share a food plant, they also have to adapt their diet so that they do not compete directly with each other. Over the course of millions of years, an unbelievable variety of mouthparts and ultimately also species develops.” The results of the study are reflected above all in today’s tropical forests, where most herbivorous insect species live, the researchers say: That’s where the different plant families feed a variety of species at the same time.
In conclusion, Wappler says: “Our study underscores that fossil evidence can be used to test fundamental theories about the origin of biodiversity. The results of our study are also an important benchmark for finding out which factors determine the diversity of herbivorous insects in today’s ecosystems,” says the scientist.
Source: Senckenberg Society for Natural Research, specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2300514120