Résumé
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
the regional scale might not necessarily be distinct at the 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 1,350
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, local and community scales
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 variations of 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 scales only. These results suggest the role played by
environmental filtering on species' distinctiveness at the local
scale, but the determinant of distinctiveness variations at the community
scale still needs 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.