Carlos Martínez del Rio

STABLE ISOTOPES IN ANIMAL ECOLOGY: ASSUMPTIONS, CAVEATS, AND A CALL FOR MORE LABORATORY EXPERIMENTS

For decades, plant ecologists have used naturally occurring stable isotope ratios to disentangle ecological and physiological processes. The methodology can also become a very powerful tool in animal ecology. However, the application of the technique relies on assumptions that are not widely recognized and that have been rarely tested. The purpose of this communication is to identify these assumptions, to characterize the conditions in which they are not met, and to suggest the laboratory experiments that are needed to validate them. The ease with which isotopic data can be gathered and the growing popularity of the method are generating a large amount of data on the isotopic ecology of animals. The proper interpretation of these data demands that we identify the assumptions on which these inferences are based, and that we conduct comparative laboratory experiments to assess their validity.

A niche for isotopic ecology

Fifty years ago, GE Hutchinson defined the ecological niche as a hypervolume in n-dimensional space with environmental variables as axes. Ecologists have recently developed renewed interest in the concept, and technological advances now allow us to use stable isotope analyses to quantify these niche dimensions. Analogously, we define the isotopic niche as an area (in δ-space) with isotopic values (δ-values) as coordinates. To make isotopic measurements comparable to other niche formulations, we propose transforming δ-space to p-space, where axes represent relative proportions of isotopically distinct resources incorporated into an animals tissues. We illustrate the isotopic niche with two examples: the application of historic ecology to conservation biology and ontogenetic niche shifts. Sustaining renewed interest in the niche requires novel methods to measure the variables that define it. Stable isotope analyses are a natural, perhaps crucial, tool in contemporary studies of the ecological niche.

The effect of cold-induced increased metabolic rate on the rate of 13C and 15N incorporation in house sparrows (Passer domesticus)

Animals with high metabolic rates are believed to have high rates of carbon and nitrogen isotopic incorporation. We hypothesized that (1) chronic exposure to cold, and hence an increase in metabolic rate, would increase the rate of isotopic incorporation of both 13C and 15N into red blood cells; and (2) that the rate of isotopic incorporation into red blood cells would be allometrically related to body mass. Two groups of sparrows were chronically exposed to either 5 or 22°C and switched from a 13C-depleted C3-plant diet to a more 13C-enriched C4-plant one. We used respirometry to estimate the resting metabolic rate

IT TAKES GUTS (AND MORE) TO EAT FRUIT: LESSONS FROM AVIAN NUTRITIONAL ECOLOGY

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Physiological Correlates of Preference and Aversion for Sugars in Three Species of Birds

of birds exposed chronically to our two experimental temperatures. The allometric relationship between the rate of 13C incorporation into blood and body mass was determined from published data. The

Does Gut Function Limit Hummingbird Food Intake

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