COS 2-4
Do more niches or better niches promote species richness? Insight from local and regional drivers of lichen diversity across U.S. forests
The number of species in a local community is influenced by regional processes that alter the pool of species able to colonize a site as well as local processes that filter which species establish and persist upon arrival. A species’ local persistence will depend on whether environmental conditions meet fundamental niche requirements and the availability of sufficient niche space. Thus, both environmental optimality and habitat heterogeneity can potentially increase local richness within a group of species with comparable niche requirements. Understanding the relative influence of local versus regional processes and environmental optimality versus heterogeneity on the assembly of local communities is a long-sought ecological goal. We present the first continental-scale evaluation of local-regional and optimality-heterogeneity hypotheses for lichens by integrating data from the U.S. Forest Inventory and Analysis National Program (FIA), herbarium records, a global trait database, and several environmental data sources. We use a structural equation model and variation partitioning on generalized linear models to relate local epiphytic macrolichen richness and abundance on 1,923 forest plots to a variety of regional and local-scale variables measuring both average forest conditions as well as forest heterogeneity. The importance of different processes is inferred from model parameter estimates and explained variance.
Results/Conclusions
We find that local processes likely regulate macrolichen species richness below an upper limit set by regional processes. The regional model uniquely explained five times more variation in local richness than the local model. However most of this explanatory power was due to climate variables, which the SEM indicated were primarily influencing local richness either directly or indirectly via an effect on lichen abundance. This suggests that even though climate variables were measured at a regional scale, they acted primarily via local mechanisms, and not indirectly by regulating the regional species pool. Although forest structure variables were generally poor predictors of lichen richness, there was some evidence that habitat optimality had a stronger influence on local richness than habitat heterogeneity. The model containing variables measuring average habitat conditions uniquely explained three times more variation in local richness than the model containing variables measuring habitat heterogeneity. Furthermore, total lichen abundance was strongly correlated with species richness and most local variables were indirectly correlated with local richness via their stronger correlation with lichen abundance. We conclude that epiphytic macrolichen species richness in forest patches is most strongly regulated by local environmental filters on a set of species with similar niche requirements.