Opposing community assembly patterns for dominant and non-dominant plant species in herbaceous ecosystems globally

Biotic and abiotic factors interact with dominant plants —the locally most
frequent or with the largest coverage— and non-dominant plants
differently, partially because dominant plants modify the environment
where non-dominant plants grow. For instance, if dominant plants compete
strongly, they will deplete most resources, forcing non-dominant plants
into a narrower niche space. Conversely, if dominant plants are
constrained by the environment, they might not exhaust available resources
but instead may ameliorate environmental stressors that usually limit
non-dominants. Hence, the nature of interactions among non-dominant
species could be modified by dominant species. Furthermore, these
differences could translate into a disparity in the phylogenetic
relatedness among dominants compared to the relatedness among
non-dominants. By estimating phylogenetic dispersion in 78 grasslands
across five continents, we found that dominant species were clustered
(e.g., co-dominant grasses), suggesting dominant species are likely
organized by environmental filtering, and that non-dominant species were
either randomly assembled or overdispersed. Traits showed similar trends
for those sites (<50%) with sufficient trait data. Furthermore,
several lineages scattered in the phylogeny had more non-dominant species
than expected at random, suggesting that traits common in non-dominants
are phylogenetically conserved and have evolved multiple times. We also
explored environmental drivers of the dominant/non-dominant disparity. We
found different assembly patterns for dominants and non-dominants,
consistent with asymmetries in assembly mechanisms. Among the different
postulated mechanisms, our results suggest two complementary hypotheses
seldom explored: (1) Non-dominant species include lineages adapted to
thrive in the environment generated by dominant species. (2) Even when
dominant species reduce resources to non-dominant ones, dominant species
could have a stronger positive effect on some non-dominants by
ameliorating environmental stressors affecting them, than by depleting
resources and increasing the environmental stress to those non-dominants.
These results show that the dominant/non-dominant asymmetry has ecological
and evolutionary consequences fundamental to understand plant communities.