When does diversity matter? Species functional diversity and ecosystem functioning across habitats and seasons in a field experiment

Abstract

Summary1. Despite ample experimental evidence indicating that biodiversity might be an important driver of ecosystem processes, its role in the functioning of real ecosystems remains unclear. In particular, the understanding of which aspects of biodiversity are most important for ecosystem functioning, their importance relative to other biotic and abiotic drivers, and the circumstances under which biodiversity is most likely to influence functioning in nature, is limited. 2. We conducted a field study that focussed on a guild of insect detritivores in streams, in which we quantified variation in the process of leaf decomposition across two habitats (riffles and pools) and two seasons (autumn and spring). The study was conducted in six streams, and the same locations were sampled in the two seasons. 3. With the aid of structural equations modelling, we assessed spatiotemporal variation in the roles of three key biotic drivers in this process: functional diversity, quantified based on a species trait matrix, consumer density and biomass. Our models also accounted for variability related to different litter resources, and other sources of biotic and abiotic variability among streams. 4. All three of our focal biotic drivers influenced leaf decomposition, but none was important in all habitats and seasons. Functional diversity had contrasting effects on decomposition between habitats and seasons. A positive relationship was observed in pool habitats in spring, associated with high trait dispersion, whereas a negative relationship was observed in riffle habitats during autumn. 5. Our results demonstrate that functional biodiversity can be as significant for functioning in natural ecosystems as other important biotic drivers. In particular, variation in the role of functional diversity between seasons highlights the importance of fluctuations in the relative abundances of traits for ecosystem process rates in real ecosystems.

Functional diversity: a review of methodology and current knowledge in freshwater macroinvertebrate research

“…Although we do not state that the conclusions drawn from these studies are incorrect, we argue following Verberk et al (2013) that ecological traits describing habitat preferences of macroinvertebrates should not be used for assessing functional diversity because ecological traits should be regarded as response traits (Violle et al, 2007), and response traits are not directly linked to ecosystem functions (Fig. 1, but see indirect effects in Frainer et al, 2014). We should note, however, that the idea of effect and response traits was developed in the context of plant ecology, and the categorization of traits strongly depends on the actual situation.…”

“…Response traits macroinvertebrates drives ecosystem functioning (Cardinale et al, 2002;Frainer et al, 2014), whereas recent reviews have drawn a more complex picture based on field studies (Lecerf & Richardson, 2010;Vaughn, 2010;Dolédec & Bonada, 2013). In particular, the link between ecosystem functioning and biodiversity depends on a cascade effect among species' dominance and identity, the existence of positive interactions among species, sequence of species loss, species traits and the environmental context (e.g.…”

“…Both measures have rarely been used in quantifying the functional diversity of freshwater macroinvertebrate assemblages (e.g. Frainer et al, 2014).…”

“…This finding is surprising, because experimental evidences suggest that the diversity of suspension feeders influences the filtering of suspended particulate material from water (Cardinale et al, 2002) and because the effects of detritivore diversity on leaf litter processing has become a flagship of stream ecology research (Jonsson & Malmqvist, 2000, 2003Gessner et al, 2010;Lecerf & Richardson, 2010;Frainer et al, 2014;Frainer & McKie, 2015). The most likely explanation is that the authors using the term ''functional diversity'' were not interested in concrete ecological functions, and focused only on aspects of diversity that influence ecosystem functions in general.…”

Species traits reveal effects of land use, season and habitat on the potential subsidy of stream invertebrates to terrestrial food webs

“…dredging and agricultural channelization (Burcher et al 2007), with these impacts particularly prominent in our study region in central Sweden. These modifications often result in a stream profile characterised by long pool sequences separating short riffle sequences without meanders (Petersen 1992), in contrast with the more regular pool-riffle sequence which characterizes many more natural systems flowing through forest (Leopold et al 1964;Frainer et al 2014). This contrast was apparent in our study, where riffle habitats were very rare and short in length in the agricultural streams (pers obs), whereas in the forested streams, there was a more even distribution between pool and riffle habitats.…”