This review covers more than twenty-five years of research listing and discussing the biogeochemical, mineralogical, and physical impacts of fungus-growing termites (or FGT, Macrotermitinae ...sub-family) on savanna sediments and landscapes. The main mechanisms by which FGT transform the surficial geological formations in tropical and sub-tropical environments is investigated from a geological perspective and the potential FGT legacies in the sedimentary facies are identified. In order to sustain a twenty million-year symbiosis with the fungus, in which fungi provide digestible food to termites, FGT must optimize the living conditions of the fungus for it to thrive. To do so, they build a biogenic structure maintaining a constant humidity of 80% and a temperature of 30 °C in any kind of environment and all year long. Indeed, FGT adapt to their environment by (i) modifying the grain-size distributions of sediments and soils where they develop, (ii) forming clay horizons below their mounds enabling water to be stored for long period of time in dry environments, (iii) increasing the alkalinity by an order of magnitude of two to three, (iv) mineralizing around 20% of all organic carbon in dry savannas, thus making them the predominant decomposing organisms and crucial actors in the carbon cycle, and (v) concentrating vital nutrients for plants and animals, creating patches of fertile land in sandy semi-deserts. Through their mound-building activities, termites substantially increase the clay fraction compared to the adjacent soil and alter 2:1 clay properties, particularly after the removal of potassium, leading to the formation of smectite layers, demonstrating their biogeochemical effects on silicate mineralogy. Through the binding of aggregates, FGT increase the strength of the mound by a factor of ten and provide exceptional weathering resistance to their mounds. Therefore, termites impact their environment from sub-millimetric transformations to solid voluminous landmarks. The water-holding capacity of a FGT mound leads to an array of positive feedbacks to the savanna landscape by enhancing protection from fires, delaying desertification, supporting rebounds by seedlings and reinforcing dryland resistance and recovery from drought. Termite bioturbation allows sediments to accumulate at a rate averaging 1 mm.y−1.ha−1, and tends to mitigate physical and chemical processes of soil degradation, boosting the heterogeneity at the landscape scale, providing it more resilience. Some of the modifications brought by FGT will remain in the landscape for long periods, testifying to past environmental conditions, and making these mounds potential proxies for paleoenvironmental reconstructions. To conclude, FGT are not only biological actors of the savanna ecosystem, but they act as a geological force by their impact on landscapes as well as by their major role in biogeochemical cycles. Finally, further research is recommended regarding the role of termite's saliva as a binding agent, as well as the age and the evolution of mounds over time.
•Termites modify the grain-size distributions of sediments and soils where they develop.•They modify the chemical properties of their environment.•They play a major role in the organic carbon cycle of dry savannas.•They generate patches of fertile land in sandy semi-deserts by concentrating vital nutrients.•Termitariae constitute potential palaeoenvironmental proxies
Fungus-growing termites have long been considered as ecosystem engineers for the modifications they make to the soil, through their ability to concentrate nutrients and create patches of fertile ...land. However, few studies have highlighted the degree to which, in building their mounds, they are able to modify the grain-size distributions of the sediments and soils in their surroundings. Therefore, the aim of this study is to document the potential impact of fungus-growing termites on sediment sorting of their environment. The assessment is based on comparisons between two different mounds and their associated control soils developed in contrasting grain-size settings, one on sand and the other on fine material (diatomite). The sedimentary modifications carried out by termites between these two parent materials and associated constructed mounds are addressed using techniques mostly based on grain-size distributions (performed with End-Member Mixing Analyses) and soil micromorphology. In order to conduct this investigation, two fungus-growing termites’ mounds were selected in the Chobe Enclave District, northern Botswana. The key questions of this study center on the capacity of fungus-growing termites to (i) adapt to any kind of parent material to build their mounds, and (ii) enrich or deplete this parent soil to meet their texture requirements in terms of mound stability and other mechanical properties to insure the success of the colony. This study demonstrated that fungus-growing termites assemble the mandatory texture required for the functions and properties of their mounds, whatever a given parent material, by selecting, transporting, and mixing the various grain sizes. To conclude, the sorting by fungus-growing termites must be considered when reconstructing paleoenvironment based on particle-size distributions.
