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The effect of cadavers on soil ecology: biotic and functional responses (CADAVER)

Titre du projet
The effect of cadavers on soil ecology: biotic and functional responses (CADAVER)
Description
Cadaver decomposition has a clear impact on the soil ecosystem due to large, localised inputs of nutrients, especially C, N & P, as well as microclimatic effects. Cadaveric materials are rapidly introduced into the soil environment and assimilated by soil communities to form a “cadaver decomposition island (CDI)”. This “pulse of resources” affects the soil biota and functions drastically; yet these effects are not well understood. This natural perturbation contributes to biological and functional diversity through the contrasted responses of different taxa and associated processes and the spatial and temporal heterogeneities caused by the presence of cadavers. Once the decomposition process is completed the soil ecosystem is thought to gradually return to a “normal” condition. These patterns and processes are relevant to general soil biology and application in forensic science, yet there is little data on cadaver decomposition effects on the soil biota and functions and the patterns and pace of secondary succession leading back to the initial condition. We plan to study the effects of cadavers on the soil biota in both fundamental (disturbance and nutrient pulses impact on soil biota) and applied (potential of soil biota and functions as indicators for forensic applications) perspectives. A key aspect of forensic research is to develop tools to determine if a cadaver has decomposed in a certain place and how long ago the death occurred (the post-mortem interval – PMI). Forensic pathology uses several tools (e.g. body temperature, rigor mortis, insect infestation) but is usually not precise beyond 3-4 weeks. Hypothesised dynamic changes in the soil biota over a longer period (months to years) both in the disturbance and the recovery phases should allow the developing new tools in forensic taphonomy for estimating the time since death and the location of graves (e.g. if corpses were removed). We plan to conduct a fully factorial experiment to assess how a pig (standard model in forensic science) cadaver influences the soil biota and functioning and how the soil environment recovers from this perturbation. In addition to the cadavers and control treatments, fake pigs (i.e. filled cloth bags of equivalent size, but without cadaveric fluids) and pigs decomposing 2m above the soil (to test the influence of cadaveric fluids without the weight of the corpse) will be used to separate microclimatic and chemical effects, the latter being hypothesised as having a stronger influence. Soil samples from real forensic cases (if available) and nearby control soil will also be studied to test the broader applicability of the newly developed indicators and expand the range of natural conditions studied. As insects are supposed to be a major driving force regarding the speed and amount of decomposition (and much better understood than the soil biota), classical forensic entomology analyses will also be performed for comparison with the changes in the soil biota. This research addresses fundamental soil ecology (disturbance, resilience and succession – linkages among taxa, abiotic variables and functions) and applied forensic science (bioindication, PMI estimation) that have never been addressed in the same study. The project is divided into 5 tasks. 1) Insect succession on cadavers in relation to microclimate. 2) Soil physico-chemical characteristics. 3) Molecular diversity and community structure of soil Bacteria, micro-Eukaryotes and mesofauna assessed by pyrosequencing. 4) Morphological characterisation of testate amoebae and mesofauna communities. 5) Community and ecosystem ecology modelling (species/species traits-environment-function correlations, path analysis to explore cause/effects relationships, and testing if the nutrient pulse leads to phylogenetic clumping in soil communities) and definition of abiotic and biotic (bio)indicators of post-mortem interval.
Chercheur principal
Statut
Completed
Date de début
1 Août 2012
Date de fin
31 Juillet 2015
Chercheurs
Organisations
Identifiant interne
14988
identifiant
https://libra.unine.ch/handle/123456789/1430
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  • Publication
    Accès libre
    Impact of vertebrate cadavers on soil communities and soil chemistry to develop new forensic indicators for estimating the time after death
    (2017)
    Szelecz, Ildikò 
    ;
    Mitchell, Edward 
    La décomposition des cadavres dans l'écosystème sol créée des "hot spots" éphémères particuliers, en introduisant de l'eau, des communautés microbiennes et de grandes quantités de liquides riches en éléments nutritifs dans le sol. Ceci a un impact fort, quoique spatialement et temporellement limité, sur la chimie et les organismes du sol. La recherche des impacts des cadavres sur l'environnement du sol est à l'interface entre les sciences écologiques et les sciences forensiques ; le premier domaine se focalise sur leur importance en tant que ressource dans les écosystèmes et le second sur l'estimation de l’intervalle post-mortem ou post-mortem interval (PMI) qui est le temps écoulé depuis la mort. Cependant, la recherche sur les cadavres est encore limitée et, en particulier, les connaissances sur leurs impacts sur les organismes du sol. En science forensique, l'estimation des intervalles post-mortem longs (plusieurs semaines, mois, voire années) reste un défi, parce que les méthodes actuelles comme l'examen médical du défunt ou l'utilisation de preuves d'insectes (entomologie forensique) perdent leur précision après quelques jours ou plusieurs semaines respectivement. Il y a donc une demande croissante de méthodes nouvelles et applicables en science forensique. Cette thèse de doctorat se focalise sur trois sujets clés: (1) caractériser les changements dans les caractéristiques chimiques des sols en réponse aux cadavres en décomposition, (2) évaluer