In 2001, two American researchers published an article in the journal Conservation Biology in which they took up the theory of the ‘dilution effect’, which comes to say that in an ecosystem with greater biodiversity there is less possibility of disease transmission to humans. In the context of the COVID-19 pandemic, and considering that around 70% of infectious diseases in humans have an animal origin, the theory of the dilution effect has once again taken center stage, and we wonder if this suggestive hypothesis is applicable in all cases.
“After the publication of this article, many researchers verified the theory of the dilution effect in other systems, but almost all were correlational studies, that is, they measured the number of hosts of a parasite and the percentage of the population with the disease, but they did not delve into in the causes, which is something fundamental in science ”, explains Mario Garrido, a researcher in disease ecology at the Ben-Gurion University in Israel. “In addition, it must be taken into account that Ostfeld and Keesing – the authors of the aforementioned article-, tested the hypothesis in a very specific ecosystem, which are the deciduous forests of North America, and with a very specific disease that is Lyme”, we Explain. One of the articles most critical of the universalization of the theory , published in 2012 in the journal Parasitology , also argued that the appearance of the dilution effect or its opposite, that of amplification, depends more on the specific composition than on biodiversity. per se .
Garrido’s team has just published an article in the journal Proceedings of the Royal Society B that addresses the study of the dilution effect in multi -host systems: “the classical models of parasite-host systems were very compartmentalized: you have a parasite and a host and you study their direct relationships, but in nature everything is connected and the complexity is greater: the same parasite infects several hosts and the same host is infected by several parasites, which has consequences at an evolutionary level. We are trying to integrate this natural complexity into our models ”, he explains. The results of their study, carried out in dune systems with three gerbil species that function as hosts, suggest that one of them would function as an amplifier of pathogens and another two as diluents.
“In the end it is very difficult to generalize, we have to go on a case-by-case basis and study the characteristics of the host and those of the parasite, that is another important issue,” Garrido explains. “For example, it is not the same that the parasite is a specialist – it only affects a very specific type of host – or a generalist – it affects all species in the community equally. In the latter case, biodiversity gives us exactly the same, because the parasite is going to spread as well, but if the composition of species in the ecosystem were generalist, it would be relevant ”. We must also consider the mechanisms of transmission of the disease : “If it were shown that SARS-CoV-2 is transmitted by direct contact to the entire human population equally, that would mean that although we have populations with different genetic profiles, the virus it is going to transmit everywhere ”, he gives us as an example. But what if it were transmitted through a vector that infects more men than women? In that case, the fact of having a population in which there are only men or women would be decisive ”.
So doesn’t biodiversity protect us from disease?
As we have already seen, the possibility that the theory of the dilution effect can be extrapolated to all situations is nowadays the subject of heated debate, and many of its detractors criticize the adoption of this mantra of “nature allows us cure”.
However, there are many nuances in this matter: “it must be taken into account that, at first, the dilution effect speaks to you of biodiversity, not of loss of biodiversity , because there we are already putting situations that disturb the natural balance of an ecosystem , whether this is more or less biodiverse ”, indicates Garrido. “Perhaps there is no real relationship between diversity and diseases, but the relationship that does exist and is proven is the one between attacks on the environment and the appearance of new diseases , we all agree on that”.
For example, in 2018 an article published in the journal Frontiers in Microbiology spoke of the relationship between coronavirus, bats and deforestation, and warned that environmental pressures increased the probability that a new coronavirus potentially dangerous for the human population would emerge in Asia. , an area of the world heavily affected by forest clearing and habitat fragmentation. Other work published last year in the journal Proceedings of the Royal Society B also indicated that the same processes that accelerate the extinction of species also favor the transmission of viruses from animals to humans.
Protect nature without expecting anything in return
For many people, the protection of the biosphere is an ethical question: that of recognizing that we cohabit on the planet with other species that we do not want to harm. “The intrinsic value of protecting biodiversity and the function of ecosystems exceeds this questionable utilitarian justification (nature protects our health and that is why we take care of it)”, indicate the authors of the aforementioned article in the journal Parasitology , very critical of the popularization and mediatization of the dilution effect.
Garrido agrees when describing this vision as utilitarian and anthropocentric but, even so, it reminds us of the value that the preservation of biodiversity has for our species : “It seems that there is only a practical feeling of conserving nature, and that we are only going to preserve what is convenient for us. Personally, to those people who need utilitarian arguments I would say that there are a lot of species in nature that we do not know and that could be useful to us, and that if we destroy them we will never know. And, on the contrary: we do not know what the real scope of our attacks on the environment is going to be and how serious its consequences for our survival as a species ”, he says. “The dilution effect may or may not work, that is another debate, but as we have already seen with COVID-19, attacks on the environment have effects that increase risks, that is unquestionable.”
