Showing papers by "Thomas J. Valone published in 1997"
TL;DR: This work documents the reorganization of an arid ecosystem that has occurred since the late 1970s, when the density of woody shrubs increased 3-fold and several previously common animal species went locally extinct, while other previously rare species increased.
Abstract: Natural ecosystems contain many individuals and species interacting with each other and with their abiotic environment. Such systems can be expected to exhibit complex dynamics in which small perturbations can be amplified to cause large changes. Here, we document the reorganization of an arid ecosystem that has occurred since the late 1970s. The density of woody shrubs increased 3-fold. Several previously common animal species went locally extinct, while other previously rare species increased. While these changes are symptomatic of desertification, they were not caused by livestock grazing or drought, the principal causes of historical desertification. The changes apparently were caused by a shift in regional climate: since 1977 winter precipitation throughout the region was substantially higher than average for this century. These changes illustrate the kinds of large, unexpected responses of complex natural ecosystems that can occur in response to both natural perturbations and human activities.
454 citations
TL;DR: The ability of path analysis to characterize direct and indirect interactions in a natural ecological system and a path diagram reflecting presumed relationships between kangaroo rats and two other small rodent species is evaluated.
Abstract: We evaluated the ability of path analysis to characterize direct and indirect interactions in a natural ecological system. Based on previous experimental work, we constructed a path diagram reflecting presumed relationships between kangaroo rats (Dipodomys spp.) and two other small rodent species. The technique was applied to examine the direct effects of competition and indirect effects mediated through vegetation. Path analysis gave varying and sometimes uninterpretable results when applied to data collected from unmanipulated and manipulated systems. The variation most likely can be attributed to some combination of inadequate specification of the path diagram, differences between unmanipulated and manipulated systems, varying responses to experiments of different duration, and natural temporal variation. Because these issues are applicable to many kinds of complex systems, we suggest caution before applying and when interpreting the results of path analysis.
62 citations