Niche breadth and niche position explain species distribution and abundance relationships across large-scale diversity gradients

Background/Question/Methods:

The positive relationship between species occupancy and local abundance is one of the most ubiquitous patterns in ecology. Two prominent mechanisms proposed to explain occupancy-abundance relationships are interspecific differences in niche breadth (range of environmental conditions where a species occurs) and niche position (the preference of species to particular environments). Yet little is known about how or why the relative importance of niche breadth and niche position varies across broad-scale diversity gradients spanning temperate and tropical ecosystems. We tested the importance of niche breadth and niche position in explaining changes in species occupancy and mean local abundance across three regions with contrasting species diversity: temperate oak-hickory forests in the Missouri Ozarks, USA (38 species); premontane sub-tropical forests in the Argentinian Andes (51 species); and lowland tropical forests in the Bolivian Amazon (297 species). We calculated niche breadth and niche position using multivariate distance methods including 16 environmental variables (climate, soils & topography). Within each region, we tested the influence of niche breadth and position on local abundance (mean abundance in plots where a species was present) and occupancy (number of plots in which a species occurred) using linear regressions.

Results/Conclusions:

We found strong support for both the niche-breadth and niche-position hypotheses in all three forests. Niche breadth and niche position were more strongly correlated with species occupancy than with mean local abundance. Species occupancy was positively correlated with niche breadth (Missouri Ozarks: P < 0.0001, R²=0.51; Bolivian Amazon: P < 0.0001, R²=0.69; Argentinian Andes, P < 0.0001, R²=0.51) and negatively correlated with niche position (Missouri Ozarks: P < 0.0001, R²=0.86; Bolivian Amazon: P < 0.0001, R²=0.81; Argentinian Andes: P < 0.0001, R²=0.30). In contrast, species mean abundance was negatively correlated with niche position (Missouri Ozarks: P = 0.002, R²=0.30; Bolivian Amazon: P < 0.0001, R²=0.05; Argentinian Andes: P < 0.0001, R²=0.37), but not niche breadth. Moreover, the occupancy-niche breadth relationship was steeper in the temperate region compared to the tropical region. Our results suggest that (1) niche breadth and niche position are stronger predictors of regional species distributions, but weaker predictors of local species abundances, across low- and high-diversity regions; and (2) niche-based mechanisms may be more important in low-diversity regions.