Course:CONS200/2023WT1/The recovery of Bald Eagle populations in North America

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Introduction

Bald Eagle (Haliaeetus leucocephalus)

The Bald Eagle (Haliaeetus leucocephalus) is native to most of North America, from the east to the west coast and from northern Canada to northern Mexico.[1] They have a wingspan of 2-2.5 meters and eat a variety of prey, including mainly fish, but also smaller animals such as other birds, small mammals, and reptiles.[2] They rely on a diverse range of ecosystems for habitat, including freshwater, estuarine, and coastal environments,[3] as well as large trees for nesting.[1] Since Bald Eagles occupy a wide range of ecosystems in various locations, Bald Eagles are exposed to more factors that threaten their population.

Population

Decline

During the 1950s to 1960s, Bald Eagles experienced a major decrease in population across North America, largely due to the destruction of their habitats, anthropogenic-induced disease, and their exposure to dichloro-diphenyl-trichloroethane (DDT) and other biomagnifying environmental contaminants.[3] The Bald Eagle was listed as ‘endangered’ under the Endangered Species Act (ESA) in most U.S. States in 1978,[3] with the number of nesting pairs reaching a historical low of 417 pairs in 1963.[1] The World Wildlife Fund defines a species that is “endangered” as a species that is considered to be facing a very high extinction risk in the environment.[4]

Bald Eagle populations remained prosperous in most of Canada and Alaska during their time of endangerment in the U.S.[5] The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) has classified Bald Eagles as 'not at risk', but recommends civilians keep their distance from the birds due to their high mortality rates, as nearly 50% of eaglets do not survive their first year.[6] Other classifications include 'least concern' by the International Union for Conservation of Nature (IUCN), 'secure' by Wild Species (Canada), and 'priority species' by Bird Conservation Region Strategy (Canada).[7] Although the current primary threat to Bald Eagles is the disturbance of their coastal and shoreline habitats, the species has proven to succeed in habituating human-dominated environments.[7]

Recovery & Carrying Capacity

Due to recent recovery efforts including legal protections and pesticide bans,[8] the number of nesting pairs increased to 10,000 as of 2007.[1] As populations recovered, the Bald Eagle’s status was updated from endangered to threatened in 1995, and removed from both lists in 2007.[1] Their rise in population is expected to plateau as they reach their carrying capacity and habitat becomes more scarce.[3] The World Wildlife Fund also defines 'threatened' as a species that is likely to become endangered if there is nothing done to reverse the factors contributing to the extinction or extirpation of the species.[9]

A population’s carrying capacity is explained by National Geographic as the increase of a population size until it fails to meet environmental requirements, such as adequate water, shelter, or food, which prevents further increases of the population size.[10] Unsuitable environmental conditions will continue to decrease the population until the necessary resources rebound. The carrying capacity is essentially the largest population size that an ecosystem is able to support without the population decreasing due to limiting factors.

Problems

Dichloro-diphenyl-trichloroethane

DDT

Dichloro-diphenyl-trichloroethane (DDT) is a synthetic insecticide developed in the 1940s and was used mainly as an insecticide in agriculture, livestock production, and gardening throughout North America prior to its ban in 1972 by the United States Environmental Protection Agency (EPA).[8] DDT has been proven to have negative physiological impacts on Bald Eagle reproduction and was a major contributor to the decline of the Bald Eagle population from the 1950s to 1960s.[3] Following heavy DDT use as an agricultural insecticide in the 1940s, DDT and its chemical residues eventually leached from agricultural lands into aquatic ecosystems,[11] where the chemicals were absorbed by aquatic plants and fish. DDT rapidly accumulated in Bald Eagle populations due to the process of biomagnification, which refers to the progressive increase in chemical contaminant concentrations with increasing animal trophic status.[12] Bald Eagles are apex predators and consume a diet mainly composed of fish,[3] and through consuming large amounts of DDT-contaminated fish, Bald Eagle populations accumulated a higher concentration of DDT than any other trophic level.[12]

Specific metabolites of DDT have been found to inhibit enzymes that are responsible for calcium transport in the plasma membrane of bird eggs which can lead to decreased eggshell weight and thickness in Bald Eagle eggs.[13] As a result, Bald Eagle eggshells affected by DDT would often break during incubation or otherwise fail to hatch,[11] which was especially impactful given that female Bald Eagles only lay one to three eggs per nesting attempt.[3] The thinning effect of DDT on eggshells contributed greatly to the historic decline of Bald Eagle populations across North America. Additionally, DDT is known to be highly persistent in organisms and aquatic environments due to its long half-life.[14] The eggshell thinning effects of DDT have been found to persist in birds for long periods of time after the individuals resume an uncontaminated diet,[14] further slowing Bald Eagle population recovery.

