Ontogenetic functional diversity: Size structure of a keystone predator drives functioning of a complex ecosystem

Abstract

Abstract. A central challenge in community ecology is to understand the connection between biodiversity and the functioning of ecosystems. While traditional approaches have largely focused on species-level diversity, increasing evidence indicates that there exists substantial ecological diversity among individuals within species. By far, the largest source of this intraspecific diversity stems from variation among individuals in ontogenetic stage and size. Although such ontogenetic shifts are ubiquitous in natural communities, whether and how they scale up to influence the structure and functioning of complex ecosystems is largely unknown. Here we take an experimental approach to examine the consequences of ontogenetic niche shifts for the structure of communities and ecosystem processes. In particular we experimentally manipulated the stage structure in a keystone predator, larvae of the dragonfly Anax junius, in complex experimental pond communities to test whether changes in the population stage or size structure of a keystone species scale up to alter community structure and ecosystem processes, and how functional differences scale with relative differences in size among stages.We found that the functional role of A. junius was stage-specific. Altering what stages were present in a pond led to concurrent changes in community structure, primary producer biomass (periphyton and phytoplankton), and ultimately altered ecosystem processes (respiration and net primary productivity), indicating a strong, but stage-specific, trophic cascade. Interestingly, the stage-specific effects did not simply scale with size or biomass of the predator, but instead indicated clear ontogenetic niche shifts in ecological interactions. Thus, functional differences among stages within a keystone species scaled up to alter the functioning of entire ecosystems. Therefore, our results indicate that the classical approach of assuming an average functional role of a species can be misleading because functional roles are dynamic and will change with shifts in the stage structure of the species. In general this emphasizes the importance of accounting for functional diversity below the species level to predict how natural and anthropogenic changes alter the functioning of natural ecosystems.

Population structure determines functional differences among species and ecosystem processes

“…There is also no reason to expect that differences among species remain constant over ontogeny. For instance, dragonfly larvae in our system switch in their microhabitat use (from vegetation to leaf litter) during ontogeny 21 , although we did not find a clear indication that this also occurs for beetle larvae. Such differential shifts in microhabitat use could explain why functional differences between species changed during ontogeny.…”

“…Consequently, in systems where the effects of individuals on the ecosystem are largely driven by body size, there is a high risk of misinterpreting results when using traditional substitutive designs 37,40 . Therefore, we refrained from keeping initial biomass or density constant and followed previous suggestions and designs that recommend using natural size-abundance relationships 21,37,38,40 . This approach allowed us to estimate the actual impact of each size class in natural populations and the relative impact of each size class by separating quantitative differences from qualitative differences among size classes.…”

“…Indeed, ecological differences (for example, diet, habitat use) among stages within species can rival or even exceed differences between species [18][19][20] . Given such differences among stages, the demographic structure of populations should strongly determine the functional roles of species, thereby influencing ecosystem functioning 21 . Yet, there is considerable debate about whether and how it should be incorporated into current ecological models and conservation efforts 2,[22][23][24][25] because it is generally assumed that ontogenetic functional variation within species is insignificant compared with functional variation among species at the ecosystem level, although this remains to be tested.…”

Trait- and size-based descriptions of trophic links in freshwater food webs: current status and perspectives

“…Temperature strongly affects predation rates (Rall et al, 2012;Dell et al, 2013) and could be the main driver after accounting for changes in body size, ST and FV traits. However, individual ontogeny and species phenology also play significant roles in the variation of trophic interactions (Rudolf and Rasmussen, 2013). Organisms have higher metabolic requirements during periods of growth, and fast-growing organisms may thus have much larger mass-specific prey consumption rates as larvae than as adults (e.g., diving beetles: Klecka and Boukal, 2012).…”

“…Body size underpins biomass growth and energy transfer in aquatic habitats (Edgar, 1990) and size-based metrics describe well the structure and function of entire aquatic ecosystems (Hildrew et al, 2007;Rudolf and Rasmussen, 2013). Community size spectra are also sensitive to natural and human-driven disturbances (Brucet et al, 2005;Solimini et al, 2005;Emmrich et al, 2011) and can be used in environmental monitoring (Basset et al, 2012;Garcia et al, 2012).…”