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The extremely high diversity of Collembola in relict forests of Primorskii Krai of Russia

Abstract Background The diversity of soil animals of relict forest ecosystems in East Asia continues to be insufficiently studied and almost not represented in international databases, including GBIF. This article is based on 7550 records of 175 species which were collected in Ussuriiskii, Sikhote-Alinskii and Kedrovaya Pad’ natural reserves of Russian Far East in 2016–2017. A multi-scale sampling design allowed us to estimate population densities and local species richness of Collembola at areas of different sizes. The work continues the digitization of the collections of the Moscow Pedagogical State University (MPSU) and their publication through GBIF.org, which began in 2019. This article is based on original data including 2377 specimens of springtails from eight forests and 648 soil cores. New information Within the framework of modern taxonomy, this work represents the first publication of lists of Collembola species of forests of Primorsky Krai. The work focused on the relict protected cedar-deciduous forests. Nine species new to science were described and data on the fauna of the region were significantly revised. Considerable contribution was made to the biogeography of Collembola of East Asia. The design of the sampling allowed us to draw conclusions about the species saturation of springtails at various spatial scales within the habitat: from a few square cm to 100 sq. m. Number of species reached record high values reflecting the benchmark state of Collembola communities of undisturbed old temperate forest ecosystems.


Introduction
Collembola, or springtails, is one of the most abundant and diverse groups of soil microarthropods that play an important role in the processes of destruction of organic residues (Petersen andLuxton 1982, Hopkin 1997). We aimed to estimate the upper limit of the local species diversity of Collembola in a region known for its high species richness of various groups of organisms (Latham and Ricklefs 1993). Part of this region is located in the south of the Russian Far East, where broad-leaf -cedar pine forest ecosystems are distributed. The highest diversity of insects in the temperate zone is described for these forests (Chernov et al. 2011). UNESCO included this area in the World Heritage List (Central Sikhote-Alin) noting that "the combination of glacial history, climate and relief has allowed the development of the richest and most unusual temperate forests in the world". Regular research of Collembola in the south of the Primorskii Krai of Russia began about half a century ago showing a great originality of the fauna (e.g. Kutyreva 1979, Kutyreva 1984, Martynova 1988. During the last decades, new approaches to taxonomy have led to the taxonomic revision of many genera of springtails (Pomorski and Sveenkova 2006, Deharveng et al. 2011, Jie et al. 2011, Jordana et al. 2011, Huang and Potapov 2012, Smolis et al. 2012. However, these works did not focus on the issues of local species richness. We collected springtails in the region in the years 2016-2017. The data from 2016 was published ). However, the species list was limited to abundant species only. The present study includes the full species list, based on samples from 2016 and 2017 taking into account the recently described new species for science. A specific multi-scale sampling design allowed us to estimate a local species richness of Collembola at areas of different sizes.

General description
Purpose: The purpose of the paper is to present information on species composition and abundances of Collembola in relict broad-leaf -cedar pine forest ecosystems of the Russian Far East. Diversity and аbundance are presented in the most detailed form of sampling-evidence.

Sampling methods
Study extent: The dataset  provides information on the number of individuals of springtail species collected in broad-leaf -cedar pine forests of three natural reserves in July 2016 and August 2017 (7550 occurrences). Three forests were examined in Ussuriiskii natural reserve, 2 -in Kedrovaya Pad', 2 -in Sikhote-Alinskii natural reserve and 1 -in Chuguevsky District. The sampling plots "Fauri" and "Grabovaya" were in mountain forest, plots "Turova" and "Kedrovaya" were on a slope, plots "Pikhtovaya", "Kema", "Chuguev" and "Anikin" were in the river valleys. The material includes about 24 thousands individuals of 175 species from eight sampling series and 648 soil cores. They were collected by Natalia Kuznetsova, Mikhail Potapov, Anna Geraskina, Alexander Kuprin, Anastasia Korotkevich and were identified by Natalia Kuznetsova, Mikhail Potapov, Babenko Anatoly, Shveenkova Yulia.

