Direct and Indirect Drivers of Grassland Bird Population Declines and Settlement Decisions Over Broad Spatial and Temporal Scales PDF Download

Author: Dylan J. Smith
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

View

Book Description
Many North American bird populations are declining, and of those, grassland bird populations are declining most rapidly, having been reduced by about half since 1970. Such declines are difficult to study, both because grasslands are characterized and maintained by disturbance, and because grassland birds have a high propensity for dispersal. The primary cause for population declines is habitat loss. For example, only ~14% of pre-European colonization eastern tallgrass prairie remains intact. Even where habitat remains, many species continue to decline, and these declines may be due to reductions in reproductive success. One potential cause of these local declines is the encroachment of woody vegetation on grasslands, which for grassland birds reduces the overall amount and degrades the quality of usable habitat. In addition, local demographic changes cause-and are caused by-regional- or continental-scale patterns. Thus, broad spatial approaches are needed to best understand the drivers of demographic change. In my first chapter, I sought to identify whether woody encroachment, via reductions in reproductive success, can explain changes in population abundance. I compiled and analyzed data on nest contents and nest success collected at Konza Prairie Biological station over nearly five decades. Nest success did not change since 1971, but the frequency of Brown-headed Cowbird (Molothrus ater) brood parasitism increased. In addition, grassland obligates were harder hit than species that prefer woodier habitat, supporting the idea that grassland degradation most strongly affects birds that rely on pristine prairie. In chapter 2, I tested alternative mechanisms linking the high degree of climatic variability in the Great Plains to inter-annual fluctuations breeding distribution and local abundance of a common grassland bird species, Grasshopper Sparrows (Ammodramus savannarum perpallidus). I used 11 years of citizen science data from eBird spanning the entire Great Plains to determine whether overwinter mortality, temperature during migration, or precipitation during the preceding year's growing season and vegetation phenology best predicted local abundance. The start date of the growing season and the total precipitation in the preceding year's growing season correlated most strongly with sparrows' interannual settlement decisions. Local abundance was highest in areas where the growing season started before March and where the preceding year's growing season precipitation was low. The drivers of population declines and movement decisions in grassland birds are complex and often indirect or delayed. As grasslands face ongoing anthropogenic threats ranging from agricultural development to fire suppression and climate change, identifying the distributional and demographic responses of grassland birds will be crucial to conserving this declining group of species.

Author: Tracey N. Johnson
Publisher:
ISBN:
Category : Grassland ecology
Languages : en
Pages : 161

