Is the Relationship between Body Size and Trophic Niche Position Time-Invariant in a Predatory Fish? First Stable Isotope Evidence

Abstract

Characterizing relationships between individual body size and trophic niche position is essential for understanding how population and food-web dynamics are mediated by size-dependent trophic interactions. However, whether (and how) intraspecific size-trophic relationships (i.e., trophic ontogeny pattern at the population level) vary with time remains poorly understood. Using archival specimens of a freshwater predatory fish Gymnogobius isaza (Tanaka 1916) from Lake Biwa, Japan, we assembled a long-term (>40 years) time-series of the size-dependence of trophic niche position by examining nitrogen stable isotope ratios (δ 15N) of the fish specimens. The size-dependence of trophic niche position was defined as the slope of the relationship between δ 15N and log body size. Our analyses showed that the slope was significantly positive in about 60% of years and null in other years, changing through time. This is the first quantitative (i.e., stable isotope) evidence of long-term variability in the size-trophic relationship in a predatory fish. This finding had implications for the fish trophic dynamics, despite that about 60% of the yearly values were not statistically different from the long-term average. We proposed hypotheses for the underlying mechanism of the time-varying size-trophic relationship.

Isotopic niches of sympatric native and exotic fish species in a Neotropical floodplain

“…Some researchers have demonstrated that changes in the diet during the ontogenetic development may result in alterations in trophic relationships both at population and species level (Nakazawa et al 2010). The native H. sp2 showed a significant difference in δ 15 N values between juveniles and adults although they are in the same trophic level.…”

Individual interaction data are required in community ecology: a conceptual review of the predator–prey mass ratio and more

“…The PPMR measures the body mass ratio of interacting predators and prey. Body size can be linked with interaction strength, which allows us to describe food webs mediated by size-dependent predation (Brose 2010;Nakazawa et al 2011). In this review, body size represents individual size, including the concept of age or developmental stage, rather than species-specific representative body size, because this review concerns the community consequences of ontogenetic growth.…”

“…This represents the simplest possible scenario (i.e., trophic module) for a stage-structured community. However, the concept applies to various community contexts, such as aquatic food webs, in which ontogenetic diet shifts of predatory fish from planktivore to benthivore couple energy flows in the surface and bottom waters (Nakazawa et al 2010, Briones et al 2012, plant-insect interactions, in which stage structures entangle herbivory and pollination networks (Altermatt and Pearse 2011;Ke and Nakazawa, unpublished data), and interface areas, in which metamorphosis of amphibians or aquatic insects connect aquatic and terrestrial ecosystems (Baxter et al 2005;Nakazawa 2015b). …”

“…Nearly all species, including plants ( Barton and Koricheva 2010;Boege et al 2011), exhibit ontogenetic niche shifts. Furthermore, ontogenetic niche shifts may occur more than once or continuously in an organism's lifespan, irrespective of maturation, due to increasing body mass (e.g., Nakazawa et al 2010, Briones et al 2012). In such situations, an ecological community can be considered a network of interactions among individuals rather than of species because each interspecific interaction is redefined as a network of individuals at different ontogenetic stages (Fig.…”

“…Predator-prey mass ratio as a tool for describing sizebased food webs A promising approach for describing individual interaction-mediated food webs is to analyze the PPMR in a focal system (Brose 2010;Nakazawa et al 2011). This idea is reasonable because a predator cannot efficiently utilize excessively large prey because of physical constraints on feeding (e.g., gape limitation) and because a predator should not target excessively small prey because of their limited nutritional values (Brose 2010).…”