• A study led by the Doñana Biological Station (EBD-CSIC) and the Pyrenean Institute of Ecology (IPE-CSIC) concludes that stopping winter floods of rice field in the Ebro Delta, a management to control the invasion of the apple snail, have not had an impact to date on the abundance of many waterbirds species wintering in the area. 
• This apple snail control has been highly controversial due to its potential side effects on waterbird conservation. However, by assessing long series of censuses in the Ebro Delta and l'Albufera, this study shows that recently observed abundance declines of some species could be mostly associated with other factors. 
• The scientists point out that these results should not be considered in isolation when making decisions on wetland conservation and they highlight the importance of carrying out new studies to evaluate the side effects of these managements.

A scientific team from the Doñana Biological Station (EBD-CSIC) and the Pyrenean Ecology Institute (IPE-CSIC) has concluded that stopping winter flooding of rice fields in the Ebro Delta to control invasive apple snail could not be the main reason for the observed waterbird abundance declines. Abundance trends of waterbirds wintering in the area were expected to decrease in the Ebro Delta due to the lack of flooding, but not in l'Albufera where flooding has not been interrupted. The results show that 12 of the 27 species studied have had recent declines in the abundance trend in the Ebro Delta or l'Albufera. However, the expected pattern was only observed in 4 species, suggesting that at least most of the temporal variations could be due to other factors.

In 2009, after an escape of an aquarium wildlife import company, the apple snail (Pomacea insularum) invaded the Ebro Delta. The voracity of this species along with its rapid population growth affected rice productions, the main economic activity in the region. To control crop damage, among many other managements, farmers were forced to stop flooding their rice fields in winter, an agri-environmental scheme, applied for more than 20 years in Europe to favor flora and fauna from wetlands, including wintering waterbirds. "The apple snail has caused a huge economic damage, and the managements carried out, such as the winter drying of rice fields, could collide with conservation aims", explains Rubén Bernardo Madrid, a researcher at the Doñana Biological Station-CSIC and lead author of the study. "Staff from the Ebro Delta Natural Park have warned about the decline of birds and their possible relationship with this apple snail control. Now, more than 10 years after the first limitations, we have studied for the first time the impact of this management on the waterbird abundance".

The study, published in the journal Frontiers in Ecology and Evolution, has analyzed the abundance trends of 27 waterbird species in the Ebro Delta over 35 years. Species with different ecological requirements have been studied, such as species feeding in low, shallow, or pooled waters as well as in fresh and salt water habitats. To control possible local effects, the researchers also studied the tendency of these waterbird species in l'Albufera, a nearby wetland. In this wetland, closely linked to rice cultivation and with a similar agri-environmental aid framework, winter flooding has not been interrupted since it was not invaded by the apple snail.

"We have not found that the observed abundance declines are only or mainly due to the cessation of winter flooding to control the apple snail", confirms the researcher Rubén Bernardo. These conclusions could be partially explained by the complementary results of the study showing that previous flooding practices did not affect waterbird abundance in the Ebro Delta and l'Albufera in a positive and systematic way.   

The study also emphasizes that rice fields have great importance beyond the number of birds they attract, as well as for many other species not studied in this work. Therefore, the team points out that these results should not be taken in isolation to make management decisions. It does highlight the importance of studying the effects of agri-environmental managements in detail and the compliance with the conservation objectives of these wetlands. "Due to the threatened status of a large part of the world's wetlands and the limited resources for their conservation, we need to optimize our efforts. For this reason, it is necessary to study in detail under what circumstances, such as from what minimum extent of available natural water, the wintering flooding of ricefields are a complement that favors waterbird abundance, a management carried out for different reasons in North America, Europe and Japan", concludes the researcher.

Reference:

Rubén Bernardo-Madrid, Pablo Vera, Belinda Gallardo, Montserrat Vilà. Stopping winter flooding of rice fields to control invasive snails has no effect on waterbird abundance at the landscape scale. Frontiers in Ecology and Evolution. https://doi.org/10.3389/fevo.2021.688325