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 × 50 cm) plots in gypsum outcrops from central Spain. Each plot was divided into 100 (5 cm × 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.