Although how rare species persist in communities is a major ecological question, the critical phenotypic dimension of rarity is broadly overlooked. Recent work has shown that evaluating functional distinctiveness, the average trait distance of a species to other species in a community, offers essential insights into biodiversity dynamics, ecosystem functioning, and biological conservation. However, the ecological mechanisms underlying the persistence of functionally distinct species are poorly understood. Here we propose a heterogeneous fitness landscape framework, whereby functional dimensions encompass peaks representing trait combinations that yield positive intrinsic growth rates in a community. We identify four fundamental causes leading to the persistence of functionally distinct species in a community. First, environmental heterogeneity or alternative phenotypic designs can drive positive population growth of functionally distinct species. Second, sink populations with negative growth can deviate from local fitness peaks and be functionally distinct. Third, species found at the margin of the fitness landscape can persist but be functionally distinct. Fourth, biotic interactions (either positive or negative) can dynamically alter the fitness landscape. We offer examples of these four cases and some guidelines to distinguish among them. In addition to these deterministic processes, we also explore how stochastic dispersal limitation can yield functional distinctiveness.

Global change drivers such as anthropogenic nutrient inputs simultaneously alter biodiversity, species composition, and ecosystem functions such as aboveground biomass. These changes are interconnected by complex feedbacks among extinction, colonization, and shifting relative abundance. Here, we use a novel temporal application of the Price equation to quantify the functional contributions of species that are lost, gained, and persist under ambient and experimental nutrient addition in 59 global grasslands. Under ambient conditions, compositional and biomass turnover was high, but species losses (i.e., local extinctions) were balanced by gains (i.e. colonization). There was biomass loss associated with species loss under fertilization. Few species were gained in fertilized conditions over time but those that were, and species that persisted, contributed to net biomass gains, outweighing biomass loss. These components of community change are key to understanding the relationship between change in composition, diversity and functioning.

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.

How historical and concurrent drought regulate plant diversity-productivity relationships through altering soil microbial communities remains a key knowledge gap. We addressed this gap with plant diversity-productivity relationship experiments under drought and ambient conditions over two phases (Phase I: soil conditioning, and Phase II: plant response). Our results reveal that plant diversity and drought interacted and caused divergent soil microbial communities in Phase I, leading to soil microbial legacies. These soil legacies interacted and caused more pronounced plant diversity-productivity relationships in Phase II, reflecting increased net biodiversity effects over time. Complementarity effects were most positive in plant communities with highest plant richness and in the Drought-Ambient (Phase I-II) treatment, and selection effects were most negative in these communities. Our results highlight the importance of soil microbial communities in driving positive plant diversity effects, and future rainfall changes can cause complicated patterns in the biodiversity-ecosystem functioning relationships through soil microbial legacy.

1. The dissimilarity and hierarchy of trait values that characterize niche and fitness differences, respectively, have been increasingly applied to infer mechanisms driving community assembly and to explain species co-occurrence patterns. Here, we predict that limiting similarity should result in the spatial segregation of functionally similar species, while functionally similar species will be more likely to co-occur together either due to environmental filtering or competitive exclusion of inferior competitors (hereafter hierarchical competition). 2. We used a fully mapped 50-ha subtropical forest plot in southern China to explore how pairwise spatial associations were influenced by trait dissimilarity and hierarchy between species in order to gain insight into assembly mechanisms. We assessed pairwise spatial associations using two summary statistics of spatial point patterns at different spatial scales and compared the effects of trait dissimilarity and trait hierarchy of different functional traits on the interspecific spatial associations. These comparisons allow us to disentangle the effects of limiting similarity, environmental filtering and hierarchical competition on species co-occurrence. 3. We found that trait dissimilarity was generally negatively correlated with interspecific spatial associations, meaning that species with similar trait values were more likely to co-occur together and thus supporting environmental filtering or hierarchical competition. We further found that leaf area, wood density and maximum height had stronger trait hierarchy effects on the pairwise spatial associations relative to their corresponding trait dissimilarity effects, which suggests that hierarchical competition played a more (or at least equally) important role in structuring our forest community compared to environmental filtering. 4. This study employed a novel method to disentangle the relative importance of multiple assembly mechanisms in structuring co-occurrence patterns, especially the mechanisms of environmental filtering and hierarchical competition, which lead to indistinguishable co-occurrence patterns. This study also reinforced the importance of trait hierarchy rather than trait dissimilarity in driving neighborhood competition.

Species invasion represents one of the major drivers of biodiversity change globally, yet there is widespread scientific and popular confusion and controversy about the nature of non-indigenous species (NIS) impact. This confusion stems from differing notions and understanding of what constitutes invasive species ‘impact’ and the scales at which it should be assessed. I argue that the proximate mechanisms determining invasive species impact happen at smaller scales where species interact, and by understanding these mechanisms, we can scale up to a broader understanding of how invasive species impact biodiversity. The mechanisms of NIS impact on potential competitors can be classified into four scenarios: 1) minimal impact from NIS inhabiting unique niche space; 2) neutral impact spread across the community and proportional to NIS abundance; 3) targeted impact on a small number of competitors with overlapping niches; and 4) pervasive impact that is disproportionate to NIS abundance and ostensibly caused by ecosystem modification that filters out other species. I develop a statistical test to distinguish these four mechanisms based on community rank-abundance curves. Using an example dataset from plant communities invaded by the dominant invasive vine, Vincetoxicum rossicum, I show that in long-term plots that had high native plant diversity and where V. rossicum increased, impact resulted in either targeted extirpations (scenario 3) or widespread biodiversity loss (scenario 4). Regardless of whether NIS impact is neutral, targeted or pervasive, the net outcome will be the homogenization of ecosystems and reduced biodiversity at larger scales, perhaps reducing ecosystem resilience.