Intuition, mathematical models and field observations have suggested that loss of native vegetation and all of its associated organisms may promote the transmission of viruses, bacteria and other disease-causing agents. The problem is that other scientific studies have argued the opposite. A new experiment has now strengthened the first possibility—the greater the number of species, the lower the transmission rate of a disease—by showing that the transmission of a fungus that caused the extinction of amphibians in several countries was 66% lower among groups of frogs with greater species diversity, compared to groups with a single species.
“Diversity alone, regardless of species composition, deters disease transmission,” concludes biologist Carlos Guilherme Becker, a researcher at São Paulo State University (Unesp) at Rio Claro and principal investigator for the study. Biological richness, therefore, measured by the number of plant and animal species, seems to have a protective effect by hindering the transmission of disease-causing agents.
On the basis of this reasoning, we can associate the loss of native vegetation, along with the organisms it hosts, with the emergence of viral diseases such as those responsible for the AIDS epidemic in the 1990s and the present-day outbreaks of Ebola. The viruses that cause these diseases were initially found in natural reservoirs, the wild animals that the poorest inhabitants of Africa kill for food. “Recent years have seen intensive deforestation and hunting in the countries with the largest number of Ebola cases at present,” Becker says.
In the study that led to these findings, Becker used about 200 frogs from seven species with differing habits. Two species lived in the water, three were arboreal and two reproduced only in terrestrial environments. With the appropriate permits from environmental agencies, all the animals were taken in October 2012 from areas of Serra do Mar State Park where they are abundant. Becker prepared 53 plastic boxes, in which he covered half of the bottom with soil and the other half with water. In each box he placed four animals, in two different groups: 28 boxes contained animals of the same species, and 25 held animals of different species, randomly combined and without repetition.
While the animals adapted to the new space, Becker grew a sample of the fungus Batrachochytrium dendrobatidis, or Bd, responsible for killing amphibian populations around the world. He prepared 53 solutions, each with about one million zoospores—the infectious stage of the fungus—and seven days after the experiment began he poured them into the water in each box, which was visited frequently by the animals. After 18 days, he collected samples of the animals’ skin secretions using a swab, organized the 212 samples and quantified the fungus infection load by means of laboratory analyses at Cornell University in the U.S., where he was working on his doctorate.
The longer, the better
The molecular and statistical analyses, described in detail in a paper published in the Proceedings of the Royal Society of London B: Biological Sciences in November 2014, indicated that for most species, the contamination levels were lower when the animals were mixed in with representatives of other species. The aquatic animals displayed a higher contamination rate than the terrestrial animals, which avoided the water but were also infected. On average, the animals in the mixed boxes of four species showed a 66% lower fungal load on the skin than those in the boxes containing animals of a single species.
“Species with different habits avoid one another and occupy the space more efficiently, which must have reduced the transmission of the fungus,” Becker observes. He notes that sometimes species composition tends to influence transmission of the fungus. In 14 boxes there were representatives of Brachycephalus pitanga, a terrestrial species that the other animals avoided, perhaps because of its orange color and toxic secretions. As a result, its companions went into the water more and became more infected with the fungus than it did.
“Species diversity in and of itself is already a cause of reduced fungal transmission, regardless of species composition,” concludes Becker, who has been based at the laboratory headed by Célio Haddad at Unesp in Rio Claro since October 2014. In his opinion, that conclusion could be expanded upon: “The greater the diversity of species living in their own spaces, the lower the risk of disesase transmission.”
His reasoning is that, in a forest, animals such as frogs live relatively isolated in distinct spaces, trees and other plants, near the water or in burrows in firm soil. “In environments with a high degree of deforestation, there are generally a few arboreal, a few terrestrial and a few aquatic species that are overabundant and have high population density.” And densification facilitates the proliferation of disease-causing microorganisms.
The conclusion now experimentally supported is that preserving natural environments will likely help reduce the transmission of infectious disease, as researchers in the United States noted in a paper published in Nature in 2010. According to that study, loss of biodiversity can alter the abundance or behavior of the host or the vector. In fact, the number of cases of Lyme disease—which is caused by bacteria and transmitted by a tick—has risen in the United States, possibly as a consequence of the elimination of wild animals such as small birds that served as natural reservoirs for the virus and thereby contained its transmission, as well as an increase in the population of rodents, which transmit the bacteria that cause the disease. “If we maintain the biodiversity,” Becker concludes, “there is a lower risk of problems for wild animals and for ourselves.”
Speciation of frogs in high-altitude environments (nº 08/50928-1); Grant mechanism Thematic project; Principal Investigator Célio Fernando Baptista Haddad (Unesp); Investment R$1,407,985.13 (FAPESP).
BECKER, C. G. et al. Partitioning the net effect of host diversity on an emerging amphibian pathogen. Proceedings of the Royal Society of London B: Biological Sciences. 281, n. 1.795. 2014.
KEESING, F. et al. Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature. 468, n. 7.324, p. 647-52. 2010.