A few years ago I read an op-ed piece in the journal Nature that celebrated the potential demise of mosquitoes as scientists prepared to release genetically modified mosquitoes in Brazil in an attempt to eradicate populations carrying malaria. What most struck me about the piece was that the author concluded that mosquitoes performed no ecological function and that the world would be a better place without these pestiferous nuisances. This statement left me feeling a little uneasy. How certain could we actually be that mosquitoes performed absolutely no ecological function?
“Eradicating any organism would have serious consequences for ecosystems — wouldn’t it? Not when it comes to mosquitoes…”
In 2014 I listened to a podcast produced by Radiolab that reiterated the pointlessness of mosquitoes and again I wondered whether this could really hold entirely true. Apart from David Quammen’s valiant effort to convince us of the mosquito’s general innocence (it is after all only the females that bite, and even this is only in order to produce young). He also asks us to imagine just how quickly deforestation and exploitation of the tropics would have progressed without the relative protection afforded by the mosquito and all of it’s diseases.
According to the World Health Organisation (WHO) 17% of the global estimate of all infectious diseases are vector-borne. Of these, mosquito-borne diseases constitute the majority, with malaria causing an estimated 627,000 deaths in 2012 and infecting 1.5 to 2.7 million people a year. Some of the other mosquito-borne diseases that affect humans are Dengue fever, West Nile virus, Yellow fever, Lymphatic filariasis, Japanese encephalitis, Rift Valley fever, and Chikungunya; causing death, suffering and both social and economic hardship.
There are approximately 3,500 named mosquito species in the world. They are found in a variety of habitats in every biogeographic region apart from the Antarctic.Of these, only 40 Anopheles species are known to be effective transmitters of human malarial infection and only around 350 species are regarded as effective in all mosquito-borne human disease transmission. The catholic nature of mosquitoes in relation to habitat selectivity is best illustrated in the breadth of the geographic area covered by dominant malarial Anopheles mosquitoes. Mosquitoes are highly speciose, with the greatest species diversity being found in the Neotropical regions as shown in the map below.
This preponderance of mosquitoes to cause such human hardship has led to a variety of campaigns designed to control and eradicate them; from the use of DDT in the 1940s to attempts to sterilise males through exposure to radiation. Though there has been some success with these methods in the past, elimination of mosquitoes in the tropics has always proven difficult due to mosquito resistance, pathogen resistance to treatments, the lack of infrastructure and financial support. Conventional means of avoiding infection from mosquito-borne diseases have been to prevent being bitten through the use of mosquito nets and chemical repellents. I was therefore rather intrigued to hear about the work of Oxitec, the Alphey Lab and others in relation to developing genetic controls to exterminate this “winged scourge”.
The ecological niche filled by mosquitoes is little understood and has been poorly studied. In 2010, at the British Ecological Society’s annual meeting, the chair, Professor Charles Godfray said:
“We know very little about the [mosquito] community ecology… and this is significant because if you were to knock it out then you want to know what would take its place. […] And we don’t know enough, not for the want of trying, about the dispersal of the mosquitoes; how they move from one place to another.”
I simply couldn’t believe that such a large knowledge-gap existed with regards such an ubiquitous insect, so I decided to survey the scientific literature to figure out what is currently understood to be the ecological function performed by mosquitoes. I found that a very small number of papers actually concerned themselves with this topic directly and those that did were generally in relation to highly specific niches like larval processing of detritus chain interactions within pitcher-plants, the pollination of orchids, or focused on other species entirely, such as reindeer and caribou whose migration behaviour is influenced by the predation of mosquitoes and other biting flies. Understandably, most papers concentrated on the mosquito as disease vector – especially in relation to humans – but, apart from noting that mosquitoes constitute an enormous biomass, are found in both freshwater and terrestrial ecosystems at different life stages, and that they are highly speciose; there has been little scientific research into their ecological significance.We can extrapolate that they must be an important food source for a number of other insects, birds, reptiles, fish, amphibians and even mammals, but the data is lacking to support this – we need more research to be conducted to be certain. There is also a possibility that mosquitoes contribute to a disease dilution effect, but further study would be required to support any such claim.
So, is it a good idea to locally exterminate mosquitoes if we really don’t have any idea what will happen to their ecosystems? I would suggest that it probably isn’t the greatest idea. Possible scenarios are a reduction in available food for predators that will cause greater predation on other food sources thereby decreasing these at a faster rate and increasing competition. Increased competition can in turn lead to lower reproductive success and in the worst-case scenarios population collapse of apex predators. At least, I think it would be safe to assume that those ecosystems would no longer operate in the same way – their species composition would shift to a greater or lesser degree and with that the functional ecology.
And what about the disease dilution effect? Well, if it holds true in the case of mosquitoes then we may witness an intensification of disease virulence and higher infection rates. An alternative hypothesis is that the pathogens might move into other host species and we would be left scrabbling for new control mechanisms.
As someone with a desire to understand the intricately interlinked nature of our world and all the living creatures in it, I couldn’t support the deliberate extinction of any species (despite the detrimental effects it can have on humanity) without first knowing what the knock-on effects of that extinction would be. In doing something that we hope would benefit humanity, we may in fact be creating new and more complex problems.
An American entomologist, Jeremy Lockwood, wrote of the need to establish an ethical basis of “philosophically sound, scientifically consistent” considerations with regards our relationship to insects. He proposed that we refrain from taking actions that would kill or cause nontrivial pain to insects, but not if by avoiding those actions there would be nontrivial costs to human welfare. Genetically modified mosquitoes, and by association other mosquito control mechanisms, would presumably be considered acceptable to most people within this anthropocentric ethical framework. The irony of this position however is, as Lockwood points out, that a person considered a humanitarian is often referred to as, “one who wouldn’t hurt a fly”.
This blogpost is based on my final-year research paper. For those of you wanting a bit more in-depth information, you can read the full paper here.
Alphey, L. (2014). Genetic Control of Mosquitoes. Annual Review of Entomology, 59(1), 205–224. doi:10.1146/annurev-ento-011613-162002