Niche shifts lead to hybridization between twoAmazonaparrot sister-species introduced to Southern California

Abstract

Hybridization involving introduced species is becoming more common as humans modify landscapes and ecosystems. When two closely related species are introduced to the same area, their niche dynamics will dictate the level of sympatry and potential for hybridization. Amazona parrots offer a rare case where multiple closely related species have established as breeders in Southern California. Red-crowned Parrots (A. viridigenalis) and Lilac-crowned Parrots (A. finschi) are particularly interesting because they are sister species (Russello and Amato, 2004) with allopatric native ranges in Mexico. In Southern California, where they established in the 1980s and have since grown in numbers, they appear to occupy the same urban habitat. We sought to test whether introduced Red-crowned and Lilac-crowned parrots have shifted their niches compared to their native ranges, and if so, whether sympatry has led to hybridization. Using broad-scale environmental data collected from weather stations and orbiting satellites, we found that Red-crowned and Lilac-crowned parrots have partially divergent environmental niches in their native ranges, but now occupy the same environmental niche in their introduced range in Southern California. This new niche is largely different from what they experience in their respective home ranges, supporting a niche-shift model of species introduction. Due to this niche shift, the two species now come into contact across Southern California, leading to hybridization. Genomic markers support the existence of some recent hybrids as well as advanced backcrosses resulting from older hybridization events closer to the time of first introduction. Photographs from community scientists included as part of the Free-flying Los Angeles Parrot Project (FLAPP) also document hybrids, but underestimate their frequency compared to genetic data. Despite evidence for ongoing hybridization, the bimodal distribution of ancestry among the introduced population points to the existence of reproductive isolating mechanisms keeping the two species distinct. Further study is needed to understand if reproductive isolating mechanisms result from behavioral factors, such as conspecific flocking preferences, suggested by the community science data, or from genomic incompatibilities built up from a long history of isolation in Mexico. The integrity of these genetic lineages in Southern California carries important conservation implications, as both species are listed as endangered in their native ranges due to trapping for the pet trade and habitat loss.