Determinants of tree species preference of birds in oak–acacia woodlands of Central America

Abstract

In mid-elevation areas of Mesoamerica, Acacia pennatula commonly occurs in mixed woods with various species of oak. During a 1-y study in Nicaragua, we found the abundance of birds in acacia was far higher than the representation of this species in the mix of trees, whether this is estimated by number of individual trees, canopy cover or foliage cover. This higher abundance was probably related to the fact that acacias supported approximately three to four times the abundance of arthropods that were found in oaks and twice the biomass. Although oak foliage supported fewer arthropods, relatively more of them were large (>1 cm). The greater preponderance of small arthropods in acacias versus oaks was probably related to the small leaflet size of acacias. However, it is likely that the higher abundance of arthropods in acacias, particularly herbivorous species, was related to the higher nutritional content of the acacia foliage (crude protein, minerals, non-structural carbohydrates) and lower content of digestion-inhibiting compounds (structural carbohydrates, total phenolics, condensed tannins). The major defensive mechanisms of acacia are mechanical (thorns) or qualitative-defence chemicals (cyanogenic glucosides) that are apparently more effective against vertebrate than invertebrate herbivores. These observations support the hypothesis that the anti-herbivore defences of acacia are primarily directed against large mammalian herbivores, rendering the foliage highly palatable to arthropods.