Shorebirds in a changing landscape: Assessing the top-down and bottom-up drivers of American oystercatcher (<i>Haematopus palliatus</i>) reproductive success in the Virginia barrier islands

Abstract

Understanding the factors that affect population growth is a vital component of the conservation of imperiled species. On the United States (U.S.) Atlantic and Gulf coasts, the American oystercatcher (Haematopus palliatus) is largely managed within a conservation framework that aims to identify and manipulate factors that limit reproductive success. Coordinated research and monitoring across the species' range have identified several key threats to the survival of nests and chicks, including predation, habitat loss and degradation, and human disturbance. However, the relative importance of those threats can change, and new threats may emerge, as coastal ecosystems dynamically respond to global climate change. In the Virginia barrier islands, where American oystercatcher reproductive success has been historically high due to the widespread availability of undeveloped coastal habitat, declines in the average annual productivity rate of American oystercatchers since 2016 may signal evolving threats to reproduction. To provide insight into changing threats to American oystercatcher productivity in Virginia, and to inform adaptive management, we investigated the drivers of American oystercatcher reproductive success on the Virginia barrier islands. We considered three components of American oystercatcher conservation and management: (1) interactions between American oystercatchers, their conspecifics, and a complex predator community; (2) altered availability of nesting habitat due to climate-driven changes within barrier island ecosystem; and finally, (3) community support for widespread protection of the barrier island landscape and participation in conservation initiatives designed to protect breeding American oystercatchers, such as adhering to island access restrictions and recreation guidelines. We first investigated nest and chick survival on Metompkin Island and Fisherman Island—two islands that represent a range of geomorphological and ecological conditions present within the Virginia barrier island system—in 2021–2023 using a combination of field-based surveys and automated radio telemetry methods. Next, we used remotely sensed data to explore how climate-driven ecosystem state change on the Virginia barrier islands may be driving changes in the abundance and distribution of American oystercatcher nesting habitat across the barrier island system from 2004–2021. Finally, we evaluated the short-term outcomes of a field-based high school environmental education program using pre- and post-program surveys of students to assess how environmental education may promote changes in knowledge and public attitudes related to shorebird conservation initiatives in Virginia. Through routine surveys of nests and radio-marked chicks, we found that the cumulative probability of a nest surviving the incubation stage was greater than 90%, while the cumulative probability of a chick surviving from hatch to fledging was approximately 51%. Thus, chick survival, rather than nest survival, may be limiting reproductive success in the Virginia barrier islands. Flooding was a primary cause of nest loss (32% of failed nests), and storm-driven overwash in late April and early May forced peak nesting to shift later in the breeding season as breeding pairs renested, particularly in 2022. Despite ongoing management of mammalian predators through lethal removal, reproductive success still appears to be limited through the top-down effects of predation on both stages of American oystercatcher reproduction, as both nests (22% of failed nests) and chicks (46.4% of marked chicks) were lost from non-mammalian predators, including raptors (e.g., peregrine falcons [Falco peregrinus] and great horned owls [Bubo virginianus]) and Atlantic ghost crabs (Ocypode quadrata). Remotely sensed data provided an opportunity to investigate factors for which we lacked data collected in situ. Using aerial orthoimagery of the barrier islands collected by the National Agriculture Imagery Program (U.S. Department of Agriculture), we found that habitat abundance increased by 228.26 ha from 2004–2016, and then decreased by 124.38 ha from 2016–2021. Across the 2004-2021 timespan, abundance of American oystercatcher nesting habitat (defined as sites where the relative probability of nesting is ≥ 90%) was both temporally and spatially variable within the Virginia barrier island systems, with changes on only a few islands (e.g., Metompkin Island, Cedar Island, and Cobb Island) driving most of observed system-wide trends. We suggest that temporal variability may be a result of patterns of storminess along the Virginia coast 2004–2021, while spatial variability is likely due to localized differences in the geomorphological and ecological processes that control habitat availability, including sediment dynamics (e.g., sand erosion versus accretion), dune-building processes, and vegetative succession. Finally, we used an evaluation of The Nature Conservancy's field-based environmental education program for high school students as a case study to demonstrate the value of incorporating environmental education into a multidisciplinary conservation program. Using surveys that assessed components of environmental literacy—including knowledge of ecological concepts, feelings of connection and stewardship toward the local ecosystem, and behavioral intentions—we examined short-term educational outcomes for students who participated in a formal field trip to one of the Virginia barrier islands. By comparing responses on surveys delivered immediately before and immediately after the field trip, we found that participation in the field trip increased students' knowledge about barrier island ecosystems and sense of place attachment. Additionally, students self-reported positive environmental literacy outcomes on the post-trip assessment, including increased interest in scientific learning, increased likelihood of participating in stewardship behaviors, and increased intentions to participate in conservation behaviors. Given the pace at which ecosystems are changing in response to climate change and anthropogenic drivers, it is expected that the factors limiting American oystercatcher population growth will also continuously change. To protect against population declines, shorebird managers will need to continuously monitor for change. They will also need to consider the use of creative management approaches, including ecosystem-based management to address complex predation and climate threats, and environmental education and outreach programs to promote broad community support for conservation initiatives.