title: Naturally rare versus newly rare: demographic inferences on two timescales inform conservation of Galápagos giant tortoises.
abstract: Long-term population history can influence the genetic effects of recent bottlenecks. Therefore, for threatened or endangered species, an understanding of the past is relevant when formulating conservation strategies. Levels of variation at neutral markers have been useful for estimating local effective population sizes (N e ) and inferring whether population sizes increased or decreased over time. Furthermore, analyses of genotypic, allelic frequency, and phylogenetic information can potentially be used to separate historical from recent demographic changes. For 15 populations of Galápagos giant tortoises (Chelonoidis sp.), we used 12 microsatellite loci and DNA sequences from the mitochondrial control region and a nuclear intron, to reconstruct demographic history on shallow (past ∽100 generations, ∽2500 years) and deep (pre-Holocene, >10 thousand years ago) timescales. At the deep timescale, three populations showed strong signals of growth, but with different magnitudes and timing, indicating different underlying causes. Furthermore, estimated historical N e of populations across the archipelago showed no correlation with island age or size, underscoring the complexity of predicting demographic history a priori. At the shallow timescale, all populations carried some signature of a genetic bottleneck, and for 12 populations, point estimates of contemporary N e were very small (i.e., < 50). On the basis of the comparison of these genetic estimates with published census size data, N e generally represented ∽0.16 of the census size. However, the variance in this ratio across populations was considerable. Overall, our data suggest that idiosyncratic and geographically localized forces shaped the demographic history of tortoise populations. Furthermore, from a conservation perspective, the separation of demographic events occurring on shallow versus deep timescales permits the identification of naturally rare versus newly rare populations; this distinction should facilitate prioritization of management action.
authors.affiliation: Department of Biology, University of Mississippi Oxford, Mississippi, 38677.
authors.forename: Ryan C
authors.initials: RC
authors.lastname: Garrick
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University New Haven, Connecticut, 06520.
authors.forename: Brittney
authors.initials: B
authors.lastname: Kajdacsi
authors.affiliation: Department of Biology, University of British Columbia Okanagan Campus, Kelowna, British Columbia, V1V 1V7, Canada.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Russello
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University New Haven, Connecticut, 06520.
authors.forename: Edgar
authors.initials: E
authors.lastname: Benavides
authors.affiliation: Department of Biology, University of Mississippi Oxford, Mississippi, 38677.
authors.forename: Chaz
authors.initials: C
authors.lastname: Hyseni
authors.affiliation: College of Environmental Science and Forestry, State University of New York Syracuse, New York, 13210.
authors.forename: James P
authors.initials: JP
authors.lastname: Gibbs
authors.affiliation: Department of Applied Research, Galápagos National Park Service Puerto Ayora, Galápagos, Ecuador ; Biodiver S.A. Consultores Km 5 Vía a Baltra, Isla Santa Cruz, Galápagos, Ecuador.
authors.forename: Washington
authors.initials: W
authors.lastname: Tapia
authors.affiliation: Department of Ecology and Evolutionary Biology, Yale University New Haven, Connecticut, 06520.
authors.forename: Adalgisa
authors.initials: A
authors.lastname: Caccone
keywords: Conservation; Galápagos giant tortoise; demographic history; genetic diversity; population size