Extreme climatic events shape arid and semiarid ecosystems

tally a climatic and oceanographic phenomenon, but it has profound effects on terrestrial ecosystems as well. Although the ecological effects of ENSO are becoming increasingly known from a wide range of terrestrial ecosystems (Holmgren et al. 2001; Wright 2005), their impacts have been most intensively studied in arid and semiarid systems. Because inter-annual variability in precipitation is such a strong determinant of productivity in dry ecosystems, increased ENSO rainfall (Panel 1) is crucial for plant recruitment, productivity, and diversity in these ecosystems. Several long-term studies show that this pulse in primary productivity causes a subsequent increase in herbivores, followed by an increase in carnivores, with consequent changes in ecosystem structure and functioning that can be quite complex and pervasive. We begin by discussing the effects on plant communities, then move to studies of populations of higher-level consumers and entire food webs. Where appropriate, we discuss the implications of ENSO events for land-use practices, natural resource management, and humans. We conclude with a brief synthesis, including suggestions for additional work needed in this important area.