INSIGHTS INTO THE MAINTENANCE OF HIGH SPECIES DIVERSITY IN THE NEOTROPICS: HABITAT SELECTION AND FORAGING BEHAVIOR IN UNDERSTORY BIRDS OF TROPICAL AND TEMPERATE FORESTS

Abstract

For almost three decades, structural habitat complexity has been regarded as a primary ecological factor responsible for maintaining high species diversity of trop- ical bird communities. However, differences in habitat complexity between temperate and tropical forests have not been documented sufficiently. Differences between tem- perate and tropical forests were quantified by measuring 36 variables related to under- story habitat structure. Structural components of temperate and tropical habitats differed significantly in several features. However, no differences were found in overall habitat heterogeneity (i.e. complexity) between temperate and tropical forests. To search for "bird-related" factors important in maintaining high tropical species diversity, we com- pared habitat selection between foliage-gleaning insectivorous birds in temperate and tropical forest understories. The tropical species were more specialized in horizontal and vertical habitat selection, and had lower "niche breadths" in foraging substrate and in foraging height. Tropical species also showed less interspecific overlap in most foraging variables than did temperate species. Therefore, higher species diversity, at least within this guild of birds in tropical and temperate forest understories, can be attributed at the proximate level to greater specialization and "tighter species packing," and may be more independent of greater habitat complexity than previously thought. RESUMEN.--Por casi tres decadas, la complejidad estmctural del habitat se ha consi- derado como el principal factor ecologico responsable de mantener la alta variedad de especies encontrada en comunidades de pajaros tropicales. Sin embargo, la diferencia en complejidad del habitat entre bosques templados y bosques tropicales no ha sido sufi- cientemente quantificada. Nosotros quantificamos diferencias entre bosques templados y bosques tropicales midiendo 36 variables relacionadas a la estructura del habitat bajo el dosel y encontramos varias diferencias significativas en componentes estructurales entre habitats tropicales y templados. Sin embargo, no encontramos diferencias en la hetero- geneidad del habitat (complejidad) entre bosques templados y tropicales. Buscando fac- tores que rueran importantes en mantenet la alta diversidad de las especies de pajaros tropicales, comparamos la seleccion del habitat entre pajaros insectivoros que usan el follaje bajo el dosel en bosques templados y tropicales. Las especies tropicales estaban mas especializadas en su seleccion de habitats al nivel horizontal y vertical, teniendo pequenos nichos en el substrato y altitud del area donde forrajeaban. En la mayoria de las variables relacionadas al forraje, las especies tropicales tambien demonstraron menos traslapo entre-especies que las especies templadas. Concluimos pues que la alta variedad de especies, pot lo menos dentro de este grupo de pajaros que viven bajo el dosel en bosques templados y tropicales, se puede atribuir inicialmente a mayor especializacion y concentracion de especies y puede que sea mas independiente de la total complejidad del habitat de lo que actualmente se cree.