Abiotic, Biotic, and Evolutionary Control of the Distribution of C and N Isotopes in Food Webs

Abstract

Online enhancement: appendix. Dryad data: http://dx.doi.org/10.5061/dryad.g4p92.abstract: Ecosystem functioning depends on nutrient cycles and their responses to abiotic and biotic determinants, with the influence of evolutionary legacies being generally overlooked in ecosystem ecology. Along a broad elevation gradient characterized by shifting climatic and grazing environments, we addressed clines of plant N and C : N content and of d 13 C and d 15 N in producers (herbs) and in primary (grasshoppers) and secondary (birds) consumers, both within and between species in phylogenetically controlled scenarios. We found parallel and significant intra-and interspecific trends of isotopic variation with elevation in the three groups. In primary producers, nutrient and isotope distributions had a detectable phylogenetic signal that constrained their variation along the environmental gradient. The influence of the environment could not be ascribed to any single factor, and both grazing and climate had an effect on leaf stoichiometry and, thus, on the resources available to consumers. Trends in consumers matched those in plants but often became nonsignificant after controlling for isotopic values of their direct resources, revealing direct bottom-up control and little phylogenetic dependence. By integrating ecosystem and mechanistic perspectives, we found that nutrient dynamics in food webs are governed at the base by the complex interaction between local determinants and evolutionary factors.

The evolutionary convergence of avian lifestyles and their constrained coevolution with species' ecological niche

“…We point to competition among similar, closely related species as a candidate factor capable of segregating species that share habitat associations and intrinsic traits. This is because congeners occupy different elevation bands in our study area [20], as in other mountain regions where competition is an important community structuring mechanism [19], but often share life histories [13]. Undoubtedly, we have made simplified assumptions on competition Table 1.…”

Life-History Responses to the Altitudinal Gradient

“…At high elevations, low temperatures (or a short duration of mild temperatures) negatively affect soil enzymatic activities, the rate of nutrient mineralisation and turnover, thus reducing nutrient availability for primary producers with consequences for upper trophic levels (Laiolo et al 2015a). In some mountain areas, shorter grazing seasons and reduced nutrient inputs from herbivores also contribute to lower productivity, and the permanence of nutrients shortens in steep and shallow soils (Mariotti et al 1980;Huber et al 2007).…”

The intensity of male-male interactions declines in highland songbird populations

“…size; Brodmann et al 1997), although early-settling females show a longer time lag between settlement and clutch initiation than late ones ). In our study area, grassland productivity markedly declines with elevation (Laiolo et al 2013(Laiolo et al , 2015, and the cover of habitat elements that reduce the quality of habitat for pipits, such as rocks and snow patches, increases with it Bollmann et al 1997;Rauter and Reyer 1997;Rauter et al 2002;Laiolo and Melendez 2014). The breeding season is also inevitably delayed and more synchronous, following patterns of snow melting (Melendez and Laiolo 2014).…”

“…Due to historical forest clearance, the study area is covered by grasslands and pastures with interspersed limestone and low shrubs from 800-1300 m a.s.l. to the upper alpine belt (Laiolo et al 2015). This broad elevational range supports high densities of the water pipit Anthus spinoletta, a ground-nesting, insectivorous bird that is territorial during breeding but partially erratic outside of reproduction (Rehsteiner et al 1998).…”

“…This has important implications for the expression of costly sexual traits: Badyaev (1997b) and Snell-Rood and Badyaev (2008) found less pronounced plumage dimorphism and shorter, less elaborate songs in high elevation Cardueline finches. The density and distribution in time and space of resources also largely change along elevations and may play a role in the spatial variation of behaviours displayed during male-male conflicts (Laiolo 2013;Laiolo et al 2015).…”