Size matters: implications of the loss of large individuals for ecosystem function

Abstract

Size is a fundamental organismal trait and an important driver of ecosystem functions. Although large individuals may dominate some functions and provide important habitat structuring effects, intra-specific body size effects are rarely investigated in the context of BEF relationships. We used an in situ density manipulation experiment to explore the contribution of large, deep-burrowing bivalves to oxygen and nutrient fluxes across the sediment-water interface. By manipulating bivalve size structure through the removal of large individuals, we held species identity constant, but altered the trait characteristics of the community. The number of large bivalves was the best predictor of ecosystem functioning. Our results highlight that (a) accounting for body size provides important insights into the mechanisms underpinning biodiversity effects on ecosystem function, and (b) if local disturbances are recurrent, preventing individuals from reaching large sizes, the contribution of large adults may be lost, with largely unknown implications for ecosystem functionality.T here is clear evidence that losses in biodiversity reduce the efficiency of ecosystem functions, including productivity and nutrient cycling 1 , but the actual mechanisms that underpin the positive biodiversityecosystem function (BEF) relationships remain an area of intense research 2,3 . A few species (with unique traits) may in fact dominate certain ecosystem processes [4][5][6] , and recent meta-analyses have indeed shown that species identity effects may be as important as richness effects per se 7 . Improving the mechanistic understanding of BEF relationships and allow prediction of the magnitude of change in ecosystem function following the loss of particular traits 2,8 thus depends on the nature of the ecosystem function(s) of interest and trait composition not only across the resident community but also within species. Trait-based approaches have, however, largely focused on assigning traits to species rather than individuals 9 . Hence, while interspecific differences in trait composition and the subsequent functional contribution to ecosystem processes have been acknowledged, differences in intra-specific trait characteristics are rarely addressed 10 (but see e.g. [11][12][13] ).Body mass is a fundamental organism trait that affects metabolic rate, energy demand and uptake rate 9,14,15 , and is an important characteristic of overall population and community structure through density-mass allometric relationships 14 . Even though high numbers of small individuals may dominate specific ecosystem functions through rapid turnover rates, the fewer large individuals may dominate other functions and provide important habitat structuring effects 16 . Particularly where ecosystem functions relate to the generation of biogenic habitat or organisms mediation of the nature and flux of energy and matter, size matters. This is potentially very important in marine sediments where large organisms can be expected to displace more sediment, pump more water an...

Mapping functional groups can provide insight into ecosystem functioning and potential resilience of intertidal sandflats

“…Similarly, functional groups with low abundances and high occurrence may be more resilient to small-scale stressors because they can readily re-colonise following a disturbance, e.g. after a predation (Thrush et al 1994) or localised sedimentation event (Norkko et al 2013). Although they are not locally abundant per se, they are likely to have a high degree of redundancy provided by their broad-scale occupancy.…”

“…On average, 75% of the individuals in a functional group harboured the dominant/ reported traits. Size was included for only those taxa for which it has been recognised to be important to ecosystem functioning, such as large bioturbating bivalves (Norkko et al 2013). This approach resulted in 26 functional groups ( Table 2, Table S1 at www.…”

Disentangling the influences of mean body size and size structure on ecosystem functioning: an example of nutrient recycling by a non‐native crayfish

“…2012; Toscano and Griffen 2012; Norkko et al. 2013; Rudolf and Rasmussen 2013b), a perennial challenge involves the logistical constraints of experiments or natural field observations where size structure is often confounded with mean body size. For any function of interest that is predicted to scale allometrically (e.g., foraging rate, nutrient recycling, productivity), varying the distribution around the mean body size will deterministically alter the aggregate sum of that function at the population level following Jensen's inequality.…”

Size diversity and species diversity relationships in fish assemblages of Western Palearctic lakes

“…morphological traits related to diet and habitat niche) very often do not include intraspecific variability but rather use average trait values for species (Cianciaruso et al 2009). The sizebased approach as applied here overcomes this issue because body size is easily measured for all individuals in a population and hence helps to detect the potential intraspecific functional trait variation, which may have large effects on the dynamics and structure of aquatic communities (Jansson et al 2007, Persson and De Roos 2013, Norkko et al 2013.…”