Dwindling biodiversity could cause more humans to contract infectious diseases such as Lyme disease and West Nile virus, according to scientists who have reviewed the results of 24 separate studies.
Biodiversity hotspots must be protected to prevent the transmission of dangerous diseases from increasing, they warn.
According to the review of research published since 2005, loss of species from a range of ecosystems, including forests, savannahs and coral reefs, leads to a boost in the transmission of infectious diseases.
“What we’re finding out is that the protection of human health is one of many major ecosystem services provided by biodiversity,” said lead author Prof Felicia Keesing at Bard College, New York. High levels of biodiversity also help ecosystems to resist drought and store carbon, reducing climate change.
Extinction rates are 100 to 1,000 times higher today than they were before humans existed, according to the International Union for Conservation of Nature. Until recently, however, it was not clear how this loss of biodiversity would affect disease transmission. “It was possible that the species most responsible for transmitting pathogens would be the first ones to be lost as biodiversity declined, reducing disease transmission,” said Keesing. “But that’s not what we see.”
Most of the studies included in Keesing’s analysis, which is published in tomorrow’s edition of Nature, suggest that the first species to go extinct in an ecosystem tend to be the ones that reduce disease transmission.
“The species that persist or even thrive when diversity is lost tend to be the ones that amplify the disease,” says Dr Rick Ostfeld, a disease ecologist based at the Cary Institute of Ecosystem Studies in Millbrook, New York. “These critters include various kinds of mice, urban-adapted birds, some kinds of snails, toads and grasses. When we humans do things that erode biodiversity, we’re effectively getting rid of the helpful species and favouring the nasty ones.”
The white-footed mouse in eastern North America is one example, says Keesing. The mouse is the most popular host for the bacteria that cause Lyme disease and the ticks that transmit it. In contrast, Virginia opossums are rarely used as hosts by the bacteria, and they successfully kill most of the ticks that try to feed on them. Yet as biodiversity declines in the forests of eastern North America, numbers of the white-footed mouse have remained constant, while Virginia opossums have declined.
The researchers are not sure why species that tend to transmit diseases appear less likely to go extinct but they speculate that their lifestyle and how they fit into the ecosystem may have an influence on both.
Biodiversity loss can also increase disease transmission in plants by making the environment more favourable to fungi. According to a study of rice plants published in the journal Phytopathology in 2005, genetically diverse groups have drier leaves because they alter the microclimate. As a result, biodiverse communities of rice plants are less likely to become infected with rice blast fungus than single crops.
Prof Os Schmitz, an infectious disease ecologist based at Yale University in New Haven, Connecticut, agrees with Keesing that loss of biodiversity is likely to increase the transmission of disease. But he worries that there is a trend among conservationists to protect “good” species – but dismiss pathogens as “bad” species.
“We celebrate diversity for its ecological services to human welfare, but we only celebrate it if it enhances human welfare,” said Schmitz. “Disease species are also part of global biodiversity, and in fact diseases may be agents that spur the evolutionary processes that lead to biodiversity.”
According to Andrew Dobson, a co-author of the report, the review shows the need to protect plant biodiversity. “Protecting plant and fungal diversity will provide the best hope of finding novel new drugs for fighting not only new diseases, but also the old ones like malaria, TB and HIV,” he said.
(Photo: Michie Yasuda)
(Republished with permission)