Ressources numériques en sciences humaines et sociales OpenEdition Nos plateformes OpenEdition Books OpenEdition Journals Hypothèses Calenda Bibliothèques OpenEdition Freemium Suivez-nous

Fossil insects unveil the Holocene history of a Pyrenean forest


The study of palaeoclimates and ancient environments through the analysis of subfossil insect assemblages extracted from Quaternary sediments has become increasingly important in palaeoecological disciplines over the last few decades (Elias, 1994). This success is a consequence of several characteristics that are specific to insects, and in particular to beetles.


The solidity of the chitinous integuments of beetles (Fig. 1) means that fossil remains can be preserved for a long time in sediments in very good conditions (Fig. 2), making beetles one of the dominant orders in palaeoecological assemblages. Fossil Quaternary beetles are morphologically and ecologically identical to their modern analogues, so they can be identified fairly easily by comparison with modern reference specimens. What’s more, their palaeoecological significance can be revealed by consulting the abundant bibliography available for this order, which is much studied by ‘actualist’ entomologists.


The numerical importance of this order of insects is amazing. For example in France, Coleoptera are represented by almost 11,000 species, occupying almost every available environment, whether natural or anthropogenic. This spectacular taxonomic and biological diversity of beetles explains the variety of palaeoecological information that can be obtained by studying fossil assemblages.


Species that indicate particular environments (stenotopic species) are invaluable in refining palaeoecological reconstructions. For example, many phytophagous species are linked to a single plant species. These monophagous species are particularly interesting to study for reconstructing past vegetation composition. Aquatic insects associated with running or standing water provide information on the nature of the watercourse, i.e. the speed of the current and the water temperature, while those associated with riparian and aquatic plants help to reconstruct the hygrophilous vegetation. Ground-dwelling insects reveal the particular nature of the substrate (sand, gravel, silt, etc.) or a certain degree of openness in the plant cover.


Quaternary entomology research is still relatively restricted in France due to the very small number of specialists in this very specific field of research. However, over the last thirty years or so, there has been growing number of studies, for example in the north-east and centre of France, and in southern mountains, mainly the Alps and Massif Central. In the Pyrenees, the only significant study concerns a well-defined period at the transition between the Allerød and the Younger Dryas, around 12 900 years ago (Ponel et al., 1999). This situation is all the more regrettable as various publications have highlighted the importance of the Ariège/Pyrénées-Orientales region in understanding the evolution of the climate and vegetation of the Pyrenees since the last glaciation (Reille, 1990, 1993; Reille & Andrieu, 1993; Reille & Lowe, 1993; Ponel et al., 1999).


The aim of La Restanque project is therefore to analyse the sedimentary sequence of a peat bog located in the upper Bruyante valley at an altitude of around 1620 m (Reille & Andrieu, 1993). The Restanque peat bog is ideal for a palaeoecological study of insect macroremains because this depositional site is located at the confluence of several small watercourses, which have certainly helped to collect the macroremains from a vast catchment area in the upper valley. This glaciolacustrine sequence is contemporaneous, at the base, with the retreat of the Würmian Bruyante glacier and is continuous for the last 12 millenia (Late-glacial-Holocene). This easily accessible sequence allows the use of a specially-built large-diameter (Ø 20 cm) hand corer (Fig. 3), which is essential for extracting the large quantities of sediment required for palaeoentomological analysis.

The main aims of the investigations based on this data source, which is independent of other palaeoecological markers, are 1. to reassess the climatic and hydrological conditions of the Younger Dryas-Holocene transition, 2. to establish the nature of the forest cover and its altitudinal variations during the Late-glacial Interstadial and the Younger Dryas, and then during the Holocene, 3. to analyse the effects of human impact on the vegetation through associated insect fauna.

