Gut shuttle service: endozoochory of dispersal-limited soil fauna by gastropods

Community metabarcoding reveals the relative role of environmental filtering and spatial processes in metacommunity dynamics of soil microarthropods across a mosaic of montane forests

Disentangling the relative role of environmental filtering and spatial processes in driving metacommunity structure across mountainous regions remains challenging, as the way we quantify spatial connectivity in topographically and environmentally heterogeneous landscapes can influence our perception of which process predominates. More empirical data sets are required to account for taxon- and context-dependency, but relevant research in understudied areas is often compromised by the taxonomic impediment. Here we used haplotype-level community DNA metabarcoding, enabled by stringent filtering of amplicon sequence variants (ASVs), to characterize metacommunity structure of soil microarthropod assemblages across a mosaic of five forest habitats on the Troodos mountain range in Cyprus. We found similar β diversity patterns at ASV and species (OTU, operational taxonomic unit) levels, which pointed to a primary role of habitat filtering resulting in the existence of largely distinct metacommunities linked to different forest types. Within-habitat turnover was correlated to topoclimatic heterogeneity, again emphasizing the role of environmental filtering. However, when integrating landscape matrix information for the highly fragmented Quercus alnifolia habitat, we also detected a major role of spatial isolation determined by patch connectivity, indicating that stochastic and niche-based processes synergistically govern community assembly. Alpha diversity patterns varied between ASV and OTU levels, with OTU richness decreasing with elevation and ASV richness following a longitudinal gradient, potentially reflecting a decline of genetic diversity eastwards due to historical pressures. Our study demonstrates the utility of haplotype-level community metabarcoding for characterizing metacommunity structure of complex assemblages and improving our understanding of biodiversity dynamics across mountainous landscapes worldwide.

Do mites eat and run? A systematic review of feeding and dispersal strategies

Dispersal is an important process affecting the survival of organisms and the structure and dynamics of communities and ecosystems in space and time. It is a multiphase phenomenon influenced by many internal and external factors. Dispersal syndromes can be complicated, but they are vital to our knowledge of the biology of any organism. We analysed dispersal ability in mites (Acariformes and Parasitiformes), a highly diverse group of wingless arthropods, taking into consideration various modes of dispersal, feeding strategies, body size and the number of articles published for each species. Based on 174 articles summarized for this study, it appears that mites are opportunistic when it comes to dispersal, regardless of their feeding habits, and are often able to adopt several different strategies as needs arise. Moreover, we find a significant positive relationship between the amount of research effort that was put into studying a given species and the number of modes of dispersal that were described. The most salient conclusion to be drawn from this positive correlation is that additional studies are needed, especially on a broader set of mite taxa, until the aforementioned correlation is no longer demonstrably significant.

Towards an integrative understanding of soil biodiversity

Soil is one of the most biodiverse terrestrial habitats. Yet, we lack an integrative conceptual framework for understanding the patterns and mechanisms driving soil biodiversity. One of the underlying reasons for our poor understanding of soil biodiversity patterns relates to whether key biodiversity theories (historically developed for aboveground and aquatic organisms) are applicable to patterns of soil biodiversity. Here, we present a systematic literature review to investigate whether and how key biodiversity theories (species-energy relationship, theory of island biogeography, metacommunity theory, niche theory and neutral theory) can explain observed patterns of soil biodiversity. We then discuss two spatial compartments nested within soil at which biodiversity theories can be applied to acknowledge the scale-dependent nature of soil biodiversity.

Taxi drivers: the role of animals in transporting mycorrhizal fungi

Dispersal of mycorrhizal fungi via animals and the importance for the interacting partners' life history as well as for ecosystems is an understudied topic. In this review, we describe the available evidence and the most important knowledge gaps and finally suggest ways to gain the missing information. So far, 33 articles have been published proving a successful transfer of mycorrhizal propagules by animals. The vast majority of research on invertebrates was focused on arbuscular mycorrhizal (AM) fungi, whereas papers on vertebrates (mainly rodents and artiodactyls) equally addressed ectomycorrhizal (ECM) and AM fungi. Effective dispersal has been mostly shown by the successful inoculation of bait plants and less commonly by spore staining or germination tests. Based on the available data and general knowledge on animal lifestyles, collembolans and oribatid mites may be important in transporting ECM fungal propagules by ectozoochory, whereas earthworms, isopods, and millipedes could mainly transfer AM fungal spores in their gut systems. ECM fungal distribution may be affected by mycophagous dipterans and their hymenopteran parasitoids, while slugs, snails, and beetles could transport both mycorrhizal groups. Vertebrates feeding on fruit bodies were shown to disperse mainly ECM fungi, while AM fungi are transported mostly accidentally by herbivores. The important knowledge gaps include insufficient information on dispersal of fungal propagules other than spores, the role of invertebrates in the dispersal of mycorrhizal fungi, the way in which propagules pass through food webs, and the spatial distances reached by different dispersal mechanisms both horizontally and vertically.

