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Soil animals and archaeological site formation processes, with a particular focus on insects

Doyle McKey
p. 37-44

Résumés

Les sites archéologiques sont formés non seulement par les activités humaines, mais aussi par des processus naturels agissant sur les sédiments pendant leur dépôt et depuis lors. Pour traduire les données archéologiques en déductions sur le comportement humain, il faut tenir compte de ces processus. La bioturbation est la plus importante des nombreuses façons dont les animaux du sol affectent la formation des sites. Les animaux bioturbateurs sont divers ; je me concentre sur les fourmis, les termites et les vers de terre, les bioturbateurs les plus répandus et les plus importants dans les milieux terrestres. Selon les propriétés du bioturbateur et les caractéristiques du site, la bioturbation peut augmenter ou diminuer l’érosion, et donc accélérer ou freiner la dégradation des sites archéologiques. La bioturbation peut également affecter la stratigraphie des sols et des sédiments, obscurcissant ou détruisant parfois la relation entre la profondeur et l’âge que la stratigraphie peut refléter. Par le biais du processus de « biosorting », c’est-à-dire le déplacement sélectif vers le bas des plus grosses particules qui caractérise la pédogenèse dans les biomantles, les animaux du sol créent également des « lignes de pierre ». Celles-ci peuvent être interprétées à tort comme des strates d’origine géogénique ou – si les plus grosses particules comprennent des artefacts – comme des horizons d’occupation. Les animaux du sol créent également d’autres types de « pseudo-éléments ». Dans certains environnements, les rétroactions positives mutuelles entre les animaux du sol et la topographie aboutissent à la création de paysages de monticules de terre qui ont été interprétés à tort comme des terrassements d’origine humaine. La confusion peut également aller dans le sens inverse, à savoir le scepticisme tenace de certains écologues quant à l’origine anthropique des vestiges de certains types de champs surélevés. Enfin, par les mêmes types de rétroactions positives mutuelles entre leur activité et la topographie qui conduisent à la création de paysages naturels de monticules de terre, les animaux du sol vivant dans les vestiges de terrassements tels que les champs surélevés ou les barrages à poissons en terre peuvent préserver ces vestiges contre l’érosion. Les terrassements créés par l’homme et réaménagés par les animaux du sol sont de véritables éléments de paysage bioculturels.

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Acknowledgments

I thank Stéphen Rostain for inviting me to contribute to this special issue. I also thank Diana Johnson and Prof. Randall Schaetzl (Michigan State University) for graciously sending me a treasure trove of images of stone-lines and biomantles, and I wish there had been space for more than two photographs. Finally, I thank numerous colleagues who have been partners in our studies of earth-mound landscapes, particularly Rumsaïs Blatrix, Delphine Renard, Leonor Rodrigues, and Anne Zangerlé. Our research on this topic has been funded by support from the Institut universitaire de France and by grants from the Tosca committee (Terre Solide, Océan, Surfaces Continentales, Atmosphère) of the Cnes (French National Center for Space Research), project FLOODSCAPE; from the Institut Ecologie et Environnement (Inee)/Cnrs (Projets Exploratoires Pluridisciplinaires Tohmis); and from the LabEx CeMEB (Centre Méditerranéen Environnement et Biodiversité, Montpellier), an Anr “Investissements d’avenir” program (ANR-10-LABX-04-01).

