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

Abstract

Intraspecific trait variability (ITV) maintains functional diversity in populations and communities, and plays a crucial role in ecological and evolutionary processes such as trophic cascades or speciation. Furthermore, functional variation within a species and its populations can help buffer against harmful environmental changes. Trait variability within species can be observed from differences among populations, and between‐ and within individuals. In animals, ITV can be driven by ontogeny, the environment in which populations live and by within‐individual specialization or variation unrelated to growth. However, we still know little about the relative strength of these drivers in determining ITV variation in natural populations. Here, we aimed to (a) measure the relative strength of between‐ and within‐individual effects of body size on ITV over time, and (b) disentangle the trophic changes due to ontogeny from other sources of variability, such as the environment experienced by populations and individual preferences at varying temporal and spatial scales. We used as a model system the endangered marble trout Salmo marmoratus, a freshwater fish living in a restricted geographical area (<900 km2) that shows marked changes in diet through ontogeny. We investigated two trophic traits, trophic position and resource use, with stable isotopes (δ15N and δ13C), and followed over time 238 individually tagged marble trout from six populations to estimate the trophic changes between and within individuals through ontogeny at three different time‐scales (short term: 3 months, medium term: 1 year and long term: 2 years). We found that the relative strength of between‐ and within‐individual effects of body size on trophic position and resource use change strongly over time. Both effects played a similar role in ITV over medium‐ and long‐term time‐scales, but within‐individual effects were significantly driving trophic variability over short‐term scales. Apart from ontogenetic shifts, individuals showed variability in trophic traits as big as the variability estimated between populations. Overall, our results show how the relative strengths of ITV drivers change over time. This study evidences the crucial importance of considering effects of time‐scales on functional variability at individual, population and species levels.

Biological traits approaches in benthic marine ecology: Dead ends and new paths

“…Plant ecology has much to share in terms of approaches for better elucidating intraspecific variability, with methods for comparing trait expression across the entirety of environmental gradients and for combining field observations with transplant experiments (“common garden experiments” in plant ecology terms) to tease apart genetic variation and trait plasticity (Ahrens et al, 2021 ; Anderegg et al, 2021 ). Vertebrate ecology also offers methodological insight, with researchers adapting functional diversity indices to incorporate intraspecific data (Manna et al, 2019 ) and statistical modeling of temporal multi‐population studies to disentangle ontogeny from environmental effects (Musseau et al, 2020 ). There are financial and logistical challenges in applying some of these methods to benthic ecology, where assemblages can be very diverse, physically remote, and species often rather small and difficult to study, but work in these other disciplines can guide further developments.…”

On the evolution of trophic position

“…The variation in trophic position relevant to understanding its evolution is that which arises from heritable phenotypic differences among individuals. There is growing evidence that variation in trophic position within populations can be correlated with heritable traits that are associated with foraging performance, such as body size, aspects of the foraging apparatus and behaviours (Cucherousset et al, 2011 ; Dumont et al, 2016 ; Matthews et al, 2010 ; McCarthy et al, 2004 ; Musseau et al, 2020 ; Post, 2003 ; Wagner et al, 2009 ). For instance, Matthews et al ( 2010 ) showed a correlation between trophic position and gill raker morphology in Gasterosteus aculeatus (threespine stickleback)—a heritable trait that is relevant for foraging performance in the pelagic habitat of lakes (Robinson, 2000 ).…”

Linking brain size in wild stream-dwelling brown trout with dietary supply of omega-3 fatty acids

“…δ 13 C values were mathematically corrected for lipid content following methods as described by Post et al (2007). The baseline 13 C value was further corrected to account for variability in basal resources across the sampling sites (Olsson et al 2009;Musseau et al 2020) using the following equation:…”