Asymmetric Contribution of Isotopically Contrasting Food Sources to Vertebrate Consumers in a Subtropical Semi‐arid Ecosystem

Abstract

Habitat heterogeneity is a primary ecological factor that is particularly pronounced in arid ecosystems. The Tehuacán valley is a subtropical semi‐arid ecosystem in which several species of columnar cacti and agave (i.e., CAM plants) constitute the dominant elements accompanied by patches of trees and shrubs (i.e., C3 plants). Vegetation in Tehuacán is isotopically heterogenous because CAM plants have less depleted δ13C values than C3 plants. Fruits and flowers of cactus and agaves offer abundant food to vertebrates, but their leaves might be less attractive to insects than the leaves of C3 plants. Therefore, we use carbon and nitrogen stable isotope analysis to test the hypothesis that C3 and CAM food would contribute asymmetrically to different guilds of birds and bats. We predict that granivorous and frugivorous birds and nectarivorous and frugivorous bats will consume a CAM diet, whereas insectivorous birds and bats will consume a C3 diet. Due to omnivory of bird and bat consumers, we predict that the importance of CAM food will decrease as the trophic level of the animal increases. Our results showed that CAM food predominated in plant‐eating birds and in some flower‐visiting bats, whereas C3 food predominated in insect‐eating bats and birds and frugivorous bats. Habitat heterogeneity in Tehuacán is important for conservation due to the asymmetric role of CAM and C3 food in the nutrition of different feeding guilds of vertebrates. Our study provides basic information to evaluate the potential impact of habitat loss on functional groups of consumers in a semi‐arid ecosystem.

Columnar cacti as sources of energy and protein for frugivorous bats in a semi‐arid ecosystem

“…Dietary reconstruction using δ 13 C values of blood samples confirmed the predominance of C 3 ‐derived food previously found in frugivorous bats in the study site (Herrera et al . ). The predominant use of CAM‐derived food to build tissue by bats seems to be limited to nectarivorous species as found previously in the Tehuacán Valley (Herrera et al .…”

“…The predominant use of CAM‐derived food to build tissue by bats seems to be limited to nectarivorous species as found previously in the Tehuacán Valley (Herrera et al . ). Pollen represents a rich source of protein provided by cactus to bats but its digestive processing is not optimal in frugivorous bats (Herrera & Martínez del Rio , Mancina et al .…”

“…Carbon stable isotope analyses of whole blood showed that cacti contributed significantly to tissue building in nectarivorous bats, but they were of low importance for frugivorous bats, which in turn relied heavily on C 3 ‐derived food sources (Herrera et al . ). However, the study also revealed the presence of remains of cactus fruits in the feces of some frugivorous bats (Herrera et al .…”

“…However, the study also revealed the presence of remains of cactus fruits in the feces of some frugivorous bats (Herrera et al . ). Other studies based on visual inspection of food remains in feces have revealed that frugivorous bats feed on cactus fruits and flowers in the Tehuacán Valley (Valiente‐Banuet et al .…”

“…Based on previous work in the study area (Herrera et al . ), we expected that bats relied primarily on C 3 ‐derived food to construct tissues and fuel oxidative metabolism. We focused our study to spring–summer when most flowering and fruiting of cactus and agaves occur, and complemented our stable isotope analysis approach with the examination of food remains in fecal samples.…”