Untangling the importance of niche breadth and niche position as drivers of tree species abundance and occupancy across biogeographic regions

Aim: Ecological niches shape species commonness and rarity, yet the
relative importance of different niche mechanisms within and across
ecosystems remains unresolved. We tested the influence of niche breadth
(range of environmental conditions where species occur) and niche position
(marginality of a species’ environmental distribution relative to the mean
environmental conditions of a region) on tree-species abundance and
occupancy across three biogeographic regions. Location: Argentinian Andes;
Bolivian Amazon; Missouri Ozarks. Time period: 2002–2010. Major taxa
studied: Trees. Methods: We calculated abiotic-niche breadths and
abiotic-niche positions using 16 climate, soil, and topographic variables.
For each region, we used model selection to test the relative influence of
niche breadth and niche position on local abundance and occupancy in
regional-scale networks of 0.1-ha forest plots. To account for
species-environment associations caused by other mechanisms (e.g.,
dispersal), we used null models that randomized associations between
species occurrences and environmental variables. Results: We found strong
support for the niche-position hypothesis. In all regions, species with
higher local abundance and occupancy occurred in non-marginal
environments. Observed relationships between occupancy and niche position
also differed from random species-environment associations in all regions.
Surprisingly, we found little support for the niche-breadth hypothesis.
Observed relationships between both local abundance and niche breadth, and
occupancy and niche breadth, did not differ from random
species-environment associations. Main conclusion: Niche position was more
important than niche breadth in shaping species commonness and rarity
across temperate, sub-tropical, and tropical forests. In all forests, tree
species with widespread-geographic distributions were associated with
environmental conditions commonly found throughout the region, suggesting
that niche position has similar effects on species occupancy across
contrasting biogeographic regions. Our findings imply that conservation
efforts aimed at protecting populations of common and rare tree species
should prioritize conservation of both common and rare habitats.