Genetic lineages and ecological gradients co-determine the trophic niches of earthworms

Trophic niche partitioning is a fundamental mechanism explaining species coexistence and diversity. Trophic niches vary with elevational gradients, but the consequences of genetic divergence for trophic niche shifts are little studied in particular for belowground communities. Many earthworm species comprise various genetic lineages, however, the trophic niche variations of these co-existing lineages under gradually changing environmental conditions is still unknown. This study barcoded 280 individuals of earthworm Eisenia nordenskioldi sampled from an elevational gradient (800–1700 m) on Changbai Mountain in northeastern China and divided them into two lineages (i.e., large and small bodied). Using normalized stable isotope ratios (Δ), we evaluated the trophic niche shifts of the two distinct lineages across seven elevations. The results showed that genetic lineage and elevation together explained the variations of isotopic compositions (72.9% for Δ¹³C and 95.4% for Δ¹⁵N). Ignoring the elevations, the two lineages showed no significant difference in their trophic niches (P > 0.05) with a 27% of overlap; while when they co-occurred at certain elevations, the two lineages overlapped by 7–24% in trophic niches, and showed significant differences. The trophic niche sizes of two lineages changed significantly but in an opposite way with elevations. The significant differences in trophic niches indicated that the two lineages had different physiologies, supporting their status as cryptic species. For both lineages, the Δ¹³C values showed a hump-shaped relationship with body mass, while the Δ¹⁵N values decreased linearly with body mass; litter C/N ratio and soil moisture explained variations of Δ¹³C values, and litter C/N ratio and soil pH explained the variations of Δ¹⁵N values. Overall, the findings highlight the power of integrating the genetic and ecological information and the novel fact that intraspecific variations in trophic traits can be of similar importance to those among different species when evaluating the responses to environmental changes.