A global database of soil nematode abundance and functional group composition.

As the most abundant animals on earth, nematodes are a dominant component of the soil community. They play critical roles in regulating biogeochemical cycles and vegetation dynamics within and across landscapes and are an indicator of soil biological activity. Here, we present a comprehensive global dataset of soil nematode abundance and functional group composition. This dataset includes 6,825 georeferenced soil samples from all continents and biomes. For geospatial mapping purposes these samples are aggregated into 1,933 unique 1-km pixels, each of which is linked to 73 global environmental covariate data layers. Altogether, this dataset can help to gain insight into the spatial distribution patterns of soil nematode abundance and community composition, and the environmental drivers shaping these patterns.

Soil nematode abundance and functional group composition at a global scale.

Soil organisms are a crucial part of the terrestrial biosphere. Despite their importance for ecosystem functioning, few quantitative, spatially explicit models of the active belowground community currently exist. In particular, nematodes are the most abundant animals on Earth, filling all trophic levels in the soil food web. Here we use 6,759 georeferenced samples to generate a mechanistic understanding of the patterns of the global abundance of nematodes in the soil and the composition of their functional groups. The resulting maps show that 4.4 ± 0.64 × 1020 nematodes (with a total biomass of approximately 0.3 gigatonnes) inhabit surface soils across the world, with higher abundances in sub-Arctic regions (38% of total) than in temperate (24%) or tropical (21%) regions. Regional variations in these global trends also provide insights into local patterns of soil fertility and functioning. These high-resolution models provide the first steps towards representing soil ecological processes in global biogeochemical models and will enable the prediction of elemental cycling under current and future climate scenarios.

Six new and one known species of Geomonhystera (Nematoda, Monhysteridae) from moss, an epiphytic plant and soil in México and Ecuador.

Six new species of Geomonhystera (Nematoda, Monhysterida: Monhysteridae) Andrássy, 1981, are described and illustrated, five from México and one from Ecuador. The new species have many features in common and in common with at least several other species of the genus. Consequently, those features individually are not very useful for species-level diagnosis. They include the cuticular striation which, under light microscopy, ranges from fine to apparently smooth; however, scanning electron micrographs reveal very fine cuticular striation on all the new species. All the new species have a few body setae scattered along the length of the body and the vulval lips of females are slightly protruding. As for other species of the genus, there is considerable overlap in the ranges of standard morphometric characters so a combination of standard and additional morphometrics, as well as qualitative characters, is necessary for species separation. Geomonhystera mexiquense sp. n., collected from moss on a rock and on the trunk of Quercus crassipes in Juchitepec, México State, México, has outer labial setae that are unsegmented and 44-67% of the head width, the cephalic setae are thin, unsegmented and 31-50% of the head width; the vulval lips protrude slightly. Geomonhystera michoacana sp. n., collected from moss on a rock in Tzararacua National Park, Uruapan, Michoacan State, México, has a very finely-striated cuticle which appears almost smooth and bears rare cervical and caudal setae; the tail is long, 15-17% of the body length, and the rectum also is long, 23-32 (27±2.0) µm. Geomonhystera longispiculata sp. n., collected from an epiphytic plant in the botanical garden of the Ecology Institute in Jalapa, Veracruz State, México, is distinguished from all the species of the genus by the long, slender spicules, 55-85 (75±3.0) µm. The cuticle has conspicuous fine striation and there are somatic setae scattered along the body. Geomonhystera ecuatoriana sp. n., collected from moss on the trunk of Scalesia pedunculata growing on the twin volcanoes of Isla Santa Cruz, Galapagos, Ecuador, has fine cuticular striation and a few small body setae; the unsegmented outer labial setae are 42-54% of the head width and the cephalic setae are thin, unsegmented and 33-45% of the head width; the vulval lips protrude slightly and the vulva-anus distance is 37-48 µm or 2.1-2.5 times anal body diameter. Males of G. ecuatoriana have slightly arcuate spicules, 35 µm long. Geomonhystera galindoi sp. n., collected from moss on the trunk of Quercus peduncularis in San Pablo Ixzayo, Texcoco, México State, México, has fine cuticlar striation and a few fine body setae, the vulval lips are slightly protruding; the vulva-anus distance is 35-50 (41±0.9) µm, 1.5-2.2 (1.9±0.2) times anal body diameter. Males of G. galindoi are slightly ventrally curved with the posterior end strongly curved, giving the body a hook-shaped appearance. Geomonhystera chiautzingoensis sp. n., collected from moss on the trunk of Crataegus mexicana in Chiautzingo, Puebla State, México, has exceedingly fine striation of the cuticle and outer labial and cephalic setae that are less than 50% of the head width. Additionally, G. dubia Siddiqi Shahina, 2004 was sollected in soil samples around a plum tree (Prunus sp.) in the garden of Montecillo Campus, Colegio de Postgraduados, Texcoco, México State, México This species has a small body with fine cuticular striation and a few sparsely-distributed body setae; the outer labial and cephalic setae are unsegmented, 5-8 and 3-5 µm long, 56-80% and 33-50% of the head width, respectively; the vulval lips protrude slightly but are sometimes flush with the body contour.

