A preliminary checklist of soil ants (Hymenoptera: Formicidae) of Colombian Amazon

Abstract Background This paper presents an updated list of soil ants of the Colombian Amazon collected in three different river basins: the Amazon, the Caquetá and the Putumayo. The list includes 10 subfamilies, 60 genera and 218 species collected from TSBF monoliths at four different depths (Litter, 0 - 10 cm, 10 - 20 cm and 20 - 30 cm). This updated list increases considerably the knowledge of edaphic macrofauna of the region, due to the limited published information about soil ant diversity in the Colombian Amazon region. New information This is the first checklist of soil ant diversity of the Colombian Amazon region. Six new records of species for Colombia are exposed: Acropyga tricuspis (LaPolla, 2004), Typhlomyrmex clavicornis (Emery, 1906), Typhlomyrmex meire (Lacau, Villemant & Delabie, 2004), Cyphomyrmex bicornis (Forel, 1895), Megalomyrmex emeryi (Forel, 1904) and Myrmicocrypta spinosa (Weber, 1937), most of them corresponding to subterranean ants.


Introduction
In tropical forests, the abundance and diversity of ants is usually high, which brings out the importance of ants for these ecosystems (Floren and Linsenmair 2005, Floren et al. 2002, Dunn et al. 2007, Davidson et al. 2007, Jaffe et al. 2007. Ants, together with earthworms and termites, are known as "ecosystem engineers" due to the positive effect of their activity on ecosystems (Decaëns et al. 1999, Decaëns 2010, Lavelle et al. 1997, Luke et al. 2014, Griffiths et al. 2017. Physical, chemical and biological soil properties are positively affected by the presence of ant nests, chambers, galleries and mineral aggregates that ants create (Seybold et al. 1999, Barros et al. 2001, Sanabria et al. 2014, Wu et al. 2015 Santander. Different natural and anthropic land uses were included in the sampling: primary and secondary forests, young secondary forests, pastures and indigenous slashand-burn agricultural plots (Table 1)  Study area, sampling localities.

Sample collection and analysis
Soil ant collection took place between September 2015 and July 2017. Soil ants were collected using the methodology suggested by the Tropical Soil Biology and Fertility Program (TSBF) for soil macrofauna collection (Anderson and Ingram 1993). In each sampling site, a plot of 60 x 60 metres was selected. There, five monoliths of 25 x 25 x 30 cm of depth were done: one in each corner of the 60 x 60 m delimited square plot and one in the centre of it. In each monolith, macrofauna samples were collected at four depths: litter, 0 -10 cm, 10 -20 cm and 20 -30 cm. Macrofauna collection in each monolith depth was undertaken in the field manually. Recovered samples were preserved in ethanol at 75% until their arrival to the SINCHI Institute laboratories in Leticia, Colombia, where specimens were vouchered and preserved in the CATAC collection.
In the laboratory, samples were cleaned and classified into morphotypes and species. All samples were identified by using the keys of recent revisions, verifying the species with the diagnosis and in some cases comparing with photos of type material in AntWeb (Brandão 1990, Kugler 1994, De Andrade and Baroni 1999, Palacio 1999, Fernández 2003, Longino and Fernández 2007, Jiménez et al. 2008, Mackay and Mackay 2010, Ortiz and Fernández 2011, Pacheco and Mackay 2013, Lenhart et al. 2013, Ješovnik and Schultz 2017, AntWeb 2018, LaPolla 2004, Snelling and Longino 1992, Brandão 2003, Fernández et al. 2015, Lattke et al. 2007, Lattke 1997, Longino 2013, Sosa-Calvo et al. 2018, Longino 2003. Camponotus, Brachymyrmex and Pheidole were identified through the comparison of material identified by specialists and reference collection. All data were organised alphabetically by subfamily, genus and species in an ant checklist following the nomenclature suggested in the Bolton online catalogue of the ants of the world (AntCat, Bolton 2018).

