Small but Mighty: Insects as Proxies

By Wayne Heideman

Insects have been on the Earth for a long time and their presence can affect their surrounding environment. It is important to look at insects in the past as they can provide us with insight on how they can act in the present and in the future. In a paleoecological sense, insects can be studied in a number of ways. One way is to look at plant-insect interactions through plant fossils (herbivory) and peatlands (habitat). Another technique is through amber and observing the insect in a snapshot of time. Lastly, sediment cores in lakes can capture insect presence, notably Chironomidae (non-biting midges) larvae and Coleoptera (beetles). All three are viable means of observing past insect use but they all have their strengths and weaknesses which should be assessed before using a specific method.

A picture of three different types of insect damage on plants. A) Shows a frass trail as well as an oviposition site marked by the arrow. B) Shows a high degree of herbivory, only leaving fine veins and C) shows areas of leaf case shelter sites. From Wilf 2008.

A picture of three different types of insect damage on plants. A) Shows a frass trail as well as an oviposition site marked by the arrow. B) Shows a high degree of herbivory, only leaving fine veins and C) shows areas of leaf case shelter sites. From Wilf 2008.

Insect presence in an area reveals what happened in that location. Insects, notably aquatic insects, can be particular with their habitat and as a result, are great proxies for what the landscape looked like at a given point in time. Aquatic insects require specific conditions for their larvae and have the benefit of winged adults to oviposit in the appropriate environment. Some species prefer lentic (lake-like) environments whereas others prefer lotic (flowing) habitats. Substrate matter can also be a determinant for where a species will oviposit, as well as other specifications. In a paper written by Whitehouse et. al., peatlands in Britain were successfully identified through its formation, expansion, to present day conditions based on the beetle exoskeletons left behind in the peat. The surrounding area was also identified from starting as an oak-beech-pine forest at about 3618 BC, to a pine dominated forest around 2921 BC, to complete bog conditions at 2445 BC. All of these areas had specific beetle species that used their microhabitats. At this particular site, 30% of the beetle fauna found in the fossil record are still present at the bog, meaning the bog has been desiccated over time (Whitehouse et. al. 2008).

Insect damaged plants are also a great way to interpret the surrounding environment. Throughout time, there have been specific time periods in which insect herbivory increased. These seem to be more episodic throughout time, but its presence is noted. According to Labandeira, C.C., there have been 5 major insect herbivory events throughout history, the most recent occurring at the Paleocene–Eocene Thermal Maximum (PETM). These are correlated with large extinction events, radiations of new species (Labandieria 2013), and drastic changes in climate and ecosystems (Wilf, P. 2008). Higher temperatures were found to be one of the largest insect-driving forces on herbivory. Decreases in insect herbivory occurred when there’s less rain but insect herbivory increased as plant diversity increased (Wilf, P. 2008). Plant fossils provide insight on the surrounding environment based on the amount of insect herbivory on the plant.

A sample of an amberized insect. From Wikimedia Commons.

A sample of an amberized insect. From Wikimedia Commons.

Amber, as made famous by Jurassic Park, is another way to identify insects and make inferences about their surrounding habitat. There are a few hot spots of amber mining which include the Baltic region, Dominican Republic, the Middle East and even the Mid-Atlantic States. The oldest of these dates back to 135 million years with the Middle East samples and the more recent amber deposits are from the Dominican Republic dating back to 25-40 million years ago. Insects in amber have given us the opportunity to study topics such as biogeography, mimicry, behavior, extinction, and paleosymbiosis. There are remains of copulating insects in amber, mate guarding, ant mimicry from a beetle, and even parasitism of insects are recorded. Another notable insect-plant relationship can heighten amber production. Research completed by McKellar et. al. suggests that insect outbreaks increases plant amber production. This was seen in a modern day example with mountain pine beetle in western Canada. The same defenses were seen in amber received from New Jersey (Late Cretaceous) as well as the Dominican Republic (25-40 million years ago). This time span allows us to see that these tree defenses have been active for years.