View

Book Description
In grasslands, grazing by large ungulates can influence vegetation structure, composition, primary productivity, and ultimately, ecosystem functioning. While grazing represents a complex disturbance, grazing intensity largely determines the effects of grazing on vegetation. Structural and compositional changes in the plant community caused by grazing could have bottom-up effects on species and interactions at higher trophic levels. Thus, particular management strategies for domestic livestock in rangeland systems could exert a strong affect on grassland wildlife. Grassland-dependent songbirds may be particularly susceptible to the effects of domestic grazers because they depend on grassland vegetation for foraging and nesting. Domestic livestock may influence grassland-breeding bird populations by affecting settlement decisions, resource availability, or reproductive success. We investigated the effects of grazing intensity on grassland vegetation structure and songbird demography in a northwestern bunchgrass prairie using paddocks with experimentally-manipulated cattle stocking rates. We compared effects of four stocking rates (0, 14.4, 28.8, and 43.2 animal unit months) on songbirds using a randomized complete block design with four replicates of each stocking rate to address hypotheses regarding demography of grassland songbirds. Overall paddock-level vegetation structure decreased and structural heterogeneity of vegetation increased with higher stocking rates, and those effects carried over one-year post-grazing. However, most bird species were able to locate nesting sites with similar vegetation structure regardless of paddock-level effects of stocking rate. The exceptions were western meadowlarks and vesper sparrows; nests of these species in paddocks with higher stocking rates had less vegetative cover. Apparent nest density for grasshopper sparrows was negatively affected by higher stocking rates. Grazing treatment effects on songbird population density were restricted to negative effects of higher stocking rates on savannah sparrows, but this relationship was not observed until the post-treatment year. Songbird community composition differed between control and heavily-grazed paddocks, but diversity was not affected by stocking rate. Nest fates were evaluated to determine whether stocking rate influenced nest survival or cause-specific nest failure. Other variables such as vegetation structure and predator community, date, year, and nest age were included to help clarify which mechanisms might be responsible for differences in nest survival or failure rates among treatments. For our analysis, we introduce the use of a novel software package, McNestimate, to estimate the daily probability of nest survival and failure from specific causes. McNestimate estimates the probability of nest failure from competing causes when the exact dates of failure are unknown using a Markov Chain framework, and incorporates a model selection approach which allows the use of covariates to help identify variables important in explaining variation in the daily probability of nest failure. Nest predation rates increased with the age of the nest and throughout the breeding season, but were not affected by stocking rate. The probability of nest failure from adverse weather declined throughout the season, but the rate of decline depended on year. Nest failure rates due to trampling were higher in paddocks with higher stocking rates, but also depended on the number of days cattle were present during the nesting period. Patterns of overall probability of nest success were driven by predation patterns in the first year, but in the second year were strongly influenced by the chances of weather-related nest failure. Although starvation was not identified as a significant source of nest failure, grazing-induced changes to vegetation structure and composition could influence food availability for breeding songbirds, ultimately affecting the composition of nestling diets and nestling condition. To better understand the relationship between grazing intensity, nestling diet composition, and subsequent effects on nestling condition, we examined the invertebrate composition of nestling fecal samples. All species showed strong preferences for Lepidoptera (moths and butterflies) larvae, and partial preferences for Coleoptera (beetles) and Araneae (spiders). The proportion of preferred prey items was not affected by stocking rate. There were effects of bird species on the proportion of Araneae and Coleoptera and the proportion of Acrididae (short-horned grasshoppers) in the diet of western meadowlark nestlings decreased with high stocking rates. Growth rates for western meadowlarks and vesper sparrows were negatively affected by higher stocking rates. These results suggest that stocking rates can have variable effects on grassland songbird population and nest density depending on each species' habitat requirements. However, negative effects of high stocking rates on nest survival and nestling condition could have consequences for juvenile survival and recruitment. Overall, low-to-moderate stocking rates are likely compatible with many grassland bird species in northwest bunchgrass prairie, and although heavier livestock grazing may help create suitable vegetation structure for some songbird species, high stocking rates may influence grassland songbird diet quality, or have negative effects on nestling condition. We hypothesized that grazing intensity could influence the grassland songbird community through "bottom-up" effects on vegetation, but effects of grazing at different intensities did not translate directly through the food web to influence songbird populations as strongly as lower trophic levels. Processes responsible for changes in community composition such as immigration or emigration may not have had time to ensue during our short-term experiment; alternatively, sufficient spatial or temporal heterogeneity remained in the system, even at the highest grazing intensity, such that grazing-induced changes in lower trophic levels were irrelevant for most songbird species. Our results contribute to understanding grassland songbird demographic responses to different grazing intensities and identify specific mechanisms by which conservation measures for declining grassland bird populations can be improved.

Author: Sarah Jean Kiuama Frey
Publisher:
ISBN:
Category : Bird communities
Languages : en
Pages : 138

