Great smoky mountain ant community composition

Disentangling the drivers of diversity gradients can be challenging. The
Measurement of Biodiversity (MoB) framework decomposes scale-dependent
changes in species diversity into three components of community structure:
the species abundance distribution (SAD), the total community abundance,
and the within-species spatial aggregation. Here we extend MoB from
categorical treatment comparisons to quantify variation along continuous
geographic or environmental gradients. Our approach requires sites along a
gradient, each consisting of georeferenced plots of abundance-based
species composition data. We demonstrate our method using a case study of
ants sampled along an elevational gradient of 28 sites in a mixed
deciduous forest of the Great Smoky Mountains National Park, USA. MoB
analysis revealed that decreases in ant species richness along the
elevational gradient were associated with decreasing evenness and total
number of species which counteracted the modest increase in richness
associated with decreasing spatial aggregation along the gradient. Total
community abundance had a negligible effect on richness at all but the
finest spatial grains, SAD effects increased in importance with sampling
effort, while the aggregation effect had the strongest effect at coarser
spatial grains. These results do not support the more-individuals
hypothesis, but they are consistent with a hypothesis of stronger
environmental filtering at coarser spatial grains. Our extension of MoB
has the potential to elucidate how components of community structure
contribute to changes in diversity along environmental gradients and
should be useful for a variety of assemblage-level data collected along
gradients.