David N. Ewert

Bio: David N. Ewert is an academic researcher from The Nature Conservancy. The author has contributed to research in topics: Endangered species & Population. The author has an hindex of 15, co-authored 35 publications receiving 855 citations. Previous affiliations of David N. Ewert include Connecticut College.

Conserving stopover sites for forest-dwelling migratory landbirds

Abstract: 'The Nature Conservancy, Migratory Bird Program, 1303 Rio Grande Boulevard NW, Suite 5, Albuquerque, New Mexico 87104, USA; 2Department of Biological Sciences, University of Southern Mississippi, P.O. Box 5018, Hattiesburg, Mississippi 39406, USA; 'The Nature Conservancy, Great Lakes Program, 2840 East Grand River Avenue, Suite 5, East Lansing, Michigan 48823, USA; 4The Nature Conservancy, Migratory Bird Program, 76 Emery Street, Portland, Maine 04102, USA; 'The Nature Conservancy, Migratory Bird Program, 633 West Main Street, Madison, Wisconsin 53703, USA; 6The Nature Conservancy, Southern U.S. Region, 6114 Fayetteville Road, Suite 109, Durham, North Carolina 27713, USA; and 7U.S. Fish and Wildlife Service, 6578 Dogwood View Parkway, Suite B, Jackson, Mississippi 39213, USA

Positive relationships between association strength and phenotypic similarity characterize the assembly of mixed-species bird flocks worldwide.

Abstract: Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrices from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a heterospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly.

Seasonal survival estimation for a long-distance migratory bird and the influence of winter precipitation

Abstract: Conservation of migratory animals requires information about seasonal survival rates. Identifying factors that limit populations, and the portions of the annual cycle in which they occur, are critical for recognizing and reducing potential threats. However, such data are lacking for virtually all migratory taxa. We investigated patterns and environmental correlates of annual, oversummer, overwinter, and migratory survival for adult male Kirtland’s warblers (Setophaga kirtlandii), an endangered, long-distance migratory songbird. We used Cormack–Jolly–Seber models to analyze two mark–recapture datasets: 2006–2011 on Michigan breeding grounds, and 2003–2010 on Bahamian wintering grounds. The mean annual survival probability was 0.58 ± 0.12 SE. Monthly survival probabilities during the summer and winter stationary periods were relatively high (0.963 ± 0.005 SE and 0.977 ± 0.002 SE, respectively). Monthly survival probability during migratory periods was substantially lower (0.879 ± 0.05 SE), accounting for ~44% of all annual mortality. March rainfall in the Bahamas was the best-supported predictor of annual survival probability and was positively correlated with apparent annual survival in the subsequent year, suggesting that the effects of winter precipitation carried over to influence survival probability of individuals in later seasons. Projection modeling revealed that a decrease in Bahamas March rainfall >12.4% from its current mean could result in negative population growth in this species. Collectively, our results suggest that increased drought during the non-breeding season, which is predicted to occur under multiple climate change scenarios, could have important consequences on the annual survival and population growth rate of Kirtland’s warbler and other Neotropical–Nearctic migratory bird species.

Mapping tropical dry forest height, foliage height profiles and disturbance type and age with a time series of cloud-cleared Landsat and ALI image mosaics to characterize avian habitat

Abstract: Remote sensing of forest vertical structure is possible with lidar data, but lidar is not widely available. Here we map tropical dry forest height (RMSE = 0.9 m, R2 = 0.84, range 0.6–7 m), and we map foliage height profiles, with a time series of Landsat and Advanced Land Imager (ALI) imagery on the island of Eleuthera, The Bahamas, substituting time for vertical canopy space. We also simultaneously map forest disturbance type and age. We map these variables in the context of avian habitat studies, particularly for wintering habitat of an endangered Nearctic-Neotropical migrant bird, the Kirtland's Warbler (Dendroica kirtlandii). We also illustrate relationships between forest vertical structure, disturbance type and counts of forage species important to the Kirtland's Warbler. The ALI imagery and the Landsat time series are both critical to the result for forest height, which the strong relationship of forest height with disturbance type and age facilitates. Also unique to this study is that seven of the eight image time steps are cloud-cleared images: mosaics of the clear parts of several cloudy scenes. We created each cloud-cleared image, including a virtually seamless ALI image mosaic, with regression tree normalization. We also illustrate how viewing time series imagery as red–green–blue composites of tasseled cap wetness (RGB wetness composites) aids reference data collection for classifying tropical forest disturbance type and age. Our results strongly support current Landsat Program production of co-registered imagery, and they emphasize the value of seamless time series of cloud-cleared imagery.

