Impact of Non-Native Birds on Native Ecosystems: A Global Analysis

Abstract

Introduction and naturalization of non-native species is one of the most important threats to global biodiversity. Birds have been widely introduced worldwide, but their impacts on populations, communities, and ecosystems have not received as much attention as those of other groups. This work is a global synthesis of the impact of nonnative birds on native ecosystems to determine (1) what groups, impacts, and locations have been best studied; (2) which taxonomic groups and which impacts have greatest effects on ecosystems, (3) how important are bird impacts at the community and ecosystem levels, and (4) what are the known benefits of nonnative birds to natural ecosystems. We conducted an extensive literature search that yielded 148 articles covering 39 species belonging to 18 families -18% of all known naturalized species. Studies were classified according to where they were conducted: Africa, Asia, Australasia, Europe, North America, South America, Islands of the Indian, of the Pacific, and of the Atlantic Ocean. Seven types of impact on native ecosystems were evaluated: competition, disease transmission, chemical, physical, or structural impact on ecosystem, grazing/ herbivory/ browsing, hybridization, predation, and interaction with other non-native species. Hybridization and disease transmission were the most important impacts, affecting the population and community levels. Ecosystem-level impacts, such as structural and chemical impacts were detected. Seven species were found to have positive impacts aside from negative ones. We provide suggestions for future studies focused on mechanisms of impact, regions, and understudied taxonomic groups.

Fig 1. Naturalized and studied number of species in each region.

Number of naturalized species (black bars) and of studied species (grey bars) in the nine regions considered. Numbers between parentheses indicate the number of articles analyzed.

Fig 2. Relation between mean impact value and the number of studies conducted for every impact category.

Number of studies conducted for every impact category (x axis) and the corresponding impact mean (y axis). Bars represent the standard error. Vertical and horizontal lines are located at the mean of x and y axes respectively and separate points in four quadrants according to the relative number of studies and the level of impact. Categories “chemical, physical or structural impact on the ecosystem”, “grazing/herbivory/browsing”, and “interaction with other non-native species”, are abbreviated to “physical impacts”, “grazing”, and “interactions”, respectively.

Fig 3. Impacts of avian families on natural ecosystems.

Bars represent global impacts of the family and black points the average impact of the family. Only families with reported impacts are shown. Categories “chemical, physical or structural impact on the ecosystem”, “grazing/herbivory/browsing”, and “interaction with other non-native species”, are abbreviated to “physical impacts”, “grazing”, and “interactions”, respectively.

Fig 4. Relation between known impact of a species and its residence time.

Each point represents the event of introduction of one species to one region of the world. Multiple R-squared: 3.30e-05, Adjusted R-squared: -0.02, p = 0.97.

Fig 5. Relation between mean impact score by category and mean residence time.

Points represent the mean values and error bars the standard error. This graph shows only the four categories that have at least seven data points.