Print PageOne of the greatest challenges facing ecology is understanding shifts in ecosystems that are being reported across the globe. However, understanding whether a particular shift in an ecosystem is the result of directional anthropogenic influences is complicated by the discovery of long-term climate cycles, such as the Pacific Decadal Oscillation (PDO), that fluctuate between alternative phases on decadal timescales. Because many currently operating long-term studies were implemented in the 1970s, 1980s, and 1990s, many of these studies have collected data only under one phase of these decadal-scale climate cycles, making it unclear whether the long-term trajectories documented in some ecosystems represent long-term lagged responses to large-scale shifts in climate due to naturally occurring climate cycles. The PDO, which shifted phases in 1977 and again in 1998, has a large impact on precipitation across the arid southwest United States. The shift in 1977 brought increased winter precipitation, whereas the new phase is expected to bring significant decreases in winter precipitation. Using data on plants and rodents, we examine the long-term dynamics of a well-studied desert ecosystem near Portal, AZ to understand how it has responded to the transitions in the PDO that occurred in 1977 and 1998.
Results/Conclusions
From 1977 to 1998, many aspects of this desert ecosystem exhibited responses consistent with increased winter precipitation: higher abundance of annual plants in the winter vs. summer communities, a three-fold increase in shrub cover, and a major reorganization of the rodent community caused by the loss of grassland species and increases in shrubland species. As expected from the 1998 phase shift in the PDO, precipitation across the region has decreased. Since 1998, the Palmer Hydrological Drought Index for 10 of 12 years has indicated drought for the state of Arizona. As expected from this decline in precipitation, there are indications of a major reorganization. We document declines in the abundance of shallow-rooted shrubs, increased relative abundance of the summer annual community, declines in total rodent abundance, and in the relative abundance of rodent species that were dominant prior to the 1998 shift in the PDO (e.g., Bailey’s pocket mouse). However, other aspects of the ecosystem, such as the species richness and relative abundance distribution of the rodent community appear to be fairly resilient. While additional years of data are needed, early indications are consistent with predictions that long-term changes at this site have been generated by lagged responses to a multi-decadal climate cycle.
