The apple snail could have massive consequences for biodiversity if it establishes in the freshwater wetlands of southern Europe. The risk is high for species such as amphibians and for some already threatened species, as well as for the diversity of native species and habitats. A snail invasion could also jeopardise ecosystem services of the affected wetlands such as the supply of good quality freshwater and the regulation of pests and diseases. These are some of the conclusions of EFSA’s environmental risk assessment of apple snails from the genus Pomacea.
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, and Pomacea is now considered a threat to the freshwater wetlands of southern Europe.
EFSA carried out the risk assessment using for the first time its Guidance on the Environmental Risk Assessment (ERA) of Plant Pests, which focuses on possible risks to both biodiversity and ecosystem services. The Panel on Plant Health performed two assessments: one for the short term (five years) and one for the long term (30 years).
On biodiversity, the Panel’s main conclusions are:
- For genetic diversity and native species diversity the risk is major both in the short and the long term.
- For native habitats the risk is massive in the short term and major in the long term.
- For threatened species and habitats of high conservation values the risk is massive in both the short and the long term.
- The overall risk to biodiversity is massive in the short term and major in the long term.
For ecosystem services, the Panel concludes:
- The risk for genetic resources, climate regulation, pest and disease regulation, and pollination is moderate for the short and long term.
- The risk for food is moderate in the short term and major in the long term.
- For water and erosion regulation the risk is major in both the short and the long term.
- The risk for freshwater is massive in both the short and long term.
- For nutrient cycling and photosynthesis and primary production of macrophytes (see below) the risk is massive in the short term and major in the long term.
- The overall risk to ecosystem services is major in both the short and long term.
In some cases the long-term threat is lower than for the short term because it is expected that in time macrophytes non-palatable to Pomacea will emerge, as will natural predators of Pomacea.
What are ecosystem services?
Humans benefit in many ways from ecosystems. These benefits are known collectively as ecosystem services. The concept was popularised in the early 2000s by the Millennium Ecosystem Assessment. This grouped ecosystem services into four broad categories: provisioning, such as the production of food and water; regulating, such as the control of climate and disease; supporting, such as nutrient cycles and crop pollination; and cultural, such as recreational benefits. The assessment of the Panel did not cover the latter category as it is not within its remit.
Why are macrophytes so important?
Apple snails feed voraciously on macrophytes. These are aquatic plants that play a key role in nutrient cycling, acting as natural “biofilters” that ensure water quality in freshwater ecosystems. They do this by restricting phytoplankton growth, thus preventing the development of toxic algal blooms. Macrophytes also play an important role in assimilating nitrogen, phosphorous and heavy metals and maintaining the pH balance of water.
Sharp reductions in macrophyte species lead to these functions being significantly impaired. This can have serious consequences for the survival of fish such as carp, pike and tench, which feed on macrophytes and the microinvertebrates which they host, and hunting birds such as kingfisher and osprey.
Intense phytoplankton production may also raise the pH of the water to toxic levels that threaten fish and amphibians; and lower oxygen levels – caused by decaying plant matter – may increase fish mortality levels and inhibit reproduction. In addition, it is likely that apple snails will predate directly on amphibian eggs and aquatic invertebrates such as worms.