This study analyzes the abundance, diversity, trophic structure, ecological indices, and functional traits of nematode communities along a continentality gradient within the Moroccan Argan Forest Biosphere Reserve. A total of 130 soil samples were collected from three distinct bioclimatic zones: insular, coastal, and semi-continental.
Bacterivorous nematodes are the most abundant trophic group in all zones, followed by herbivores, fungivores, omnivores, and predators. At the trophic group level, the insular zone stands out due to a greater representation of four groups (bacterivores, herbivores, fungivores, and omnivores-predators), whose proportions are higher there than in the coastal and semi-continental zones—indicating a more diverse functional composition, which should not be confused with evenness among taxa, which follows an opposite trend (see below).
Total abundance and taxonomic richness of nematodes are significantly higher in the insular zone. In contrast, Shannon's diversity index (a measure of environmental biodiversity) and Simpson's index (a measure of dominance) show no significant differences between zones. This result suggests that the increase in richness in the insular zone is offset by a less even distribution of abundances among taxa: while the overall quantity and number of taxa decrease with continentality, the overall diversity structure remains relatively stable, whereas the relative distribution of species varies significantly.
This compensation is confirmed by Pielou's evenness index, which is significantly lower in the insular zone (0.71) than in the coastal (0.81) and semi-continental (0.77; $F = 18.73, p < 0.001$) zones. In other words, while the insular zone harbors more trophic groups and taxa, these taxa are dominated by a few highly abundant species—unlike the coastal and semi-continental zones, where the less rich communities are more evenly distributed among species.
Non-metric multidimensional scaling (NMDS) analysis reveals a significant geographical structure of trophic communities along the continentality gradient, with a clear differentiation of the coastal zone, which is characterized by high inter-site variability and a high proportion of fungivores, predators, and omnivores. The insular and semi-continental zones exhibit structures that are more similar to each other. Principal component analysis (PCA) confirms these trends: fungivores and predators contribute the most to differentiation between zones, while bacterivores, dominant everywhere, are relatively homogeneously distributed.
Community-Weighted Mean (CWM) values for body weight, which indicate the size structure of the communities, are systematically higher in the insular zone across all trophic groups; the coastal and semi-continental zones show lower and similar values. A comparable trend is observed for carbon use efficiency (CUE), which is higher in the insular zone for most groups, except for herbivores, where no significant difference is observed. Nevertheless, these CUE values remain generally low (less than 0.5) across all zones, suggesting a significant allocation of energy to respiration rather than growth—a potential sign of environmental stress, particularly thermal, or metabolic strategies adapted to local conditions.
The analysis of environmental responses indicates that nematode biomass and productivity progressively decrease with increasing continentality. The metabolic activity of the main trophic groups is highest in the insular zone, intermediate in the coastal zone, and lowest in the semi-continental zone. Temperature range emerges as a major stress factor, showing a negative correlation between temperature fluctuations and metabolic activity: herbivores are the most sensitive to this, followed by bacterivores, fungivores, and then predators.
Finally, analyses linking soil properties to nematode communities show that their abundance is favored by warm conditions and high soil moisture, as well as adequate levels of pH, nitrogen, and calcium carbonate. Conversely, high heavy metal concentrations (copper, zinc, manganese) are associated with a decrease in their abundance. A negative relationship is also observed with altitude and precipitation—a seemingly counterintuitive result given the positive link with soil moisture, which could be explained by indirect effects such as nutrient leaching at higher altitudes or hypoxic conditions caused by excess water. In contrast, community diversity and carbon use efficiency appear to be little influenced by these soil parameters.
Overall, these results confirm that increasing continentality is accompanied by a decrease in nematode abundance and richness, as well as modifications in the functional structure and ecological balances within communities—without major alteration of global diversity indices, but with a clear shift toward greater dominance of certain taxa in the insular zone.
Réf. Braimin A., Benjlil H., Filali Alaoui I. et al., 2026 - Soil Nematode-Mediated Carbon and Energy Fluxes Along a Continental Gradient in Arid Ecosystems. Soil Syst. 2026, 10, 73 - https://doi.org/10.3390/soilsystems10070073
Posted by Jean-Paul Peltier.