Introduction

1The concerns of archaeoentomology treated by other articles in this issue include the material and symbolic uses of insects in past cultures (cf. Rostain), how insects affected past cultures (e.g., as pests of stored products: cf. Mayeux et al.; Toriti et al.), how insects can help us interpret human remains (forensic archaeoentomology: cf. Kirgis & Huchet), and how insects in archaeological contexts can be used to reconstruct environments and climates (cf. Moret et al.; Yvinec et al.). This paper explores a different kind of interface between insects and archaeology: how soil insects (and other animals) affect site formation processes. Archaeological sites are formed not only by human activities, but also by natural processes acting on sediments during their deposition and ever since (Schiffer 1983; Stein 2001). The effects of these natural processes—for example, the effects of soil animals on archaeological sediments and on artifacts themselves—must be understood if we are to accurately translate archaeological data into inferences about human behavior (Wood & Johnson 1978). Methods for studying these effects thus form part of the toolkit of geoarchaeology, particularly the budding of that field termed geoethnoarchaeology (Shahack-Gross 2017), which enables the simultaneous study of cultural and non-cultural formation processes in ethnographic contexts (Friesem 2016). The diversity of ways that soil animals affect site formation is not widely recognized. They can affect archaeological materials by direct ingestion, by ingestion of materials altered by bacteria or fungi, by their effects on soil chemistry and biology, and by the disturbance or movement of soils and sediments (Wicksten 1989). Earthworms can ingest small seeds, degrading or destroying them, and may selectively remove them from the archaeobotanical record (Stein 1983; Tryon 2006). Earthworms may also contribute to the enzymatic breakdown of starch grains in soils and sediments (Haslam 2004). Ants move seeds and other plant materials. While carbonized seeds can be easily distinguished from recent seeds in many deposits, distinguishing ancient and recent seeds, pollen, and phytoliths in waterlogged or dry cave sites may be problematic (Miksicek 1987). Termite infestations in ancient dwellings may degrade sites and remove architectural details (Sutton 1995). Termites can gnaw and destroy bones, and colonies of some species may even nest preferentially, or survive and grow better, in bone-rich archaeological sites because their requirements for calcium and other minerals are better satisfied. Furthermore, the effects of termites on soil chemistry and biology can also accelerate the dissolution of bones (Backwell et al. 2012). But termites can also pre-serve evidence of ancient human activities. For example, localized activity of termites has preserved casts of early African cultigens (Fowler et al. 2004).

2But the way in which soil animals most strongly affect site formation is through bioturbation—the re-working of soils and sediments by all kinds of organisms (Wood & Johnson 1978), including not only burrowing animals but also microbes and rooting plants (e.g., Schaetzl et al. 1990). Bioturbation is a key process in ecosystem functioning and has played key roles in the diversification of life (Meysman et al. 2006) and in the fashioning of the earth’s surface (Wilkinson et al. 2009).

3Many animals burrow in soil. Mammals such as badgers (Mallye 2011), armadilloes (Álvarez et al. 2020), and various rodents (e.g., Sapir & Faust 2016; Bocek 1986) can locally move and mix large quantities of earth, but the widespread earth-moving activities by invertebrate animals, particularly two groups of social insects (ants [Johnson & Johnson 2010; Robins & Robins, 2011; Viles et al. 2021] and termites [McBrearty 1990 ; Williams et al. 2021a]), but also other insects (Bétard 2021) and earthworms (Darwin 1881; Cunha et al. 2016), dwarf those of other bioturbators. These three groups of invertebrates are commonly recognized as “ecosystem engineers” (Jones et al. 1994) owing to the extent and the ecological importance of their earth-mixing activities. Some other animals can have comparably large effects in particular environments, such as gophers in prairies (see figure 9.1 in Wood & Johnson 1978) and crabs in tropical littoral ecosystems (Specht 1985; Graham et al. 2016).

4The movement of archaeological materials by soil animals can have both positive and negative effects on site integrity. These effects result from the redistribution of archaeological materials both horizontally (via erosion) and vertically (affecting stratigraphy).

Bioturbation and erosion

5Erosion can redistribute archaeological materials horizontally, degrading archaeological sites by obscuring their lateral limits (Johnson 1990; Armour-Chelu & Andrews 1994) or even destroying sites. Bioturbation can increase erosion rates by bringing erodible sediments to the surface, increasing the sediment supply; it can also counter erosion by increasing porosity and water infiltration and reducing runoff, or by creating water-stable surfaces (Wilkinson et al. 2009). The balance between these two opposing effects can vary, depending on the properties of the bioturbator, the characteristics of the site, or both.

6In southern Tunisia, where ants, rodents and termites are the main bioturbators, the first two excavate individual sand grains, accelerating wind erosion, whereas termites deposit soil sheetings made of sand grains linked together by bridges of organic matter and small mineral particles. These aggregates are as stable as biological soil crusts, and greatly reduce erosion (Jouquet et al. 2021). In another environment—moist semi-deciduous forests in West Africa—termites bring fine soil particles to the surface, where they are washed downslope by water erosion from summits and hill slopes, creating gravel layers (Kristensen et al. 2019). The relationships between ant bioturbation and erosion are similarly complex and variable among species and settings (Viles et al. 2021). Thus, bioturbation can act either to destroy or to preserve archaeological features, and to hasten or to hinder the degradation of archaeological sites.

