Using Modern Rainforests to Study Fern-Insect Interactions in the Fossil Record

Fern-Arthropod Interactions from The Modern Upland Southeast Atlantic Rainforest Reveals Arthropod Damage Insights to Fossil Plant-Insect Interactions

Summarized by: Haley Vantoorenburg is a geology major at the University of South Florida. Haley currently researches encrusting organisms on Paleozoic brachiopods and plans to work closely with fossil preparation and preservation studies in the future.

What was the hypothesis being tested (if no hypothesis, what was the question or point of the paper)? Ferns were some of the first plants to have evolved broad leaves (fronds) in the fossil record (the earliest known records are about 360 million years old). These broad leaves allow large areas of insect damage from insects present while the plant was alive to be preserved. Modern and fossil ferns can be compared against one another to understand what insect interactions were present throughout geologic time, and the ways these interactions have either changed or remained constant. 

What data were used?: This study examined 17 types of damage (grouped into categories by the method used to cause the damage or by the area of the leaf affected; see Methods below) caused by insects, using both fossil ferns from multiple collection sites and modern ferns from a rainforest in southern Brazil. Ferns were chosen because, as opposed to other plant types, their broad leaves increase access for insect predation and modern broad-leafed ferns are very similar to some of their fossil relatives. Ferns became abundant in the Carboniferous (359.2–299 Mya). In the Carboniferous, records of arthropod (spider and insect) damage to plants also became more frequent. While insects are often not preserved with the fossil ferns, the types of damage that prehistoric insects caused are very similar to the damage types observed today, even if we don’t know if the types of insects that made the damage are or aren’t similar. Because fossil ferns are so similar to their living relatives, and because ferns are one of the first broad-leaved plants, scientists can use modern ferns as models to study the oldest plant-arthropod interactions. 

Methods: This study used an area of rainforest with high humidity, many fern species, and high fern density to study modern ferns. A census of the ferns present and any records of insect-fern interactions were collected over a transitional area from the lower broad-leaf forest to the upland grassland. The damage type, richness per leaf, and damage size were recorded using hand lenses, calipers, and macroscopic and microscopic photography. Functional feeding groups (FFG) were made to categorize the types of insect damage. Damage from egg-laying and traces were also recorded. Damage was recorded using a damage type guide that described 413 different damage types. This was compared to compiled fossil fern data from many sites. 

Results: Even with 413 pre-established damage types, one new damage type was discovered in this study. This new damage type is a sub-type of surface feeding that features a series of rounded damage marks that was observed in both modern ferns and in multiple fossil ferns. Some types of damage were found rarely in modern ferns, but never in fossil ferns (hole feeding – the creation of separate holes in the leaf tissue – and galling – the development of waxy or swollen layers). Margin feeding (consuming only the edges of a leaf) was found in both fossil and modern ferns and included the most common damage types (46% of the damage observed). Surface feeding (damaging but not completely breaking through the leaf tissue) was recorded on both fossil and modern ferns (10%). Some types were found in modern and fossil plants, but some types were only found in angiosperms (i.e., flowering plants) in the fossil record and not fossil ferns (piercing and sucking, small points of damage or swollen leaf sections, 15%, and mining, creating subsurface damage, 8%). 

A bar chart with the number of observed instances on the left y-axis to match the bars and the types of functional feeding groups on the x-axis. It is overlain by a line representing the cumulative percentage. From left to right: Margin feeding, 220 instances and 46% of the total. Piercing-and-sucking, 73 instances, the cumulative total 61%. Hole feeding, unlabeled but about 52 instances, 72% the cumulative total. Surface feeding, 50 instances and 83% of the cumulative total. Mining, unlabeled but around 40 instances, 91% of the cumulative total. Hole feeding, 34 instances, 98% of the cumulative total. Galling, nine instances, 100% of the cumulative total.
Figure: A bar chart of the recorded damage types by functional feeding group, showing the dominance of margin feeding in the modern ferns in the Sao Francisco de Paula National Forest, municipality of Sao Francisco de Paula, Rio Grande do Sul, southern Brazil.

Why is this study important?: This study showed that modern ferns can provide a better understanding of the marks that different insect feeding methods cause and of the fossil record of these marks on similar ferns. Researchers found that the levels of precipitation impacted the amount and types of fern-insect interactions in modern ferns. This means that studying modern ferns can create models for studying past environmental conditions using fossil fern data. Additionally, there are fossil and modern instances of insect interactions that show a specialized association with specific ferns.

Broader Implications beyond this study: The similar rates of predation by insects on both modern and fossil plants show that ferns were important to herbivorous (plant-eating) arthropods throughout history. All FFGs identified in the fossil record were found in modern ferns, so understanding interactions in modern environments can be used to determine the environmental conditions of different fossil assemblages, such as the projected precipitation level of their environment. The prevalence of fern-arthropod interactions throughout history means that it can be used to study changes in these fern-arthropod relationships in geologic time and we may be able to use them to model the influence of climate change. 

Citation: Cenci, R., & Horodyski, R. S. (2022). Fern-Arthropod Interactions from the Modern Upland Southeast Atlantic Rainforest Reveals Arthropod Damage Insights to Fossil Plant-Insect Interactions. Palaios, 37(7), 349–367.

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