8 May 2014, Mongabay news - Deforestation is wiping out habitat for plants and animals around the world. It  is linked to reductions in air and water quality, hastening climate change, and  is contributing to increased rates of drought and fire. Now, for the first time,  researchers have found that deforestation may also lead to a heightened risk of  human disease

The study, published in Proceedings of the National  Academy of Sciences, links a rise in disease to reductions in large mammals  and the consequential colonization of deforested areas by rodent populations  harboring pathogens.

"We are in the middle of an extinction crisis," the  authors write. "Among mammals, for which population trends are known, more than  50 percent of species are currently declining."

Of these, large-bodied species are more  likely to experience decline. This is primarily due to their need for larger  habitats and the fact that their populations have a more difficult time  recovering when reduced because generational times tend to be long and offspring  few.

Deforestation can have a one-two punch effect on ecosystems  inhabited by large animal populations. As forest cover is removed from an area,  large species leave -- a situation termed "defaunation" by the study's authors.  As large mammals vacate an area, small ones move in to fill the void and take  advantage of resources such as grasses and seeds normally eaten by large  species.

Large animals act as  ecosystem stabilizers. Herds of ungulates such as bison keep grasses in check,  while their manure fertilizes the soil and keeps vegetation growing.

"The systematic decline of large species, both herbivores and predators, is thus  often associated with pronounced effects on other aspects of community  composition and structure, ecosystem function, and even evolutionary  trajectories," write the authors.

These effects can include the emergence  of diseases transmissible from animals to humans, called "zoonoses." According  to the study, at least 60 percent of all human diseases originated in other  animal species. For example, humans can become infected by a form of bovine  tuberculosis, and most seasonal strains of influenza start off in flocks of  ducks and other poultry in Asia.

Zoonotic infection most commonly occurs  when humans live in close proximity to large numbers of other animals. Usually  this happens in agricultural situations, when humans are tending to domestic  breeds. However, defaunation caused by deforestation can upset natural  ecosystems and result in colonization of an area by large numbers of wild  animals – often, rodents.

"Rodents are common reservoir hosts (long-term  source hosts for a pathogen) for many human zoonotic pathogens, such as Borrelia burgdorferi (Lyme disease), hantaviruses...Yersinia pestis,  and Bartonella (bartonellosis).," write the authors. "They are  particularly important hosts for flea-borne diseases, which are absent or low in  prevalence in most larger wild animals, most of which do not carry fleas."
Bartonella is a type  of bacteria that can be transmitted through flea saliva from rodents to humans.  Infection produces a wide range of symptoms such as fever, cough, and  inflammation of heart tissue. While treatment with antibiotics is usually  effective, those with compromised immune systems may have difficulty recovering.

For their study, researchers examined the effects of large wildlife  removal on the risk of Bartonella infection by isolating a tract of land  in Kenya from its normal array of large mammal fauna. They found that the number  of rodents roughly doubled in their experimental tract after large mammals were  removed. Of these rodents, more than 95 percent carried fleas. Because of the  higher number of rodents and, thus, the higher number of fleas, the researchers  found that their experimental tract contained a much higher Bartonella  presence than in a control area where rodent populations were kept in check by  large mammals.

"These results provide strong experimental evidence that  the effects of differential loss of large wildlife can cascade to cause  increases in the abundance of zoonotic pathogens across a landscape, via an  increase I the abundance of hosts and vectors," the study states.

This  study lends a new perspective to deforestation. Human-caused forest loss for  timber, agricultural, and other purposes, is felling the world's trees at a rate  of nearly 50,000 hectares every day. The existences of innumerable species are  threatened by deforestation through habitat loss and degradation. Now, humans  themselves may be at greater risk of disease due of deforestation.

"As  large wildlife continue to decline globally, " the authors write, "ecologists,  conservation scientists, and health practitioners are challenged to understand  and interpret the implications of these changes for ecological communities and  the people who inhabit these spaces."

Source: http://news.mongabay.com/2014/0508-morgan-deforestation-disease.html?n3ws1ttr