View

Book Description
Population trends and patterns in species distributions are the major currencies used to examine responses by biodiversity to changing environments. Effective conservation recommendations require that models of both distribution dynamics and population trends accurately reflect reality. However, identification of the appropriate temporal and spatial scales of animal response, and then obtaining data at these scales present two major challenges to developing predictive models. In heterogeneous forested mountain landscapes I examined: A) the relative drivers of climatic variability at fine spatial scales under the forest canopy ('microclimate'), B) the influence of microclimate on local-scale occupancy dynamics of bird communities, and C) the effects of spatial scale and imperfect bird detection on long-term avian population trends. Climate change has been predicted to cause widespread biodiversity declines. However, the capacity of climate envelope models for predicting the future of biodiversity has been questioned due to the mismatch between the scale of available data (i.e., global climate models) and the scales at which organisms experience their environment. Local-scale variation in microclimate is hypothesized to provide potential 'microrefugia' for biodiversity, but the relative role of elevation, microtopography, and vegetation structure in driving microclimate is not well known. If the microrefugia hypothesis is true, I expected to see areas on the landscape that remained relatively cooler (i.e., buffered sites). To test this, I collected temperature data at 183 sites across elevation and forest structure gradients in complex terrain of the H.J. Andrews Experimental Forest in the Cascade Mountains of Oregon, USA (Chapter 2). I used boosted regression trees, a novel machine learning approach, to determine the relative influence of vegetation structure, microtopography, and elevation as drivers of microclimate and mapped fine-scale distributions of temperature across the landscape. Models performed extremely well on independent data - cross-validation correlations between testing and training data were 0.69 - 0.98 for ten selected climate variables. Elevation was the dominant driver in fine-scale microclimate patterns, although vegetation and microtopography also showed substantial relative influences. For instance, during the spring-summer transition, maximum monthly temperatures observed in old-growth sites were 2.6°C (95% CI: 1.8 - 3.3°C) cooler than plantation sites and minimum temperatures during winter months were 0.6°C (95% CI: 0.4 - 0.8°C) warmer. This suggests that older forest stands mediate changes in temperature by buffering against warming during summer months and moderating cold temperatures during the winter. Climate is generally considered most influential on species distributions at large spatial scales; however much microclimate variability exists within regional patterns. I tested whether this high degree of microclimate variability has relevance for predicting species distributions and occupancy dynamics of the Andrews Forest bird community. I collected bird occurrence data in 2012 and 2013 at all 183 sites with fine-scale temperature measurements. I used dynamic occupancy models to test the effects of temperature on occupancy and apparent within-season bird movement while statistically accounting for vegetation effects and imperfect bird detection (Chapter 3). Most species (87%) exhibited within-season shifts in response to local-scale temperature metrics. Effects of temperature on within-season occupancy dynamics were as large or larger (1 to 1.7 times) than vegetation. However, individual species were almost as likely to shift toward warmer sites as toward cooler sites, suggesting that microclimate preferences are species-specific. My results emphasize that high-resolution temperature data provide valuable insight into avian distribution dynamics in montane forest environments and that microclimate is an important variable in breeding season habitat selection by forest birds. I hypothesize that microclimate-associated distribution shifts may reflect species' potential for behavioral buffering from climate change in complex terrain. Factors influencing population trends often differ depending on the spatial scale under consideration. Further, accurate estimation of trends requires accounting for biases caused by imperfect detection. To test the degree to which population trends are consistent across scales, I estimated landscape-scale bird population trends from 1999-2012 for 38 species at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire, USA and compared them to regional and local trends (Chapter 4). I used a new method - open-population binomial mixture models - to test the hypothesis that imperfect detection in bird sampling has the potential to bias trend estimates. I also tested for generalities in species responses by predicting population trends as a function of life history and ecological traits. Landscape-scale trends were correlated with regional and local trends, but generally these correlations were weak (r = 0.12 - 0.4). Further, more species were declining at the regional scale compared to within the relatively undisturbed HBEF. Life history and ecological traits did not explain any of the variability in the HBEF trends. However, at the regional scale, species that occurred at higher elevations were more likely to be declining and species associated with older forests have increased. I hypothesize that these differences could be attributed to both elevated rates of land-use change in the broader region and the fact that the structure of regional data did not permit modeling of imperfect detection. Indeed, accounting for imperfect detection resulted in more accurate population trend estimates at the landscape scale; without accounting for detection we would have both missed trends and falsely identified trends where none existed. These results highlight two important cautions for trend analysis: 1) population trends estimated at fine spatial scales may not be extrapolated to broader scales and 2) accurate trends require accounting for imperfect detection.

