The apple snail: a creeping threat to the environment?

Feature story
18 February 2014

The apple snails Pomacea maculata and P.canaliculata are some of the largest freshwater snails. They consume vast amounts of rice and can devastate the flora and fauna of natural wetlands. Native to South America, apple snails were introduced to Taiwan in the 1980s as a potential food source. However, they quickly became a pest, spreading rapidly across the region to Thailand, Cambodia, China, the Philippines and Japan. In 2010 apple snails invaded rice fields in the Ebro Delta in Spain. Until then, they were not present in the wild in the EU and were not regulated. The snail invasion is still spreading in the Ebro Delta despite the control and eradication measures in place in the rice paddies. Joop van Lenteren, Vice-Chair of EFSA’s Panel on Plant Health, explains how EFSA is assessing the threat from this invasive alien species.

Dr J.C. van Lenteren
Vice-Chair of EFSA’s Panel on Plant Health

How does the apple snail spread?

JVL: Natural spread occurs via rivers and canals, in which the snails crawl, drift and float. Extreme weather events and flooding increase spread. The snails can also spread by attaching themselves to other animals such as birds, cattle and horses. Human activity assists spread through cultivation, transport of rice seedlings, aquaculture, boats and other means of transportation, and irrigation. The risk of spread by human assistance may have decreased following a decision by the European Commission in 2012 to ban the introduction of the genus Pomacea into the EU.

How did EFSA get involved?

JVL: In 2011 EFSA evaluated a pest risk analysis of the snail carried out by the Spanish Environment Ministry. While endorsing its main findings, we concluded that further work would be needed to predict the potential consequences for the environment – biodiversity as well as ecosystem services – were the snail to establish and spread in the European Union. The Spanish analysis highlighted the serious problems related to rice production, but the Plant Health Panel was concerned about the effect that apple snails could have on European wetland ecosystems, as the snails have many plant species on their menu.

What was the first step?

JVL: First of all we had to assess the potential for the snail to establish in the EU. A preliminary climate matching exercise demonstrated that climate is not a limiting factor for spread and further establishment and that vast areas of Mediterranean Europe have climatic conditions similar to those areas where the snail is already established in Asia and the Americas. We then developed a population dynamics modelling approach to estimate the population abundance of snails and thus identify potential “hot spots”.

“Because the apple snail is so voracious, there is a high risk to biodiversity and in particular the survival of endangered species such as rare plants, fish, amphibians and birds.”

What were the key elements of the population dynamics model?

JVL: Our model produced estimates of potential snail abundance according to temperature fluctuation and availability of water. The crucial factor for the viability of eggs is air temperature because eggs are laid above water; for juveniles and adults it is water temperature. Meteorological data was therefore collected and a specific water temperature model was developed. The potential distribution and abundance of eggs, juveniles and adults for Europe was obtained by calculating the average snail abundance per year in each node of grids of 25km x 25km covering Europe.

 So what are the at-risk areas?

JVL: The area of potential establishment of the apple snails comprises European rice production areas and most of the wetlands of southern Europe and the Balkans up to the latitude of the Danube River.

What happens next?

JVL: An EFSA working group is currently performing an environmental risk assessment of Pomacea using the EFSA guidance published in 2011. The guidance includes – for the first time in a pest risk assessment scheme – methods for assessing the consequences on both the structural (biodiversity) and the functional (ecosystem services) aspects of the environment. It is particularly suitable for a plant pest such as Pomacea, which is a significant driver of change in wetland ecosystems. The results should be published in March 2014.

What are the main potential threats to the environment from the apple snail?

JVL: When the snail establishes in rivers and wetlands the consequences can be serious. Because it is so voracious, there is a high risk to biodiversity and in particular the survival of endangered species such as rare plants, fish, amphibians and birds. Ecosystem services such as the availability of fresh, good-quality water could also suffer, thereby reducing the availability of plants and fish as food, and making recreation activities less attractive due to diminished bird and fish populations and growth of algal bloom.

Why are the effects so dramatic?

JVL: Apple snailsdevour macrophytes. These are aquatic plants that play a key role in nutrient cycling, acting as natural “biofilters” that ensure minimum water quality in freshwater ecosystems. They do this by restricting phytoplankton growth, thus preventing the development of toxic algal blooms. Maintaining an abundance of macrophytes and a richness of macrophyte species are essential to preserve the self-cleaning characteristic of rivers and wetlands.

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