Community structure, trophic position and reproductive mode of soil and bark-living oribatid mites in an alpine grassland ecosystem

Abstract

The community structure, stable isotope ratios ((15)N/(14)N, (13)C/(12)C) and reproductive mode of oribatid mites (Acari, Oribatida) were investigated in four habitats (upper tree bark, lower tree bark, dry grassland soil, forest soil) at two sites in the Central Alps (Tyrol, Austria). We hypothesized that community structure and trophic position of oribatid mites of dry grassland soils and bark of trees are similar since these habitats have similar abiotic characteristics (open, dry) compared with forest soil. Further, we hypothesized that derived taxa of oribatid mites reproducing sexually dominate on the bark of trees since species in this habitat consume living resources such as lichens. In contrast to our hypothesis, the community structure of oribatid mites differed among grassland, forest and bark indicating the existence of niche differentiation in the respective oribatid mite species. In agreement with our hypothesis, sexually reproducing taxa of oribatid mites dominated on the bark of trees whereas parthenogenetic species were more frequent in soil. Several species of bark-living oribatid mites had stable isotope signatures that were similar to lichens indicating that they feed on lichens. However, nine species that frequently occurred on tree bark did not feed on lichens according to their stable isotope signatures. No oribatid mite species could be ascribed to moss feeding. We conclude that sexual reproduction served as preadaptation for oribatid mites allowing them to exploit new habitats and new resources on the bark of trees. Abiotic factors likely are of limited importance for bark-living oribatid mites since harsh abiotic conditions are assumed to favor parthenogenesis.

Fig. 1

Mean (±SD) of ¹³C and ¹⁵N values of oribatid mite species and potential resources (underlined) in Fliess (Austria) (filled diamond forest soil, cross grassland soil, filled triangle lower bark, filled square upper bark). Oribatid mite species and resources from the forest soil are shown in black, those from the grassland in green, those from the lower bark in orange and those from the upper bark in red. See Table 1 for full names of species and number of replicates

Fig. 2

Mean (±SD) of ¹³C and ¹⁵N values of oribatid mite species and potential resources (underlined) in Kaunerberg (Austria) (filled diamond forest soil, cross grassland soil, filled triangle lower bark, filled square upper bark). Oribatid mite species and resources from the forest soil are shown in black, those from the grassland in green, those from the lower bark in orange and those from the upper bark in red. See Table 1 for full names of species and number of replicates

Fig. 3

Relative abundance of six groups of oribatid mites at the four habitats (forest soil, grassland soil, lower bark, upper bark). Ptyctima were not included since their densities were low and did not differ between the four habitats. Data were arc-sin-square-root transformed prior to the analysis. Bars sharing the same letter are not significantly different (Tukey’s HSD test; P > 0.05)

Fig. 4

Discriminant function analysis (DFA) of oribatid mite communities from forest soil, grassland soil, upper bark and lower bark. All habitats differed significantly from each other except grassland and forest soil (for details see text)

Fig. 5

Principal components analysis (PCA) of oribatid mites of the forest soil, grassland soil, upper bark and lower bark. The four habitats were included as supplementary variables. Data were log-transformed prior to the analysis. See Table 1 for full names of species. Oppiidae, Quadroppiidae, Suctobelbidae and Brachychthoniidae were not determined to species level. Eigenvalues of axes one and two were 0.42 and 0.16, respectively

Fig. 6

Relative abundance of parthenogenetic individuals of oribatid mites of the forest soil, grassland soil, upper bark and lower bark. Data were arc-sin-square-root transformed prior to the analysis. Bars sharing the same letter are not significantly different (Tukey’s HSD test; P > 0.05)