Sampling description:
The sampling was based on a multi-scale design to study the structure of biodiversity at different spatial scales (Lande 1996, Azovsky et al. 2000). A fractal arrangement of cores allows us to reduce sample effort because the same core is used for the analysis at different scales (Marsh and Ewers 2012). A few cases of applying the approach in soil zoology include testate amoebae (Tsyganov et al. 2014), oribatid mites (Bolger et al. 2014) and springtails (Kuznetsova and Saraeva 2018). We used the small size of the corer (8 cm in section) to implement special attention on the diversity and spatial structure of the population at the micro level. Soil was investigated down to 20 cm. A total of 81 cores were taken in each sampling plot. Cores were placed in the corners of different-scale equilateral triangles inscribed in squares with sides 10 cm, 25 cm, 1 m and 10 m. The different-scale triangles were designed following the principles of fractal geometry. The sample design is described in detail by Saraeva et al. (2015).

Extraction of Collembola from cores:
Plastic containers were used for storage and transportation of individual cores (Fig. 1). Each core was placed in a 0.3 litre plastic container with a vent hole covered with a gas cloth (Fig. 2). The containers were transported to MPGU (Moscow). Extraction of Collembola was performed in the laboratory using Tullgren's funnels at approximately 25 C. Extraction into 70% alcohol was continued for 4-5 days until the cores were completely dry.
Laboratory processing: All the specimens were mounted on slides in Phoera liquid according to a standard procedure (Ghilarov 1975, Potapov andKuznetsova 2011).
Sampling plots: Short descriptions of some sampling plots ("Turova", "Grabovaya", "Kedrovaya", "Pikhtovaya") were published . The descriptions of the other plots are given for the first time. All the plots were located on brown soil.
The sampling plot "Anikin" is in the Ussuriyskii natural reserve, Suvorovskoye forest district, Anikinsky station, valley of Anikin River; it is valley broadleaf forest (Juglans mandshurica, Populus maximowiczii, Fraxinus mandshurica) with Pinus koraiensis; in the above-soil cover are Carex L., Leptorumohra amurensis etc. The dead cover is 50-90% and the thickness of the litter is 3-4 cm. Quality control: We used both modern taxonomic papers and keys (Martynova 1988, Babenko et al. 1994, Potapov 2001 for the taxonomic determination of Collembola. The material was checked by leading experts in taxonomy of Collembola. Scientific names were checked using the GBIF species matching tool. Step description: Data on species were digitised, standardised according to the Darwin Core (Wieczorek et al. 2012), the quality of the data was checked and errors were corrected and then published through GBIF.org .

Geographic coverage
Description: Primorskii Krai of the Russian Far East (Fig. 10).

Taxonomic coverage
Description: So far, the taxonomical knowledge of different families and genera of Collembola is highly irregular in the area under study. Our identification of particular groups of Collembola, therefore, considerably depended on taxa. The species of Neelidae, Symphypleona, Lepidocyrtinae and Entomobryini were identified, based on the appearance (body size, colour pattern, length of limbs and other easily recognisable features), other taxa -on modern taxonomy, family Tomoceridae -on traditional characters. Families Hypogastruridae, Onychiuridae and Isotomidae were identified down mostly to species level, while the genera Isotoma and Desoria still are less certain and were differentiated as morpho-species (sp. 1, sp. 2 etc). Family Odontellidae is less understood in the area and so it was mostly represented by the "sp." in the list. Some species were described as new to science in the material: Anurida -6 spp. n.

Additional information
In total, in the relict forests of the Far East, we found the highest diversity of Сollembola that has ever been observed in the ecosystems of the temperate zone and possibly the world: up to 90 species per area 10 x 10 m (sampling plot). The species saturation reaches 30 species on an area of 8 square centimetres (one core) and can exceed 60 species on 1 square metre (Fig. 11).