Habitat Loss

Aquatic Bald Eagle ecosystem

The degradation and loss of habitats pose a threat to Bald Eagles, as their populations are reaching a carrying capacity due to the lack of habitat for nesting and hunting.[3] Bald Eagles occupy a variety of ecosystems, including both freshwater and coastal environments.[3] They typically require 100 acres of undisturbed forest for nesting and roosting, as well as 50 adjacent acres of wetlands for hunting fish.[15] Nesting pairs prefer trees with sufficient branching and those in close proximity to a water source that contains potential prey.[3] Due to the scope of their habitats, recent events associated with climate change, such as sea level rise, coastal erosion, and increased hurricane intensity have affected many ecosystems that are considered important for nesting and hunting.[11] Eagle habitats have also been, and continue to be, threatened by anthropogenic development and other human disturbances,[11] such as urban expansion.

Bald Eagles tend to nest near aquatic environments, meaning their habitats tend to overlap areas where habitat destruction rates are higher.[11] The reliance of Bald Eagles on aquatic environments for hunting grounds and nesting areas puts their population at risk due to land degradation and sea level rise, both bound to increase with economic development and associated climate change effects.[11] As their available habitat decreases over time, Eagles are beginning to occupy locations unlike their natural habitats, which are less optimal as individuals will likely come into more contact with urban environments, humans, and their disturbances.[3] However, Bald Eagles are adept at living in landscapes with humans present,[7] which is one contributing factor to their recovery.

The loss of habitat, along with DDT poisoning, was a primary reason that prompted the listing of the Bald Eagle in the Endangered Species Act (ESA) in the United States,[11] and continues to be a threat today.

Disease

An american coot swimming in a lake and eating the cyanobacteria infected hydrilla.
An American coot (prey of Bald Eagles) ingesting the cyanobacteria infected Waterthyme.

First found in Bald Eagles in 1994, Vacuolar Myelinopathy (VM) or Avian Vacuolar Myelinopathy (AVM) is a disease that affects the brain with neurotoxins leading to neurological impairment, inhibiting motor function and leading to mortality.[16] The source of this disease is the cyanobacteria (Aetokthonos hydrillicola) that began to colonize riparian areas on anthropogenically introduced Waterthyme plants (Hydrilla verticillata).[16] The cyanobacteria is ingested as animals eat the plants and thus the neurotoxins are passed up the food chain to higher trophic levels.[16] It has been shown to accumulate in fish, reptiles, amphibians, waterfowl, and eventually Bald Eagles, leading to their death, similar to the bioaccumulation process that DDT is involved in.[16] A single outbreak of this disease in 1994 led to the death of over 70 Eagles in the subsequent two years around DeGray Lake in Arkansas and the very source of this mass mortality was a mystery for years.[16] The anthropogenically introduced and invasive species, commonly categorized as "Noxious weed", is now more prevalent than ever across the United States and is even spreading internationally, largely through means of shipping and aquarium usage.[17]

Solutions

Bans on Pesticides

Rachel Carson, author of 'Silent Spring', an influential book in the banning of DDT

In the late 1950s through to the 1960s, the U.S. Department of Agriculture began to take action to regulate and even prohibit DDT use as a pesticide due to growing evidence of its damaging environmental and toxicological effects.[8] The book Silent Spring, published in 1962 by conservationist Rachel Carson, played a large role in generating public awareness about the dangers of improper pesticide use.[12] While Silent Spring was met with strong opposition from pesticide and chemical companies, it created widespread public concern about the lack of regulations surrounding agricultural pesticide use throughout North America.[8] This shift in public opinion, combined with the substantial amount of evidence on the adverse environmental effects and potential harm to human health, led to the United States Environmental Protection Agency issuing a cancellation order for DDT in 1972, essentially making the general use of DDT illegal.[8]

In the years following the ban of DDT, Bald Eagle populations across North America began to slowly recover.[13] While DDT alone was not the sole contributor to the decline of Bald Eagle populations,[13] banning the use of DDT as a pesticide was an effective mitigation strategy that successfully addressed one of the main causes of the Bald Eagle population decline.

