Marshes as "Mountain Tops": Genetic Analyses of the Critically Endangered São Paulo Marsh Antwren (Aves: Thamnophilidae)

Abstract

Small populations of endangered species can be impacted by genetic processes such as drift and inbreeding that reduce population viability. As such, conservation genetic analyses that assess population levels of genetic variation and levels of gene flow can provide important information for managing threatened species. The São Paulo Marsh Antwren (Formicivora paludicola) is a recently-described and critically endangered bird from São Paulo State (Brazil) whose total estimated population is around 250-300 individuals, distributed in only 15 isolated marshes around São Paulo metropolitan region. We used microsatellite DNA markers to estimate the population genetic characteristics of the three largest remaining populations of this species all within 60 km of each other. We detected a high and significant genetic structure between all populations (overall FST = 0.103) which is comparable to the highest levels of differentiation ever documented for birds, (e.g., endangered birds found in isolated populations on the tops of African mountains), but also evidence for first-generation immigrants, likely from small local unsampled populations. Effective population sizes were small (between 28.8-99.9 individuals) yet there are high levels of genetic variability within populations and no evidence for inbreeding. Conservation implications of this work are that the high levels of genetic structure suggests that translocations between populations need to be carefully considered in light of possible local adaptation and that remaining populations of these birds should be managed as conservation units that contain both main populations studied here but also small outlying populations which may be a source of immigrants.

Fig 1. São Paulo Marsh Antwren distribution.

Locations where the São Paulo Marsh Antwren has been found along with release sites near the metropolitan region of the city of São Paulo. Letters indicate our study sites where samples were collected: São José dos Campos (SJC), Mogi das Cruzes (MC), and Salesópolis (SL). These represent the largest extant populations. The map was produced in Quantum GIS 2.2 [29]. The urban areas and reservoir contours were obtained by LANDSAT 8 imagery [30].

Fig 2. Population structure and first generation migrants.

(A) Proportional membership (Q) of each São Paulo Marsh Antwren in the genetic clusters inferred by Structure (K = 3). This graphic represents each individual as a bar, and its membership probability in each cluster. Individuals 1 to 26: Mogi das Cruzes population; Individuals 27 to 43: Salesópolis population, and individuals 44 to 57: São José dos Campos. Black arrows identify individuals indicated as first generation migrants. (B) Output graphics from Structure Harvester, indicating the higher value of Ln P(K) (left), and ΔK (right).