Five new species of the family Trischistomatidae (Nematoda: Enoplida) from North and Central America, with keys to the species of Trischistoma and Tripylina.

Three new species of Trischistoma and two new species of Tripylina (Trischistomatidae) are described. Trischistoma ripariana n. sp. was collected in the surface organic material and upper rhizosphere soil on a stream bank in Oakville, California, USA. It is characterized by a short, thin body, the vulva at 79-83%, a small index c (17-29) and a short tail, 34-57 µm. Trischistoma corticulensis n. sp. was found in moss on tree bark in a tropical forest at the La Mancha Ecological Institute, Veracruz State, México. The vulva is at 67-73% and the tail is elongate conoid (51-84 µm). Trischistoma helicoformis n. sp. was collected on lichen growing on tree bark near the Carretera Interamericana in Costa Rica. It is characterized by its spiral shape after fixation, the tail length (76-101 µm), a very small index c (10-14.5) and very small sclerotised pieces around the vagina. Tripylina rorkabanarum n. sp. was collected from moss on tree bark in a tropical forest at the La Mancha Ecological Institute. It is characterized by the presence of two cervical setae, the position of the subventral teeth posterior to the dorsal tooth, the absence of sclerotized pieces around the vagina and the distance of the dorsal tooth from the anterior, 10-15 µm. Tripylina iandrassyi n. sp. was collected from soil around a banana tree at the La Mancha Ecological Institute. It is characterized by the presence of a post-uterine sac, well-developed buccal lips, subventral teeth located posterior to the dorsal tooth, one cervical seta in females and two in males, and by spicules not completely surrounded by a muscular sheath.

Five new species of the genus Tripylella (Nematoda: Enoplida: Tripylidae).

Five new species of the genus Tripylella are described, two from México, one from Fátima, Portugal, one from Quito, Ecuador, and one from California, USA. Tripylella mexicana sp. n. is characterized by its short body (average 0.74 mm), short pharynx (average 161 µm), short tail (average 117 µm), presence of an excretory pore and small setae distributed sparsely along the body, the presence of body pores, the posterior position of the subventral teeth in relation to the small dorsal tooth with all teeth in contiguous stomal chambers, the finely-striated cuticle with many anastomoses, the non-protruding vulval lips, and the presence of sclerotized pieces in the vulval region. Tripylella muscusi sp. n. is characterized by its body length (average 0.94 mm), pharynx length (average 201 µm), tail length (average 140 µm), the anterior position of the subventral teeth in relation to the small dorsal tooth in a single stomal chamber, the presence of an excretory pore, the presence of body pores and sparse somatic setae, the finely-striated cuticle with sparse anastomoses, protruding vulval lips and sclerotized oval-shaped pieces present in the vulval region. Tripylella quitoensis sp. n. is characterized by the short body length (average 0.72 mm), the short outer labial setae, the short pharynx (average 175 µm), the location of the anterior subventral teeth and posterior dorsal tooth in the same stomal chamber, the short tail (average 98 µm), the apparent absence of an excretory pore, presence of body pores, presence of somatic setae, a finely-striated cuticle, non-protruding vulval lips, and very small oval sclerotized pieces in the vulva. Tripylella fatimaensis sp. n. is characterized by the short body, (average 0.74 mm) long, by the length of the pharynx (average 180 µm), the length of the tail (average 110 µm) and in the length of its reduced diameter portion, 45-58 µm, the presence of an excretory pore, body pores and three pairs of caudal setae (one pair each latero-ventral, latero-dorsal and ventral). Tripylella dentata sp. n. is characterized, and differs from all the species of the genus, by the presence of two adjacent stomal chambers, with two large teeth, one dorsal and one ventral, in the posterior stomal chamber and two subventral teeth in the anterior smaller chamber, short body (average 0.85 mm), pharynx length (average 209 µm), tail length (average 115 µm), the apparent absence of an excretory pore, the presence of two cervical setae in a lateral position, and by the presence of conspicuous pores along the body.