•Fungus-growing termites transform geological parent material to build their mounds.•Fungus-growing termites achieve a granulometric optimum within a given range of grain-size distributions.•Fungus-growing termites can enrich or deplete the parent soil to reach the mandatory texture adapted to the mound.•Micromorphological patterns reflect distinct changes attributed to termites compared to control soils.•There must be evidence of termite reworking in sediments and soils.
•Electrical resistivity tomography used for the first time to investigate FGT mounds.•Fungus-growing termites increase drastically soil electrical resistivity.•Micromorphology and X-ray μ-CT ...emphasize the role of microstructure in resistivity.•Particle size distributions influence the range of electrical resistivity values.•The study presents a method of determining mound structure that is non-destructive.
Fungus-growing termites (FGT) are important agents in subtropical and semiarid ecosystems for the biological and geological modifications they bring through the construction of their mounds. In order to survive and thrive, the mounds are designed to host the Termitomyces fungus in pre-set conditions of humidity and temperature. The interior structure of FGT mounds has only been scarcely studied in the scientific literature, making knowledge about mound morphology and development incomplete. In this study, a relic FGT mound in northern Botswana was analyzed using a combination of electrical resistivity tomography (ERT), particle size distribution (PSD), X-ray computed microtomography (X-ray micro-CT), and soil micromorphology methods. Specifically, we set out to investigate (i) whether changes in the measured soil resistivities reflect expected changes between the various functional compartments of the mound; (ii) what changes in soil texture and structure are associated with the resistivity variations; and (iii) whether this information can be used to better understand the mound compartments and their development. Our results suggest that ERT may eventually provide an attractive and efficient method to investigate the interior structure of FGT mounds that is non-destructive. The use of ERT does not require to remove the epigeal of the mound. The modifications brought by FGT to the texture and structure of the soil, respective to the function of each mound compartment, can increase resistivity of up to three orders of magnitude. Further, the work presented in this paper helps to (i) identify processes involved in the sediment transfer between the mound and its surroundings, (ii) increases the understanding of mound morphology, and (iii) show that fungus-growing termitaria affect durably savanna landscape. Finally, this study demonstrates that ERT is a pertinent non-destructive method to investigate mound structure.
Understanding the formation of tropical yellow to red earth (TYRE) is essential for preserving soil multifunctionality in well-drained tropical landscapes. Weathering and bioturbation mutually ...interact in TYRE evolution, whereas allochthonous materials appear restricted to distinct (paleo)landscapes. A layered appearance of TYRE can result from quasi-constant deposition of invertebrate mound debris, outcompeting diffusional mixing. Age-depth profiles from optically stimulated luminescence (OSL) and charcoal radiocarbon (14Cchar) data of TYRE sites in different tropical landscapes, both from the literature and the present study, all reveal quasi-constant soil upbuilding, in accordance with our model. The rates of soil upbuilding are mostly in the range of 100–200 mm*ka−1, which conforms with published mounding rates of termites and ants. By comparison, geochemical transformation of rock to saprolite proceeds at rates at least one order of magnitude smaller. Termites mining saprolite, sometimes even below indurated subsoil, produce TYRE, thus linking the interconnected subsystems of differing process rates. The work of the bioengineers appears essential for transforming the deep-weathering products into well-structured TYRE. Future research may extend the provided database, the spatial scale, and the use of geochronology, coupled with paleoenvironmental proxies, in order to further enhance our understanding of tropical soil and landscape evolution, as one basis for advances in sustainable land use.
