Direct and indirect effects of environmental factors on dietary niches in size‐structured populations of a wild salmonid

Abstract

Dietary plasticity of populations can be associated to ontogenetic diet preferences and depends on the size-structure of populations. Dietary niche characterizes the functional role of organisms in a food web, as it reflects both resources' diversity used by a consumer and trophic interactions in the system. Dietary niches are controlled both by biotic and abiotic factors, but their interactions in natural systems remain poorly studied. Here, we investigated the variability of dietary niche in salmonid wild populations focusing both on inter-population and intra-population (through time) trophic changes, using marble trout (Salmo marmoratus) living in Slovenian headwater streams as a model system. Stable isotope analysis showed high variability of dietary niche and trophic diversity among six of the seven remnant marble trout populations. We observed substantial differences in dietary niche width among populations and within populations through time. Results of partial least square path modelling highlighted opposite effects of immature and mature trout on trophic niche structure. Direct effects of temperature and slope (stream and watershed) were opposite; temperature narrowed dietary niches while slope increased them. Environmental factors (e.g., temperature, stream and watershed slope) had indirect effects on trophic niches after accounting for fish density. Our results showed that size-distribution and sexual maturity are key determinants of the dietary niche width in a population. Increasing density of immature trout tended to widen the dietary niche while increasing density of mature trout tended to narrow it. Environmental factors had direct effects both on resources and consumers densities and indirect effects. Direct and indirect effects were often antagonistic

Dietary niche expansion and niche shift in native marble trout (Salmo marmoratus) living in sympatry with introduced rainbow trout (Oncorhynchus mykiss)

“…Niche expansion in wild populations can be caused by different abiotic and biotic drivers such as individual specialisation (Bolnick et al, 2003), interindividual niche variation (Araújo, Langerhans, Giery, & Layman, 2014), intraspecific competition (Musseau, Vincenzi, et al, 2015;Svanbäck & Bolnick, 2007), ecological release from interspecific competition (Bolnick et al, 2010), predation pressure (Sharpe & Chapman, 2014) and ecological opportunity (Layman, Quattrochi, Peyer, & Allgeier, 2007). Although pool surface tended to increase piscivory rate in allopatric marble trout populations (Musseau, Vincenzi, et al, 2015), the highest piscivory rate in the present study was in Lower Idrijca, the sector with the lower pool surface. However, temperature was basically the same in the two sectors of Idrijca River and both stream slope and riparian forest are very similar.…”

“…Niche expansion in wild populations can be caused by different abiotic and biotic drivers such as individual specialisation (Bolnick et al, 2003), interindividual niche variation (Araújo, Langerhans, Giery, & Layman, 2014), intraspecific competition (Musseau, Vincenzi, et al, 2015;Svanbäck & Bolnick, 2007), ecological release from interspecific competition (Bolnick et al, 2010), predation pressure (Sharpe & Chapman, 2014) and ecological opportunity (Layman, Quattrochi, Peyer, & Allgeier, 2007). Niche expansion in wild populations can be caused by different abiotic and biotic drivers such as individual specialisation (Bolnick et al, 2003), interindividual niche variation (Araújo, Langerhans, Giery, & Layman, 2014), intraspecific competition (Musseau, Vincenzi, et al, 2015;Svanbäck & Bolnick, 2007), ecological release from interspecific competition (Bolnick et al, 2010), predation pressure (Sharpe & Chapman, 2014) and ecological opportunity (Layman, Quattrochi, Peyer, & Allgeier, 2007).…”

“…In recent years, the study of stable isotope niche as a proxy of dietary niches has become more popular as they reflect both resources' diversity used by a consumer and trophic interactions in the system (Layman, Araújo, Boucek, Hammerschlag-Peyer, & Harrison, 2012;Newsome, Martinez del Rio, Bearhop, & Phillips, 2007). Using δ 15 N and δ 13 C is common in fish diet analysis for quantifying the relative contribution of different types of prey in predator diet and for characterising species trophic niche variability as responses to abiotic and biotic factors (Musseau, Boulenger, et al, 2015;Musseau, Vincenzi, et al, 2015). δ 13 C is the indicator of energy source, and δ 15 N is the indicator of trophic level (Layman et al, 2012).…”

Within‐individual trophic variability drives short‐term intraspecific trait variation in natural populations

“…Causes of variation in functional traits between populations can have genetic and/or environmental sources (Albert et al, ). In our study, both environment and genetic structure are quite different among populations (Fumagalli et al, ; Musseau et al, ). Previous studies on the same marble trout populations showed a strong genetic differentiation among populations despite a very restricted geographical range, much greater than for other fish species living in larger areas (Fumagalli et al, ).…”

“…The population‐level variability describes trait (e.g. trophic position, body length) variation between populations of a single species (Musseau et al, ); the between‐individual variability reflects the variation in traits among individuals of the same population (Darimont, Paquet, & Reimchen, ) and the within‐individual variability is the variation in the traits of one individual over time (Vander Zanden, Bjorndal, Reich, & Bolten, ). Whether ITV is better explained by differences between individuals or changes within individuals is a central question in functional ecology (Bolnick et al, ; Lehmann, Mfune, Gewers, Brain, & Voigt, ).…”