Density and community structure of soil- and bark-dwelling microarthropods along an altitudinal gradient in a tropical montane rainforest

Abstract

Microarthropod communities in the soil and on the bark of trees were investigated along an elevation gradient (1,850, 2,000, 2,150, 2,300 m) in a tropical montane rain forest in southern Ecuador. We hypothesised that the density of microarthropods declines with depth in soil and increases with increasing altitude mainly due to the availability of resources, i.e. organic matter. In addition, we expected bark and soil communities to differ strongly, since the bark of trees is more exposed to harsher factors. In contrast to our hypothesis, the density of major microarthropod groups (Collembola, Oribatida, Gamasina, Uropodina) was generally low and decreased with altitude. However, as we predicted the density of each of the groups decreased with soil depth. Density of microarthropods on tree bark was lower than in soil. Overall, 43 species of oribatid mites were found, with the most abundant higher taxa being Poronota, pycnonotic Apheredermata, Mixonomata and Eupheredermata. The oribatid mite community on bark did not differ significantly from that in soil. The number of oribatid mite species declined with altitude (24, 23, 17 and 13 species at 1,850, 2,000, 2,150 and 2,300 m, respectively). Rarefaction curves indicate that overall about 50 oribatid mite species are to be expected along the studied altitudinal gradient. Results of this study indicate (1) that microarthropods may be limited by the quality of resources at high altitudes and by the amount of resources at deeper soil layers, and (2) that the bark of trees and the soil are habitats of similar quality for oribatid mites.

Fig. 1

Microarthropod densities in the three soil layers at the four different altitudes (a), and density of the four microarthropod taxa in the three soil horizons in a tropical montane rain forest in Southern Ecuador (b). For Fig. 1a the densities of all microarthropod taxa were summed up; for Fig. 1b the mean of the four altitudes in given. Bars and uneven standard errors represent back-transformed means of log-transformed data (for “Statistical analysis” see text)

Fig. 2

Density of a Oribatida, b Collembola, c Gamasina and d Uropodina in soil and on the bark of trees at four altitudes (1,850, 2,000, 2,150, 2,300 m) in a tropical montane rain forest in southern Ecuador. Bars and uneven standard errors represent back-transformed means of log-transformed data (for “Statistical analysis” see text). Note the different scales

Fig. 3

Principal component analysis (PCA) of oribatid mites at four different altitudes (1,850, 2,000, 2,150, 2,300 m) in soil. PCA was carried out with log-transformed data that were calibrated for density. The altitude was included as a passive variable. Eigenvalues of axes one and two were 0.38 and 0.19 respectively

Fig. 4

Sample-based rarefaction curves and corresponding estimators of soil and bark living oribatid mites. ICE, incidence-based coverage estimator; MMMean, asymptote of Michaelis–Menten curve estimated from the sample-based rarefaction curve; Jack 2 Mean, second-order Jackknife richness estimator. Sobs, observed species richness; Singletons, number of species present in only one samples; Doubletons, number of species present in exactly two samples