7However, any threats posed today to archaeological sites by erosion caused by bioturbating soil animals pale when compared to the threat posed by erosion accelerated by land-use changes (Meylemans et al. 2008), such as plowing (Huisman et al. 2019) and removal or degradation of vegetation (Rostain & McKey 2015).

Bioturbation and stratigraphy: the good, the bad, and the deceptive

8Bioturbation can have great effects on the stratigraphy of soils and sediments studied by archaeologists. Stratigraphy results from processes of horizonation. Pedoturbation (including bioturbation by soil animals) can result in homogenization, which can obscure or destroy stratigraphy (Wood & Johnson 1978). But bioturbation can also contribute to horizonation, producing strata that can confuse archaeologists. This diversity of effects is explained by the concept of the soil biomantle. The biomantle refers to the layers of soil in which organisms are concentrated and in which bioturbation is a dominant process (Johnson 1990, 2002; Johnson et al. 2005). Depth of the biomantle varies with environment; in many lowland tropical environments the biomantle can be several meters deep. Within the biomantle, smaller particles ingested and defecated by earthworms, or moved by ants, termites, and other burrowing animals, are homogenized. Small artifacts such as flakes or beads are sometimes deposited at the surface, in rodent mounds, anthills, etc., enabling archaeologists to find sites without digging (Wood & Johnson 1978; Sapir & Faust 2016; Trachet et al. 2017). Soil animals do not ingest or move larger particles such as gravel and most artifacts of any size. Over time, these larger objects are buried by smaller particles (Darwin 1881; Johnson 1990, 2002; Balek 2002; Johnson et al. 2005). This size-selective downward movement of larger objects has several consequences for soils and sediments and their study by archaeologists.

The good

9First, burial of objects by soil animals preserves artifacts from erosion (Van Nest 2002). As already recognized by Darwin (1881) in his last book, archaeologists owe a great debt to earthworms and other soil invertebrates (Wood & Johnson 1978; Feller et al. 2003).

The bad

10Downward movement can also alter or obscure stratigraphy and thereby the relationship between depth and age that stratigraphy may reflect. Burrowing by insects creates an unstable landsurface subject to local and uneven collapse. Some objects may tumble into termite galleries, while nearby objects remain in place (Clark & Harris 1985; McBrearty 1990). Thus, artifacts of the same age may be found at considerably different depths in the soil. This is illustrated by cases in which fragments (for example, of ceramics) buried at different depths are shown to be conjoinable pieces of a single artifact (Villa 1982). Conversely, the transport of soil particles and excavation of subterranean galleries by termites can also create a concentrated artifact horizon, as formerly vertically diffuse artifacts and stones come to rest in cavities made by these animals (Clark & Harris 1985). By altering stratigraphy, bioturbation affects all the inferences based on it. For example, movement of charcoal in soil or sediment can lead to confusing distributions of radiocarbon dates (e.g., Johnson et al. 2008). Combining charcoal dating with other analyses can help assess the effects of bioturbation (Grave & Kealhofer 1999). Bioturbation can also lead to errors in interpretation of luminescence dating (Rink et al. 2013). However, once the effects of bioturbation are recognized and taken into account, luminescence dating can also be used to estimate rates of bioturbation (Kristensen et al. 2015).

The deceptive

11Bioturbation can also produce features that, although natural in origin, can be confused with archaeological features. McBrearty (1990) cites observations by Clark & Harris (1985) of the concave bases of excavated burned termite mounds, noting (p. 132) that “if the upper portion of the mound were eroded away, the resulting concavity lined with baked clay might easily be mistaken for a hearth”. She termed such features “pseudofacts”, contrasting them with true artifacts. Nowadays, however, “pseudofacts” is a synonym for “fake news”. While termites that create structures mimicking artifacts could be accused of propagating “fake news”, the term “pseudofeatures” (Sutton 1995) is more appropriate and occupies a less crowded semantic space.