Author: Robert J. Fletcher
Publisher:
ISBN:
Category :
Languages : en
Pages : 416

View

Book Description
Tallgrass prairie has declined throughout the midwestern United States during the past two centuries, and migratory birds breeding in these habitats have also experienced precipitous population declines. One conservation strategy used to mitigate the effects of habitat loss and fragmentation is habitat restoration. I studied how both habitat fragmentation and restoration affect songbird populations breeding in grassland and wetland habitats in northern Iowa, 1999-2002. Most grassland birds tended to be less abundant near edges, yet birds avoided woodland edges more so than other types of edges. Edge avoidance could not be explained by changes in habitat structure. For Bobolinks (Dolichonyx oryzivorus), edge avoidance also increased near corners of sites, where multiple edges converged. These local patterns of edge avoidance were consistent with observed patterns of area sensitivity when scaling patterns up to fragmented landscapes using simulation modeling in neutral (randomized) landscapes. Moreover, regression modeling at landscape scales suggested that edge density metrics were better than landscape compositional metrics at explaining bird density within patches. Habitat restoration provided breeding habitat for many bird species, in which birds tended to occur at high densities in restored patches relative to other land cover types in the landscape. However, demographic data revealed a more complex pattern. In restored wetlands, reproduction was tied closely to interannual climate variation. In particular, nest predation was negatively correlated with water depth in wetlands. In restored grasslands, nest success tended to be relatively low for most species. Population projection models suggested that for Dickcissels (Spiza americana) and, to a lesser extent, Bobolinks, population growth rates were not high enough to be sustainable without immigration into the area ([mu]

Author: Paul D. Goriup
Publisher: International
ISBN:
Category : Nature
Languages : en
Pages : 268

View

Book Description

Author: Michael L. Morrison
Publisher: Johns Hopkins University Press
ISBN: 1421424711
Category : Science
Languages : en
Pages : 1017

View

Book Description
Wood, Robert M. Zink, Benjamin Zuckerberg

Author: Anika Mahoney
Publisher:
ISBN: 9781339441511
Category : Grasslands
Languages : en
Pages : 85

View

Book Description
Wind energy development is rapidly expanding in grassland habitat, but potential indirect effects to wildlife populations are largely unknown. As observed in other types of human development, wind energy infrastructure could alter plant and invertebrate communities, change soil and hydrology, generate visual and aural noise, and increase human activity in previously undeveloped areas. These changes to habitat can indirectly affect wildlife in many ways, for example, by altering trophic interactions or habitat use. Native grasslands in the United States have undergone extensive habitat loss and degradation, and subsequently, many grassland-associated bird species have experienced precipitous population declines. We assessed the indirect effects of wind energy development on the habitat use and reproductive success of two grassland songbirds, Horned Lark (Eremophila alpestris) and McCown’s Longspur (Rhynchophanes mccownii) in Wyoming. We modeled bird density in 2012 as a function of habitat characteristics and wind energy development. We estimated bird density at two scales of development- between two wind farms and two undeveloped sites, and within wind farms as a function of proximity to nearest wind turbine. Despite a general characterization of Horned Larks as disturbance-tolerant, the numbers of Horned Lark within wind farms were substantially lower near turbines. McCown’s Longspur numbers were higher near turbines in areas with high heterogeneity in bare ground, but showed no variation with heterogeneity of bare ground far from turbines. These responses indicate potential avoidance of turbines by Horned Lark, and a difference in habitat associations by McCown’s Longspur near turbines. We evaluated nest placement of Horned Lark and McCown’s Longspur in 2012 in relation to turbine proximity on three wind farm plots. The mean turbine proximity at each plot was no different than predicted by a random distribution, and the turbine-proximity values of observed nests were uniformly distributed, indicating no evidence of displacement of nests from turbines. Although Horned Lark occur in lower numbers near turbines, there was no evidence of avoidance of turbines in nest placement. We assessed the reproductive success of Horned Lark and McCown’s Longspur in 2011 and 2012 on 3 wind farms and 2 undeveloped sites. We evaluated multiple indices of nesting productivity (clutch size, size-adjusted nestling mass, daily nest survival rate, and number of young fledged per successful nesting attempt), and modeled these reproductive metrics as a function of habitat and wind energy development. We quantified wind energy development using several measures of disturbance: turbine proximity, turbine density, distance to developed edge, and the amount of developed and reclaimed area within the neighborhood of the nest, and identified which measure was most predictive of reproductive success. Turbine density was repeatedly the measure of wind energy development most strongly associated with avian reproductive success. The nestling mass and, in 2011, nest survival of Horned Lark were negatively related to increasing turbine density. Annual variation in the effect of infrastructure on nest survival indicates that infrastructure may only have negative consequences in correlation with other biotic or abiotic events, e.g., during years of high predator abundance. The nest survival of McCown’s Longspur was most strongly predicted by nest site and neighborhood habitat variables and turbine density within 1 km of the nest. Likelihood of McCown’s Longspur nest survival decreased from 54% to 5% as vegetation density increased, and the inclusion of turbine density and grassland area in the neighborhood amplified this relationship resulting in a 28% to