Lastly, sediment cores can be used to see insect presence in the landscape. Much like the peatland example from before, sediment cores can indicate specific types of environments based on insect species. Aquatic insects have specificity in their environments and because of their short lifespans; one can see change in a system over a short period of time. Chironomids and Coleoptera are the two most prominent insect taxa represented in sediment cores. There is a large focus on Chironomids because of their sclerotized heads and they are easy to identify based off of their head morphology. They are also common species in various habitats. Coleoptera make for great preservation due to their hard exoskeletons, which can sometimes resist degradation. Chironomids alone have been found to successfully indicate environmental patterns and changes in a study completed in Argentina. Chironomids in Argentina have been exposed to major natural disturbances such as volcanism but have a high resilience and return to the lake rapidly after the eruption. Chironomid species have many successional phases within the same family (Massaferro J. and Corley J. 1998). A problem with insects in sediment cores is that is site specific. Luoto, T.P. found that insects found in the cores varied depending on where in the lake the core was taken. The habitats in which the insects lived had the highest abundance of those species. Most of the insects lived in the shallower part of the lake which can pose a problem as cores tend to be taken more from the center of the lake.

A stratigraphy showing the prevalence of notable Chrionomid taxa. Head capsule accumulation rate (HCAR), total organic carbon (TOC%), Shannon’s diversity index, and equitable indices are also graphed on the right-hand side. From Massaferro and Corley 1998.

A stratigraphy showing the prevalence of notable Chrionomid taxa. Head capsule accumulation rate (HCAR), total organic carbon (TOC%), Shannon’s diversity index, and equitable indices are also graphed on the right-hand side. From Massaferro and Corley 1998.

In conclusion, there are a few ways that insects can be utilized as a proxy in paleoecology studies. Insect-plant interactions show how an environment could have developed and how climate could have affected the plant-insect relationship. One thing to consider with this proxy is the timing and prevalence of usable fossils. A knowledge of plant-insect interplay would be required as well as some insects are specific to a particular microhabitat. Amber is a famous example of a proxy but its benefits can be place and time limited as there are only a limited amount of sites that have amber reserves. Sediment cores have a large amount of usability as there are many lakes in the world but they can vary in the location in the lake the core is taken from. Insect proxies can prove to be useful in paleoecological work, but make sure to use the one most suitable!

References

Labandeira, C.C. 2013. A paleobiologic perspective on plant–insect interactions. Current Opinions in Plant Biology 16:414–421.

Luoto, T. P. 2012 Intra-lake patterns of aquatic insect and mite remains. Paleolimnology 47:141-157.

Massafferro J. and Corley J. 1998. Environmental disturbance and chironomid palaeodiversity: 15 kyr BP of history at Lake Mascardi, Patagonia, Argentina. Aquatic conservation: marine and freshwater ecosystems 8: 315–323.

Poinar Jr., G.O. 1993. Insects in Amber. Annual Review Entomology 46:145-59.

Whitehouse, N.J., Langdon, P.G., Bustin, R., and Galsworthy, S. (2008). Fossil insects and ecosystem dynamics in wetlands: implications for biodiversity and conservation. Biodiversity Conservation 17: 2055-2078.

Wilf, P. 2008. Insect-damaged fossil leaves record food web response to ancient climate change and extinction. New Phytologist 178: 486–502.

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2 thoughts on “Small but Mighty: Insects as Proxies

  1. Well done for tackling this topic! I just wanted to comment on one of your entries above that cites some of my work. The overlap between modern and palaeo fauna isn’t related to desiccation. Rather, it tells us about the ancient origins of the fauna. In essence, conditions on the peatland 4000 yrs ago create the fauna that is still extant today, including endemic taxa. This means that history of a site is hugely important to its modern biodiversity. Modern & old faunal assemblages are intimately entertwined. I hope that makes sense? Anyhow, well done for delving into this topic.

  2. It’s so interesting how much detail is preserved in fossilized leaves and amber, allowing us to learn about life histories of the insects themselves and also the surrounding environment and climate! I am curious though about how large extinction events lead to increased insect herbivory?

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