Rodent outbreaks or irruptions in semiarid Chile are associated with rainfall pulses driven by the El Niño Southern Oscillation (ENSO). During the last decade, north-central Chile has experienced an almost uninterrupted sequence of dry years, the so-called megadrought, which had led to a new ecological situation in this region. We employ a diagnostic approach to analyze the abundance of data regarding two rodent species, Phyllotis darwini and Abrothrix olivacea, using a 33-yr time series spanning from 1987 to 2019. Our population dynamic models provide evidence of competitive interactions within and among both species of rodents. This result is novel since rainfall variability influences the degree of interspecific competition and is asymmetric. The diagnostic approach used here offers a way to develop simple population models that are useful for understanding the causes of population fluctuations and for predicting population changes under a climate change scenario. 

Rodent and precipitation data

The empirical data come from a time series of 33 years for Phyllotis darwini and Abrothrix olivacea abundances in the Reserve. These rodents were inventoried with mark-release live trapping (Sherman traps) on two sites (xeric and mesic slopes, 1.1 ha each) for four days in summer, autumn, winter, and spring, from 1987 to 2019. Each grid had 7 x 7 live-trapping stations spaced 15 m, covering an area of 105 x 105 m including a peripheral boundary strip of 7.5 m.  We calculated an index of population density based on the minimum number known alive (MNKA) per ha assuming a 7.5-m buffer around trapping grids. Although the use of closed Capture-Mark-Recapture (CMR) models for estimating population size is less biased, we preferred to use the MNKA index to be consistent with previous studies (Lima et al. 1999, 2002). Both xeric and mesic grids were activated simultaneously during three consecutive nights in either slope. All traps remained permanently in the field and were closed when not used. Traps were baited with rolled oats and checked each morning. The individuals captured were identified to species and information was collected on size, sex, and weight, and coded to avoid overestimating the count (see Lima et al. 1999 for more details). Precipitation data were obtained from Chile's climate explorer (CR)² (https://explorador.cr2.cl/).