A statement from the Institute of Biology, University of the Philippines Diliman
RECIPE FOR ECOLOGICAL DISASTER: Dengue outbreak and the release of invasive species
According to the Department of Health, an 85% increase in dengue cases nationwide was recorded from January to June 2019 compared to the same period in 2018. The recent outbreak of dengue in the country has given rise to some ill-advised if not desperate measures to curb the spread of the disease. These measures involve the release of thousands of invasive alien species (IAS). IAS are non-native plants, animals, pathogens and other organisms that reproduce and spread fast and which may cause environmental or economic harm or adversely affect human health. According to the International Union for the Conservation of Nature (IUCN 2019), IAS are one of the top causes of biodiversity loss and the second most common cause of species extinctions worldwide.
POTENTIAL FOR DISASTER 1: The release of Mosquitofish (Gambusia affinis)
To address recent outbreaks in dengue, the National Integrated Fisheries Technology Development Center-Bureau of Fisheries and Aquatic Resources (NIFTDC-BFAR) released 6,000 mosquitofish (Gambusia affinis) stocks in Dagupan City, Pangasinan . Many other communities and agencies followed suit.
In 1917, the initial release of mosquitofish as potential predator for mosquito larvae in the Philippine Islands was published in the Philippine Journal of Science (Seale 1917). According to this publication, the mosquitofish was found to be an effective predator of fish larvae in Hawaii. However, it did not study the effectiveness of mosquitofish under Philippine conditions, nor did it weigh on the negative impacts of introducing mosquitofish to Philippine aquatic ecosystems and biodiversity.
Are mosquitofish effective predator of mosquito larvae? The short answer based on many publications to date is NO. The mosquitofish is not the most effective predator of mosquito larvae (Rupp 1996; Pyke 2005; Pyke 2008). It is an opportunistic predator of insects, invertebrates, and plankton. It is also an aggressively competitive fish, killing and injuring smaller fishes and other aquatic vertebrate larvae that overlap with their feeding grounds (Meffe 1985; Pyke 2005). Because of its broad diet and ability to outcompete many fish, invertebrate, vertebrate larvae, the negative impacts of introduced mosquitofish on native fauna is much greater than its intended use as Mosquito control agent (Rupp 1996).
To date, the mosquitofish is counted as one of the 100 worst invasive species globally (Polverino et al. 2013). It is known to alter ecosystems (Hurlbert et al. 1972) and are implicated in the decline of fish species and more than 10 species of frogs in Australia (Gomon and Bray 2019). It also affected larval survival of the endangered Fire Salamander Salamandra infraimmaculata in Northern Israel (Segev et al. 2009). They compete and prey on the larvae of native fish species (Lydeard and Belk 1993; Laha and Mattingly 2007). Other exotic fishes introduced to the Philippines for mosquito control such as the guppy species Poecilia reticulata, P. latipinna, and P. sphenops have similar adverse effects on the environment (El-Sabaawi et al. 2016; Servick 2017).
Given the many scientific evidence on the negative ecological impacts of introducing mosquitofish in aquatic systems, the practice must be stopped. As early as 1996 and 2016, scientists have called for the stop in the release of mosquitofish to control mosquitoes (Rupp 1996; Servick 2016). Government agencies and concerned public must heed these calls and prevent further ecological disasters. In addition, research must be done to monitor and assess the impacts of introduced mosquitofish and other introduced fish species on Philippine aquatic systems and biodiversity in order to (1) inform the public, and (2) help concerned government agencies craft effective and environmentally sound policies to stop dengue and other mosquito-related disease outbreaks (Guerrero 2014).
POTENTIAL FOR DISASTER 2: The release of Cane Toads (Rhinella marina)
Another government-sponsored move to curb dengue was the release of at least a thousand cane toads (Rhinella marina) in Barangay Matandang Balara in Quezon City. The cane toad, locally called baki, kamprag, or bullfrog, is likewise one of the 100 worst invasive species worldwide according to the IUCN Invasive Species Specialist Group (Global Invasive Species Database, 2019). Cane toads are poisonous in all its life stages; from egg, tadpole, froglet to adult. It preys on and outcompetes native amphibians and reptiles for food and breeding sites and also causes declines in local predators such as snakes and lizards that are not adapted to the bufotoxin it secretes (Hilgris, 2001). It can cause illness and death in domestic animals like dogs and cats that come into contact with it. Human fatalities have even been recorded following ingestion of the eggs or adults (Invasive Species Specialist Group, 2006).