Laws and Regulations

The federal legislation enacting protection for Bald Eagles includes the Migratory Bird Treaty Act in 1918, the Endangered Species Act (ESA) in 1973, and the Bald and Golden Eagle Protection Act in 1940, among others.[3]

The Migratory Bird Treaty Act is a collaboration between the U.S., Canada, Mexico, Japan, and Russia that prohibits killing, capturing, selling, trading, and transporting (collectively referred to as 'taking') protected migratory birds, which includes Bald Eagles, without prior authorization from the U.S. Fish & Wildlife Service (USFWS).[18] The Endangered Species Act, developed by the USFWS in 1973, requires federal agencies to consult with the USFWS to ensure species listed under the act cannot be jeopardized by activities funded, authorized, or carried out by federal agencies themselves.[19] The Bald and Golden Eagle Protection Act was enacted in 1940 and states that anyone who violates a Bald Eagle individual (known as an American Eagle), nest, or egg will either be fined up to $5,000 or imprisoned for up to one year.[14] Both punishments could be sentenced simultaneously in certain cases, and in repeating cases, the penalties would increase.[14]

These pieces of legislation were vital and highly successful in terms of increasing the Bald Eagle population. The estimated nesting pairs in the United States increased from 417 pairs in 1963 to 9,789 in 2007.[14] These laws and regulations addressed some of the primary threats to Bald Eagles, such as hunting and habitat loss, leading to their change of classification under the ESA from 'endangered' to 'threatened' in 1995, and their complete removal of listing under the ESA in 2007.[14]

Way Forward

Private Property

Private property poses both a threat and an opportunity to Bald Eagle recovery. Since the Bald Eagle’s endangerment in the 1960s, regulatory agencies have classified areas of 660 feet around active Eagle nests to be ‘critical habitat’.[20] One study found that 80% of Bald Eagle nests and their associated buffer areas exist on privately owned property,[20] so the management of private lands is critical to the recovery of the species. In areas of critical habitat, certain restrictions on human activity are enforced, especially the prohibition of “taking” (pursuing, killing, capturing, or disturbing) Eagles, as enforced by the ESA.[20] The buffer zones around critical habitats provide Bald Eagles with a competitive advantage while ignoring jointly recovering species, presenting a pitfall of single-species recovery strategies,[21]

Bald Eagle nest in an urban environment

Restricting activities on private property in the name of conservation can diminish landowners’ support for conservation efforts, especially where compensation for such restrictions is lacking.[20] The use of DDT and AVM disease are no longer significant threats to Bald Eagles due to targeted legislation that has since been implemented, so the primary responsibilities of private landowners are to prevent further habitat loss, especially since Bald Eagles have shown success in habituating in urban areas.[20] Private landowners are encouraged to conserve mature forest stands and dead snags that may otherwise be cut down, especially along shorelines and water reservoirs,[3] as these are the favored habitats of Bald Eagles.

Due to the widespread occupation of the Bald Eagle, individual landowners reducing population-threatening actions is not as significant as it would be for species with localized distributions.[20] Thus, it is more vital to the conservation of Eagles to maintain public-private trust so as not to erode public support for endangered species policy.[20] Further restrictions on private land would need to be reconsidered if the Bald Eagle population were to become threatened or endangered once again.

Habitat Restoration

Bald Eagle hunting an Osprey

In the early 1980s, only four active Eagle nests were reported in the entire Great Lakes region.[15] In 2013, the first eaglets to hatch in the Canadian Great Lakes region occurred in a marsh area called Cootes Paradise.[15] Natural habitat restoration efforts enabled the re-entry of Eagles into the area, including temporary fencing around marshes to protect native species and prevent the entry of invasive species, the sorting of fish to ensure invasive species do not enter waterways under restoration, and planting trees in damaged Eagle habitat.[15] Other habitat restoration efforts have included the mapping of high-priority nesting habitat and the constructing of nesting platforms within those areas[22] to provide Eagles with a place to nest in the face of deforestation.

