Browsing by Author "Shriver,W. Gregory"
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Item Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone(Biomed Central Ltd, 3/22/16) Walsh,Jennifer; Shriver,W. Gregory; Olsen,Brian J.; Kovach,Adrienne I.; Jennifer Walsh, W. Gregory Shriver, Brian J. Olsen and Adrienne I. Kovach; Shriver, W G.Background: Evolutionary processes, including selection and differential fitness, shape the introgression of genetic material across a hybrid zone, resulting in the exchange of some genes but not others. Differential introgression of molecular or phenotypic markers can thus provide insight into factors contributing to reproductive isolation. We characterized patterns of genetic variation across a hybrid zone between two tidal marsh birds, Saltmarsh (Ammodramus caudacutus) and Nelson's (A. nelsoni) sparrows (n = 286), and compared patterns of introgression among multiple genetic markers and phenotypic traits. Results: Geographic and genomic cline analyses revealed variable patterns of introgression among marker types. Most markers exhibited gradual clines and indicated that introgression exceeds the spatial extent of the previously documented hybrid zone. We found steeper clines, indicating strong selection for loci associated with traits related to tidal marsh adaptations, including for a marker linked to a gene region associated with metabolic functions, including an osmotic regulatory pathway, as well as for a marker related to melanin-based pigmentation, supporting an adaptive role of darker plumage (salt marsh melanism) in tidal marshes. Narrow clines at mitochondrial and sex-linked markers also offer support for Haldane's rule. We detected patterns of asymmetrical introgression toward A. caudacutus, which may be driven by differences in mating strategy or differences in population density between the two species. Conclusions: Our findings offer insight into the dynamics of a hybrid zone traversing a unique environmental gradient and provide evidence for a role of ecological divergence in the maintenance of pure species boundaries despite ongoing gene flow.Item Long-term dynamics in local host-parasite interactions linked to regional population trends(Wiley-Blackwell, 8/18/16) Ladin,Zachary S.; D'Amico,Vincent; Baetens,Jan M.; Roth,Roland R.; Shriver,W. Gregory; Zachary S. Ladin, Vincent D�Amico, Jan M. Baetens, Roland R. Roth, and W. Gregory Shriver; Shriver, W G.;Roth, Roland RayTemporal changes in the relative abundances of host-parasite populations can influence the magnitude of the effects of corresponding interspecific interactions. When parasite populations are at relatively low abundance, the negative effects on host populations may be insignificant, but when parasite abundance increases beyond critical thresholds, they can have population limiting effects on the host. Here, we used data from a 40-yr demographic study on breeding Wood Thrushes (Hylocichla mustelina) and avian brood parasitic Brown-headed Cowbirds (Molothrus ater) in the mid-Atlantic United States to disentangle host-parasite interactions. The relative abundance for these two species has changed both locally and regionally over this time period with a reduction in host abundance coincident with an increase in the parasite population. We detected a fivefold increase in Brown-headed Cowbird parasitism rates of Wood Thrushes over the 40-yr time period leading to a reduction in Wood Thrush fitness (i.e.,adult survival, fecundity, and recruitment). After accounting for the effects of Wood Thrush age, individual, and annual and within-season variation in reproduction, we found that Wood Thrushes exhibited increased reproductive effort (produced more nests per year) as nest parasitism rates increased. Additionally, we found that as parasitism rates increased, both Wood Thrush clutch size and fecundity declined. In conjunction with widespread habitat loss and land use change on both wintering and breeding ranges, increasing rates of Brown-headed Cowbird parasitism are reducing Wood Thrush fitness, and are likely contributing to observed regional Wood Thrush population declines. Coordinated local and regional efforts to reduce Brown-headed Cowbird populations, particularly in fragmented landscapes, may help reduce the decline for Wood Thrushes, and likely other parasitized Neotropical migratory species.Item Scale-dependent effects of nonnative plant invasion on host-seeking tick abundance(Wiley-Blackwell, 