Biotic homogenization and differentiation of soil faunal communities in the production forest landscape: taxonomic and functional perspectives.

Biotic homogenization has been reported worldwide. Although simplification of communities across space is often significant at larger scales, it could also occur at the local scale by changing biotic interactions. This study aimed to elucidate local community processes driving biotic homogenization of soil faunal communities, and the possibility of biotic re-differentiation. We recorded species of oribatid mites in litter and soil layers along a gradient of forest conversion from monoculture larch plantation to primary forests in central Japan. We collected data for functional traits of the recorded species to quantify functional diversity. Then we quantified their taxonomic/functional turnover. Litter diversity was reduced in the larch-dominated stands, leading to habitat homogenization. Consequently, litter communities were biologically homogenized and differentiated in the plantations and in the natural forest, respectively. Turnover of functional traits for litter communities was lower and higher than expected by chance in the plantations and in the natural stand, respectively. This result suggests that the dominant assembly process shifts from limiting similarity to habitat filtering along the forest restoration gradient. However, support for such niche-based explanations was not observed for communities in the soil layer. In the monocultures, functional diversity expected from a given regional species pool significantly decreased for litter communities but not for those in the soil layer. Such discrepancy between communities in different layers suggests that communities more exposed to anthropogenic stresses are more vulnerable to the loss of their functional roles. Our study explains possible community processes behind the observed patterns of biological organization, which can be potentially useful in guiding approaches for restoring biodiversity.

Concordance and discordance between taxonomic and functional homogenization: responses of soil mite assemblages to forest conversion.

The compositional characteristics of ecological assemblages are often simplified; this process is termed "biotic homogenization." This process of biological reorganization occurs not only taxonomically but also functionally. Testing both aspects of homogenization is essential if ecosystem functioning supported by a diverse mosaic of functional traits in the landscape is concerned. Here, we aimed to infer the underlying processes of taxonomic/functional homogenization at the local scale, which is a scale that is meaningful for this research question. We recorded species of litter-dwelling oribatid mites along a gradient of forest conversion from a natural forest to a monoculture larch plantation in Japan (in total 11 stands), and collected data on the functional traits of the recorded species to quantify functional diversity. We calculated the taxonomic and functional ß-diversity, an index of biotic homogenization. We found that both the taxonomic and functional ß-diversity decreased with larch dominance (stand homogenization). After further deconstructing ß-diversity into the components of turnover and nestedness, which reflect different processes of community organization, a significant decrease in the response to larch dominance was observed only for the functional turnover. As a result, there was a steeper decline in the functional ß-diversity than the taxonomic ß-diversity. This discordance between the taxonomic and functional response suggests that species replacement occurs between species that are functionally redundant under environmental homogenization, ultimately leading to the stronger homogenization of functional diversity. The insights gained from community organization of oribatid mites suggest that the functional characteristics of local assemblages, which support the functionality of ecosystems, are of more concern in human-dominated forest landscapes.

Evolution of a key trait greatly affects underground community assembly process through habitat adaptation in earthworms.

Underground community assemblies have not been studied well compared with aboveground communities, despite their importance for our understanding of whole ecosystems. To investigate underground community assembly over evolutionary timescales, we examined terrestrial earthworm communities (Oligochaeta: Haplotaxida) in conserved mountainous primary forests in Japan as a model system. We collected 553 earthworms mostly from two dominant families, the Megascolecidae and the Lumbricidae, from 12 sites. We constructed a molecular taxonomic unit tree based on the analysis of three genes to examine the effects of a biogeographic factor (dispersal ability) and an evolutionary factor (habitat adaptation) on the earthworm community assembly process. The phylogenetic distance of the earthworm communities among sites was positively correlated with geographic distance when intraspecific variation was included, indicating that the divergence within species was affected by biogeographic factors. The community assembly process in the Megascolecidae has also been affected by environmental conditions in relation to an evolutionary relationship between habitat environment and intestinal cecum type, a trait closely related to habitat depth and diet, whereas that in the Lumbricidae has not been affected as such. Intestinal cecum type showed a pattern of niche conservatism in the Megascolecidae lineage. Our results suggest that investigating the evolution of a key trait related to life history can lead to the clear description of community assembly process over a long timescale and that the community assembly process can differ greatly among related lineages even though they live sympatrically.

