Lecture 15 - Silent Killers: Toxins, Chemicals, and Wildlife Conservation

Introduction: The Human Population and the Chemical Management Revolution

• The Rise of the Human Population and Resource Demand:

  • The human population has experienced enormous growth, correlating with a significantly increased need for resources.

  • This growth was facilitated by developments that allowed humans to battle diseases and intensify agricultural practices.

  • Solutions were required to manage species that compromised human lifestyles, including those affecting agricultural productivity (rodents, insects) and those acting as disease vectors (mosquitoes carrying malaria).

• The Chemical Management Revolution:

  • Human affluence and the current population size are linked to the ability to synthesize and manage chemicals.

  • The 1940s marked the rise of the first synthetic chemicals designed to protect and intensify agriculture.

  • These chemicals were vital for increasing productivity and decreasing outbreaks of diseases significant to human development.

• Emerging Risks:

  • Despite the benefits to human development, risks associated with these chemicals began to emerge regarding both human health and wildlife health.

Rachel Carson and the Silent Spring

• The Warning:

  • American marine biologist Rachel Carson was one of the first to raise concerns about synthetic chemicals.

  • Her primary concern was DDT, one of the earliest synthetic insecticides, which was initially regarded as a "wonder chemical."

• DDT Applications:

  • DDT was used to nearly eradicate malaria in certain regions.

  • It was sprayed over agricultural crops, effectively wiping out insect pests.

  • It was persistent, meaning it did not require frequent re-application.

• Silent Spring (1962):

  • Carson published this book in 1962 to highlight the massive human and ecosystem health issues associated with DDT usage.

  • Consequently, DDT was banned throughout much of the world by the 1980s, though it retains limited usage in specific areas.

• Empirical Success of DDT in Malaria Control:

  • Pre- and post-usage data for mosquito control in Cuba illustrates the effectiveness of the drug:

  • Pre-DDT (1962): $3,500$ deaths.

  • Post-DDT (1969): $3$ deaths.

  • This represents a reduction in cases of between $99\%$ and $100\%$.

Toxicology and Bioaccumulation of DDT

• Chemical Stability:

  • DDT is extremely stable and does not break down quickly through biological activity or water exposure.

  • It persists in the environment for 20 to 30+ years.

• Bioaccumulation and Biomagnification:

  • Bioaccumulation: Because DDT does not break down, it is deposited in the fatty tissues of animals and is not easily excreted.

  • Biomagnification: Predators receive a concentrated dose when they consume prey containing DDT, meaning the chemical is magnified as it moves higher up the food chain.

• Aquatic Food Chain Transmission:

  • DDT is not soluble in water. It moves through river systems and accumulates in lakes.

  • The process follows this path: $\text{Low concentration in water} \rightarrow \text{phytoplankton} \rightarrow \text{zooplankton} \rightarrow \text{small fish} \rightarrow \text{large fish} \rightarrow \text{top-order predators (e.g., eagles)}$.

The Case of the Bald Eagle in the United States

• Population Impact:

  • The Bald Eagle, a national symbol of the United States, suffered high concentrations of DDT because it is a fish-eating predator.

  • The chemical caused widespread breeding failure across the Continental United States and parts of Northern America into Canada.

• Mechanism of Failure: Eggshell Thinning:

  • DDT impacted the birds' capacity to produce healthy eggs.

  • The shells thinned to a point where they could not support the weight of the adult bird during incubation, leading to broken eggs.

• Population Trends in Chesapeake Bay:

  • 1930s: $600$ to $800$ breeding pairs.

  • 1940s: DDT production and distribution begin.

  • 1959: DDT usage reaches its peak.

  • 1970s: The breeding population dropped to just $60$ pairs.

  • 1972: The U.S. bans DDT usage.

  • 2001: After significant conservation effort, the population recovered to $646$ breeding pairs.

  • 2020: The population has experienced a "boom" due to modern protection and the removal of DDT impacts.

• Persistence in Australia:

  • Tests on Power Flowers (likely reference to Powerful Owls) in 2021/2022 still detected DDE (a breakdown product of DDT).

