On the trail of the work of caterpillars, beetles and co: Researchers report that plants have never been eaten up by insects as much as they are today since the end of the Cretaceous period. This is suggested by comparing insect damage to modern leaves from different forest ecosystems and fossil specimens up to around 67 million years ago. The currently particularly intense infestation might be linked to the age of man. So far, however, it remains unclear to what extent rapid anthropogenic climate change, complex ecosystem changes or other factors play a role, say the scientists.
Since time immemorial, insects have been attacking the enormous biomass that is formed by the terrestrial flora: In the course of evolution, herbivorous insects developed parallel to plants and feed on them in various ways. The basic plant and insect groups that characterize our nature today developed before the end of the Cretaceous period. It is therefore assumed that the actors in the system have at least not changed fundamentally in the last 67 million years. However, it is unclear how the level of infestation of plants changed during this period of time, which was characterized by climatic changes.
At least as far as modern times are concerned, a noticeable trend is already emerging: herbarium specimens from the early 2000s are significantly more likely to show insect damage than specimens collected in the early 1900s. This pattern has been linked to global warming. Because herbivorous insects often reproduce better at higher temperatures and are also hungrier. In the last 67 million years, however, global temperature increases have occurred from time to time. Among other things, the researchers led by Lauren Azevedo-Schmidt from the University of Wyoming in Laramie are now investigating the extent to which an increase in insect infestation of plants can also be determined during these times.
Gnawed leaves in sight
Azevedo-Schmidt and her colleagues examined the traces of insect damage on fossilized leaves from the late Cretaceous to the Pleistocene – and thus the period from regarding 67 million years to two million years ago. They then compared the evaluation results with comparable leaf samples collected in three different forest ecosystems today. The researchers differentiated between different forms of damage associated with certain groups of insects: such as lateral gnawing, holes or mines in the leaves. In addition, basic characteristics of the plant biodiversity of the respective sample systems were recorded.
The extensive analysis results revealed: “The difference in insect damage between the modern era and the fossil finds is striking,” says Azevedo-Schmidt, summing up the central result. Because the scientists found that all types of damage to today’s leaves are significantly more common. In comparison with the level in the last 67 million years, they reach an average of regarding twice as high. Although the world changed significantly over this period of time, the basic level of insect damage in the forests apparently fluctuated only comparatively little, the evaluations showed. Even with the known climatic warming of the past, only small increases were recorded, the researchers report.
Presumably complex human effect
The results thus prove that the change in plant-insect interaction is linked to the age of man. However, what exactly is behind the effect has so far been unclear and may be complex, the researchers emphasize. According to them, the increased feeding damage is at least not directly attributable to the increase in temperature as part of climate change. Because then there would have been strong increases in the past. However, the scientists warn that the current climate change is happening very quickly compared to past episodes. Azevedo-Schmidt and her colleagues explain that the longer change times may have once given plants more opportunities to adapt to the increased infestation by predatory insects through defensive strategies.
However, other man-made factors might also play a decisive role: “It is possible that the extent of human influence on plant-insect interactions is not controlled solely by climate change, but rather by the way in which humans interact with the environment.” terrestrial landscape,” say the scientists. Because in nature, as is well known, a lot has gotten mixed up in a complex way. The researchers write that the increasing fragmentation of forests, the loss of biodiversity among the animal enemies of chewing insects and the spread of invasive species might play a specific role. According to them, however, further investigations should now clarify the exact causes of the damage caused by feeding on plants, which is apparently unprecedented in geological terms.
Source: University of Wyoming, journal article, PNAS, doi: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2202852119
