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Recent work has shown that evaluating functional trait distinctiveness, the average trait distance of a species to other species in a community offers promising insights into biodiversity dynamics and ecosystem functioning. However, the ecological mechanisms underlying the emergence and persistence of functionally distinct species are poorly understood. Here, we address the issue by considering a heterogeneous fitness landscape whereby functional dimensions encompass peaks representing trait combinations yielding positive population growth rates in a community. We identify four ecological cases contributing to the emergence and persistence of functionally distinct species. First, environmental heterogeneity or alternative phenotypic designs can drive positive population growth of functionally distinct species. Second, sink populations with negative population 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 (positive or negative) can dynamically alter the fitness landscape. We offer examples of these four cases and guidelines to distinguish between them. In addition to these deterministic processes, we explore how stochastic dispersal limitation can yield functional distinctiveness. Our framework offers a novel perspective on the relationship between fitness landscape heterogeneity and the functional composition of ecological assemblages. [ABSTRACT FROM AUTHOR]

Beyond the local abundance of species, their functional trait distinctiveness is now recognized as a key driver of community dynamics and ecosystem functioning. Yet, since the functional distinctiveness of a species is always relative to a given species pool, a species distinct at regional scale might not necessarily be distinct at local or community scale, and reciprocally. To assess the importance of scale (i.e. the definition of a species pool) when quantifying the functional distinctiveness of species, and how it might distort the ecological conclusions derived from it, we quantified trait distinctiveness of 1350 plant species at regional, local and community scales over ca 88 000 grassland plots in France. We measured differences in functional distinctiveness of species between regional (mainland France), local (10 × 10 km cell) and community (10 × 10 m plot) scale, and tested the influence of environmental predictors (climate and nitrogen input) and contexts (environmental distinctiveness, frequency and heterogeneity) on these variations. In line with theoretical expectations, we found large variation in functional distinctiveness (in particular between regional and community scales) for many species, with a general tendency of lower distinctiveness at smaller scales. We also showed that nitrogen input – a key aspect of high land use intensity – and environmental frequency partly explained the differences between local and regional scale only. These results suggest the role played by environmental filtering on species distinctiveness at local scale, but the determinant of distinctiveness variations at community scale still need to be elucidated. Our study provides robust empirical evidence that measures of ecological originality are strongly scale‐dependent. We urge ecologists to carefully consider the scale at which they measure distinctiveness, as ignoring scale dependencies could lead to biased (or even entirely wrong) conclusions when not considered at the scale of interest for the respective research question. [ABSTRACT FROM AUTHOR]