12A much more widespread—and more persistent—source of confusion results directly from the size-selective downward movement of larger particles that characterizes pedogenesis in biomantles the world over. As burrowing animals churn the soil, the small particles that ants and termites excavate, or that earthworms ingest and defecate, bury the larger particles that they do not move. Over time, these larger particles—gravel, pebbles, and stones, including stone artifacts—tend to concentrate at the bottom of the biomantle, where they often form a distinct layer of stones, termed a stone-line or a stonelayer, buried under clay, silt and sand (Johnson 1990, 2002; Johnson & Balek 1991; Balek 2002). These subsurface layers or mats of stones resulting from this process of “biosorting” are evident as a “line of stones” in exposures such as road cuts and soil or archaeological trenches (fig. 1). Such stone-lines or stonelayers characterize a wide variety of soils in tropical, subtropical and mid-latitude regions. Stone-lines frequently include numerous artifacts. In tropical Africa, for example, lithic assemblages are frequently associated with stone-lines (Mercader et al. 2002). Thus stone-lines of pedogenic origin may be erroneously interpreted as archaeological occupation surfaces (Wood & Johnson 1978; Clark & Harris 1985; McBrearty 1990; Williams 2019; Williams et al. 2021a).

Fig. 1. Stone-lines. a: stone-line buried by the activities of termites, Kenya. b: a stone-line in soil developed on granitic uplands in western Transvaal, South Africa, with the author of the pedogenic hypothesis for the origin of stone-lines, the late Prof. Donald L. Johnson. Both photographs courtesy of Diana Johnson and Prof. Randall Schaetzl.

Fig. 1. Stone-lines. a: stone-line buried by the activities of termites, Kenya. b: a stone-line in soil developed on granitic uplands in western Transvaal, South Africa, with the author of the pedogenic hypothesis for the origin of stone-lines, the late Prof. Donald L. Johnson. Both photographs courtesy of Diana Johnson and Prof. Randall Schaetzl.

13In contrast to this pedogenic hypothesis for the origin of stone-lines, some geologists and soil scientists postulate a geogenic origin, seeing stone-lines as evidence of strong erosion that moved large detrital particles, for example during glaciation (Wood & Johnson 1978). In relatively stable landscapes of the lowland tropics, where stone-lines are often deepest and most marked, the strong erosional phases implied by this hypothesis have usually been considered to be related to dramatic climatic fluctuations (Mercader et al. 2002). Thus, misinterpreting a pedogenic stone-line as one produced geogenically could also lead to mistaken conclusions about the paleoecological context of artifact horizons.

14The weight of evidence increasingly favors the pedogenic origin of stone-lines in most environments. However, both geogenic and pedogenic processes can disturb the archaeological record, altering our interpretation of the behavior of the humans who left behind the artifacts found in stone-lines, and Mercader et al. (2002: p. 74) noted that “the precise extent to which sheet erosion, loss of fines, or bioturbation could have disturbed the archaeological record remains to be quantified”.

15Stone-lines continue to generate controversy among archaeologists. An example showing how high the stakes can be, in terms of our understanding of human history, is the controversy around surprisingly old estimated dates (65 000 BP) given by Clarkson et al. (2017) for artifacts in two sites in northern Australia. This estimate, which would represent a 30 % increase in the previously accepted length of human occupation of the continent, was based on stratigraphic association of artifacts with radiocarbon-dated organic material and with sandy sediments dated using optically stimulated luminescence. Williams (2019) and Williams et al. (2021a) contested this interpretation; stone-lines and other products of termite activity led him to question the stratigraphic integrity of the sites. Smith et al. (2020) countered by proposing criteria to distinguish termite stone-lines from artifact horizons, and concluded that the features at the two sites in question were true examples of the latter. Williams et al. (2021b) found these criteria, and the conclusions based on them, unconvincing. The latest volley in this exchange is a further reply by Smith et al. (2021).

