Individual specialists in a generalist population: results from a long-term stable isotope series

Abstract

Individual variation in resource use has often been ignored in ecological studies, but closer examination of individual patterns through time may reveal significant intrapopulation differences. Adult loggerhead sea turtles (Caretta caretta) are generalist carnivores with a wide geographical range, resulting in a broad isotopic niche. We microsampled scute, a persistent and continuously growing tissue, to examine long-term variation in resource use (up to 12 years) in 15 nesting loggerhead turtles. Using stable isotopes of nitrogen and carbon, we examined the resource use patterns (integration of diet, habitat and geographical location) and demonstrate that individual loggerheads are long-term specialists within a generalist population. We present our results in the context of a conceptual model comparing isotopic niches in specialist and generalist populations. Individual consistency may have important ecological, evolutionary and conservation consequences, such as the reduction of intraspecific competition.

Figure 1.

Conceptual model of three population patterns of isotope signatures representing resource use through time. Arrows track individuals, and each circle represents the δ¹⁵N value for a layer of inert tissue, which reflects resource use (integration of diet, habitat and geographical location). See text for discussion of the three strategies. (a) Specialist population–specialist individuals; (b) generalist population–generalist individuals and (c) generalist population–specialist individuals.

Figure 2.

(a) δ¹⁵N values of successive scute layers from 15 loggerheads. Each line represents all layers for one individual, noted with a unique symbol. (b) δ¹³C values with the same format and the same individual symbols as (a). Starting points and intervals vary for some individuals because layers were combined to provide sufficient samples for analysis. The number of layers reflects the thickness of the sample.