•TYRE (tropical yellow to red earth) widely covers well-drained tropical landscapes.•Luminescence ages from different continents corroborate quasi-constant TYRE upbuilding.•Layering may outcompete homogenization in the presence of mounding invertebrates.•Mounding (∼100–200 mm*ka−1) is one order of magnitude times faster than weathering; both processes are mutually connected.•Improvement of soil-landscape evolution models and support for sustainable land use.
•Termites are commonly differentiated into feeding and building groups.•Sheeting properties are mainly explained by the properties of the environment.•Building strategy better explains sheeting ...properties.•Redefining groups using a trait-based approach is proposed as perspective.
Termites are key soil bioturbators in tropical ecosystems. Apart from mound nests constructed by some advanced lineages, most of the species use their faeces, oral secretions, debris, or soil aggregates to protect themselves from predators and desiccation when they go out to forage. Although this soil ‘sheeting’ is considered to play a key role in soil functioning, the properties of this termite-made material has been poorly studied. The few available data showed that sheeting properties are highly variable with positive, neutral or negative impacts on soil C and clay content, and consequently on soil aggregate stability. Therefore, the objective of this study was to determine the factors controlling the physical (particle size fractions and structural stability) and chemical (pH, electrical conductivity and carbon content) properties of soil sheeting produced by termite species encompassing all feeding and building categories using a dataset representative of an important diversity of biotopes coming from 21 countries from all continents colonized by termites. We showed that sheeting properties were explained by the properties of their environment, and especially by those of the bulk soil (linear relationships), followed in a lesser extent by the mean annual precipitation and biotope. Classic hypotheses related to termite feeding and building strategies were not hold by our analysis. However, the distinction of termites into fungus-growing and non-fungus growing species was useful when differentiating the impact of termites on soil electrical conductivity, C content, and structural stability. The large variability observed suggests the need to redefine termite functional groups based on their impacts on soil properties using a trait-based approach from morphological, anatomical and/or physiological traits.
Human consumption of seafood can be promoted because of its positive health effects. Conversely, it is a source of chemical contaminants. Due to this dilemma, a probabilistic intake assessment of ...nutrients and contaminants via seafood is of interest to provide more detailed information. A key component of such an assessment is the selection of the most appropriate input distributions to describe the consumption and concentration data. This paper describes the construction of a nutrient database, pooling vitamin D and omega-3 fatty acid concentrations in seafood from different publications and the encountered problems related to a lack or inconsistency of information given in these publications: food description, number of samples, sampling plan, sources of the values, limit of quantification (LOQ), etc. Different solutions have been proposed and the study resulted in a huge database allowing the description of distributions of vitamin D, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) concentration and their variability in 34 seafood species relevant for Belgian consumption. The distribution fitting and selection procedure resulted in different models for the different species and nutrients. The normal and lognormal distributions are most frequently used, followed by the uniform, beta and loglogistic distribution.
Seafood consumption is related to both favorable health benefits of nutrients and to potential adverse health impacts of chemical contamination. To quantify the magnitude of this dilemma, ...probabilistic intake assessments can be performed. One step in such a procedure involves the determination of nutrient and contaminant concentrations in seafood for which data need to be collected. This article describes the process of building up a database containing previously published contaminant concentrations in seafood, and defining input distributions characterizing the variability. During the constitution of the database, several problems influencing the comparability of the data were encountered related to (1) sampling plans of the published data; (2) sample handling prior to analysis; (3) analytical methodologies; (4) the format of reporting results; and (5) missing data. Different solutions for these methodological problems have been developed. Contaminant concentrations ranges per gram fresh weight of 2.4-4390.0 ng for mercury, 0.1-5736.6 ng for the sum of indicator PCB, 0.002-115.000 pg TEQ for the sum of all PCBs, 0.002-34.400 pg TEQ for dioxins, and 0.006-126.000 pg TEQ for total of dioxin-like compounds were found. This work confirms that more guidelines are needed to standardize the analytical methodologies to be used and the format for result reporting in order to improve the comparability of data critical to conduct a human intake and risk-benefit assessment.
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