A 4 m core was extracted and divided into 77 samples. Samples for insect analysis were processed using the standard method (Coope, 1986) : disintegration in water, then sieving over a 300 µm sieve, followed by separation and concentration of insect fragments by paraffin flotation. The insect fragments were identified by comparison with a modern reference collection and then preserved in alcohol. A total of 322 insect taxa belonging to 7 different orders were counted, Coleoptera being by far the dominant taxa within these assemblages. A summary figure showing the evolution over time of the main ecological categories and a few remarkable taxa is provided (Fig. 4).


Contrary to pollen data, which indicate the existence of a pioneer pine forest before the Holocene, the abundance of cold-adapted species (Fig. 4 a) until the beginning of the Preboreal period indicates that no forest cover was present at that time and that the vegetation cover was comparable to a high-altitude grassland (Fig. 4 b). Average summer temperatures (Tmax) did not exceed 10°C. Among the cold-adapted species, Helophorus glacialis (Fig. 5) is particularly representative : this beetle can only complete its life cycle when snow cover persists late into the summer, a condition found at sea level in northern Scandinavia and above 2000 m in southern regions. However, the rare occurrences of Pinus stomata make it likely that isolated specimens of this tree are present on the sun-exposed slopes of the mountains. Isolated occurrences of beetles associated with cold climates after the Younger Dryas are probably due to passive transport by run-off from higher regions.


At the beginning of the Holocene, the disappearance of cold-adapted beetle species was sudden (Fig. 4 c). This was accompanied by a spectacular double peak in the numbers of the Trichoptera Drusus discolor (Fig. 4 d), which today lives in the cold torrents of high-altitude regions (Décamps & Pujol, 1975). This species can be clearly recognised by the highly distinctive ornamentation on its head, and more particularly on its frontoclypeus (the part of the exoskeleton that is best preserved in sediment) (Fig. 6). This double peak of Drusus discolor corresponds exactly to two gravel beds in the sedimentary profile, which testifies to a torrential regime of the watercourse during a brief period. This taxon in turn disappeared abruptly, in response to rapid climatic warming at the beginning of the Holocene.

The rarefaction of running-water beetles (Fig. 4 e), just before the peak of Drusus discolor, is evidence for an intensification of the cold correlated with the Younger Dryas. The resulting cooling certainly caused the environment to freeze over, leading to a reduction and then a cessation in run-off. The massive appearance of Drusus discolor indicates a sudden resumption of a seasonal torrential regime, marking the end of the Younger Dryas and the beginning of the Holocene.

This warming was followed by a long period characterised by regular occurrences of beetles associated with deciduous and coniferous trees during the first half of the Holocene (from 9000 to 4700 years before present, approximately). The proximity of a forest is likely, although very few truly forest-dwelling ground beetles are present, suggesting that the canopy must have been sparse. The high temperatures are suggested by the presence of several taxa, including the ant Dolichoderus quadripunctatus (Fig. 7), a rather southern, lowland species that was unexpected at this altitude (Fig. 4 f).

The Subboreal period (4700-2700 BP) was marked by a series of clear-cut events : 1. decline in beetles associated with conifers (Fig. 4 g) and collapse in Pinus frequencies, 2. reappearance of beetles associated with open environments (Fig. 4 h) and a decline in tree pollen associated with the increase in Poaceae pollen (grasses), 3. a maximum of Camponotus (Fig. 4 i), an ant genus mainly associated with dead trees, certainly favoured by the availability of felled trunks and stumps.


At the same time, the beech forest becomes denser, as can be seen from the large number of specimens of the weevil Rhynchaenus fagi (Fig. 4 j) and the high percentage of beech pollen. Rhynchaenus fagi is a small, monophagous Curculionidae beetle that lives exclusively on Fagus (Fig. 8). The larva gnaws at the leaf parenchyma, while the adult uses its rostrum to riddle the leaves with stings, causing damage during outbreaks of the insect. Both indicators peak at the same time. Another weevil, Orobitis cyanea (Fig. 9) which lives on Viola in the undergrowth, suggests the developement of a forested environment nearby. The Restanque site records the transition between a coniferous forest in the process of being destroyed and the expansion of a beech forest. The upheavals recorded by the insects are likely due to deforestation linked to human activity, as there are no thermal markers to indicate any clear climatic change during this period.