Changes in radiocesium concentrations in epigeic earthworms in relation to the organic layer 2.5 years after the 2011 Fukushima Dai-ichi Nuclear Power Plant accident.

We reported previously that radiocesium ((137)Cs) concentrations in earthworms increased with those in litter and/or soil in Fukushima Prefecture forests 0.5 y after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. This study provides further results for 1.5 and 2.5 y after the accident and discusses temporal changes in (137)Cs concentrations and transfer factors (TF) from litter to earthworms to better understand the mechanisms by which (137)Cs enters soil food webs. The concentration of (137)Cs in accumulated litter on the forest floor rapidly decreased, and the concentration in soil (0-5-cm depth) increased over time from 0.5 to 1.5 y, but changed only moderately from 1.5 to 2.5 y. The concentration of (137)Cs in earthworms consistently decreased during the study period; values 2.5 y after the accident were 18.8-68.5% of those 0.5 y after the accident. The TFs from accumulated litter to earthworms decreased over time: 0.24 ± 0.08 (mean ± SD) at 0.5 y and 0.16 ± 0.04 at 2.5 y. This decrease may be a result of decreases in the bioavailability of (137)Cs in litter and the surface soil layer. Changes in (137)Cs bioavailability should be continuously tracked to determine any changes in the relationship between radiocesium concentrations in earthworms and that in accumulated litter or soil.

Radiocesium concentrations in epigeic earthworms at various distances from the Fukushima Nuclear Power Plant 6 months after the 2011 accident.

We investigated the concentrations of radiocesium in epigeic earthworms, litter, and soil samples collected from forests in Fukushima Prefecture 6 months after the Fukushima Dai-ichi Nuclear Power Plant accident in 2011. Radiocesium concentrations in litter accumulated on the forest floor were higher than those in the soil (0-5 cm depth). The highest average (134+137)Cs concentrations in earthworms (approximately 19 Bq g(-1) of wet weight with gut contents and 108 Bq g(-1) of dry weight without gut contents) were recorded from a plot that experienced an air dose rate of 3.1 µSv h(-1), and earthworm concentrations were found to increase with litter and/or soil concentrations. Average (134)Cs and (137)Cs concentrations (with or without gut contents) were intermediate between accumulated litter and soil. Different species in the same ecological groups on the same plots had similar concentrations because of their use of the same habitats or their similar physiological characteristics. The contribution of global fallout (137)Cs to earthworms with gut contents was calculated to be very low, and most (137)Cs in earthworms was derived from the Fukushima accident. Transfer factors from accumulated litter to earthworms, based on their dry weights, ranged from 0.21 to 0.35, in agreement with previous field studies.

Variability of sexual organ possession rates and phylogenetic analyses of a parthenogenetic Japanese earthworm, Amynthas vittatus (Oligochaeta: Megascolecidae).

Although earthworms are hermaphroditic animals with biparental sexual reproduction, some parthenogenetic species have been found. Evolutionary trends in parthenogenetic earthworms revealed a reduction in the reproductive organs. To clarify the phylogenetic relationships of parthenogenetic earthworms with different degree of degraded reproductive organs, we conducted a morphological analysis of the reproductive organs and molecular phylogenetic analyses of Amynthas vittatus which usually degraded a part of reproductive organs. Morphological analysis revealed that almost all individuals collected around Mt. Aobayama, Sendai city of northeastern Japan, possessed male pores, while individuals collected from areas located across Hirose River did not. Phylogenetic analysis using mitochondrial DNA sequences of 48 individuals representing 20 populations indicated that almost all individuals collected around Mt. Aobayama belonged to a different lineage from the other populations collected around Sendai, and that almost all individuals collected from across Japan belonged to the latter lineage. We suggest that the difference in the male pore possession rate was caused by histories of each population, but the A. vittatus population found on Mt. Aobayama belongs to a different lineage as compared to the other Japanese populations and not the primitive population. Thus, the parthenogenetic earthworm A. vittatus has undergone at least two morphological evolutionary processes.