16Natural earth-mound landscapes resulting from the earth-moving activities of soil animals offer another example of confusion between archaeological features and structures of natural origin. Numerous insects make mounds of varying size (Bétard 2021), but only termites and ants (Troll 1936; Whitford & Eldridge 2013; Viles et al. 2021) construct large mounds. Along with their mounds, those made by some earthworms (Zangerlé et al. 2016) and burrowing mammals (Finney 2012) are large enough to be confused with archaeological features (figure 2). The confusion-inducing mounds are usually round, varying in size from earthworm mounds that can be < 1 m in diameter and a few dm in height, but often much larger (Zangerlé et al. 2016), to termitaria and anthills that can be up to 20 m or more in diameter and 10-15 m high (Bétard 2021). Natural earth-mound landscapes are particularly frequent in poorly drained environments, where construction of a locally raised surface allows ants and termites to avoid flooding of their nests (Troll 1936; Whitford & Eldridge 2013), and allows earthworms to come to the surface to respire between bouts of foraging in waterlogged soils (Zangerlé et al. 2016). In these environments, natural self-organizing mechanisms—explained in the following section of this paper—result in regular spacing of the numerous mounds. This regular spacing has led to confusion with archaeological vestiges of some kinds of agricultural raised fields, which are also most frequent in poorly drained environments. While platform fields, ridge fields, etc., are unambiguously of human origin, groups of round raised fields (“mound fields”) such as those found in one region of Bolivia’s Llanos de Mojos (Rodrigues et al. 2018) and in seasonally flooded coastal savannas of French Guiana (McKey et al. 2010) can be easily confused with natural earth-mound landscapes. For example, we believe that structures described as raised fields in the Orinoco Llanos in Colombia by Reichel-Dolmatoff & Reichel-Dolmatoff (1974) are in fact surales landscapes made by earthworms (Zangerlé et al. 2016; see figure 2). The anthropogenic origin of structures described as vestiges of “agricultural mounds” in Guyana by Shearn & Heckenberger (2020) may be similarly called into question, in the absence of firm archaeological evidence for their origin and their use by humans.

17Earth mounds of natural origin also sowed confusion for archaeologists in North America, where “mima mounds” were widespread before they disappeared under the plow (Finney 2012). Today, the weight of evidence supports the hypothesis that they were built by gophers, but in the Upper Midwest of the US, in the 19th and early 20th centuries they were studied mostly by archaeologists interested in “Moundbuilder” cultures. “Every mound encountered in the Upper Midwest during this early period was viewed as a potential burial mound that might contain artifacts, and thus a potential antiquities repository to be exploited. As a result, archaeologists discovered thousands of presumed Indian mounds in the Upper Midwest, many of which proved to be natural prairie mounds” (ibid.: p. 86).

18Confusion may also go in the opposite direction: some ecologists are generally loath to consider a hypothesis of human origin for landscapes they consider potentially explicable by natural mechanisms (McKey et al. 2014). In fact, both natural and anthropogenic earth-mound landscapes exist, and the interactions between them are a fascinating theme, treated in the next and final section of this paper.

Soil animals and the preservation of archaeological earthworks in seasonal tropical wetlands

19The frequency of natural earth-mound landscapes in seasonally flooded and other poorly drained environments can be explained by reciprocal positive feedbacks between soil engineer animals and the islands of well-drained soil they create (McKey et al. 2014; Zangerlé et al. 2016), feedbacks that are generated only in these environments. Archaeologists who study earthworks in these environments should understand these feedbacks, because raised fields, or their vestiges, also form islands of well-drained soil during the high-water season, and can be affected by the same interactions that link soil engineers and natural islands (McKey et al. 2014, 2021).

20How do these reciprocal feedbacks work? Soil engineers create islands of well-drained soil (feedback from engineer activity to topography). They then continue to concentrate their activities in these islands (feedback from topography to engineer activity). For example, nests of mound-building ground-nesting ants and termites escape seasonal flooding in their self-made islands. In the low-water season, these central-place foragers continually bring mineral and organic matter to the mound, which grows over time (feedback once again from engineer activity to topography). New colonies can establish only in mounds (feedback once again from topography to engineer activity), which thus persist and grow over generations of occupants. For earthworms, mechanisms generating feedbacks are different but the effects are the same (Zangerlé et al. 2016). Seeking refuge deep in the soil during the dry season, earthworms forage in the waterlogged topsoil during the high-water season. Because they need to respire, they make towers with their castings to gain access to air (feedback from engineer activity to topography). Between bouts of foraging, they continually return to these islands to breathe (feedback from topography to engineer activity), each time depositing more castings, so that each tower eventually becomes a mound (feedback once again from engineer activity to topography). The feedback from topography to engineer activity only functions when well-drained soil is a patchy resource. If the entire savanna is well-drained, there is no force driving engineers to build new mounds or to concentrate their activities in mounds.