Under pressure: force resistance measurements in box mites (Actinotrichida, Oribatida)

Background

Mechanical defenses are very common and diverse in prey species, for example in oribatid mites. Here, the probably most complex form of morphological defense is known as ptychoidy, that enables the animals to completely retract the appendages into a secondary cavity and encapsulate themselves. The two groups of ptychoid mites constituting the Ptyctima, i.e. Euphthiracaroidea and Phthiracaroidea, have a hardened cuticle and are well protected against similar sized predators. Euphthiracaroidea additionally feature predator-repelling secretions. Since both taxa evolved within the glandulate group of Oribatida, the question remains why Phthiracaroidea lost this additional protection. In earlier predation bioassays, chemically disarmed specimens of Euphthiracaroidea were cracked by the staphylinid beetle Othius punctulatus, whereas equally sized specimens of Phthiracaroidea survived. We thus hypothesized that Phthiracaroidea can withstand significantly more force than Euphthiracaroidea and that the specific body form in each group is key in understanding the loss of chemical defense in Phthiracaroidea. To measure force resistance, we adapted the principle of machines applying compressive forces for very small animals and tested the two ptyctimous taxa as well as the soft-bodied mite Archegozetes longisetosus.

Results

Some Phthiracaroidea individuals sustained about 560,000 times their body weight. Their mean resistance was about three times higher, and their mean breaking point in relation to body weight nearly two times higher than Euphthiracaroidea individuals. The breaking point increased with body weight and differed significantly between the two taxa. Across taxa, the absolute force resistance increased sublinearly (with a 0.781 power term) with the animal's body weight. Force resistance of A. longisetosus was inferior in all tests (about half that of Euphthiracaroidea after accounting for body weight). As an important determinant of mechanical resistance in ptychoid mites, the individuals' cuticle thickness increased sublinearly with body diameter and body mass as well and did not differ significantly between the taxa.

Conclusion

We showed the feasibility of the force resistance measurement method, and our results were consistent with the hypothesis that Phthiracaroidea compensated its lack of chemical secretions by a heavier mechanical resistance based on a different body form and associated build-up of hemolymph pressure (defensive trade-off).

Slow-moving soil organisms on a water highway: aquatic dispersal and survival potential of Oribatida and Collembola in running water

BACKGROUND

Oribatida and Collembola are an important part of the soil food web and increase soil fertility by contributing to the recycling of nutrients out of dead organic matter. Active locomotion enables only limited dispersal in these tiny, wingless arthropods, while passive dispersal plays an important role for long-distance dispersal. Previous investigations have focused on passive transport by wind, other animals, or sea currents, whereas studies on transport via running water are missing. However, previous observation of the long survival of submerged terrestrial microarthropods makes passive dispersal with running water very likely.

METHODS

By combining field and lab experiments, we studied the potential for passive dispersal of oribatid mites with running water. We investigated terrestrial Oribatida and Collembola: (1) along a stream taking soil and moss samples, (2) in a stream using sticky covers and aquarium nets, and (3) studied their ability to colonise new soil after aquatic transport with the help of floating islands. Furthermore, we investigated the survival of submerged Oribatida species and their floating capabilities in lab experiments to predict dispersal distances with running water. We tested for differences between species using Kruskal-Wallis test for equal medians and Mann-Whitney pairwise-comparison and χ²-test for the influence of body size on aquatic dispersal.

RESULTS

Soil and moss samples revealed a pool of 52 oribatid mite species at the stream bank. Within the stream, we caught 180 individuals from 36 oribatid mite species. Only 14 of those species were also found in the soil and moss samples, whereas the remaining 22 were of unknown origin. Based on material caught on sticky covers, an average of 63.9 (± 54.6) oribatid mite individuals fell on one m² stream water per week. Four species of Collembola (27 individuals) and 21 species of oribatid mites (47 individuals) were collected with aquarium nets. Eight microarthropod species (Oribatida + Collembola) successfully colonised new soil in the floating islands after aquatic dispersal. Lab experiments showed that Oribatida can float for at least 14 hours at the surface of running water and may survive for more than 365 days when submerged. The floating abilities and survival rates were largely species-specific.

CONCLUSION

This is the first study to demonstrate successful passive dispersal with running water for two groups of terrestrial soil microarthropods, including subsequent colonisation of new soil. We show that submersion survival, as well as floating abilities, and therefore dispersal capability, are not only high in oribatid mites, but also species-specific. Running waters obviously serve as long-distance dispersal highways for many of these less mobile soil-living animals.

Land Use Influences Antibiotic Resistance in the Microbiome of Soil Collembolans Orchesellides sinensis

Numerous studies have investigated the composition and diversity of antibiotic resistance genes (ARGs) in multiple environments but the pattern of ARGs in field-collected soil fauna remains poorly understood. In the present study soil collembolans were collected from six sites with three different land use types (parkway land, park land, and arable land) and 285 ARGs and 10 mobile genetic elements (MGEs) in the microbiome of these "wild" collembolans were quantified by high-throughput quantitative PCR. A total of 76 unique ARGs and 5 MGEs were detected. There were significant differences between collection sites in the antibiotic resistome in the collembolans. Land use significantly altered the distribution patterns of collembolan ARGs. Thirty shared ARGs and three shared MGEs were identified. The co-occurrences of shared resistomes were largely random, and more positive relationships were found in the coassociation network. Partial redundancy analysis confirms that the changes in bacterial communities explained 27.77% of the variation in ARGs. These findings suggest that resistance genes are pervasive in the microbiome associated with the field collembolan and the activity of the collembolans may contribute to the spread and dissemination of resistance genes in the environment, an aspect of ARGs that has until now been largely overlooked.