Checklist of the soil ant species of Colombian Amazon
A total of 1341 specimens and 4318 individuals were analysed. From the total soil macrofauna, ants were the most abundant and species-richest organisms collected. Ants dominated litter and 0 -10 cm depths (Barros et al. 2002, Mathieu et al. 2005, Rossi et al. 2006, Velásquez et al. 2012, Suárez Salazar et al. 2015. Litter had the highest species richness with 129 species, followed by the 0 -10 cm depth with 110 species. Layers from 10 -20 cm depth and 20 -30 cm depth had 77 and 45 species, respectively, showing a decreasing ant richness structure in the soil profile with depth. The preliminary checklist of soil ants from the Colombian Amazon region (  Checklist of the soil ant species of the Colombian Amazon. The list is organised alphabetically by subfamily, genus and species. Species names in bold characters refer to species recorded for the first time in Colombia. River basins corresponded to: A = Amazon river; C = Caquetá river; P = Putumayo river. Depth of species collection: 1 = Litter; 2 = 0 -10 cm; 3 = 10 -20 cm; 4 = 20 -30 cm. Land use corresponded to PF = Primary forest; SF = Secondary forest; P = Pasture; R = Young secondary regeneration forest. Ant richness in this report is remarkable when compared with previous reports from the Amazon region. The study done by Ryder Wilkie et al. 2010, which is recognised as the most complete work on ant diversity in the Amazon region, recorded at the Tiputini Reserve in Ecuador 66 genera and more than 300 species between subsoil and canopy. The high diversity reported in our work was certainly the effect of the broad area sampled (which includes three river basins) and the wide range of altitude included (Marsh et al. 2018).
The following are new records for Colombia:  Comments. New record for Colombia. This species has been recorded in Bolivia, French Guiana, Guyana, Peru and Suriname (Type Locality) (Brandão 2003, Brandão 1990).

Discussion
The Caquetá river basin showed the highest number of soil ant species amongst basins (149 species, which corresponded to 68% of the total ants recorded), followed by the Amazon river basin (86 species, 40%) and the Putumayo river basin (71 species, 33%). From all species recorded, 89 species were exclusively registered in the Caquetá river basin, which was twice the number of species reported exclusively in the Amazon river basin (36 species) and in the Putumayo river basin (25 species). The high diversity of the Caquetá river basin may be areflection of the geographic conditions of the area and the sampling effort used there. The Caquetá river basin includes the Andean-Amazonian transition where a high turnover of species might occur, but additionally, it was the one with the greatest sampling effort.
From all the ant species recorded, 20 species were found in the four soil depths (  et al. 2009) highlighted the diversity of these genera in the region. Canopy and arboreal ant species can be an important part of the ant density in the upper strata of soil (leaf litter and the depth of 0 -10 cm) as occurred in this study where most of these ants were collected in litter. Results inferred that these ants use the soil as a way for transportation or for food provisioning, although they do not live in the soil such as ants of the genera Pheidole, Acropyga, Cryptopone or Hypoponera.
Some arboreal species of Crematogaster, Camponotus, Myrmelachista, Procryptocerus and Pseudomyrmex were found in soil deep horizons, even at 20-30 cm depth such as Azteca and Pseudomyrmex. Ant collection was done manually in the field. During this process, some arboreal ants could fall down and contaminate monolith samples when the bags were not well closed. However, arboreal ants may realistically be away from their common substrate or nest, as little is known about their biology, even more so when this is not the first time they have been recorded in soil samples (Rosumek et al. 2008, Vasconcelos et al. 2003, Delabie and Fowler 1995. Ants are the most diverse soil macrofauna group in the Amazon region (Mathieu et al. 2005Barros et al. 2008) and represent a high density (Table 3). In the Caquetá basin, they are the most dense organisms of the soil macrofauna. In the other two river basins, ants are only exceeded by termites. Differences in ant and termite densities might be a reflection of the land use sampled. Termites tend to be more abundant in less disturbed ecosystems (Mboukou-Kimbatsa et al. 1998, Velásquez et al. 2012, while ants tend to be more abundant in disturbed or degraded ecosystems of the Amazon region. In our study, the Caquetá basin is where the most disturbed coverings, such as pastures and young secondary forests, are found (Table 1) (Aquino et al. 2008, Barros et al. 2002, Marichal et al. 2014, Pinzón et al. 2014, Rousseau et al. 2014).  The Neotropics (including the Amazon basin) have been recognised as a region of speciation and conservation of multiple lineages of ants (Moreau and Bell 2013). Results presented here increase the knowledge of soil ants from the Amazon region and suggest that ant species richness may increase considerably when sampling effort increases and combined methodologies are used to capture ants in different habitats.