Soil Nematode Communities Vary among Populations of the Iconic Desert Plant, Welwitschia Mirabilis

Scattered throughout the Namib Desert of Namibia are populations of Welwitschia mirabilis, a unique, low-lying, and slow-growing gymnosperm plant. We studied soils under Welwitschia plants and in adjacent interplant areas along a 400-km range to examine the potential of these plants as resource islands supporting nematode communities. We found significant differences in nematode density and community structure among the sites that were correlated to differences in climate, edaphic factors, and plant size and density. Soils from the Torra Conservancy site, which receives the most precipitation and had the highest density of Welwitschia plants, contained the highest organic matter and the most diverse nematode communities, with the broadest representation of nematode trophic groups. The largest and likely oldest Welwitschia plants occurred in the Messum Crater, the site with the least rainfall, which hosted the densest nematode communities (mean = 14,683 kg-1 soil). These communities consisted almost entirely of the bacterial-feeding nematode Panagrolaimus sp. Two other sites, Welwitschia Plain, a well-known tourist destination, and Hope Mine, the southernmost known population, contained the fewest nematodes with moderate levels of diversity. Differences in nematode abundance between Welwitschia soils and interplant soils were not discernable at three of the four field sites, suggesting the resource island effect is not very strong. Interplant spaces also support diverse and abundant nematode communities, perhaps due to the growth of cryptobiotic crusts or ephemeral rainfall-induced vegetation.