A section of scientists say there is a ‘biodiversity dilution effect’ in which declining biodiversity results in increased infectious-disease transmission.
Scientists have observed a clear link between decrease in disease frequency with increase in host diversity.
Incidence of West Nile Disease and Lyme Disease has been linked to the biodiversity dilution effect.
However, another section of scientists say the issue of biodiversity dilution effect is the subject of ongoing research and is still unresolved.

Do biodiversity losses aggravate transmission of infectious diseases spread by animals to humans? The jury is still out but several scientists say there is a ‘biodiversity dilution effect’ in which declining biodiversity results in increased infectious-disease transmission.

The rationale is that greater host diversity in a biodiversity-rich region provides range of hosts, many incompetent, for the pathogen. The presence of a range of hosts and incompetent reservoir hosts ‘dilutes’ the risk of exposure to zoonotic infections spread by animals and the transmission chain to humans, say scientists.

Among those who see clear links between biodiversity losses and increased risk of transmission is Rajan Patil, associate professor of epidemiology at SRM University, Chennai who says that “biodiversity has great influence on magnitude and impact of epidemics.”

Scientists have observed a clear link between decrease in disease frequency with increase in host diversity, says Patil, citing examples of West Nile Disease and Lyme Disease whose incidence has been linked to the biodiversity dilution effect.

In a 2018 report in the Indian Journal of Community Medicine, Patil’s group attributed an outbreak of anthrax in Chhattisgarh state to the loss of biodiversity.

Patil says that biodiversity and human health are inter-connected because man is part of ecological systems comprising diverse flora and fauna. Any human action that alters the ecological balance and puts some species at risk of extinction can directly impact human health, he says.

Patil and colleagues had earlier explained in the Annals of Tropical Medicine and Public Health that biodiversity can be described in terms of ‘species richness’ which represents the diversity or different types of species, and ‘species evenness’ which represents the proportional representation of each of these species, explains Patil. In the case of disease transmission dynamics, species evenness is important as it indicates the total distribution of vectors available for a pathogen to feed from, he says.

In the case of Lyme disease which is transmitted by ticks from white-footed mice to humans, species-poor region with few other alternative hosts other than white-footed mice results in greater contact between tick and white-footed mouse.

The West Nile Virus is a mosquito-borne disease in which the primary reservoir is wild ‘passerine’ birds or perching birds that comprise half of the total bird species. When alternate hosts including non-passerine birds, crows, horses and even crocodiles are available for the mosquitoes, the vector get less infected, leading to fewer transmissions among humans, he says.

Richard Ostfeld, a disease ecologist at the non-profit Cary Institute of Ecosystem Studies, New York, says his team’s research shows a “consistent, general pattern that loss of biodiversity is associated with increases in transmission of infectious diseases.” Read the full article following the lik above