Thibaud Decaëns

5 - Earthworms as Key Actors in Self-Organized Soil Systems

This chapter describes the drilosphere, which is defined as an earthworm population, all the biogenic structures that they build in the soil, and the communities of smaller organisms that inhabit these structures. The chapter shows that drilospheres have all the characteristics of self-organizing systems. It also discusses the theoretical and practical meaning of this organization. The chapter presents the interactions with micro-organisms and other inhabitants of the drilosphere that allow earthworms to derive energy from decomposing organic matter. The chapter describes the physical domains created in soils by the accumulation and spatial array of biogenic structures (earthworm casts, galleries, voids, and middens), addressing the effects of this system on the soil environment and the possible positive feedbacks provided in return to earthworms and inhabitants of the drilosphere. It also discusses how effective management of the drilospheres relates to sustained provision of soil ecosystem services, such as water infiltration and storage, C-sequestration, and nutrient cycling.

Stay Out (Almost) All Night: Contrasting Responses in Flight Activity Among Tropical Moth Assemblages

Variations in diel activity among hyperdiverse tropical communities of moths, despite representing a key component of niche partitioning between species, have barely been studied so far. Using light trapping from dawn to sunset over a 1-year period in French Guiana, we investigated these variations within and between two families of moths (Sphingidae and Saturniidae). Our results revealed contrasting patterns in flight activity at night between Sphingidae and Saturniidae. Sphingidae reached their peak in species richness and abundance between 7:00 and 8:00 p.m., followed by a decrease around 11:00 p.m. to midnight, whereas Saturniidae were continuously present throughout the night, with a peak around midnight. In addition, we found changes in diel activity among some of the most common genera in each family, highlighting distinct behavioral, physiological, and functional traits among taxa. Given differences in flight activity at different taxonomic levels, it is strongly recommended to monitor by light trapping throughout the night to effectively sample saturniid and sphingid assemblages, even though the activity of Sphingidae sharply declines after midnight. These results improve the general natural history information of tropical moths and reinforce the need of further research on the ecological and taxonomic consequences of differences in diel activity.

Dissecting the multi-scale spatial relationship of earthworm assemblages with soil environmental variability

BackgroundStudying the drivers and determinants of species, population and community spatial patterns is central to ecology. The observed structure of community assemblages is the result of deterministic abiotic (environmental constraints) and biotic factors (positive and negative species interactions), as well as stochastic colonization events (historical contingency). We analyzed the role of multi-scale spatial component of soil environmental variability in structuring earthworm assemblages in a gallery forest from the Colombian “Llanos”. We aimed to disentangle the spatial scales at which species assemblages are structured and determine whether these scales matched those expressed by soil environmental variables. We also tested the hypothesis of the “single tree effect” by exploring the spatial relationships between root-related variables and soil nutrient and physical variables in structuring earthworm assemblages. Multivariate ordination techniques and spatially explicit tools were used, namely cross-correlograms, Principal Coordinates of Neighbor Matrices (PCNM) and variation partitioning analyses.ResultsThe relationship between the spatial organization of earthworm assemblages and soil environmental parameters revealed explicitly multi-scale responses. The soil environmental variables that explained nested population structures across the multi-spatial scale gradient differed for earthworms and assemblages at the very-fine- (<10xa0m) to medium-scale (10–20xa0m). The root traits were correlated with areas of high soil nutrient contents at a depth of 0–5xa0cm. Information on the scales of PCNM variables was obtained using variogram modeling. Based on the size of the plot, the PCNM variables were arbitrarily allocated to medium (>30xa0m), fine (10–20xa0m) and very fine scales (<10xa0m). Variation partitioning analysis revealed that the soil environmental variability explained from less than 1% to as much as 48% of the observed earthworm spatial variation.ConclusionsA large proportion of the spatial variation did not depend on the soil environmental variability for certain species. This finding could indicate the influence of contagious biotic interactions, stochastic factors, or unmeasured relevant soil environmental variables.