21Feedbacks between engineer animals and topography can also explain the regular spacing of mounds, the feature of these landscapes that has most been considered to be suggestive of an anthropogenic origin. A spectacular example is offered by the surales landscapes of the Orinoco Llanos (fig.2). Large expanses of seasonally flooded savanna are covered by densely packed, regularly spaced mounds made by earthworms. Mounds vary in size across sites, some reaching 4-5 m in diameter and 2 m from base to summit. Individual earthworms have a limited foraging radius, so soil deposited as casts on the mounds comes from the area immediately surrounding the mound. As the mound grows, the surrounding valley becomes deeper, making it impossible for worms to initiate another mound too close to an already existing one. This threshold minimum distance results in regular spacing (Zangerlé et al. 2016). The combination of a positive effect of engineer activity on topography at short scale and a negative effect at a slightly longer scale—a Turing mechanism—suggests that surales landscapes are formed by self-organizing mechanisms similar to those that have produced spatially regular landscapes in many other environments (Rietkerk & Van de Koppel 2008).

Fig. 2. Large earth mounds made by earthworms in surales landscapes of the Orinoco Llanos, Colombia. a: ground view (photograph courtesy Anne Zangerlé). b: aerial view (photographed using a drone, courtesy Delphine Renard). In both photographs, mounds are 1.5-2 m diameter. Such mounds have sometimes been mistakenly considered by archaeologists to be vestiges of agricultural raised fields. See Zangerlé et al. (2016) for details.

Fig. 2. Large earth mounds made by earthworms in surales landscapes of the Orinoco Llanos, Colombia. a: ground view (photograph courtesy Anne Zangerlé). b: aerial view (photographed using a drone, courtesy Delphine Renard). In both photographs, mounds are 1.5-2 m diameter. Such mounds have sometimes been mistakenly considered by archaeologists to be vestiges of agricultural raised fields. See Zangerlé et al. (2016) for details.

22As mentioned above, raised fields and their vestiges can “plug into” these feedbacks between engineer animals and topography in wetland savannas (fig. 3). In seasonally flooded coastal savannas of French Guiana, all ant and termite nests are found on abandoned raised fields; plants and earthworm activities are also concentrated on these islands of well-drained soil (McKey et al. 2010). Bioturbation is much more active in the abandoned raised fields than in the valley floor between these mounds, as shown by both the vertical distribution of fallout radionuclides (Pfahler et al. 2015) and the abundance of biogenic soil aggregates (Renard et al. 2013). The material accumulated by these organisms, and stabilized against erosion by the biogenic structures they build, helps explain how they have escaped being effaced by erosion during the centuries since abandonment (McKey et al. 2010). As shown in figure 3a, accumulation by soil engineers is not always sufficient to counter erosion. Where conditions do not favor activities of engineers, vestiges of raised fields can be effaced by erosion. Rates at which mounds (both human-made and natural) erode if they are abandoned and not re-engineered have rarely been measured, but can be considerable (Whitford & Eldridge 2013). Rates of erosion of abandoned mounds of the termite Amitermes vitiosus measured under exposure to natural rainfall in northern Australia would lead to their disappearance in about 30 years (Bonell et al. 1986). Present-day agricultural raised fields in the Congo Republic lose about two-thirds of their height a few decades after abandonment (Rodrigues et al. 2020). The effects of re-engineering on erosion rates probably vary greatly depending on soil type, climate, and traits of the engineers and the biogenic structures they make.

Fig. 3. Vestiges of pre-Columbian raised fields in a seasonally flooded coastal savanna of French Guiana. a: aerial view (drone photograph, courtesy Delphine Renard). Soil engineers, including termites and ants (debris pile of Acromyrmex ants, c) but also earthworms (d) and plant roots (b) constantly bring organic and mineral matter to vestiges of raised fields, and their biostructures stabilize the soil. Soil engineers thereby compensate for the loss of material by erosion, thereby contributing to preservation of the physical vestiges of raised fields. Photographs b-d: Doyle McKey.

