Frogs are uniquely susceptible to numerous threats, leading to widespread mortality events globally. Amphibians possess highly permeable skin, which they use for respiration and water absorption, making them direct monitors of environmental health. Their complex life cycle requires them to transition from an aquatic larval stage (tadpole) to a terrestrial adult, exposing them to dangers in both water and land. This dual exposure means mortality often arises from a combination of stressors, leading to rapid population declines.
Infectious Pathogens and Disease
Infectious agents are a significant and sudden cause of death for frog populations worldwide. The fungal disease Chytridiomycosis, caused by Batrachochytrium dendrobatidis (\(Bd\)), has caused declines and extinctions in hundreds of amphibian species. This fungus infects the keratinized layers of the skin, which is the primary site for gas exchange and electrolyte regulation. The infection disrupts the transport of sodium and potassium ions, leading to severe electrolyte imbalance and cardiac arrest.
Another pervasive biological threat is Ranavirus, a highly contagious viral pathogen known to cause large-scale die-offs. Ranavirus targets internal organs, causing systemic necrosis and hemorrhaging in the liver, spleen, and kidneys. This infection spreads rapidly through crowded larval populations, leading to mass mortality events that wipe out entire cohorts of tadpoles and newly metamorphosed frogs.
Chemical Exposure and Environmental Toxins
Chemical substances introduced by human activities pose a persistent threat to amphibian survival. Pesticides and herbicides are particularly harmful due to the frog’s permeable skin and aquatic breeding grounds. For example, the herbicide atrazine is an endocrine disruptor that interferes with the hormonal system of developing amphibians. Exposure can cause the feminization of male frogs, leading to reproductive failure and population collapse.
Heavy metals, such as lead and cadmium from industrial runoff, accumulate in aquatic environments and are readily absorbed through the skin or ingested by tadpoles. These metals are toxic to the nervous system and can impair development, causing deformities or direct mortality. Even road salt (sodium chloride) used for de-icing roads creates toxic conditions in nearby breeding ponds. Elevated salinity interferes with the osmotic balance of embryos and larvae, preventing successful development and causing widespread death of the young.
Physical Trauma and Predation
Acute physical events, both natural and human-caused, account for a steady level of mortality within frog populations. Natural predators, including snakes, wading birds, raccoons, and large insects, regularly consume frogs and their eggs as part of the normal food web. Predation contributes to the baseline mortality rate that populations must overcome to remain stable.
Human activity introduces sudden, traumatic causes of death that frogs are poorly equipped to avoid. Vehicle strikes are a major source of mortality, especially for frogs migrating to and from breeding ponds during rainy seasons. Accidental deaths also occur from lawnmowers, landscaping machinery, or equipment used near wetland edges. Collection for the pet trade or biological supply can also cause death through handling trauma or confinement stress.
Climate and Habitat Stressors
Large-scale systemic environmental changes and the deterioration of natural habitats significantly compromise frog health and survival. Changes in precipitation patterns leading to prolonged drought cause the temporary wetlands and ephemeral pools used for breeding to dry out prematurely. This results in the mass death of eggs and tadpoles that cannot complete metamorphosis.
Temperature extremes pose a constant threat because frogs are ectotherms, meaning they rely on external sources to regulate their body temperature. Sudden, unseasonal freezes can kill hibernating adults, while intense heat waves cause desiccation and lethal overheating when suitable shade or water is unavailable.
Increased exposure to Ultraviolet-B (UV-B) radiation, exacerbated by ozone depletion, also directly harms amphibians. UV-B radiation damages the DNA of unprotected eggs and larvae in shallow water, leading to developmental defects and reduced survival rates.
Habitat fragmentation, where natural areas are broken into smaller, isolated patches, limits a frog’s ability to survive. Isolated populations cannot easily migrate to find better breeding sites or escape local disasters, leading to inbreeding and reduced genetic diversity. The lack of connectivity prevents necessary movement for foraging, hibernation, and reproduction.