Diversity and distribution of soil micro-invertebrates across an altitudinal gradient in a tropical montane rainforest of Ecuador, with focus on free-living nematodes
Introduction
Nematodes are the most abundant and one of the most species-rich metazoan phyla on Earth (Wilson, 2000, Hodda et al., 2009) and play a key role as intermediaries in soil ecosystems (Ettema, 1998, Scheu, 2002, Bardgett and van der Putten, 2014). Communities of free-living nematodes are also functionally diverse, as shown by the presence of marked feeding specialization and a wide range of body-sizes (e.g. Ettema, 1998). The former include bacterial- and algal-feeders, predators of small invertebrates and protozoans, and stylet-bearing species able to suck out the inner contents of plant roots or fungal hyphae (Yeates et al., 1993, Traunspurger, 1997, Yeates and Bongers, 1999).
Procter (1984) suggested that the species richness, density, and biomass of free-living soil nematodes is higher at high latitudes than in tropical zones, due to abundant microbial prey and a lack of specialized invertebrate competitors. However, for diversity this statement does not seem to hold true, as tropical regions contain a very high diversity of nematode species: In their study of a primary rainforest in the Republic of Cameroon, Lawton et al. (1996) identified 204 morphospecies among 1009 specimens and estimated that a sample of 200 individuals would yield 72 different species. Based on a comparison of 24 sites along a gradient of forest disturbance also in Cameroon, Bloemers et al. (1997) detected 431 species from 5000 individuals (although 90% could not be assigned to known species), with samples of 200 individuals yielding 61 species on average. Interestingly, those authors found that forest disturbance only moderately reduced nematode diversity, as slash-and-burn or clear-cut sites showed ∼40% fewer species than primary forest sites. Powers et al. (2009) used molecular barcoding to estimate that 500 nematode species could be expected in 1600-m2 rainforest plots in Costa Rica. Tropical litter and understory habitats harbor the most diverse assemblages, with some species forming unique associations with other soil invertebrates (e.g., nine species of termites are typically associated with eight nematode species). So far, however, free-living nematode communities have been evaluated in much more detail in temperate than in tropical regions, which could have led to an under-appreciation of the species richness and basic distribution patterns.
Both the density/biomass patterns and the structure of nematode communities are affected by soil characteristics, such as soil type, pH, and nutrient content, but presumably more intensively by climatic constraints, such as temperature and rainfall regimes (Yeates and Bongers, 1999, Ruess, 2003, Nielsen et al., 2014, Bhusal et al., 2015, Tsiafouli et al., 2017). For example, Yeates and Bongers (1999) reported a consistent reduction of nematode density with increasing altitude in grassland soils forming a climosequence in New Zealand (from 4195 ind. 50 g−1 soil at 300 m asl, to only 120 ind. 50 g−1 soil at 1500 m). Tsiafouli et al. (2017) recently concluded that, among the landscape properties that affect nematode community indices, altitude rather than vegetation cover was the most predictable because altitudinal climatic conditions strongly constrain the availability and turnover of basal resources.
In this study we analyzed the changes in the density, diversity, and community structure of soil invertebrates, especially free-living nematodes, along an altitudinal gradient (1000, 2000 and 3000 m) in a tropical rainforest of the Andes of Ecuador. We specifically asked (1) whether the density of soil micro-invertebrates declines with altitude and soil layer (2) whether and how nematode species richness, feeding-types, and community structure respond to altitudinal and/or soil layer gradients, and (3) how nematode species richness in tropical montane rainforests compares with that of high-latitude forest ecosystems.
Section snippets
Study sites and sampling
This study was conducted on the eastern slopes of the Andes of southern Ecuador, along an altitude transect of 1000, 2000, and 3000 m asl. Samples were collected in June-July 2007. The maximum distance between sampling sites was 30 km. The sites at 1000 m were located in Bombuscaro (S4°6′54”, W78°58′2”), those at 2000 m in the Reserva Biologica San Francisco (S3°58′18”, W79°4′45”), and those at 3000 m in Podocarpus National Park (S4°6′711”,W79°10′581”). The soil types were alumic acrisol at 1000 m,
Micro-invertebrate community
Neither the absolute density of nematodes nor the total density of micro-invertebrates differed significantly across the altitudinal gradient or across the two different soil layers. However, based on absolute and relative densities, tardigrades and mites were significantly more abundant at 1000 m, whereas harpacticoid copepods and their larvae (nauplii) were more abundant at 2000 and 3000 m (Table 1). In terms of relative density, the dominance of nematodes decreased significantly with
Changes in density of soil micro-invertebrates with altitude and soil depth
The micro-invertebrates at the study sites comprised many groups typical of aquatic or semi-aquatic habitats, such as rotifers and harpacticoid copepods. Their contributions increased with increasing altitude—as was the case for the testate amoebae species characteristic of the submerged habitats in the study area (Krashevska et al., 2007)—and were consistent with the greater rainfall in forests at higher altitude (Röderstein et al., 2005). Altitude also impacts the amount and composition of
Conclusion
A diverse assemblage of nematodes was found dwelling the soil of a montane rainforest in Ecuador. Neither altitude nor soil horizon affected nematode density nor diversity. However, the structure of the nematode assemblage and the distribution of feeding-types changed along the altitudinal gradient, mirroring changes in soil C/N content and fungal biomass. The unusually high proportion of numerous species of omnivores and predators suggested the existence of a complex and reticulated soil food
Acknowledgements
We thank Stefanie Gehner for technical assistance and two anonymous reviewers for their constructive criticism. The study was carried out within the framework of the Research Unit “Biodiversity and sustainable management of a megadiverse mountain ecosystem in South Ecuador,” subproject “Soil fauna: Diversity and functioning.” Financial support from the German Research Foundation (DFG) is gratefully acknowledged (RU 816).
References (50)
- et al.
Litter mixture effects on decomposition in tropical montane rainforests vary strongly with time and turn negative at later stages of decay
Soil Biol. Biochem.
(2014) - et al.
Nematode community structure in relation to soil and vegetation characteristics
Appl. Soil Ecol.
(1994) - et al.
Linking aboveground and belowground diversity
Trends Ecol. Evol.
(2005) - et al.
Testate amoebae (protista) of an elevational gradient in the tropical mountain rain forest of Ecuador
Pedobiologia
(2007) - et al.
Microorganisms as driving factors for the community structure of testate amoebae along an altitudinal transect in tropical mountain rain forests
Soil Biol. Biochem.
(2008) - et al.
More sex at higher altitudes: changes in the frequency of parthenogenesis in oribatid mites in tropical montane rain forests
Pedobiologia
(2013) - et al.
The soil fauna of beech forests: comparison between a mull and a moder soil
Pedobiologia
(1990) The soil food web: structure and perspectives
Eur. J. Soil Biol.
(2002)- et al.
Nematode community indices for microhabitat type and large scale landscape properties
Ecol. Indic.
(2017) - et al.
Nematode diversity in agroecosystems
Agric. Ecosyst. Environ.
(1999)