The role of distance and habitat specificity in bryophyte and perennial seed plant metacommunities in arid scrubland fragments

QuestionsWhat are the relative roles of (1) environmental niche partitioning (especially along soil chemistry gradients), (2) effective dispersal, and (3) landscape characteristics in structuring species assemblies of bryophyte and seed plant metacommunities? LocationEbro river basin, NE Spain. MethodsWe sampled biological soil crust (BSC) bryophytes and perennial seed plants from 54 plots in ten fragments of native vegetation from two contiguous landscapes with contrasting agricultural matrix types. Each plot was characterized with respect to soil electrical conductivity (EC, a proxy for presence of gypsum and halite), pH, vegetation cover and aspect. We used unconstrained ordinations to find the main floristic gradients and evaluated their relationship with environmental variables using Kendall correlations. We studied the spatial structure of floristic dissimilarities for bryophyte and seed plant communities by using multiple regression models on distance matrices (MRM), and individual species' responses by estimating species' densities along environmental gradients. We estimated effective isolation between plots within landscapes by means of partial MRM. ResultsWe found 60 bryophyte and 57 seed plant species. Compositional dissimilarities were spatially structured. DCA showed clear floristic segregation of the two landscapes. The soil EC gradient was the main variable explaining compositional variation in both plant metacommunities, followed by soil pH. The effect of geographic distance between plots differed significantly between landscapes and matrix types, and was larger for seed plants than for bryophytes. ConclusionsOnly a handful of BSC bryophyte species can be regarded as either true gypsofuges or true gypsophiles. Gypsophilous bryophytes are fewer and less strongly associated with gypsum-rich sites than co-existing perennial seed plants. Overall, BSC bryophyte communities vary less over similar distances than corresponding seed plant communities, most likely due to higher dispersal ability. We argue that the topographically complex landscape of a less modified agricultural matrix imposes more friction' to bryophyte dispersal than a smoother landscape with a more intensely human-modified matrix. Effective conservation of fragmented arid agricultural landscapes must take into account the sensitivity of the biota to fragment intrinsic conditions, the properties and the management practices that shaped the landscape.