Feature Review
Functional trait effects on ecosystem stability: assembling the jigsaw puzzle

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Highlights

  • The main mechanisms by which biodiversity affects the stability of ecosystem functions (dominant species, compensatory dynamics, and insurance effects) all act through the functional traits of organisms that form local communities.

  • These mechanisms can be assessed using different components of trait value distributions within and between species [collectively referred as trait probability density (TPD)].

  • Variation in local populations can result in overall changes in community structure, which may or may not be propagated into changes in ecosystem functions. The extent of such propagation depends on: (i) the extent of TPD shifts, and (ii) the overlap between response and effect traits, and these effects will vary depending on the biotic mechanisms being assessed.

Under global change, how biological diversity and ecosystem services are maintained in time is a fundamental question. Ecologists have long argued about multiple mechanisms by which local biodiversity might control the temporal stability of ecosystem properties. Accumulating theories and empirical evidence suggest that, together with different population and community parameters, these mechanisms largely operate through differences in functional traits among organisms. We review potential trait-stability mechanisms together with underlying tests and associated metrics. We identify various trait-based components, each accounting for different stability mechanisms, that contribute to buffering, or propagating, the effect of environmental fluctuations on ecosystem functioning. This comprehensive picture, obtained by combining different puzzle pieces of trait-stability effects, will guide future empirical and modeling investigations.

Section snippets

Biotic mechanisms of stability: a jigsaw puzzle

As biodiversity is declining at an unprecedented rate, a particularly urgent scientific challenge is to understand and predict the consequences of biodiversity loss on multiple ecosystem functions [1., 2., 3.]. Temporal stability of the functioning of ecosystems is critical to both intrinsic and human purposes (Box 1, Figure 1). Temporal stability can be defined as the ability of a system to maintain, through time, multiple ecosystem properties (see Glossary) in relation to reference

Puzzle piece 1: effects of dominant species’ traits

According to the ‘mass-ratio hypothesis’ [36], dominant species in a community, through their traits, exert the strongest effect on ecosystem functions at a given time (called ‘immediate’ effects). Such dominant species' traits do not have only immediate effects. One of the two main drivers of constancy is how stable populations are within a community, expressed as average species-level population stability weighted by species’ relative abundances [28]. The constancy of species populations has

Puzzle piece 2: compensatory dynamics through species dissimilarity

Constancy is also influenced by the synchrony in the fluctuations of different populations within communities [28]. While it is generally accepted that a decrease in species synchrony increases stability of ecosystem properties, the mechanisms generating synchrony, or a lack thereof, are more controversial. Synchrony between species is generally attributed to similar species responses to environmental fluctuations [51,52]. Hence, species with similar adaptations to the environment (i.e.,

Puzzle piece 3: redundancy and the insurance effect

The ‘insurance effect’ requires the presence of multiple species with a similar effect on ecosystem functioning but different sensitivities to specific perturbations. Authors have thus stressed the importance of functional redundancy (i.e., the presence of multiple species with a similar effect on a given ecosystem function) as an important recovery and resistance mechanism [16]. A quick recovery can be obtained, for example, when a subordinate species, with similar effects on ecosystem

Puzzle piece 4: response and effect traits

A decisive puzzle piece modulating different trait-stability mechanisms is the trait response–effect framework [77], as originally suggested by Oliver et al. [16]. Which species will increase or decrease in response to both environmental fluctuations, including disturbances, and biotic interactions depends on their ‘response traits’ (i.e., traits that affect the fitness of species for given ecological conditions, including prevailing interactions) [10]. For example, plant traits related to

The puzzle comes together: buffering versus propagating

Oliver et al. [16] already stressed the central importance of the interplay between response and effect traits for the insurance effect. Here, by expanding this, we illustrate how this interplay represents the centerpiece for translating, via TPD, population and community changes to ecosystem functioning, integrating different stability components and their underlying mechanisms. The central concept bringing this puzzle together is that environmental fluctuations and perturbations cause changes

Connecting the pieces with TPD: data analysis considerations

Connecting different biotic drivers of stability is a tall challenge and remains a critical gap in our understanding of trait effects on ecosystem stability. Approaches similar to path analysis can provide a way forward for testing causal and cascading connections among the functional make-up of populations and communities and those of communities and ecosystem properties [17,31,55,61,89]. Existing studies have already considered a selection of TPD components to explain specific components of

Concluding remarks

Several biotic mechanisms affect the different components of ecosystem stability. Theoretical and empirical evidence is accumulating suggesting that these biotic mechanisms are affected by different components of the trait-probability distributions within local species pools. Future studies therefore need to consider differences in trait values within and between species when assessing how different biotic mechanisms affect stability (see Outstanding questions). We argue that conceptual and

Acknowledgments

This study is the result of an international workshop financed by the Valencian government in Spain (Generalitat Valenciana, reference AORG/2018/) and was supported by Spanish Plan Nacional de I+D+i (project PGC2018-099027-B-I00). E.V. was supported by the 2017 program for attracting and retaining talent of Comunidad de Madrid (no. 2017-T2/ AMB-5406).

Declaration of interests

No interests are declared.

Glossary

Asynchrony
deviation from a perfect synchrony in species’ fluctuations.
Antisynchrony
prevailing negative covariance between species’ fluctuations (i.e., negative synchrony).
Averaging/portfolio effect
link between an increase in species number and the decrease in the coefficient of variation (CV) of community abundance in the case of independent species fluctuations.
Buffering
the ability of a system to maintain given ecosystem functions, even despite species turnover. It is the opposite of

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