Quantifying species distribution within the functional space

Multidimensional representations of functional diversity help visualize
species and organism distribution in functional space, providing insights
into the mechanisms regulating community assembly and ecosystem
functioning. Typically, the species distributions in functional space are
represented by richness, divergence (or dispersion) and evenness (or
regularity). While these dimensions quantify the overall structure of
species distribution, they overlook how exactly species are scattered
within the functional space, particularly the fine-scale spatial patterns
that may reveal additional ecological or evolutionary dynamics. We
introduce a novel framework to quantify species’ functional arrangement
within the functional space, capturing patterns across broad and fine
scales. We detail the construction of a functional space and propose
statistics to assess species functional arrangement - the cumulative
proportion of pairwise neighbours (PNcp) and the cumulative proportion of
nearest neighbours (NNcp). We also outline guidelines for evaluating
departures from randomness using null models, including random
displacement and random selection from species pool. To demonstrate this
framework, we tested it in simulated scenarios and analysed the functional
arrangement patterns of vascular plant species on Surtsey Island. In the
simulated scenarios, the framework correctly identified simulated patterns
at multiple scales. As for the colonization of Surtsey Island, our
findings showed a gradual reorganization of species within the functional
space. Initially random, the distribution shifted towards clustering,
reflecting increasing habitat complexity and the formation of ecological
relationships among plant species. This pattern aligns with succession
theory, where early arrivals create conditions that favour later
colonizers, leading to a more interconnected and structured ecosystem. Our
framework allows to advance from global (richness, dispersion and
evenness) to fine-scale (arrangement) functional distribution patterns,
adopting rigorous, quantitative methods that can test specific ecological
hypotheses and uncover deeper insights into community assembly and the
functioning of ecosystems. By assessing functional arrangement, it enables
researchers to address a broader range of functional diversity questions
than previously possible. The methods presented here offer a powerful
approach to uncover and interpret complex patterns in functional diversity
occurring at multiple scales.