Effects of a litter-disturbing bird species on tree seedling germination and survival in an Australian tropical rain forest

Abstract

The spatial and temporal variation in litter disturbance by a medium-sized bird, the chowchilla (Orthonyx spaldingii Ramsay, Orthonychidae) was documented, and its potential impacts on tree seed germination and early seedling survival in an Australian tropical rain forest experimentally investigated. Chowchilla disturbances occurred on ridges, slopes and drainages in all four seasons surveyed. In some areas litter was completely turned over as frequently as every 5 wk. Tethered seeds exposed to disturbance were moved farther and experienced significantly greater variation in litter cover than those protected from disturbance. When seeds of two canopy trees were placed in litter environments simulating those created by chowchillas, the seeds of one species showed significant effects of litter treatment on germination while those of the other showed no significant effects. Seedling cohorts of four tree species were followed for 6 wk soon after germination and all showed higher survival in vertebrate exclosures. The two most abundant species suffered 68% and 35% mortality in six wks, with the majority of that unambiguously due to chowchillas. It is argued that litter-disturbing birds increase litter heterogeneity and significantly impact early seedling mortality. These effects could be important in maintaining evenness and diversity of the seedling community.