Overall, the major impacts are on predatory species that attempt to eat toads and then die; in particular, species that preferentially feed on amphibians. And while some have touted frogs and toads as natural mosquito predators, the fact is that they do not consume significantly enough to control mosquito populations. In rice fields, they consume significantly more beneficial insects than pests (Shuman-Goodier et al., 2019). Furthermore, their tadpoles largely eat plant material and rarely prey on insect larvae (Department of Entomology, 2019).
The proximity of the cane toad release site to the UP Diliman campus, one of the last remaining green spaces in Metro Manila, is a cause for concern. UP Diliman supports a plethora of native and endemic wildlife within its varied natural habitats in the campus. These wildlife can very well disappear, disrupting the ecosystem functions they provide if their habitat is overrun with cane toads.
We are calling for a halt to any further release of cane toads. Apart from their detrimental impact on ecosystem services, human economy and wellbeing, evidence actually points to them being ineffective in curbing mosquito populations. A recent study demonstrated that mosquitoes made up less than 1% of the diet of over two thousand adult frogs in a three-year study in Germany (Rowley, 2016). Far more effective were the tadpoles that prey on and compete with the mosquito larvae. However, since cane toad tadpoles prefer to eat plant material, using them in our context of controlling mosquitoes against dengue may not be applicable.
According to R.A. 9147 or the Wildlife Resources Conservation and Protection Act of 2001, before any invasive species introduction, there must be a careful study of the status, suitability, bioecology, socioeconomic and related aspects of the area where the invasive species will be introduced is needed to determine its environmental impact. However, in these cases, neither the life history and ecology of the mosquitofish and cane toad nor their interaction with mosquitoes they were meant to control were fully considered before their introduction. There is likewise no prior informed consent from the affected community, since no public consultation of stakeholders was conducted before the release, which is required in R.A. 9147. Violators of R.A. 9147 could potentially result in imprisonment of up to 8 years and a fine of up to five million pesos.
To prevent the further spread of dengue virus infection, everyone is reminded to keep our surroundings/environment clean and to remove possible breeding grounds of mosquitoes (empty cans, rubber tires, uncovered pails/drums). As much as possible, always use mosquito repellant to discourage mosquito bites. Should you experience a sudden onset of fever accompanied by headache and general body weakness, please consult a health professional as soon as possible. Please remember that dengue mortality is greatly reduced with early detection and appropriate medical management (Department of Health, 2019).
CONCLUSION: RELEASING INVASIVE SPECIES CAUSES MORE DAMAGE IN ECOSYSTEMS AND BIODIVERSITY, AND IS NOT THE SOLUTION TO DENGUE OUTBREAK.
CLEAR CALL TO ACTION:
(1) STOP THE RELEASE OF INVASIVE SPECIES.
(2) STUDY THE IMPACTS OF mosquitofish and cane toads in Philippine ecosystems and find ways to stop their damage.
(3) FIND ALTERNATIVE WAYS TO PREVENT dengue outbreaks such as (1) maintaining healthy aquatic ecosystems, and (2) finding ecosystem approach to eradicate denque-carrier mosquitoes in Philippine aquatic ecosystems (especially, in urban water systems).
(4) DEVELOP EFFECTIVE MONITORING AND SURVEILLANCE SYSTEM for invasive species in the Philippines and find ways to stop their negative impacts on Philippine Biodiversity.
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Shuman-Goodier, M.E., M.I. Diaz, M.L. Almazan, G.R.. Singleton, B.A.R. Hadi, and C.R. Propper (2019) Ecosystem hero and villain: Native frog consumes rice pests, while the invasive cane toad feasts on beneficial arthropods. Agriculture, Ecosystems & Environment 279: 100-108
The Official Gazette of the Republic of the Philippines: Republic Act 9147