Nematode Fauna of Tropical Rainforest in Brazil: A Descriptive and Seasonal Approach.

Studies of nematode assemblages in natural ecosystems can contribute to better understanding of the occurrence, relevance, and ecology of plant-parasitic and other soil nematodes. Nematode assemblages and environmental parameters (organic matter, water content (WC), bulk density (BD), total porosity (Po), soil respiration, and soil texture) were investigated in two seasons (rainy and dry) in two forest areas of the Zona da Mata, Pernambuco State. The aim of our research was to evaluate the heterogeneity between two locations and seasons in the Brazilian Atlantic Forest. Structure and composition of the nematode assemblages differed between areas and across time. Rhabditidae dominated the rainy season in both forest soils. Rarefaction curves (RC) suggest that sampling to detect more nematode taxa should be more intensive in the rainy season. The forest soils have complex, stable soil food webs with high connectance and decomposition channels dominated by bacteria. The predator-prey relationships were not affected by changes in soil properties that fluctuate with time.

Ellsworth C. Dougherty: A Pioneer in the Selection of Caenorhabditis elegans as a Model Organism.

Ellsworth Dougherty (1921-1965) was a man of impressive intellectual dimensions and interests; in a relatively short career he contributed enormously as researcher and scholar to the biological knowledge base for selection of Caenorhabditis elegans as a model organism in neurobiology, genetics, and molecular biology. He helped guide the choice of strains that were eventually used, and, in particular, he developed the methodology and understanding for the nutrition and axenic culture of nematodes and other organisms. Dougherty insisted upon a concise terminology for culture techniques and coined descriptive neologisms that were justified by their linguistic roots. Among other contributions, he refined the classification system for the Protista.

An Aegilops ventricosa Translocation Confers Resistance Against Root-knot Nematodes to Common Wheat.

Root knot nematodes (RKN; Meloidogyne spp.) cause severe losses worldwide to a wide range of crops. Crop rotations with resistant hosts can be used to control losses, but the wide host range of RKN limits this option. In this study, we found that the wheat cultivar Lassik is resistant to several isolates of the RKN species M. incognita and M. javanica, including those that can reproduce on tomato with the resistance gene Mi-1. Comparison of near-isogenic lines of wheat showed that the wheat resistance gene(s) is localized within a segment of the short arm of chromosome 2N from Aegilops ventricosa (Zhuk.) Chennav translocated into common wheat (Triticum aestivum L.) chromosome arm 2AS and is associated with a highly significant decrease in RKN eggs in the roots. This RKN resistance gene has been assigned the name Rkn3. While wheat itself is tolerant of RKN infection, a microplot experiment coupled with tomato bioassays showed less RKN root galling in the tomato samples grown in soil from the previous microplots including RKN resistant wheat varieties than in those including a susceptible wheat isogenic line. This result suggests that rotation with Rkn3 resistant wheat cultivars has the potential to be a valuable component of nematode management for crops that are highly susceptible to nematode damage and for which alternative strategies are limited.

Reflections on plant and soil nematode ecology: past, present and future.

The purpose of this review is to highlight key developments in nematode ecology from its beginnings to where it stands today as a discipline within nematology. Emerging areas of research appear to be driven by crop production constraints, environmental health concerns, and advances in technology. In contrast to past ecological studies which mainly focused on management of plant-parasitic nematodes, current studies reflect differential sensitivity of nematode faunae. These differences, identified in both aquatic and terrestrial environments include response to stressors, environmental conditions, and management practices. Methodological advances will continue to influence the role nematodes have in addressing the nature of interactions between organisms, and of organisms with their environments. In particular, the C. elegans genetic model, nematode faunal analysis and nematode metagenetic analysis can be used by ecologists generally and not restricted to nematologists.

Atypical development in plant and soil nematodes.

Observations of atypical developmental and anatomical characteristics have been recorded for many taxa of soil nematodes. They include the unusual occurrence of extra feeding structures, aberrant configuration of features of both male and female reproductive systems, and the occurrence of intersexes assumed to be functionally female, functionally male, or non-functional. In many cases, hypotheses have been advanced regarding the genetic or developmental mechanisms and environmental stimuli that control, regulate, or facilitate abnormalities, but many are quite speculative and lack experimental verification. Further, the fitness costs or advantages, and the heritability of aberrant characters are largely unknown, except where they clearly preclude reproduction, either apomictic or amphimictic. Underlying mechanisms and ecological consequences may be difficult to study in organisms that are not readily cultured under axenic or sterile laboratory conditions, however information on developmental processes in Caenorhabditis elegans represents an important resource in which to seek homologies.

