As exotic plant species invade ecosystems, ecologists have been attempting to assess the effects of these invasions on native communities and to determine what factors influence invasion processes. Although much of this work has focused on aboveground flora and fauna, structurally and functionally diverse soil communities also can respond to and mediate exotic plant invasions. In numerous ecosystems, the invasion of exotic plant species has caused major shifts in the composition and function of soil communities. Soil organisms, such as pathogenic or mutualistic fungi, have direct effects on the establishment, growth, and biotic interactions of exotic plants. An integrated understanding of how aboveground and belowground biota interact with exotic plants is necessary to manage and restore communities invaded by exotic plant species.

The last two decades have seen a virtual explosion in empirical research on the role of social interactions in the development of animals' behavioral repertoires, and a similar increase in attention to formal models of social learning. Here we first review recent empirical evidence of social influences on food choice, tool use, patterns of movement, predator avoidance, mate choice, and courtship, and then consider formal models of when animals choose to copy behavior, and which other animals' behavior they copy, together with empirical tests of predictions from those models.

Creative approaches at the interface of ecology, statistics, mathematics, informatics, and computational science are essential for improving our understanding of complex ecological systems. For example, new information technologies, including powerful computers, spatially embedded sensor networks, and Semantic Web tools, are emerging as potentially revolutionary tools for studying ecological phenomena. These technologies can play an important role in developing and testing detailed models that describe real-world systems at multiple scales. Key challenges include choosing the appropriate level of model complexity necessary for understanding biological patterns across space and time, and applying this understanding to solve problems in conservation biology and resource management. Meeting these challenges requires novel statistical and mathematical techniques for distinguishing among alternative ecological theories and hypotheses. Examples from a wide array of research areas in population biology and community ecology highlight the importance of fostering synergistic ties across disciplines for current and future research and application.

Using a carefully chosen set of examples, we illustrate the importance and ubiquity of quantitative reasoning in the biological sciences. The examples range across many different levels of biological organization, from diseases through ecosystems, and the problems addressed range from basic to applied. In addition to the overall theme that mathematical and statistical approaches are essential for understanding biological systems, three particular and interacting mathematical themes emerge. First, nonlinearity is pervasive; second, inclusion of stochasticity is essential; and third, issues of scale are common to all applications of quantitative approaches. Future progress in understanding many biological systems will depend on continued applications and developments in these three areas, and on understanding how nonlinearity, stochasticity, and scale interact.

The extensive construction of reservoirs over the past century has radically altered the environmental landscape on a global scale. Construction of dams on most large rivers has interrupted the connectivity of water flow and greatly increased the abundance of standing freshwater habitats. Reservoirs act as stepping-stones for the dispersal of exotic species across landscapes. A variety of passively dispersing species have invaded reservoirs, spread through interconnected waterways, and been transported to nearby disconnected habitats. We hypothesize that reservoirs are more readily invaded than natural lakes, because of their physiochemical properties, greater connectivity, and higher levels of disturbance. Here we summarize properties of reservoirs that would make them prone to invasions and discuss cases in which reservoirs have facilitated rapid range expansion. Our overview illustrates linkages between two important forms of global environmental change: the widespread manipulation of river flows and the accelerating spread of exotic species.

A recent Forum article by Vermeire and colleagues (BioScience 54: 689–695) presents a critique of claims made by various authors who advocate heightened prairie dog conservation. Vermeire and colleagues assert that the only existing historic rangewide estimate of prairie dog occupancy circa 1900 is “artificially high” as a result of human activities and drought during the decades 1880–1900. The authors' contention that prairie dog abundance was artificially high during those decades ignores data on long-term climatic patterns and the historic presence of bison, which suggest the equally likely hypothesis that if prairie dogs were expanding in the late 19th century, it was a rebound to historic levels, falling within an expected range of population fluctuation for this species. Moreover, given the highly variable and unstable environment in which prairie dogs exist, evidence of possibly large historic fluctuations in prairie dog populations should be taken into account as part of any conservation assessment and strategy for this species.