Impact of Hurricane Hugo on bird populations on St. John, U.S. Virgin Islands

Abstract: Bird populations were surveyed in Virgin Islands National Park, St. John, U.S. Virgin Islands in 1987 (two years before Hurricane Hugo hit the Virgin Islands) and in 1990 (four months after the hurricane). Abundance was estimated using the fixed-radius point count method. The average number of individual permanent residents per survey point was significantly lower after the hurricane in both moist forest and dry evergreen woodland. Most of the resident species that showed substantial population declines feed primarily on fruit or nectar, a pattern which is consistent with the results of several other recent studies of the effect of hurricanes on forest bird communities. Also, an insectivorous winter resident (Parula americana) had significantly lower densities after the hurricane. Although many areas within the park were defoliated after the hurricane, most trees remained standing and they began to produce new leaves within a few weeks. Our results suggest that even relatively mild storm damage can result in a marked reduction in the numbers of some species of birds due either to mortality or dispersal from the affected area.

Opening the archive: How free data has enabled the science and monitoring promise of Landsat

Abstract: Landsat occupies a unique position in the constellation of civilian earth observation satellites, with a long and rich scientific and applications heritage. With nearly 40 years of continuous observation – since launch of the first satellite in 1972 – the Landsat program has benefited from insightful technical specification, robust engineering, and the necessary infrastructure for data archive and dissemination. Chiefly, the spatial and spectral resolutions have proven of broad utility and have remained largely stable over the life of the program. The foresighted acquisition and maintenance of a global image archive has proven to be of unmatched value, providing a window into the past and fueling the monitoring and modeling of global land cover and ecological change. In this paper we discuss the evolution of the Landsat program as a global monitoring mission, highlighting in particular the recent change to an open (free) data policy. The new data policy is revolutionizing the use of Landsat data, spurring the creation of robust standard products and new science and applications approaches. Open data access also promotes increased international collaboration to meet the Earth observing needs of the 21st century.

Analyzing animal movements using Brownian bridges.

Abstract: By studying animal movements, researchers can gain insight into many of the ecological characteristics and processes important for understanding population-level dynamics. We developed a Brownian bridge movement model (BBMM) for estimating the expected movement path of an animal, using discrete location data obtained at relatively short time intervals. The BBMM is based on the properties of a conditional random walk between successive pairs of locations, dependent on the time between locations, the distance between locations, and the Brownian motion variance that is related to the animal's mobility. We describe two critical developments that enable widespread use of the BBMM, including a derivation of the model when location data are measured with error and a maximum likelihood approach for estimating the Brownian motion variance. After the BBMM is fitted to location data, an estimate of the animal's probability of occurrence can be generated for an area during the time of observation. To illustrate potential applications, we provide three examples: estimating animal home ranges, estimating animal migration routes, and evaluating the influence of fine-scale resource selection on animal movement patterns.

Lidar sampling for large-area forest characterization: A review

Abstract: The ability to use digital remotely sensed data for forest inventory is often limited by the nature of the measures, which, with the exception of multi-angular or stereo observations, are largely insensitive to vertically distributed attributes. As a result, empirical estimates are typically made to characterize attributes such as height, volume, or biomass, with known asymptotic relationships as signal saturation occurs. Lidar (light detection and ranging) has emerged as a robust means to collect and subsequently characterize vertically distributed attributes. Lidar has been established as an appropriate data source for forest inventory purposes; however, large area monitoring and mapping activities with lidar remain challenging due to the logistics, costs, and data volumes involved. The use of lidar as a sampling tool for large-area estimation may mitigate some or all of these problems. A number of factors drive, and are common to, the use of airborne profiling, airborne scanning, and spaceborne lidar systems as sampling tools for measuring and monitoring forest resources across areas that range in size from tens of thousands to millions of square kilometers. In this communication, we present the case for lidar sampling as a means to enable timely and robust large-area characterizations. We briefly outline the nature of different lidar systems and data, followed by the theoretical and statistical underpinnings for lidar sampling. Current applications are presented and the future potential of using lidar in an integrated sampling framework for large area ecosystem characterization and monitoring is presented. We also include recommendations regarding statistics, lidar sampling schemes, applications (including data integration and stratification), and subsequent information generation.

