Why Frogs Are Sounding the Alarm About Herbicide Spraying in Wetlands

Amphibians are among the most vulnerable species when herbicides like glyphosate reach wetland ecosystems, with recent research showing that even low concentrations can cause serious developmental problems in developing frogs. Scientists describe amphibians as "bioindicators" of ecosystem health because their highly permeable skin and dual-environment life cycle make them exceptionally sensitive to chemical contamination. When herbicides are sprayed in or near wetlands, rainfall and runoff carry these chemicals into shallow pools and waterways where frogs breed and develop, potentially affecting entire populations.

What Happens to Frogs Exposed to Glyphosate?

A recent study published in Scientific Reports examined how glyphosate-based herbicides affect developing frogs of the species Physalaemus gracilis, commonly called the weeping frog. Researchers exposed frog embryos and hatchlings to concentrations of glyphosate-based herbicide similar to levels detected in real-world environments. The results revealed multiple concerning changes in the developing frogs:

• Physical deformities: Reduced body size and malformations of the mouth and intestines
• Cardiovascular stress: Increased heart rates, a condition called cardiotoxicity that can affect survival
• Behavioral impairment: Hatchlings were less able to respond to predator-like stimuli, meaning they were slower to flee from danger
• Cellular damage: Elevated oxidative stress and biochemical changes that indicate cellular injury

Perhaps most striking was the behavioral impact. In the wild, frogs that cannot respond quickly to predators face dramatically reduced survival rates. Importantly, some of the concentrations tested in the study were within ranges that have been detected in environmental water samples, meaning these effects could occur in nature.

Are All Frog Species Equally at Risk?

Research shows that amphibian larvae are significantly more sensitive to herbicides than embryos, and sensitivity varies considerably between species. In experimental studies examining the glyphosate-based herbicide Roundup UltraMax, researchers found that larvae showed higher mortality rates and increased developmental abnormalities compared to embryos. Notably, larvae that pump larger volumes of water through their mouths while feeding may inadvertently increase their exposure to contaminants dissolved in the water, making feeding behavior a risk factor.

This highlights an important point: toxicity does not affect all species equally, and the ecological consequences depend on which species are present in a particular habitat. New Zealand's native frogs develop on land rather than as free-swimming tadpoles, but they still have highly permeable skin and remain sensitive to environmental contaminants. Introduced frog species with aquatic larvae are also present in parts of the country and may face direct exposure in wetland habitats.

Why Commercial Herbicide Products May Be More Toxic Than Expected

A critical issue that complicates risk assessment is the difference between pure glyphosate and the commercial herbicide products actually used in real-world spraying. Commercial glyphosate herbicides typically contain multiple ingredients, including surfactants designed to help the chemical penetrate plant tissues. A 2014 review examining glyphosate and freshwater ecosystems noted that these formulations often contain proprietary components whose toxicity is not always fully understood or disclosed. Surfactants used in herbicide formulations have been linked in some studies to adverse effects in non-target aquatic species, meaning that environmental risk assessments based solely on the active ingredient may not capture the full ecological impact of the product being used.

Steps to Better Protect Amphibians in Wetland Ecosystems

• Improve environmental monitoring: Establish routine testing of both amphibian populations and herbicide contamination levels in aquatic environments, since actual contamination levels in amphibian habitats are currently poorly monitored
• Expand toxicity testing requirements: Include amphibians more routinely in environmental toxicity testing for herbicides before products are approved for use near sensitive ecosystems
• Assess full product formulations: Conduct risk assessments on complete commercial herbicide products rather than just the active ingredient, accounting for surfactants and other additives that may increase toxicity
• Consider alternative weed management: Evaluate non-chemical methods for controlling invasive weeds in wetlands, particularly in areas where native amphibians breed and develop

What Makes Amphibians Such Sensitive Indicators?

Amphibians possess unique biological characteristics that make them particularly vulnerable to chemical exposure. Unlike mammals or birds, amphibians have highly permeable skin that allows water and dissolved chemicals to pass through their bodies more easily. Their life cycle also exposes them to two different environments: aquatic habitats during their early development and terrestrial habitats as adults. This dual exposure means they encounter contaminants in both settings, amplifying their overall risk.

Globally, amphibians are already in serious trouble. More than one-third of species are considered threatened with extinction, and scientists believe that habitat loss, disease, climate change, and chemical contamination all play a role in their decline. Because of their sensitivity, researchers often study amphibians to understand how chemicals may be affecting ecosystems more broadly.

The Challenge of Assessing Real-World Risk

Despite decades of herbicide use worldwide, researchers note that actual contamination levels in amphibian habitats are still poorly monitored. A review examining the potential impacts of glyphosate-based herbicides on amphibians concluded that risk assessments are difficult because environmental concentrations in amphibian habitats are not well documented, different herbicide formulations vary in toxicity, amphibian species differ widely in their sensitivity, and environmental stressors can amplify the effects of chemical exposure.

The issue has taken on renewed relevance in West Auckland following reports of drone-based aerial spraying of glyphosate formulations within parts of the Te Henga wetland system. Supporters of the program say the spraying targets invasive weeds threatening native biodiversity, while critics argue that applying herbicides in a wetland environment carries ecological risks that deserve closer scrutiny. Understanding how herbicides interact with wetland ecosystems and with the species that depend on them is therefore an important part of the wider conversation about balancing conservation goals with chemical safety.

The bottom line: when herbicides are sprayed in or near wetlands, they do not simply affect the weeds being targeted. Rainfall, runoff, and groundwater movement can carry these chemicals into surrounding waterways and shallow pools where wildlife lives and breeds, potentially harming the very ecosystems these programs aim to protect.