As habitat loss is currently one of the primary threats to Bald Eagles,[3] habitat restoration is one of the most optimal strategies to protect current Bald Eagle populations going forward. Without restoration, over 3.6 million hectares of swamp, marsh, and shoreline regions are expected to be lost in the United States over the next 50 years,[11] which serve as critical habitat for the Bald Eagle. Scientific research has found that over the next 20 to 50 years, the likelihood of Eagle nest occurrence is significantly higher with restoration efforts than with no action.[11] Restoration action will also expand the range of nesting Eagles, whereas no restoration effort further decreases available nesting habitat and makes existing habitat less suitable for nesting.[11] These findings can contribute to future management and decision-making opportunities.[11]

Nature Conservancy Canada (NCC) is another organization working towards the protection of Bald Eagle habitat since the 1990s.[23] Their efforts focus on Bald Eagle wintering grounds, particularly the small community of Brackendale just north of Vancouver, which sees the largest concentrations of wintering Eagles in North America.[23] Since 1986, NCC has also been educating the public about appropriate ways to observe Bald Eagles.[23]

Bald Eagles are a top/apex predator, which is defined as a species at the top of the food chain without any predators of its own.[24] Restoration efforts aiming to recover populations of the Bald Eagle show success in regulating species in lower trophic levels.[21] For example, Eagle recovery benefitted fisheries as Eagles regulate predators that target fisheries.[21] However, the increase in Bald Eagle populations has also reduced populations of other birds of prey, such as ospreys and herons,[21] which is important to consider in future restoration initiatives.

Future Trajectory

Bald Eagles in a nest.

Bald Eagle populations have since made an immense recovery due to pesticide bans, legal protection, and other recovery efforts, with populations increasing to approximately 10,000 nesting pairs as of 2007. However, recent trends have indicated that Bald Eagle populations may slow their growth future years as they approach their carrying capacity.[3]

Some Bald Eagle populations have shown signs that they may be reaching their carrying capacity,[3] which is the maximum size that a species’ population can remain stable at, given the available resources and environmental factors in their habitat. The amount of suitable nesting habitat and foraging habitat available appear to be the main factors limiting Bald Eagle population growth.[3] In particular, the distance between neighboring Bald Eagle nesting territories has been decreasing steadily over the past few decades and has reached a plateau in the most recent years,[3] suggesting that Bald Eagles may be approaching their carrying capacity as their population becomes limited by environmental factors. As Bald Eagle populations plateau and reach their carrying capacity, they may move towards density-dependent population regulation instead.[3] Furthermore, as anthropogenic development and urban expansion progresses throughout North America, Bald Eagle populations are being forced to expand their range into less optimal habitat,[3] which may decrease the carrying capacity of Bald Eagles and further limit population size.

Conclusion

Bald Eagle populations in North America have been severely impacted by human activity in the past, with the species nearing the brink of extinction before efforts began to preserve Bald Eagle populations. Due to the bioaccumulation of DDT causing the thinning of eggshells,[13] habitat loss and degradation as a result of urban development and environmental stressors,[11] and Avian Vacuolar Myelinopathy disease outbreaks caused by invasive species, Bald Eagle populations reached a historical low in 1963 with only 417 nesting pairs existing.[1]

However, Bald Eagle populations in North America have made a drastic recovery in recent decades and may even be approaching the natural carrying capacity of the species.[3] Following the banning of DDT as an agricultural insecticide in 1972, Bald Eagle populations slowly began to recover from the eggshell thinning effects of DDT.[13] Federal legislation, including the Migratory Bird Treaty Act in 1918, the Bald Eagle Protection Act in 1940, and the Endangered Species Act in 1973, have also played a significant role in increasing Bald Eagle populations.[3]

Going forward, it is essential to encourage private landowners to make decisions that support the growth of Bald Eagle populations, given the volume of Bald Eagle nests and the associated buffer zones existing on private property.[20] Furthermore, habitat restoration could also help protect Bald Eagle populations from the environmental degradation threatening high-priority nesting areas.[11] While there is still action that can be taken to further protect Bald Eagles, previous efforts have been successful in increasing Bald Eagle populations to the point where the species is no longer classified as endangered.

