Even evenness is important: the direct and mediating effects of species abundances for ecosystem services

This consistency across multiple indices demonstrates that evenness captures a meaningful and reliable aspect of community structure, supporting its usefulness as an ecological concept. Recently, there has been a debate on the usefulness of evenness as a concept in ecology, and it has been argued by Alroy (2025) that evenness is only an abstract concept, which is not reflecting a useful community property. The results of the study by Zhou et al. contribute to resolving this debate by showing that the relative abundances of species in a community, calculated by different indices showing similar results, do affect multiple ecosystem services. By definition, communities are composed of species occurring in specific abundances, influenced by the available species pool and abiotic and biotic filters (Meier et al., 2010). An even community consists of species with equal abundances, while an uneven community consists of species with unequal abundances (e.g. a dominant species with a high abundance and rare species with low abundances). One of the few general rules in ecology posits that communities always consist of a few dominant and many rare species (McGill et al., 2007). This indicates that there are only a few ‘winners’ in every community – due to, for example, the capability of a species to adapt to the environment, use resources or competitive ability – but also that relative abundances between species are always somewhat uneven (Hordijk et al., 2024). There are different ways to calculate evenness; every evenness index has its own mathematical focus and therefore interpretation. Zhou et al. used Hill's evenness in the main text (Chao Hordijk et al., 2023). The results of Zhou et al. contribute to a growing body of literature that suggests it is not only richness which drives the provisioning of services, but also that abundances of species are an important component of the biodiversity–ecosystem functioning (BEF) relationship. This resonates with the concept that it is truly biodiversity, richness and evenness in concert, which drive ecosystem functioning and ultimately the provisioning of services in a community (Fig. 1). It is therefore not only the number of species driving ecosystem functioning but also the abundances of the species relative to each other. At different richness levels, evenness can have a different effect on the relationship between richness and ecosystem functioning, emphasizing the multiple mechanisms playing a role. In a study including global tree inventory data, at low richness, uneven tree communities are more productive, while at high richness, even tree communities are more productive (Hordijk et al., 2023). This can be interpreted as productivity generally increases with species richness, until reduced evenness limits the overall increases in community diversity. The mediating effect of evenness has been researched mainly in the context of how evenness mediates the relationship between richness and a single ecosystem service (e.g. Zhang et al., 2012). When shifting from a single ecosystem service to multiple services, Zhou et al. show that low tree evenness results in a negative relationship between richness and multiple ecosystem services, but high tree evenness results in a positive relationship between richness and multiple ecosystem services. This indicates that it is not only the dominant species that provisions ecosystem services but also that every species in this boreal–temperate ecotone contributes to the provision of services in the system. Evenness by itself also has a direct effect on ecosystem services, and not only a mediating role in relation to richness. Zhou et al. show that tree evenness has a positive effect on both nutrient cycling (available soil phosphorus and nitrogen) and microbial habitat (soil microbial biomass), and that evenness by itself also has a positive effect on multiple ecosystem services. This implies that the relative abundances of species alone affect the functioning of the community and that the degree of dominance and rarity of the species plays an important role in ecosystem functioning. These results are in line with the study outcomes of Le Bagousse-Pinguet et al. (2021), in which increasing evenness among functionally dissimilar species promoted higher multiservices in a litter decomposition experiment. However, it contradicts the results by Maestre et al. (2012), in which evenness decreases multiservices in a microcosm experiment with lichens. A possible explanation for this contrasting result is that particular lichen species might affect multiple ecosystem services, and increasing their relative abundance could promote multiple ecosystem services. By contrast, the study by Zhou et al. suggests that complementarity plays a key role in the boreal–temperate ecotone and that when species have similar abundances, complementarity effects are enhanced. The relationship between evenness and multiservices, as reported by Zhou et al., was not dependent on key species, implying a more general mechanism independent of the identity of the dominant species. However, the mechanisms and conditions that determine the direction of the relationship between evenness and multiservices are still to be explored. State-of-the-art research on evenness highlights that ecologists should certainly consider ecosystem services beyond biomass and productivity, allowing them to elucidate how and why evenness affects ecosystem services and multiservices in different systems. There is not even a consensus on how evenness affects the most-studied ecosystem services, biomass and productivity, as neutral, positive, and negative relationships are reported (Hillebrand et al., 2008; Zhou et al.). Besides evenness as a measure of community structure, species' traits (such as height or rooting depth) affect a species' performance and the delivery of ecosystem services at the community level (Le Bagousse-Pinguet et al., 2021). Especially in the case of functional redundancy, it might not be the number of species defining ecosystem functioning, but rather the trait values of functionally unique dominant and rare species (Hordijk et al., 2025). Therefore, including functional evenness in the equation might shed light on the mechanisms behind the relationship between evenness and ecosystem functioning. Additionally, the effect of the relative abundances of species on resilience is particularly relevant in light of global change, as a growing body of literature suggests that evenness has an effect on community resilience (e.g. Wang et al., 2021). Living communities provide ecosystem services, and it is not only the number of species in a system but also the relative abundances of species that affect single services and multiservices. Understanding how community structure is influenced by global change and also affects functioning, and therefore the provision of services, is vital for sustainable management, restoration, and conservation. The author thanks Dr Joe Wan for valuable comments on a previous draft. 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