Contribution of nematodes to the structure and function of the soil food web.

As carbon and energy flow through the soil food web they are depleted by the metabolic and production functions of organisms. To be sustained, a "long" food web, with a large biomass at higher trophic levels, must receive a high rate of rhizodeposition or detrital subsidy, or be top-populated by organisms of slow growth and long life cycle. Disturbed soil food webs tend to be bottom heavy and recalcitrant to restoration due to the slow growth of upper predator populations, physical and chemical constraints of the soil matrix, biological imbalances, and the relatively low mobility and invasion potential of soil organisms. The functional roles of nematodes, determined by their metabolic and behavioral activities, may be categorized as ecosystem services, disservices or effect-neutral. Among the disservices attributable to nematodes are overgrazing, which diminishes services of prey organisms, and plant-damaging herbivory, which reduces carbon fixation and availability to other organisms in the food web. Unfortunately, management to ameliorate potential disservices of certain nematodes results in unintended but long-lasting diminution of the services of others. Beneficial roles of nematodes may be enhanced by environmental stewardship that fosters greater biodiversity and, consequently, complementarity and continuity of their services.

Plant species, atmospheric CO2 and soil N interactively or additively control C allocation within plant-soil systems.

Two plant species, Medicago truncatula (legume) and Avena sativa (non-legume), were grown in low- or high-N soils under two CO2 concentrations to test the hypothesis whether C allocation within plant-soil system is interactively or additively controlled by soil N and atmospheric CO2 is dependent upon plant species. The results showed the interaction between plant species and soil N had a significant impact on microbial activity and plant growth. The interaction between CO2 and soil N had a significant impact on soil soluble C and soil microbial biomass C under Madicago but not under Avena. Although both CO2 and soil N affected plant growth significantly, there was no interaction between CO2 and soil N on plant growth. In other words, the effects of CO2 and soil N on plant growth were additive. We considered that the interaction between N2 fixation trait of legume plant and elevated CO2 might have obscured the interaction between soil N and elevated CO2 on the growth of legume plant. In low-N soil, the shoot-to-root ratio of Avena dropped from 2.63 +/- 0.20 in the early growth stage to 1.47 +/- 0.03 in the late growth stage, indicating that Avena plant allocated more energy to roots to optimize nutrient uptake (i.e. N) when soil N was limiting. In high-N soil, the shoot-to-root ratio of Medicago increased significantly over time (from 2.45 +/- 0.30 to 5.43 +/- 0.10), suggesting that Medicago plants allocated more energy to shoots to optimize photosynthesis when N was not limiting. The shoot-to-root ratios were not significantly different between two CO2 levels.

Nematode indicators of organic enrichment.

The organisms of the soil food web, dependent on resources from plants or on amendment from other sources, respond characteristically to enrichment of their environment by organic matter. Primary consumers of the incoming substrate, including bacteria, fungi, plant-feeding nematodes, annelids, and some microarthropods, are entry-level indicators of enrichment. However, the quantification of abundance and biomass of this diverse group, as an indicator of resource status, requires a plethora of extraction and assessment techniques. Soluble organic compounds are absorbed by bacteria and fungi, while fungi also degrade more recalcitrant sources. These organisms are potential indicators of the nature of incoming substrate, but current methods of biomass determination do not reliably indicate their community composition. Guilds of nematodes that feed on bacteria (e.g., Rhabditidae, Panagrolaimidae) and fungi (e.g., Aphelenchidae, Aphelenchoididae) are responsive to changes in abundance of their food. Through direct herbivory, plant-feeding nematodes (e.g., many species of Tylenchina) also contribute to food web resources. Thus, analysis of the nematode community of a single sample provides indication of carbon flow through an important herbivore channel and through channels mediated by bacteria and fungi. Some nematode guilds are more responsive than others to resource enrichment. Generally, those bacterivores with short lifecycles and high reproductive potential (e.g., Rhabditidae) most closely mirror the bloom of bacteria or respond most rapidly to active plant growth. The feeding habits of some groups remain unclear. For example, nematodes of the Tylenchidae may constitute 30% or more of the individuals in a soil sample; further study is necessary to determine which resource channels they portray and the appropriate level of taxonomic resolution for this group. A graphic representation of the relative biomass of bacterivorous, fungivorous, and herbivorous nematodes provides a useful tool for assessing the importance of the bacterial, fungal, and plant resource channels in an extant food web.

