Genetic Lineages and Ecological Gradients Co-Determine The Trophic Niches Of Earthworms

Abstract

Trophic niche partitioning of soil fauna is a fundamental mechanism explaining their coexistence and diversity. Many earthworm species comprise a number of cryptic species, but the trophic niche partitioning between co-existing cryptic species and under gradually changing environmental conditions is not known. This study barcoded 280 individuals of Eisenia nordenskioldi sampled from an elevation gradient (800–1700 m altitudes) on Changbai Mountain in northeastern China and divided them into two cryptic species (i.e., large and small bodied). Using natural abundance stable isotope ratio measurements (δ13C and δ15N), it evaluated the trophic niche shifts of the two cryptic species across elevation gradients. The results showed that the pronounced trophic niche differentiation occurred and the percentages of overlapping area ranged 7–24% when coexisting. Isotopic values closely correlated with body mass (R2=0.32 and 0.19 for 13C; R2=0.90 and 0.76 for 15N) underlining the importance of traits. Further, trophic niche breadth of large species was as appropriately twice as that of small species at low altitudes (800 and 950 m), comparable at middle altitudes (1100 and 1250 m), and less than half at high altitudes (1400, 1550 and 1700 m). The trophic positions changed with altitudes, primarily related to changes in litter quality (C/N ratio) and soil properties (i.e., pH and moisture). Overall, the findings highlight the integration of the genetic and ecological information and provide novel insight for trophic niche shifts in soil fauna. It calls for the intraspecific variations in traits may be of similar importance to those among different species, which needs to be further expanded to other taxa of soil fauna when evaluating the responses to the environmental gradients.

Keywords: soil fauna, ecosystem engineers, species coexistence, stable isotope ratio analysis, niche differentiation