Rewinding the invasion history of monk parakeets in Barcelona city: 1976-2022

Predicting the future abundance and distribution of introduced alien species is crucial to mitigate their impact on ecosystems, but this has been shown to be highly challenging. A good approach to obtain crucial clues to the root causes behind dynamic changes over time and space of invasive species is historical re-surveys. Barcelona holds one of the largest densities in Europe of monk parakeets Myiopsitta monachus, a highly successful invasive avian alien species. In this study, we evaluate population size, population growth rates and range expansion across the city, performing periodic nest and chamber counts from detection of the first nest in 1976 to 2022. Population estimates of monk parakeets during the study period showed a steady increase, reaching 6,444 ± 449 individuals in 2022. The population exhibited exponential growth with a mean population growth rate of r = 0.19 per year, which means a population doubling time of 3.7 years. Furthermore, two phases were evident: the first from 1976 to 1994 with a growth rate of 0.37 and a population doubling time of 1.9 years; and the second from 1999 to 2022 with a growth rate of 0.08 and a time to double the population of nine years. Moreover, we document the expansion of the range during our study through the colonisation of new areas, which fitted to a diffusion model for the whole period. Currently, the growth rate of the invasive monk parakeet population does not appear to be limited by resources, nest availability, disease, or predators and we expect them to continue increasing and expanding their range if no control measures are taken.