The U.S. Fish & Wildlife Service reported an estimated 316,700 Bald Eagle individuals as of 2020, including 71,400 nesting pairs, which is approximately quadruple the numbers from their previous report in 2009.[25]

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 "Bald Eagle". Nature Conservancy Canada.
  2. ↑ Kiff, Lloyd (Dec 14, 2023). "bald eagle". Britannica.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 Winder, Virginia L. (2020). "Thirty Years of Bald Eagle Population Recovery and Nesting Ecology in Kansas, 1989-2018". BioOne Digital Library. 54(3): 255–264.
  4. ↑ "What does 'endangered species' mean?". World Wildlife Fund.
  5. ↑ "Bald Eagles". Canadian Raptor Conservancy.
  6. ↑ "Bald eagle (Haliaeetus leucocephalus)". Parks Canada.
  7. ↑ 7.0 7.1 7.2 "Bald Eagle (Haliaeetus leucocephalus)". Government of Canada. Aug. 19, 2015. Check date values in: |date= (help)
  8. ↑ 8.0 8.1 8.2 8.3 8.4 "DDT - A Brief History and Status". United States Environmental Protection Agency. April 3, 2023.
  9. ↑ "Understanding Species Status Listing". World Wildlife Fund.
  10. ↑ "Carrying Capacity". National Geographic.
  11. ↑ 11.00 11.01 11.02 11.03 11.04 11.05 11.06 11.07 11.08 11.09 11.10 11.11 11.12 Percy, Katie L. (Feb 1, 2023). "Modeling current and future habitat sustainability of an iconic bird to inform restoration planning in southeastern Louisiana, U.S.A." Wiley Online Library. 31(5).
  12. ↑ 12.0 12.1 12.2 "Bald Eagle Fact Sheet". U.S. Fish & Wildlife Service. March 24, 2021.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 Grier, James W. (Dec 17, 1982). "Ban of DDT and Subsequent Recovery of Reproduction in Bald Eagles". JSTOR. 218(4578): 1232–1253.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Knobel, JC (2015). "The Bald and Golden Eagle Protection Act, species-based legal protection and the danger of misidentification". PER. 18(7).
  15. ↑ 15.0 15.1 15.2 15.3 "Preserving biodiversity: Habitat restoration and conservation". Royal Botanical Gardens Canada. Aug. 31, 2023. Check date values in: |date= (help)
  16. ↑ 16.0 16.1 16.2 16.3 16.4 Breinlinger, S.; Phillips, TJ; Haram, BN (2021). "Hunting the Eagle Killer: A cyanobacterial neurotoxin causes vacuolar myelinopathy". National Library of Medicine.
  17. ↑ "Hydrilla (Hydrilla verticillata): Ecological Risk Screening and Summary" (PDF). U.S. Fish and Wildlife. November 2020 – via U.S. Fish & Wildlife Service.
  18. ↑ "Migratory Bird Treaty Act of 1918". U.S. Fish & Wildlife Service.
  19. ↑ "Summary of the Endangered Species Act". United States Environmental Protection Agency. Sept 6, 2023. Check date values in: |date= (help)
  20. ↑ 20.0 20.1 20.2 20.3 20.4 20.5 20.6 20.7 Watts, B. D.; Byrd, M. A. (2022). "Policy and the social burden of bald eagle recovery". Conservation Science and Practice. 4.
  21. ↑ 21.0 21.1 21.2 21.3 Cruz, Jennyffer; Windels, Steve K.; Thogmartin, Wayne E.; Crimmins, Shawn M.; Grim, Leland H.; Larson, James H.; Zuckerberg, Benjamin (5 April 2019). "Top-down effects of repatriating bald eagles hinder jointly recovering competitors". British Ecological Society. 88: 1054–1065.
  22. ↑ "Eagles". Save the River.
  23. ↑ 23.0 23.1 23.2 "Bald eagle". Nature Conservancy Canada.
  24. ↑ "apex predator". Merriam-Webster.com. Retrieved December 14, 2023.
  25. ↑ "Eagle Population Status". U.S. Fish & Wildlife Service.


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