les réponses de différents groupes d'organismes (thécamoebiens, nématodes, insectes) à ces changements et évaluer leur potentiel pour l´estimation du PMI (3) tester plusieurs méthodes (analyse des os, des acariens, nématodes et des micro-Eucaryotes du sol) dans une cas réel, combinant différentes disciplines (datation au 14C, microscopie, séquençage à haut débit). L'objectif global est d'améliorer notre compréhension de l'impact des cadavres sur le sol et d'utiliser ces connaissances pour développer de nouvelles méthodes d'estimation de PMI. Par conséquent, nous avons conduit deux expériences dans les forêts à proximité de Neuchâtel, en Suisse avec trois cadavres de cochons (Sus scrofa) dans la première (2009-2010) et dix cadavres de cochons dans la deuxième expérience (2013-2014). Dans la première expérience, nous avons comparé trois traitements (trois répétitions chacun) : 1) placettes témoin (sol nu pour référence), 2) des faux cochons (sacs de coton de taille et de masse équivalente à les cadavres de cochons pour les effets microclimatiques) et 3) des cadavres de cochons (placés directement sur le sol pour des effets microclimatiques et fluides cadavériques). Dans la deuxième expérience, nous avons augmenté le nombre de répétitions (cinq) et ajouté un quatrième traitement, soit des cadavres de cochons suspendus (cadavres suspendus dans des cages à 1 m au-dessus du sol) pour les effets fluides cadavériques seulement. Dans les deux expériences, l'échantillonnage a commencé peu de temps avant que les cadavres soient mis en place et après dans des intervalles définis. Les résultats de cette thèse de doctorat ont révélé des changements clairs dans la chimie du sol et dans l'abondance et la structure des communautés de thécamoebiens et de nématodes montrent qui peuvent être liés au processus de décomposition. Ni les constituants chimiques du sol, ni la composition communautaire des organismes du sol analysés n'étaient retournés au statut initial (soit avant le processus de décomposition) après 10 ou 12 mois. Par conséquent, ces approches ont un grand potentiel pour une estimation de PMI long. Enfin, cette thèse de doctorat montre que la combinaison de disciplines - ici l'acarologie, l'anthropologie, l'entomologie, le séquençage à haut débit des Eukaryotes, et l‘étude des caractéristiques chimiques du sol – est particulièrement puissante pour les enquêtes de cas réels. Dans cette thèse de doctorat, nous avons non seulement démontré avec succès l'importance du sol en science forensique, mais aussi ouvert la voie à d'autres marqueurs forensiques pour estimer le PMI. ABSTRACT Decomposing cadavers in terrestrial ecosystem represent discrete ephemeral “hot spots” that introduce water, microbial communities and high amounts of nutrients into the soil. This has a strong, although spatially and temporally limited, impact on soil chemistry and belowground soil organisms. The study of cadaver impacts on the soil environment is at the interface between ecological and forensic sciences, the former focusing on their importance as a resource in ecosystems and the latter on the estimation of the post-mortem interval (PMI) which is the time elapsed since death. However, carrion research is still sparse and especially knowledge about cadaver impacts on soil organisms remains extremely poor. In forensic science the estimation of long post-mortem intervals (after weeks, months or years) is still a challenge, because current established methods such as the medical examination of the deceased or the use of insect evidence (forensic entomology) lose their accuracy after a few days or several weeks respectively. Hence, there is an increasing demand for new and applicable methods in forensic science. This PhD thesis focuses on three key subjects: (1) characterising the changes in soil chemical characteristics in response to decomposing cadavers, (2) assessing the response of organism groups (testate amoebae, nematodes, insects) to these changes and evaluating their potential for PMI estimation (3) testing several methods (analysis of bones, mites, nematodes and micro-Eukaryotes) in a real case investigation, combining different disciplines (14C dating, microscopy, high throughput sequencing). The overall aim is to improve our understanding of cadaver impact on the soil and use this knowledge to develop new methods for PMI estimation. Therefore we conducted two experiments, using three pig cadavers (Sus scrofa) in the first (2009-2010) and ten pig cadavers in the second experiment (2013- 2014) that were placed in forest areas near Neuchâtel, Switzerland. In the first experiment, we compared three treatments (three replicates each): 1) control (bare soil for reference), 2) fake pigs (cotton bags of the same size as the pig cadavers for microclimatic effects) and 3) ground pigs (cadavers directly placed on the ground for microclimatic and cadaveric fluids effects). In the second experiment, we increased the number of replicates (five) and added a fourth treatment i.e. hanging pigs (cadavers hanging 1 m above ground) for cadveric fluids effects only. In both experiments sampling started shortly before the cadavers were put in place and continued in defined intervals after. The results from this PhD thesis revealed that changes in soil chemistry and in the abundance and community structure of testate amoebae and nematodes show identifiable patterns that can be related to the decomposition process. Neither the chemical constituents of the soil, nor the community composition of the analysed soil organisms had returned to the initial status (i.e. before the decomposition process) after 10 or 12 months. Therefore, these approaches have a high potential for long PMI estimation. Finally, this PhD thesis shows that the combination of disciplines - in this case, acarology, anthropology, entomology and soil science – is especially powerful for real case investigations. In this PhD thesis we have not only successfully shown the importance of soil in forensic science, but also paved the way for additional forensic markers to estimate the PMI.