Quantifying the role of deterministic assembly and stochastic drift in a natural community of Arctic mosses

The interpretation of natural plant communities frequently invokes species-sorting controlled by niche differences along spatial environmental gradients. This process of niche structuring can be explained by reference to functional traits, which provide a mechanistic explanation for community structure. In contrast, models explaining species coexistence obviate the limiting effect of niche difference, by invoking processes which cause species-level drift, e.g. demographic stochasticity. This paper investigates a simple habitat with strong gradients (moss communities in a patterned arctic wetland) to identify signature-patterns under-pinning the relative importance of deterministic assembly and stochastic drift in a natural community. First, ordination analysis was used to confirm community composition structured by a range of nine carefully selected functional traits. Second, to determine whether traits explaining community composition might also explain species richness, local species richness (sR) was compared to (1) observed trait diversity and (2) expected trait diversity based on permutation tests, which are used to simulate null community assembly for different values of sR. Traits explaining species composition, consistent with deterministic niche structuring, do not appear to maintain sR. This surprising result was explained by decomposing the community into individual pair-wise comparisons, i.e. species niche-differences and association (χ²). Results support deterministic processes via the sorting of species with similar and contrasting niches, at opposite ends of a composite environmental gradient. Nevertheless, stochastic drift is apparent in the random structure of a majority of pair-wise associations; in addition, a species' abundance was in general not related to environmental distance from response-optima. We suggest therefore that spatial pattern in the moss community is a balance between deterministic forces with respect to species traits and controlling environmental gradients, and stochastic drift, which weakens this deterministic structure.