Tropical nematode diversity: vertical stratification of nematode communities in a Costa Rican humid lowland rainforest

Comparisons of nematode communities among ecosystems have indicated that, unlike many organisms, nematode communities have less diversity in the tropics than in temperate ecosystems. There are, however, few studies of tropical nematode diversity on which to base conclusions of global patterns of diversity. This study reports an attempt to estimate nematode diversity in the lowland tropical rainforest of La Selva Biological Research Station in Costa Rica. We suggest that one reason previous estimates of tropical nematode diversity were low is that habitats above the mineral soil are seldom sampled. As much as 62% of the overall genetic diversity, measured by an 18S ribosomal barcode, exists in litter and understory habitats and not in soil. A maximum likelihood tree of barcodes from 360 individual nematodes indicates most major terrestrial nematode lineages were represented in the samples. Estimated â speciesâ richness ranges from 464 to 502 within the four 40 by 40 m plots. Directed sampling of insects and their associated nematodes produced a second set of barcodes that were not recovered by habitat sampling, yet may constitute a major class of tropical nematode diversity. While the generation of novel nematode barcodes proved relatively easy, their identity remains obscure due to deficiencies in existing taxonomic databases. Specimens of Criconematina, a monophyletic group of soil-dwelling plant-parasitic nematodes were examined in detail to assess the steps necessary for associating barcodes with nominal species. Our results highlight the difficulties associated with studying poorly understood organisms in an understudied ecosystem using a destructive (barcode) sampling method.

不同生态系统间的线虫群落比较研究显示，与多数生物类群不同，热带地区的线虫群落多样性低于温带生态系统。然而，当前可用于支撑全球多样性格局结论的热带线虫多样性研究仍十分匮乏。本研究尝试对哥斯达黎加拉塞尔瓦生物研究站的低地热带雨林内的线虫多样性进行评估。我们提出，以往热带线虫多样性估算值偏低的原因之一，是极少对矿质土壤上层生境开展采样。基于18S核糖体条形码（18S ribosomal barcode）测得的整体遗传多样性中，多达62%存在于枯落物与林下生境，而非土壤中。对360个单个线虫个体的条形码构建的最大似然树（maximum likelihood tree）表明，样本覆盖了绝大多数主要陆生线虫演化支。在4块40×40米的样地中，估算的"物种"丰富度介于464至502之间。针对昆虫及其伴生线虫的定向采样获得了第二组条形码，这类条形码无法通过常规生境采样获取，但可能构成热带线虫多样性的一个主要类群。尽管获取新型线虫条形码相对简便，但由于现有分类学数据库存在缺陷，这些条形码的分类归属仍不明确。我们对环线亚目（Criconematina）——一类栖息于土壤的植物寄生线虫单系类群——的标本进行了详细分析，以评估将条形码与正式定名物种相关联所需的步骤。本研究结果凸显了在一个尚未被充分探索的生态系统中，采用破坏性（条形码）采样方法研究认知程度极低的生物所面临的诸多挑战。