Monk (Quaker) parakeets, Myiopsitta monachus, with nest.
Image: Arthur Grosset [larger view].
Invasive species are everywhere: from plants such as Scotch (English) broom, Cytisus scoparius, whose yellow flowers bloom prolifically along roadways of North America, Australia and New Zealand to mammals such as human beings, Homo sapiens, which are the ultimate invasive species because we have invaded nearly every habitat on the planet. The widespread introduction of exotic invasive species has modified habitats, reduced species biodiversity and adversely altered ecosystem functioning across the globe — as many as 80% of all endangered species are threatened due to pressures from non-native species. Economically, the annual cost to merely control the roughly 50,000 invasive species in the United States is estimated to be $120 billion — greater than the annual expenses incurred by the Iraq War. So the ecological and economic costs associated with invasive species is not trivial.
But most species lack the potential to be invasive. Thus, it is very important to learn more about the phenomenon of species invasiveness so we can better identify which attributes make some species so aggressive.
To do this work, a team of researchers led by Michael Russello, a population ecologist at the University of British Columbia, studied a species of parrot that is invading numerous habitat types across four continents: the grey-breasted parrot, Myiopsitta monachus.
The grey-breasted parrot is more commonly known as the monk or Quaker parakeet. These small green parrots are popular pets and avicultural subjects, which has led to them becoming widespread around the world. Less well-appreciated is the fact that monk parakeets are an important agricultural pest in their native range throughout the lowlands of South America.
Surprisingly, despite the rapid spread of monk parakeets around the globe and their potential as agricultural pests, little is known about the geographical history of their invasions. Such information may provide important insights into the mechanisms of invasion success and potential for future range expansions.
Currently, four subspecies of monk parakeets are formally recognized based on geographic variations in physical morphology, plumage coloration and nesting behavior (figure 1);
Distribution of Myiopsitta monachus across its native range in South America. Alternative shading denotes the individual ranges of the four subspecies including M. m. monachus (light gray), M. m. calita (black), M. m. cotorra (dark gray), and M. m. luchsi (striped). Localities of specimens sampled for this study are indicated by dots, with associated abbreviations following Table 1 (not shown). [larger view].
Russello and his colleagues plan to identify the taxonomic and geographic source of the invasive populations along the eastern seaboard of the United States, but they first assessed the genetic validity of the four recognized subspecies of monk parakeets, as detailed in the map above. To do this work, the researchers obtained tissue from 73 museum specimens from all four subspecies, and feather or tissue samples from 64 individuals from several locations along the eastern seaboard of the United States. They obtained sequence data obtained from a single fragment of the parrots’ mitochondrial DNA (mtDNA) control region (CR) and found a total of 17 distinct groups (mtDNA CR haplotypes) among these four subspecies. Using these molecular data, the researchers reconstructed a single haplotype network within which all haplotypes had a 95% probability of being parsimoniously connected (figure 2);
Network showing genealogical relationships among Myiopsitta monachus haplotypes sampled in the native range. Haplotypes are connected with a 95% confidence limit. The size of each oval is proportional to the frequency of the haplotype in the analysis. White dots represent mutational steps separating the observed haplotypes. Different shades represent the proportion of individuals of each subspecies exhibiting that particular haplotype (colors as in Figure 1). [larger view].
As you can see in the above figure, the haplotypes from one subspecies, M. m. luchsi (denoted with striped ovals), do not overlap with those from the other three subspecies haplotypes (solid colored ovals). Further, as you can see from the maze of intersecting lines, the haplotypes from the other three subspecies formed a mixed assemblage, exhibiting neither geographic structure nor clustering patterns consistent with currently described subspecies boundaries.
This gene tree indicates that M. m. luchsi is a monophyletic group, forming a distinct species (according to our current definition of what is a species), within a larger paraphyletic assemblage. Further, because M. m. luchsi can be reliably distinguished from the other subspecies only by its nesting behavior, it may be thought of as a cryptic species. These results suggest that monk parakeets merit a taxonomic revision.
When the team went on to identify single haplotypes among exotic monk parrot populations sampled in the United States, they found that only a few were represented, nearly all of which were found only in M. m. monachus (figure 3);
Bayesian haplotype tree depicting relationships among sampled Myiopsitta monachus haplotypes relative to their geographic and taxonomic distributions. The names of each haplotype are as in Figure 2. Bayesian posterior probabilities (> 50%) are indicated above the branches. Each column in the associated table is a locality sorted by country with abbreviations following Table 1. Each row is a haplotype according to its placement in the tree on the left; the number of individuals at that sampling locality exhibiting that particular haplotype is indicated in each cell. Shading represents the subspecies designation for the distribution of haplotypes according to Figure 1. Bolded italicized numbers indicate the distribution of individuals collected in the naturalized range in the United States. Total number of sampled individuals exhibiting each haplotype (N) is denoted in the last column. For illustration purposes, accurate branch lengths leading to the outgroup are not shown (indicated by dashed line). [larger view].
The the most common single haplotype found among introduced monk parakeets living along the eastern seaboard of the United States was named monachus01. In the Bayesian haplotype tree, monachus01 was the only haplotype detected among the sampled parrots from the populations in the Bridgeport, CT and Kent County, RI, and it was the most frequent haplotype in the Miami, FL and Edgewater, NJ populations. When assessed in wild populations, monachus01 was unique to populations of the M. m. monachus subspecies from Entre Rios, Argentina and Rio Grande do Sul, Brazil.
The second highest frequency haplotype, monachus02, was also unique to the M. m. monachus. This haplotype was found in the Miami, FL and Edgewater, NJ populations of monk parakeets and was recovered over a wide geographic area among wild monk parakeet populations: from those in Rio Grande do Sul, Brazil; Soriano, Uruguay; and throughout several localities in northern and central Argentina.
Preliminary morphometric analyses as well as trapping records agree with these genetic data, indicating that the vast majority of monk parakeets captured for the pet trade were M. m. monachus exported from eastern Argentina and Uruguay. The concordance between trapping records and genetic data support the widespread view that the invasion of monk parakeets has been facilitated, at least initially, by the international pet bird trade.
But once they were introduced, the small populations of monk parakeets increased. This is due to several factors. Monk parakeets have an unusual behavior for parrots: they construct large communal nests using sticks rather than nesting as individual pairs in pre-existing cavities, and they prefer to nest on man-made structures, such as telephone and utility poles. This effectively frees them from several powerful constraints that might otherwise limit their range expansion. Additionally, monk parakeets exploit bird feeders, gardens and farmland, allowing further expansion. Studying exotic populations of this invasive species provide opportunities to follow the dynamics of their geographic range expanion and secondarily, to monitor ecological and economic costs associated with an invasive species.
Of course, one thing that should be pointed out is that most of the eastern seaboard was home to another parrot species; the Carolina parakeet, Conuropsis carolinensis, which was hunted to extinction one hundred years ago. So even though the monk parakeet is an exotic species, it is viewed by some as being a reasonable east coast ecological equivalent to this long-lost native species.
Michael A Russello, Michael L Avery, Timothy F Wright (2008). Genetic evidence links invasive monk parakeet populations in the United States to the international pet trade. BMC Evolutionary Biology, 8 (1) DOI: 10.1186/1471-2148-8-217 [open access].