Population structure and mitochondrial DNA variation in sedentary Neotropical birds isolated by forest fragmentation

The current worldwide concern about tropical deforestation raises questions about the sustainability of avian populations in isolated forest fragments. One of the most important issues concerns the sizes of forest fragments necessary to maintain populations and the genetic variation within them. We address this by: (1) using mtDNA sequence variation to infer aspects of the population structure of four species of understory birds from four sites in southern Costa Rican rainforest; and (2) determining whether forest fragmentation that has occurred in the last 50 years has had an effect on the amount of within-population variation for the species in question. High levels of between-population differentiation (D,,) were found over a relatively small geographic scale (<130 km) for white-breasted wood-wren (Henicorhina leurosticta), bicolored antbird (Gynmopithys leucaspis), and gray-headed tanager (Eucometis penicillata), suggesting that these species are highly sedentary and exhibit strong female philopatry. No mtDNA variation was found in Plain Antvireo (Dysithammus mentalis). In all three of the polymorphic species there was a significant decrease in mtDNA nucleotide diversity in populations isolated by forest fragmentation as compared to populations in contiguous primary forest. Even in relatively large (250-1000 ha) forest reserves, sedentary avian species have lost roughly half (range: 43-85%) of the nucleotide diversity in mtDNA over a relatively short period of time. Our results indicate that sedentary avian species in forest fragments isolated by clearing have undergone severe reductions in effective population size due to population bottlenecks perpetuated by prolonged isolation and potential edge effects.