Fig. 3. Vestiges of pre-Columbian raised fields in a seasonally flooded coastal savanna of French Guiana. a: aerial view (drone photograph, courtesy Delphine Renard). Soil engineers, including termites and ants (debris pile of Acromyrmex ants, c) but also earthworms (d) and plant roots (b) constantly bring organic and mineral matter to vestiges of raised fields, and their biostructures stabilize the soil. Soil engineers thereby compensate for the loss of material by erosion, thereby contributing to preservation of the physical vestiges of raised fields. Photographs b-d: Doyle McKey.

23The intense bioturbation to which raised fields and other earthworks re-engineered by soil animals are subjected in seasonally flooded savannas thus affects different aspects of site integrity in opposite ways. While this re-engineering has obscured stratigraphy, the material that engineers deposit and stabilize on the mounds compensates erosion. Although much of the soil present in these vestiges today may have been put there by engineer animals rather than by humans, the actions of engineer animals preserve the outlines of the landforms made by humans (McKey et al. 2021).

24Conversely, earth mounds of natural origin can also be re-engineered by humans: termite mounds, for example, often serve as ready-made raised fields (McKey et al. 2017). Finally, natural earth-mound landscapes may interact with raised fields not only in landscapes but also in the minds of humans. Troll (1936) wondered whether the pre-Columbian farmers who made the raised fields he observed in the Llanos de Mojos in Bolivia might have tried to mimic the natural mounds of the termite savannas he studied there, and other authors have asked the same question in the years since (Cunha et al. 2016). Similarly, Finney (2012) advanced the idea that natural earth mounds in the US Midwest were first opportunistically used by prehistoric peoples as burial sites, and later served as “ideation templates” for burial mounds and other earthworks made by the “Moundbuilder” cultures of the region.

Conclusion

25The sites in which archaeologists dig contain information about the behaviour of ancient human occupants, but to interpret this information we need to know how the sites have been altered by natural processes, including the re-working of their soils and sediments by engineer animals, among which ants and termites, along with earthworms, figure prominently. Bioturbation by these animals can have widely divergent effects on site integrity: it can obscure, destroy, or preserve information about human behaviour, and it can deceive archaeologists by producing pseudofeatures. Bioturbation is a textbook example of “why ecology needs archaeologists and archaeology needs ecologists” (Briggs et al. 2006).

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Titre Fig. 1. Stone-lines. a: stone-line buried by the activities of termites, Kenya. b: a stone-line in soil developed on granitic uplands in western Transvaal, South Africa, with the author of the pedogenic hypothesis for the origin of stone-lines, the late Prof. Donald L. Johnson. Both photographs courtesy of Diana Johnson and Prof. Randall Schaetzl.
URL http://0-journals-openedition-org.catalogue.libraries.london.ac.uk/nda/docannexe/image/13754/img-1.jpg
Fichier image/jpeg, 1,8M
Titre Fig. 2. Large earth mounds made by earthworms in surales landscapes of the Orinoco Llanos, Colombia. a: ground view (photograph courtesy Anne Zangerlé). b: aerial view (photographed using a drone, courtesy Delphine Renard). In both photographs, mounds are 1.5-2 m diameter. Such mounds have sometimes been mistakenly considered by archaeologists to be vestiges of agricultural raised fields. See Zangerlé et al. (2016) for details.
URL http://0-journals-openedition-org.catalogue.libraries.london.ac.uk/nda/docannexe/image/13754/img-2.jpg
Fichier image/jpeg, 1,7M
Titre Fig. 3. Vestiges of pre-Columbian raised fields in a seasonally flooded coastal savanna of French Guiana. a: aerial view (drone photograph, courtesy Delphine Renard). Soil engineers, including termites and ants (debris pile of Acromyrmex ants, c) but also earthworms (d) and plant roots (b) constantly bring organic and mineral matter to vestiges of raised fields, and their biostructures stabilize the soil. Soil engineers thereby compensate for the loss of material by erosion, thereby contributing to preservation of the physical vestiges of raised fields. Photographs b-d: Doyle McKey.
URL http://0-journals-openedition-org.catalogue.libraries.london.ac.uk/nda/docannexe/image/13754/img-3.jpg
Fichier image/jpeg, 3,9M
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Doyle McKey

CEFE, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France

(UMR 5175 CEFE « centre d’écologie fonctionnelle et évolutive »)

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