Nematode spatial and ecological patterns from tropical and temperate rainforests

Abstract

Large scale diversity patterns are well established for terrestrial macrobiota (e.g. plants and vertebrates), but not for microscopic organisms (e.g. nematodes). Due to small size, high abundance, and extensive dispersal, microbiota are assumed to exhibit cosmopolitan distributions with no biogeographical patterns. This assumption has been extrapolated from local spatial scale studies of a few taxonomic groups utilizing morphological approaches. Recent molecularly-based studies, however, suggest something quite opposite. Nematodes are the most abundant metazoans on earth, but their diversity patterns are largely unknown. We conducted a survey of nematode diversity within three vertical strata (soil, litter, and canopy) of rainforests at two contrasting latitudes in the North American meridian (temperate: the Olympic National Forest, WA, U.S.A and tropical: La Selva Biological Station, Costa Rica) using standardized sampling designs and sample processing protocols. To describe nematode diversity, we applied an ecometagenetic approach using 454 pyrosequencing. We observed that: 1) nematode communities were unique without even a single common species between the two rainforests, 2) nematode communities were unique among habitats in both rainforests, 3) total species richness was 300% more in the tropical than in the temperate rainforest, 4) 80% of the species in the temperate rainforest resided in the soil, whereas only 20% in the tropics, 5) more than 90% of identified species were novel. Overall, our data provided no support for cosmopolitanism at both local (habitats) and large (rainforests) spatial scales. In addition, our data indicated that biogeographical patterns typical of macrobiota also exist for microbiota.

Figure 1. A comparison of total numbers of micro- and meio-faunal species between tropical (La Selva Biological Station, Costa Rica (LS)) and temperate (Olympic National Forest in WA, U.S.A. (OF)) rainforests from the 5′-end of the SSU diagnostic region.

Figure 2. Average number of reads per species within Soil, Litter, and Canopy habitats.

A) tropical rainforest at La Selva Biological Station in Costa Rica (LS), B) temperate rainforest at the Olympic National Forest in WA, U.S.A. (OF). Species are grouped into 6 trophic guilds: bacterial feeders (BF), fungal feeders (FF), root associates(RA), plant parasites (PP), omnivores (OM), and predators (PR) and reads are sorted from their highest to lowest numbers by the litter habitat within each trophic guild.

Figure 3. An overall (across all habitats) taxonomic richness of La Selva (left panel) and Olympic Forest (right panel) at different levels of taxonomic resolution.

A) species, B) genus, and C) family. At each level of taxonomic resolution taxa were grouped by their feeding habit. BF  =  bacterial feeders, FF  =  fungal feeders, RA  =  root associates, PP  =  plant parasites, OM  =  omnivores, PR  =  predators, AP  =  animal parasites, AL  =  algivores.

Figure 4. Average number of reads per species within Soil, Litter, and Canopy habitats in the temperate rainforest at the Olympic National Forest in WA, U.S.A. generated from the use of the 5′-end diagnostic locus.

For a comparison to the results from the use of 3′-end see Figure 2B. Species are grouped into trophic guilds: bacterial feeders (BF), fungal feeders (FF), plant parasites (PP), omnivores (OM) and reads are sorted from their highest to lowest numbers by the litter habitat within each trophic guild.

Figure 5. A nematode community composition at the trophic guild level depending on the choice of the diagnostic locus (5′-end or 3′-end of the SSU rDNA).