Since 1989 we have monitored small mammal populations at a semiarid site in north-central Chile with a large-scale livetrapping grid complex. Selective exclusions of vertebrate predators or putative small mammal competitors, or both, have yielded relatively small or mostly transitory effects, or both, on small mammal population dynamics and plant community composition. During the study period 5 El Niño–high rainfall episodes have occurred lasting 1–3 years. Resident or core small mammals such as Abrothrix olivaceus, Phyllotis darwini, and Octodon degus experience dramatic fluctuations during and following rainfall pulses. Temporary resident or opportunistic species such as Oligoryzomys longicaudatus and A. longipilis disappear from the thorn scrub for varying periods of time. All species persist in more mesic nearby habitats near dry stream courses (aguadas). Since a 3-year high rainfall event in 2000–2002 mean annual rainfall has increased in this region, mainly due to a lack of prolonged droughts. Under these conditions, and building on a qualitative model proposed by Noy-Meir, long-lived species might become more abundant. Changes in the small mammal assemblage are consistent with these predictions; O. degus, a caviomorph rodent with a long life span, now comprises a more constant proportion of the small mammal biomass in the thorn scrub, and we have documented reduced variation in species diversity. Increased rainfall, a predicted consequence of global climate change in this region, might be leading to changes in small mammal assemblage structure and composition and ultimately will result in a more stable, less oscillatory assemblage in the thorn scrub. Additionally, invasive groups such as introduced lagomorphs and ephemeral plants might become more abundant in this community. The long-term consequences of changes in rainfall patterns due to El Niño Southern Oscillations (ENSOs), with important teleconnections to global-scale phenomena, will lead to diverse changes at the community level here.
Long-term studies are important for identifying the importance and strength of biotic interactions and abiotic effects such as those due to global climate change (GCC—Müller et al. 2010). The former can be subtle and difficult to detect at short timescales, requiring carefully designed experiments and long-term monitoring to tease apart multiple interactions and distinguish between top-down and bottom-up control (Hunter and Price 1992; Meserve et al. 2003; Power 1992). The latter necessitates baseline data on preexisting conditions and sustained monitoring during periods of climate variation.
Whereas small-scale “pulse” studies often yield definitive results regarding the role of biotic interactions such as competition and predation, they can be less useful in predicting long-term consequences of broadscale processes such as GCC where “press” (i.e., long-term) studies are more appropriate. Although evidence for GCC is now so pervasive as to be irrefutable (Intergovernmental Panel on Climatic Change 2007; Walther et al. 2002), there remain limited numbers of long-term studies that allow for tracking organismal responses to this change, particularly in arid–semiarid parts of the Southern Hemisphere. These regions are of special interest because increased frequency, duration, and magnitude of El Niño Southern Oscillation (ENSO) events are one facet of ongoing GCC (Diaz et al. 2001; Easterling et al. 2000; Herbert and Dixon 2002; Mann et al. 2000; Timmermann et al. 1999). In western South America increasing rainfall tends to occur during ENSO warm phases, especially in southwestern Peru and north-central Chile; concurrently, low rainfall occurs elsewhere, such as in Australia and southern Africa. Although dispute remains about linkages between ENSO and GCC (e.g., Diaz et al. 2001; Kleeman and Power 2000), evidence suggests that GCC already has altered the ENSO phenomenon (Fedorov and Philander 2000; Kerr 2004). Several stepwise shifts in climate appear to have occurred in the past 30 years, including one around 1976 when the eastern Pacific Ocean became warmer (World Meterological Organization 1992). Between 1976 and 1998 El Niños were larger, more persistent, and frequent; the 2 largest El Niños of the 20th century occurred in this period (Gergis and Fowler 2009). The implications of such changes for semiarid regions are diverse (Holmgren et al. 2006; Jaksic 2001). Increased rainfall leads to dramatic changes in ephemeral plant cover (Dillon and Rundel 1990; Gutiérrez et al. 2000a), but in multiple-year El Niño–high-rainfall events ephemeral plant cover actually can decrease in subsequent years (de la Maza et al. 2009; Gutiérrez et al. 2000b). Various organismal groups increase dramatically following El Niños, including small mammals (Lima et al. 2002, 2006; Meserve et al. 1995), their vertebrate predators (e.g., Arim and Jaksic 2005; Farias and Jaksic 2007; Jaksic et al. 1997), and birds