Climate change, hurricanes and tropical storms, and rising sea level in coastal wetlands

Abstract: Global climate change is expected to affect temperature and precipitation patterns, oceanic and atmospheric circulation, rate of rising sea level, and the frequency, intensity, timing, and distribution of hurricanes and tropical storms. The magnitude of these projected physical changes and their subsequent impacts on coastal wetlands will vary regionally. Coastal wetlands in the southeastern United States have naturally evolved under a regime of rising sea level and specific patterns of hurricane frequency, intensity, and timing. A review of known ecological effects of tropical storms and hurricanes indicates that storm timing, frequency, and intensity can alter coastal wetland hydrology, geomorphology, biotic structure, energetics, and nutrient cycling. Research conducted to examine the impacts of Hurricane Hugo on colonial waterbirds highlights the importance of long-term studies for identifying complex interactions that may otherwise be dismissed as stochastic processes. Rising sea level and even modest changes in the frequency, intensity, timing, and distribution of tropical storms and hurricanes are expected to have substantial impacts on coastal wetland patterns and processes. Persistence of coastal wetlands will be determined by the interactions of climate and anthropogenic effects, especially how humans respond to rising sea level and how further human encroachment on coastal wetlands affects resource exploitation, pollution, and water use. Long-term changes in the frequency, intensity, timing, and distribution of hurricanes and tropical storms will likely affect biotic functions (e.g., community structure, natural selection, extinction rates, and biodiversity) as well as underlying processes such as nutrient cycling and primary and secondary productivity. Reliable predictions of global-change impacts on coastal wetlands will require better understanding of the linkages among terrestrial, aquatic, wetland, atmospheric, oceanic, and human components. Developing this comprehensive understanding of the ecological ramifications of global change will necessitate close coordination among scientists from multiple disciplines and a balanced mixture of appropriate scientific approaches. For example, insights may be gained through the careful design and implementation of broad-scale comparative studies that incorporate salient patterns and processes, including treatment of anthropogenic influences. Well-designed, broad-scale comparative studies could serve as the scientific framework for developing relevant and focused long-term ecological research, monitoring programs, experiments, and modeling studies. Two conceptual models of broad-scale comparative research for assessing ecological responses to climate change are presented: utilizing space-for-time substitution coupled with long-term studies to assess impacts of rising sea level and disturbance on coastal wetlands, and utilizing the moisture-continuum model for assessing the effects of global change and associated shifts in moisture regimes on wetland ecosystems. Increased understanding of climate change will require concerted scientific efforts aimed at facilitating interdisciplinary research, enhancing data and information management, and developing new funding strategies.

Visible and invisible effects of hurricanes on forest ecosystems: an international review

Abstract: Hurricanes have visible and invisible effects on forests. The visible effects are dramatic, noticeable over the short-term and relatively well documented in the literature. Invisible effects are less understood as they require well-focused research both in the short- and long-term time scales. This review of the literature on hurricane effects focuses on the Neotropics and the temperate zone of North America. The material is organized according to a heuristic model that distinguishes between immediate effects (0 to 3 years), immediate responses (0 to 20 years), trajectories of responses (0 to 100 years) and long-term legacies (>100 years). It is suggested that the ecological role of hurricanes involves six principal effects: 1. they change the ecological space available to organisms; 2. they set organisms in motion; 3. they increase the heterogeneity of the landscape and the variability in ecosystem processes; 4. they rejuvenate the landscape and its ecosystems and redirect succession; 5. they shape forest structure, influence their species composition and diversity and regulate their function; and 6. they induce evolutionary change through natural selection and ecological creativity through self-organization. A new approach to hurricane research will study hurricanes at the same scale at which they operate (i.e., across latitudes and longitudes and over disturbed and undisturbed landscapes). This research will require networks of observation platforms located along expected hurricane paths to facilitate forest structure and functioning observations across gradients of hurricane frequency and intensity. This research will also require use of remote sensing and automated wireless technology, hardened to survive hurricane-strength winds and floods to assure real time measurements of the characteristics of hurricanes and ecosystem responses. No progress will be forthcoming in the understanding of hurricane effects if we do not learn to quantify objectively the energy dissipation of hurricanes on the full grid of affected forests as the hurricane passes over a landscape.