Granivores and restoration: Implications of invasion and considerations of context-dependent seed removal.

摘要

Granivores are important components of sagebrush communities in western North America. These same regions are being altered by the invasion of the exotic annual Bromus tectorum (cheatgrass) that alters physical and biological dynamics in ways that appear to promote its persistence. This research directly relates to the restoration of B. tectorum-dominated systems in two inter-related ways. First, because these landscapes have large quantities of seeds applied during restoration, it is important to determine the major granivore communities in intact sagebrush communities and in nearby cheatgrass-dominated communities. Second, it is important to develop an understanding of patterns of seed harvest by granivores. In addition to the data chapters there are two review chapters; Chapter 1 highlights factors contributing to seed removal and Chapter 7 provides ecologically based techniques that could minimize the negative consequences of granivores during ecological restoration. Common groups of ants showed increased abundances; uncommon species and functional groups were generally negatively impacted by cheatgrass (Chapter 2). Conversely, rodents were negatively impacted by conversion to cheatgrass (Chapter 4). Ant seed removal was highly context-dependent (Chapter 3), depending on the background vegetation (large-scale among-patch effects), foraging distance from the nest mound (small-scale among-patch effects), and the presence of other seed species in mixture (within-patch effects). In addition, cheatgrass provided associational resistance to native seeds in mixture, meaning the presence of cheatgrass increased native seed survival. In Chapter 5 a novel statistical technique in the ecological sciences showed that rodents have marked preferences for some seeds over others and that more seeds were removed in sagebrush compared to cheatgrass-dominated sites, although associational effects among seed mixtures were not detected. In Chapter 6 we show that the amount of seed harvested depended on both intraspecific and interspecific seed density. B. tectorum seeds had associational susceptibility (increased harvest) in the presence of native seeds. Although the reciprocal effect may occur, we did not find statistical support for it. These sets of studies are not only of basic ecological interests, but are also important for developing management strategies for restoration of these degraded lands.