Geocenamus angelescresti n. sp., a Diagnostic Key and Compendium to the Species of the Genus Geocenamus Thorne &Malek, 1968 (Nematoda: Belonolaimidae).

Geocenamus angelescresti n. sp. (Nematoda: Belonolaimidae) was found in rhizosphere of Pinus ponderosa and Arctostaphylos patula growing along Angeles Crest Highway in the San Gabriel mountains of California. The nematode species is characterized by a round-to-hexagonal labial disc with six bulging sectors, lateral sectors of first labial annule smaller than the submedian sectors, six to eight labial annules, distinct deirids, stylet length (45-57 microm), body length (666-996 microm), lateral field with or without areolation of outer bands on tail, and a rounded, smooth tail terminus. Geocenamus angelescresti n. sp. most closely resembles G. superbus but differs from it by a shorter stylet (45-57 microm vs. 67 microm), shorter body length (666-996 microm vs. 1,200 microm), bulged sectors and smaller diameter of the labial disc (2.3-2.8 microm vs. 4.0 microm, round, smooth), longer female tail (54-68 microm vs. 41 microm), and a narrower tail terminus. An emended description of the genus and a list of valid species are provided. Geocenamus arcticus (Mulvey, 1969) Tarjan 1973 and G. uralensis Baydulova 1983 are proposed as junior synonyms of G. tenuidens Thorne &Malek 1968. An identification key to 12 species of Geocenamus and a compendium of important diagnostic morphological characters used in the identification of species are included.

Modeling direct positive feedback between predators and prey.

Predators can have positive impacts on their prey through such mechanisms as nutrient mineralization and prey transport. These positive feedbacks have the potential to change predictions based on food web theory, such as the assertion that enrichment is destabilizing. We present a model of a simple food web, consisting of a resource, a consumer, and its predator. We assume that the predator has a direct positive effect on the consumer, by increasing the rate at which the consumer acquires resources. We consider two cases: the feedback strength is a saturating function of predator density, or it is proportional to the encounter rate between predators and prey. In both cases, the positive feedback is stabilizing, delaying or preventing the onset of oscillations due to enrichment. Positive feedback can introduce an Allee effect for the predator population, yielding multiple stable equilibria. Strong positive feedback can yield counterintuitive results such as a transient increase in consumer density following the introduction of predators, and a decrease in the resource pool following enrichment.

Sensitivity of nematode life-history groups to ions and osmotic tensions of nitrogenous solutions.

Guild designation of nematodes of similar trophic function and life-history strategy provides a basis for using nematode faunal analyses in an integrative assessment of soil food web condition. Omnivorous and predaceous nematodes, categorized at the upper end of a colonizer-persister (c-p) continuum of nematode functional guilds are generally not abundant in cropped soil. These nematodes are more sensitive to heavy metal concentrations than those in other c-p groups, but whether sensitivity to agrochemicals contributes to the observed low abundance of high c-p groups in cropped soils is less well understood. An exposure assay in solution was used to compare the sensitivity of nematodes representing various guilds obtained from field soils and from laboratory culture to several nitrogen sources. Nematodes in c-p groups 4 and 5 were more sensitive to nitrogen solutions than nematodes representing lower c-p groups. There were both osmotic and specific ion effects-the latter most evident in exposure of nematodes to NaNO and (NH)SO. The RC (concentration resulting in nematode recovery of one half of that of distilled water) for (NH)SO was < 0.052 M-N for c-p groups 4 and 5 compared to much greater values (0.34 to 0.81 M-N) for c-p groups 1 to 3. In non-ionic polyethylene glycol (PEG) solutions, osmotic tensions of 0.40 to 0.43 MPa reduced the recovery of exposed nematodes by half (RT; water potential of solution resulting in nematode recovery of one half of that of distilled water) for c-p groups 4 and 5 compared to > 1.93 MPa for c-p groups 1 to 3. RT values for urea solutions, also non-ionic, were greater than for PEG. Caenorhabditis elegans N2 (c-p 1) and Meloidogyne javanica (c-p 3) reared on solid medium and in hydroponic culture, respectively, were slightly more sensitive to specific ion and osmotic effects than nematodes of similar c-p groups obtained from soil. The greater sensitivity of c-p 4 and 5 nematodes to nitrogen solutions suggests that fertilizers may contribute to the low abundance of these nematodes in annual cropping systems. This study supports the use of nematode faunal analyses as indicators of chemical stress in soil.