  • This occurred roughly 40 years after the chemical was banned in Australia in the early 1980s.

The Asian Vulture Crisis

• Dramatic Population Crashes:

  • Across India and South Asia, vulture populations have declined by as much as $99\%$ since the early 1980s.

  • Three species of Gyps vultures had a combined population of $40,000,000$ in the early 1980s.

  • By 2017, the total population of these three species dropped to approximately $19,000$.

• Detection of the Decline:

  • The decline was noted in the mid-1990s by naturalists and the Parsi religious group, who utilize vultures to consume bodies in "Towers of Silence."

  • Farmers также noticed vultures were no longer clearing dead cattle.

• The Culprit: Diclofenac:

  • In 2003, research identified Diclofenac, an anti-inflammatory drug used for humans and livestock (cattle).

  • Because cattle are often sacred in India, they are left in fields after death, where vultures consume them.

  • Vultures are highly intolerant to Diclofenac; very small doses cause kidney failure and rapid death.

• Ecological and Health Consequences:

  • Vultures are a keystone species responsible for cleaning up carrion and recycling the system.

  • Their unique digestive systems kill diseases that might otherwise spread.

  • Without vultures, carcasses rot, leading to viruses and bacteria infecting water sources.

  • Mammalian Scavengers: Dog populations increased to eat the cattle, but they lack the vulture's gut capacity to neutralize disease.

  • This has led to the emergence of health issues such as rabies carried by large feral dog populations.

• Regulatory Management:

  • Diclofenac was banned for veterinary use in India in 2006, though enforcement is difficult.

Anticoagulant Rodenticides

• History and Context:

  • Management of rodents is a massive social issue related to food security. Outbreaks historically led to human starvation.

  • Techniques involve an "arms race" where humans develop poisons and rats evolve resistance.

• Mechanism of Action:

  • Anticoagulant rodenticides block the Vitamin K cycle, which is essential for blood clotting.

  • Animals suffer internal or external hemorrhaging from slight injuries, eventually bleeding out.

• Generations of Rodenticides:

  • First Generation (1950s):

  • Based on Warfarin (a human blood thinner).

  • Required high doses and multiple feedings (e.g., twice a week) because the body could excrete it fairly readily.

  • Widespread genetic resistance eventually developed.

  • Second Generation (SGARs):

  • Very powerful; require only a single feed to be lethal.

  • Slow-acting (takes about a week for the animal to die), allowing the poisoned rodent to continue feeding and remain exposed to predators.

• Terminology of Poisoning:

  • Non-target species: Species that consume the poison directly but were not the intended target.

  • Secondary poisoning: Predators (like owls) eat a rodent that has already consumed the poison, allowing it to move through the food chain.

Rodenticide Impact on Australian Wildlife

• The Study:

  • Research conducted around 2020-2021 during lockdowns investigated deaths in Power Flowers, Tawny Frogmouths, Boobooks, and Barn Owls.

  • Livers were tested for heavy metals and over $200$ agricultural chemicals, including rodenticides.

• Toxicological Thresholds:

  • Toxicology levels are measured in milligrams per kilogram ($mg/kg$) of liver.

  • Low Toxicity/Exposure: Up to $0.1\,mg/kg$.

  • Lethal Range: Values between $0.1\,mg/kg$ and $0.5\,mg/kg$ are the ballpark where owls tend to die.

  • Definite Death: Values above $0.5\,mg/kg$.

• Findings by Species:

  • Power Flowers: Approximately $33\%$ of tested animals were in the lethal toxicity range.

  • Tawny Frogmouths: Almost $70\%$ showed exposure; $42\%$ had levels likely to cause death, despite predominantly eating insects.

  • Boobooks: Approximately $17\%$ were in the definitely lethal category.

  • Barn Owls: Showed heavy impacts similar to global trends.

• Chemical Types Detected:

  • Brodifacoum: The most common active ingredient (found in hardware stores like Bunnings).

  • Other actives include Bromadiolone and Flocoumafen.

  • Many animals showed exposure to multiple types of rodenticides simultaneously, suggesting repeated exposure events.