Estimates of dietary overlap for six species of Amazonian manakin birds using stable isotopes

Comparative isotope ecology of western Amazonian rainforest mammals

Closed-canopy rainforests are important for climate and ecology, yet identifying this ecosystem in the fossil record is challenging. An existing paradigm for identification of closed-canopy rainforests using fossil mammal carbon isotope data is the presence of highly negative δ¹³C_diet values (<−31‰) in the herbivore community, as observed in modern equatorial African rainforest ecosystems. Our data from western Amazonian mammals, however, show that the absence of these values is not evidence for absence of closed-canopy rainforests. Our results also document that the proposed relationship between carbon isotope spacing variables and traditional dietary ecological classifications is not straightforward, and that better characterizations of the mixture of nutrients in animal diets are necessary to fully understand diet-tissue isotopic fractionation patterns.

Closed-canopy rainforests are important for climate (influencing atmospheric circulation, albedo, carbon storage, etc.) and ecology (harboring the highest biodiversity of continental regions). Of all rainforests, Amazonia is the world’s most diverse, including the highest mammalian species richness. However, little is known about niche structure, ecological roles, and food resource partitioning of Amazonian mammalian communities over time. Through analyses of δ¹³C_bioapatite, δ¹³C_hair, and δ¹⁵N_hair, we isotopically characterized aspects of feeding ecology in a modern western Amazonian mammalian community in Peru, serving as a baseline for understanding the evolution of Neotropical rainforest ecosystems. By comparing these results with data from equatorial Africa, we evaluated the potential influences of distinct phylogenetic and biogeographic histories on the isotopic niches occupied by mammals in analogous tropical ecosystems. Our results indicate that, despite their geographical and taxonomic differences, median δ¹³C_diet values from closed-canopy rainforests in Amazonia (−27.4‰) and equatorial Africa (−26.9‰) are not significantly different, and that the median δ¹³C_diet expected for mammalian herbivores in any closed-canopy rainforest is −27.2‰. Amazonian mammals seem to exploit a narrower spectrum of dietary resources than equatorial African mammals, however, as depicted by the absence of highly negative δ¹³C_diet values previously proposed as indicative of rainforests (<−31‰). Finally, results of keratin and bioapatite δ¹³C indicate that the predictive power of trophic relationships, and traditional dietary ecological classifications in bioapatite-protein isotopic offset expectations, must be reconsidered.

Variability in the routing of dietary proteins and lipids to consumer tissues influences tissue-specific isotopic discrimination

RATIONALE

The eco-physiological mechanisms that govern the incorporation and routing of macronutrients from dietary sources into consumer tissues determine the efficacy of stable isotope analysis (SIA) for studying animal foraging ecology. We document how changes in the relative amounts of dietary proteins and lipids affect the metabolic routing of these macronutrients and the consequent effects on tissue-specific discrimination factors in domestic mice using SIA. We also examine the effects of dietary macromolecular content on a commonly used methodological approach: lipid extraction of potential food sources.

METHODS

We used carbon ((13) C) and nitrogen ((15) N) isotopes to examine the routing of carbon from dietary proteins and lipids that were used by mice to biosynthesize hair, blood, muscle, and liver. Growing mice were fed one of four diet treatments in which the total dietary content of C4 -based lipids (δ(13) C = -14.5‰) and C(3) -based proteins (δ(13) C = -27‰) varied inversely between 5% and 40%.

RESULTS

The δ(13) C values of mouse tissues increased by approximately 2-6‰ with increasing dietary lipid content. The difference in δ(13) C values between mouse tissues and bulk diet ranged from 0.1 ± 1.5‰ to 2.3 ± 0.6‰ for all diet treatments. The mean (±SD) difference between the δ(13) C values of mouse tissues and dietary protein varied systematically among tissues and ranged from 3.1 ± 0.1‰ to 4.5 ± 0.6‰ for low fat diets and from 5.4 ± 0.4‰ to 10.5 ± 7.3‰ for high fat diets.

CONCLUSIONS

Mice used some fraction of their dietary lipid carbon to synthesize tissue proteins, suggesting flexibility in the routing of dietary macromolecules to consumer tissues based on dietary macromolecular availability. Consequently, all constituent dietary macromolecules, not just protein, should be considered when determining the relationship between diets and consumer tissues using SIA. In addition, in cases where animals consume diets with high lipid contents, non lipid-extracted prey samples should be analyzed to estimate diets using SIA.