Author: Maiken Winter
Publisher:
ISBN:
Category : Dickcissel
Languages : en
Pages : 4

View

Book Description
Habitat loss and fragmentation on the breeding grounds appears to be a major cause of the apparent decline of many North American bird species. Habitat fragmentation can decrease bird populations through decreases of patch size, increase of edge habitat, and isolation of habitat fragments. These effects of habitat fragmentation have been well documented in forest-nesting birds. However, grassland-nesting birds are experiencing even greater and more consistent population declines, and the reasons for these declines are still poorly understood. Factors that can potentially influence density and nesting success of grassland-nesting birds on their breeding grounds can act at three spatial scales: within-patch scale (vegetation structure and management regime), local scale (patch size and proximity to edge habitat), and landscape scale (habitat surrounding the patch). Between 1995 and 1997 I investigated how factors on each of these three spatial scales affected density and nesting success of grassland-nesting birds in 13 fragments of native tallgrass prairie in southwestern Missouri, focusing on two severely declining passerines, the Henslow's Sparrow (Ammodramus henslowii) and the Dickcissel (Spiza americana). The goal of this thesis was not merely to show that environmental factors at different scales affect density and nesting success of grassland birds; the simultaneous measurement of landscape at multiple scales has additionally allowed me to demonstrate an interaction between the factors measured at these different scales, which has not previously been investigated. Further, most studies on the effects of habitat fragmentation on grassland-nesting birds have based their conclusions on census data only, although census data do not reliably indicate how a species is affected by habitat fragmentation. Low litter depth (particularly the result of haying), small patch size, close proximity (

Author:
Publisher:
ISBN:
Category : Bird populations
Languages : en
Pages : 165

View

Book Description
Grassland and shrub/scrub breeding birds have experienced severe population declines since the beginning of the Breeding Bird Survey in 1966 (Sauer et al. 2007). Habitat loss and degradation are likely the primary causes of decline (Herkert 1994, Warner 1994, Johnson and Igl 2001). Partners in Flight (PIF) continental population objectives call for managers to increase populations by up to 100% for several of the species found in the Big Barrens, north-central Tennessee and south-central Kentucky, such as Dickcissel (Spiza americana), Henslow's Sparrow (Ammodramus henslowii), and Prairie Warbler (Dendroica discolor; Rich et al. 2004). The quantity and quality of breeding season habitat are important in determining the future population trends of grassland birds throughout North America. Herkert (1995) identified the loss and degradation of breeding season habitat as the most likely cause of most population declines. It is important to determine the relative importance of habitat features for grassland birds on multiple scales in terms of population density as well as productivity. I compared population densities of breeding grassland birds in the Big Barrens with field characteristics (e.g. field size, vegetation measurements) to determine which habitat features promoted field use by each species (Chapter 2). I also monitored 39 Henslow's Sparrow nests and 122 Field Sparrow nests in the Big Barrens during the 2006 and 2007 breeding seasons (Chapter 3). Estimates of Mayfield (1961, 1975) nest success were within the documented ranges for both Henslow's Sparrow (23.8%, 95% CI: 10.7 - 40.5%) and Field Sparrow (15.4%; 95% CI: 9.5 - 23.1%). I also found evidence that Field Sparrows are at least double-brooded in the Big Barrens. In addition to my work in the Big Barrens during the breeding season, we also sampled the winter bird community in the Big Barrens and in eastern Tennessee from 2003-07 using a variety of methods, including mist netting, widely dispersed point counts, rope dragging transects, and line transects (Chapter 4). My results indicated that a variety of habitats, including disturbed fields and agricultural fields in addition to grasslands dominated by native vegetation, is necessary to support the entire winter grassland bird community in the mid-South.

Author: Mary Ann Cunningham
Publisher:
ISBN:
Category : Birds
Languages : en
Pages : 302

View

Book Description