3/25/16) Adalsteinsson,Solny A.; D'Amico,Vincent; Shriver,W. Gregory; Brisson,Dustin; Buler,Jeffrey J.; Solny A. Adalsteinsson,1,� Vincent D�Amico,2 W. Gregory Shriver,1 Dustin Brisson,3 and Jeffrey J. Buler1; Shriver, W G.;Buler, Jeffrey JohnNonnative, invasive shrubs can affect human disease risk through direct and indirect effects on vector populations. Multiflora rose (Rosa multiflora) is a common invader within eastern deciduous forests where tick-borne disease (e.g., Lyme disease) rates are high. We tested whether R.multiflora invasion affects blacklegged tick (Ixodes scapularis) abundance and at what scale. We sampled host-seeking ticks at two spatial scales: fine scale, within R.multiflora-invaded forest fragments; and patch scale, among R.multiflora-invaded and R.multiflora-free forest fragments. At a fine scale, we trapped 2.3 times more ticks under R.multiflora compared with paired traps 25m away from R.multiflora. At the patch scale, we trapped 3.2 times as many ticks in R.multiflora-free forests compared with R.multiflora-invaded forests. Thus, ticks are concentrated beneath R.multiflora within invaded forests, but uninvaded forests support significantly more ticks. Among all covariates tested, leaf litter volume was the best predictor of tick abundance; at the patch scale, R.multiflora-invaded forests had less leaf litter than uninvaded forests. We suggest that leaf litter availability at the patch scale plays a greater role in constraining tick abundance than the fine-scale, positive effect of invasive shrubs.Item Using regional bird community dynamics to evaluate ecological integrity within national parks(Wiley-Blackwell, 9/28/16) Ladin,Zachary S.; Higgins,Conor D.; Schmit,John Paul; Sanders,Geoffrey; Johnson,Mark J.; Weed,Aaron S.; Marshall,Matthew R.; Campbell,J. Patrick; Comiskey,James A.; Shriver,W. Gregory; Zachary S. Ladin, Conor D. Higgins, John Paul Schmit, Geoffrey Sanders, Mark J. Johnson, Aaron S. Weed, Matthew R. Marshall, J. Patrick Campbell, James A. Comiskey, and W. Gregory Shriver; Shriver, W G.Understanding how biological communities respond to global change is important for the conservation of functioning ecosystems as anthropogenic environmental threats increase. National parks within the United States provide unique ecological and cultural resources that can help conserve biodiversity and maintain ecological integrity, especially in heavily urbanized environments. Parks within the National Capital Region (NCRN) and Mid-Atlantic (MIDN) Networks, representing federally protected areas located within a mixed landscape of rural to urban areas, have been monitoring forest and grassland birds annually to evaluate long-term trends in bird community dynamics. Given increasing rates of decline in forest-and grassland-breeding songbirds in North America, understanding community-level trends in parks will help their preservation for future generations. We used point count data collected between 2007 and 2015 from 640 sampling locations to calculate a bird community index (BCI) to infer relative estimates of ecological integrity. Our objectives were to (1) quantify BCI in 17 national parks in the mid-Atlantic region, (2) test for relationships between BCI and the proportion of forest and Developmenteloped land cover types, (3) assess temporal variation in BCI, and (4) additionally test for differences in estimates of species detection probability between volunteer citizen scientists and paid observers. Mean BCI scores and ecological integrity ranks among parks ranged between 33.5 (low integrity) and 58.3 (high integrity), while the majority of parks had BCI scores ranging between 40.1 and 52.0 (medium integrity). For both networks, we found that BCI was positively related to the extent of forest cover, and for NCRN, the more heavily urbanized network, we found that BCI was negatively related to Developmenteloped land cover. Assessment of temporal changes in BCI within parks indicated that BCI was stable for 12 parks, increased in four parks, and decreased in one park within our study. Lastly, we detected no differences in species detection probability between citizen scientist-and paid observer-collected data which lends support for the future comparison of bird monitoring data in regional analyses across NPS I&M Networks. The continued evaluation of ecological integrity, through measuring bird community dynamics at regional scales, is important for conserving biological diversity.