Mercury exposure in an endangered songbird: influence of marsh hydrology and evidence for early breeding impairment
How does mercury pollution, which is influenced by the way water levels change in the Florida Everglades, impact the ability of the endangered Cape Sable Seaside Sparrow to successfully breed and sustain its population?
College of Health researcher(s)
Abstract
Songbird reproductive success can decline from consuming mercury-contaminated aquatic insects, but assessments of hydrologic conditions influencing songbird mercury exposure are lacking. We monitored breast feather total mercury (THg) concentrations and reproductive success in the U.S. federally listed endangered Cape Sable Seaside Sparrow (CSSS: Ammospiza maritima mirabilis) over three breeding seasons in the Florida Everglades. We used model comparison to explore the influence of annual hydrologic variation on adult CSSS THg concentrations, and tested mercury effects on individual reproductive success (individuals’ mate status, apparent nest success, and total productivity) that were scaled to estimates on population productivity using a demographic model. We identified four hydrologic models that explained annual variation in adult THg concentrations, with the top model showing a negative association between THg concentrations and drought length of the previous breeding season and a positive association between THg concentrations and dry-season water recession rate (model adjusted R2 = 0.82). Adult male mating probability declined by 63% across the range of THg concentrations observed. We found no mercury effect on CSSS nest success or total productivity. However, demographic modeling suggested the reduced mating could produce a 60% decrease in population productivity compared to a scenario with no THg impact. Our results suggest that CSSS mercury exposure is influenced by local hydrologic conditions that can increase early breeding failure (lack of breeding initiation) and potentially limit population productivity. This study is the first to describe CSSS mercury exposure and its potential reproductive costs at the individual and population levels.
FAQ: Mercury Exposure in Cape Sable Seaside Sparrows in the Florida Everglades
What is the primary focus of this study regarding the Cape Sable Seaside Sparrow (CSSS)?
This study investigates the levels of mercury exposure in the endangered Cape Sable Seaside Sparrow (CSSS) in the Florida Everglades, focusing on how marsh hydrology influences this exposure and its potential impact on the bird's reproductive success and population productivity. It is the first study to detail CSSS mercury exposure and its reproductive costs at both individual and population levels.
How do mercury levels in CSSS compare to those in other songbirds, and what does this imply?
CSSS breast feather total mercury (THg) concentrations, ranging from 1.9 to 28.6 µg/g dry weight, are notably high compared to most other songbirds, where mean insectivore THg concentrations are typically around 1.28 µg/g dry weight. While similar to some Saltmarsh Sparrows in tidal wetlands, these high and variable concentrations highlight the pervasive and complex issue of mercury contamination in the Everglades ecosystem.
What is the relationship between marsh hydrology and mercury exposure in CSSS?
The study found a strong link between CSSS THg concentrations and specific hydrologic conditions. Higher mercury levels were associated with wetter conditions in the previous year (shorter drought periods and deeper water during the pre-basic molt) and faster water recession rates prior to the breeding season. This suggests that the timing and magnitude of marsh drying and reflooding can significantly influence the availability of mercury in the food web that CSSS consume.
How does mercury exposure affect the reproductive success of individual CSSS?
Mercury exposure significantly impacts the early breeding success of male CSSS. The study found that male mating probability declined by approximately 63% across the range of observed THg concentrations. This indicates that higher mercury levels can prevent males from successfully initiating breeding, even if they establish territories and sing to attract mates. Interestingly, no significant mercury effect was found on nest success or the total number of chicks fledged after a pair had formed and started nesting.
What are the broader implications of mercury exposure on the CSSS population?
While individual nest success and total productivity post-nest initiation were not directly impacted, the reduced male mating probability due to mercury can severely limit the overall population productivity. The demographic model suggested that this mercury-induced reduction in breeding pairs could lead to a substantial 60% decrease in per-capita population growth compared to an optimistic scenario without mercury impact. This highlights the hidden but significant effect of mercury on population dynamics, particularly concerning pre-nesting behaviors.
Why are feathers used to monitor mercury exposure in CSSS, and what are the considerations?
Feathers were chosen as the monitoring matrix due to their non-invasive nature, which is ideal for a sensitive, endangered species. While blood is generally preferred for indicating recent mercury exposure, feathers can reflect long-term exposure, especially in non-migratory birds with consistently elevated THg levels like the CSSS. The study found no correlation between adult feather THg and collection date, supporting their use for long-term monitoring, though juvenile feather THg was negatively correlated with collection date, suggesting growth dilution.
What are the key takeaways for conservation efforts regarding CSSS and mercury contamination?
Conservation strategies for CSSS should not only focus on traditional hydrologic and fire management to protect habitat but also explicitly consider the impacts of mercury exposure. Hydrologic management that aims to reduce CSSS mercury exposure, such as promoting longer drought periods and slower water recession rates in the dry season, should be investigated as a complementary conservation tool. Additionally, future research should delve deeper into the spatiotemporal variability of mercury in CSSS prey and improve diet resolution to better understand the mechanisms of exposure.
Why is considering "early breeding failure" important when assessing mercury's impact on songbirds?
Studies often focus on reproductive endpoints after nest initiation, potentially underestimating the true impact of mercury. This study demonstrates that mercury significantly affects "early breeding failure" by reducing male mating probability, a stage that occurs before nest construction. This highlights that mercury-induced impairments, possibly through endocrine disruption affecting behaviors like mate recruitment, can prevent breeding from even beginning. Therefore, incorporating pre-nesting behaviors and broader demographic models is crucial for accurately assessing the full scope of mercury's detrimental effects on avian populations.