Contributions of C₃ and C₄ plants to higher trophic levels in an Amazonian savanna

Abstract

We studied the energy flow from C₃ and C₄ plants to higher trophic levels in a central Amazonian savanna by comparing the carbon stable-isotope ratios of potential food plants to the isotope ratios of species of different consumer groups. All C₄ plants encountered in our study area were grasses and all C₃ plants were bushes, shrubs or vines. Differences in δ¹³C ratios among bushes (x¯ = −30.8, SD = 1.2), vines (x¯ = −30.7, SD = 0.46) and trees (x¯ = −29.7, SD = 1.5) were small. However the mean δ¹³C ratio of dicotyledonous plants (x¯ = −30.4, SD = 1.3) was much more negative than that of the most common grasses (x¯ = −13.4, SD = 0.27). The insect primary consumers had δ¹³C ratios which ranged from a mean of −29.5 (SD = 0.47) for the grasshopper Tropidacris collaris to a mean of −14.7 (SD = 0.56) for a termite (Nasutitermes sp.), a range similar to that of the vegetation. However, the common insectivorous and omnivorous vertebrates had intermediate values for δ¹³C, indicating that carbon from different autotrophic sources mixes rapidly as it moves up the food chain. Despite this mixing, the frogs and lizards generally had higher values of δ¹³C (x¯ = −21.7, SD = 1.6; x¯ = −21.9, SD = 1.8, respectively) than the birds (x¯ = −24.8, SD = 1.8) and the only species of mammal resident in the savanna (x¯ = −25.4), indicating that they are generally more dependent on, or more able to utilise, food chains based on C₄ grasses.