The last stage in the evolution of the vegetation corresponds to the total eradication of the forest in favour of an Ericaceae heath, as shown by the appearance of the weevil Micrelus ericae (Fig. 10) associated with heather (Calluna vulgaris) (Fig. 4 k). This event also corresponds in the pollen diagram to high levels of heather.


In conclusion, the presence and abundance of cold-climate beetles such as Helophorus glacialis during the Late-glacial Interstadial and the Younger Dryas shows : 1. that Tmax did not exceed 10°C even during the Late-glacial Interstadial, 2. that the hypothesis of the local presence of pine during the Younger Dryas, based on the identification of a few stomata, needs to be qualified. The Younger Dryas-Holocene transition is characterised by a warming and a transient torrential regime, of an intensity unmatched elsewhere in the sequence, marked by very large numbers of the rheophilous Trichoptera Drusus discolor and the disappearance of cold-climate beetles. The human impact on the vegetation at the end of the Holocene (destruction of the coniferous cover, establishment of the beech forest, etc) is recorded by a combination of converging entomological evidence. Finally, this study highlights the value of cross-referencing independent biological and non-biological markers (insects/pollen/plant macroremains/sediment, in the case of La Restanque) for reconstructing paleoenvironments.

References

Coope G.R., 1986. Coleoptera analysis. In: Berglund B.E. (Ed.), Handbook of Holocene Palaeoecology and Palaeohydrology. Wiley & Sons, Chichester, pp. 703-713.

Décamps H., Pujol J.-Y., 1975. Les larves de Drusinae des Pyrénées (Trichoptères, Limnephilidae). Annls Limnol. 11 (2): 157-167.

Elias S.A., 1994. Quaternary Insects and their Environment. Smithsonian Institution Press, Washington, 284 p.

Lepneva S.G., 1971. Fauna of the U.R.S.S, Trichoptera II (2), Larvae and pupae of Integripalpia. Israël program for scientific translations, Jérusalem, 700 p. (traduit du russe).

Ponel P., Coope G.R., Andrieu-Ponel V. & Reille M., 1999. Coleopteran evidence for a mosaic of environments at high altitude in the eastern Pyrénées, France, during the climatic transition between the Allerod and Younger Dryas. Journal of Quaternary Science 14 (2): 169-174.

Reille M., 1990. The peat-bog of La Borde (Eastern Pyrénées, France): a key-site for the study of the Late-glacial in southern Europe. Comptes-Rendus Académie des Sciences, Paris 310 II, 823-829.

Reille M., 1993. New pollen-analytical researches at Freychinède, Ariège, Pyrénées, France. Dissertationes Botanicae 196, 377-386.

Reille M. & Andrieu V., 1993. Variations of the timberline in the Pyrénées (France) during the Late-glacial. Comptes-Rendus Académie des Sciences, Paris, 316 II, 547-551.

Reille M. & Lowe J.J., 1993. A re-evaluation of the vegetation history of the eastern Pyrénées (France) from the end of the last Glacial to the present. Quaternary Science Reviews 12, 47-77.

The author

Philippe Ponel is a CNRS researcher at the Institut Méditerranéen de Biodiversité et d’Écologie marine et continentale (IMBE), at Aix Marseille University.


OpenEdition vous propose de citer ce billet de la manière suivante :
Philippe Ponel (14 février 2025). Fossil insects unveil the Holocene history of a Pyrenean forest. Bioarchéologies. Consulté le 26 avril 2025 à l’adresse https://doi.org/10.58079/13bm3


Vous aimerez aussi...

Laisser un commentaire

Votre adresse e-mail ne sera pas publiée. Les champs obligatoires sont indiqués avec *

This site uses Akismet to reduce spam. Learn how your comment data is processed.