Are soil lichen communities structured by biotic interactions? A null model analysis

Question: Are soil lichen communities structured by biotic interactions? Location: Gypsum outcrops located next to Belmonte del Tajo, central Spain. Methods: We sampled a total of 68 ( 50 cm x 50 cm) plots in gypsum outcrops from central Spain. Each plot was divided into 100 ( 5 cm x 5 cm) sampling quadrats, and the presence of all lichen species in every quadrat was recorded ( 6800 quadrats in total). We used two realistic null models to generate random communities unstructured by biotic interactions, and used them to test the hypothesis that soil lichen species co-occur less often than expected by chance. Results: We found fewer species combinations and less co-occurrence than expected by chance. However, the latter result was dependent on the null model selected. The number of checkerboard pairs did not differ significantly from the null expectation. Conclusions: Overall, our results suggest that gypsiferous soil lichen communities are structured by competitive interactions. They are consistent with studies conducted with a wide variety of taxa, and fill a gap in our knowledge of the factors driving the small-scale distribution of these important organisms.