Deciphering how climatic variability modulates the interspecific interactions in natural systems is one of the most interesting and least studied ecological processes. 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 > 10 dry years, the so‐called megadrought, which had led to a new ecological benchmark in this region. We employ a diagnostic approach to analyze abundance data regarding two rodent species, Phyllotis darwini and Abrothrix olivacea , using a 33 year time series spanning from 1987 to 2019. Our population dynamic models provide evidence of competitive interactions within and between both species. This result is novel because we show that rainfall variability influences the degree of interspecific competition and that it 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.