Browsing by Author "Ladin,Zachary S."
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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 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.