The mid-domain effect in ectomycorrhizal fungi: range overlap along an elevation gradient on Mount Fuji, Japan

Abstract

Mid-domain effect (MDE) models predict that the random placement of species' ranges within a bounded geographical area leads to increased range overlap and species richness in the center of the bounded area. These models are frequently applied to study species-richness patterns of macroorganisms, but the MDE in relation to microorganisms is poorly understood. In this study, we examined the characteristics of the MDE in richness patterns of ectomycorrhizal (EM) fungi, an ecologically important group of soil symbionts. We conducted intensive soil sampling to investigate overlap among species ranges and the applicability of the MDE to EM fungi in four temperate forest stands along an elevation gradient on Mount Fuji, Japan. Molecular analyses using direct sequencing revealed 302 EM fungal species. Of 73 EM fungal species found in multiple stands, 72 inhabited a continuous range along the elevation gradient. The maximum overlap in species range and the highest species richness occurred at elevations in the middle of the gradient. The observed richness pattern also fit within the 95% confidence interval of the mid-domain null model, supporting the role of the MDE in EM fungal richness. Deviation in observed richness from the mean of the mid-domain null estimation was negatively correlated with some environmental factors, including precipitation and soil C/N, indicating that unexplained richness patterns could be driven by these environmental factors. Our results clearly support the existence of microbial species' ranges along environmental gradients and the potential applicability of the MDE to better understand microbial diversity patterns.

Figure 1

Observed richness (solid line) and the 95% confidence limits computed by the mid-domain null model based on 50 000 simulations (dotted lines). Error bars indicate the 95% confidence interval of observed richness, computed by EstimateS. Quadratic regression models were fitted to the richness and confidence limits.

Figure 2

Occurrence overlap of ectomycorrhizal fungal species across sites. (a) Cumulative number of fungal species, from Sites 1 to 4. Values in the columns are the numbers of species. Open columns indicate site-specific species and closed columns indicate site-shared species between adjacent site pairs. Shaded columns at the right end indicate two species that were found across three adjacent sites and one species found between non-adjacent sites (Sites 2 and 4). (b) Frequency of occurrence in each site. Columns correspond to (a), but values in the columns are relative frequency of occurrence in each site (values for the short columns are presented beside the columns or not shown for clarity). X axis is the cumulative frequency across sites. Supplementary Table S1 provides detailed information on species taxa and occurrence. Cg is excluded.

Figure 3

Non-metric multidimensional scaling plot showing similarity in EM fungal composition by site and host. Hosts represented by <5 soil samples were excluded from the analyses. Stress=0.017. Open circles represent Pinaceae, gray circles represent Fagaceae and black circles represent Betulaceae. Host genera are Fagus (F), Quercus (Q), Betula (B), Carpinus (C), Abies (A), Tsuga (T), Larix (L) and Pinus (P). The number following each genus indicates the sites.