Opposing community assembly patterns for dominant and non-dominant plant
species in herbaceous ecosystems globally
Abstract
Dominant and non-dominant plants could be subject to different biotic
and abiotic influences, partially because dominant plants modify the
environment where non-dominant plants grow, causing an interaction
asymmetry. Among other possibilities, if dominant plants compete
strongly, they should deplete most resources forcing non-dominant plants
into a more constrained niche space. Conversely, if dominant plants are
constrained by the environment, they might not fully deplete available
resources but instead ameliorate some of the environmental constraints
limiting non-dominants. Hence, the nature of the interactions between
the non-dominants could be modified by dominant species. However, when
plant competition and environmental constraints have similar effects on
dominant and non-dominant species no difference is expected. By
estimating phylogenetic dispersion in 78 grasslands across five
continents, we found that dominant species were clustered
(underdispersed), suggesting dominant species are likely organized by
environmental filtering, and that non-dominant species were either
randomly assembled or overdispersed. Traits showed similar trends, but
insufficient data prevented further analyses. Furthermore, several
lineages scattered in the phylogeny had more non-dominant species,
suggesting that traits related to non-dominants are phylogenetically
conserved and have evolved multiple times. We found some environmental
drivers of the dominant—non-dominant disparity. Our results indicate
that assembly patterns for dominants and non-dominants are different,
consistent with asymmetries in assembly mechanisms. Among the different
mechanisms we evaluated, the results suggest two complementary
hypotheses seldom explored: (1) Non-dominant species include lineages
adapted to thrive in the environment generated by the dominant species.
(2) Even when dominant species reduce resources to non-dominant ones,
dominant species could have a stronger effect on—at least—some
non-dominants by ameliorating the impact of the environment on them,
than by depleting resources and increasing the environmental stress to
those non-dominants. The results show that the dominant–non-dominant
asymmetry has ecological and evolutionary consequences fundamental to
understand plant communities.