GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 5F4NA07OU417MMC7K

Recently, researchers investigated the foraging profiles of bird species in two separate eucalyptus forests in Australia: Dryandra in Western Australia and the Southern Tablelands, roughly 3000 km west in New South Wales. Despite their geographical separation, there is a broad overlap in species between the two locations. However, at Dryandra, a much larger proportion of species (61 percent) than at the Southern Tablelands (34 percent) are ground foragers. The high proportion of ground foragers in Dryandra might be explained by the openness of habitats there, that is, the absence of dense ground vegetation, and the lack of a continuous shrub layer. Ground foraging appears to be facilitated by an open habitat with area of bare ground. However, the researchers found that the Tablelands were also open with sparse to dense litter layers, abundant in woody debris, and had discontinuous or absent ground and shrub layers. Thus, differences in habitat structures between these areas and Dryandra cannot entirely explain the greater abundance of ground foragers in Dryandra. The researchers offered several hypotheses to explain the difference. First, there may be important differences in habitat structure that are not revealed by casual observation. For example, differences in tree heights and canopy complexity may contribute to differences in species richness and foraging behavior among bark- and foliage-foraging birds. Second, despite structural similarities, it is possible that there are differences between habitats in the abundance or availability of litter and ground-dwelling prey. Such differences, if they exist, may indicate fundamental differences between eucalypt ecosystems in how and where energy and nutrients are cycled, as well as in overall productivity. Finally, the differences in foraging profiles between Dryandra and Tablelands may be the result of historical changes in bird species as a consequence of changed grazing and fire regimens, the impact of introduced predators, such as foxes and feral cats, and logging following European settlement. The greatest impact of these processes is on ground-foraging and ground-nesting birds. Dryandra has not been free of these changes, but the impact may have been less or more recent with the result that Dryandra may retain a more natural or complete bird diversity relative to the Tablelands.