Global-scale patterns of assemblage structure of soil nematodes in relation to climate and ecosystem properties

Aim To conduct the first global-scale investigation of soil nematode assemblages using a standardized approach to quantify how environmental and climatic variables influence family assemblage structure in nematodes and determine whether nematode families have restricted distributions. Location Global. Methods We collected soil nematodes within four 10 m x 10 m plots distributed evenly along a 900-m transect at each of 12 sites representing multiple ecosystem types across a latitudinal gradient (68 degrees N to 77 degrees S) on six continents. We assigned > 28,000 individuals to family level and trophic group morphologically. Results We recorded a total of 43 nematode families, but sites varied considerably in family richness (1-30). Families differed in their ranges with 12 families occurring at 10 or more sites, while 14 families occurred at three or fewer sites. Total nematode and trophic group abundances were generally related to soil characteristics, including bulk density and soil moisture, but we found no good predictor of family richness, diversity or evenness at the plot level. Family richness, diversity and evenness were considerably lower in the high-latitude polar desert than elsewhere, but only family diversity showed a significant, albeit weak, latitudinal gradient. Nematode assemblage composition was quite strongly related to climate: 65% and 58% of the variation in assemblage composition across sites could be accounted for by mean annual rainfall and temperature, respectively. Main conclusions Nematode families display macroecological patterns similar to other organisms, such as a positive abundance-range size relationship and restricted distribution of some families. Local nematode abundances were related to soil characteristics, but we found no relationships between family richness and environmental or climatic variables. Family composition was related to mean annual rainfall and temperature, suggesting that climate is a good predictor of local assemblage structure. As a result, climate change may have a significant impact on nematode assemblages, with potential implications for ecosystem functioning.