Species loss endangers human health

Change in natural ecosystems promotes survival of disease carriers

White-foot mouse © Jesse Brunner
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The loss of biodiversity not only changes nature - it also threatens human health. In Nature, an American research team is now showing a direct correlation between dwindling biodiversity and an increase in infectious diseases. The scientists found that important "buffer species" are the first to die out, while carriers and intermediaries of pathogens can outlast and spread more freely.

Humanity is spreading and competing more and more with animals and plants for habitats and resources. Since the 1950s, therefore, the loss of biodiversity has been progressing faster than ever before. However, the penetration into even the last undamaged habitats and the change of the ecosystems have a price, as researchers of several American universities and research institutions have found out: More and more often it comes to the emergence of new diseases or to the spread of previously rare infections.

General trend instead of individual case

"We've already seen specific cases where biodiversity decline has promoted the onset of disease, " explains Felicia Keesing, ecologist at Bard College, Annandale, and lead author of the study. "But we've now learned that this pattern is much more universal: Biodiversity loss enhances the transmission of pathogens across a broad spectrum of infectious diseases." This pattern, the researchers found, applies to a variety of pathogens - viruses, bacteria, Mushrooms - and on many different hosts, including humans, other animals, but also plants.

"Hit probability" increases

The reasons for this trend are several different mechanisms, as the researchers found out through their own experiments and the evaluation of existing studies. One of them is the "hit probability" of the pathogen: The causative agent of schistosomiasis, a tropical disease characterized by severe rupture of the skin and organ damage, is the parasitic worm Schistosoma.

He needs a freshwater snail as an intermediate host to produce infectious cercaria and release it into the water. However, when the biodiversity among freshwater organisms is very high, much of the larvae do not end up in the proper snail but mistakenly end up in another, inappropriate host. As a result, the likelihood of a person's subsequent infection is reduced by 25 to 99 percent. display

Buffer species disappear against transmission

Another factor that can lead to an increase in infections is the disappearance of so-called "buffer species" competitors or natural enemies of the intermediaries and the transgender. The researchers found that where biodiversity suffers, for example, from the destruction and fragmentation of habitats, it is precisely the animals, plants and micro-organisms that have previously served as buffers against the spread of disease that tend to disappear. The species that remain, on the other hand, are mostly species that are themselves carriers or that serve the pathogens as intermediaries.

One of these cases is Lyme disease, also known as Lyme Disease in Anglo-American countries. The disease caused by bacteria causes influenza-like symptoms in the early stages, but may later cause severe heart, joint and nerve damage. In Europe it is transmitted by the common wood buck Ixodes ricinus, in the USA by the deer tick Ixodes scapularis. Most important host of this tick in the animal kingdom is the Wei fu maus, a kind, which spreads ever more because of the decrease of its competitors.

"Strongly limiting species such as the possum are lost when forests are fragmented, but the white-bellied mouse thrives, " explains Richard Ostfeld of the Cary Institute of Ecosystem Studies. "The mice, on the other hand, promote the increase in both the deer tick and the pathogen causing Lyme disease." Experiments showed that the opossum treated most of its fur when brushing its coat Ticks are discovered and killed, only a few infected ticks survive. In contrast, the white mouse finds and kills only about half of the ticks sucking on it, while the survivors, in turn, are infected with borrelia.

Habitat protection as infection control

The researchers do not yet know why it is precisely the most resistant species that are the vectors of the pathogens. However, in her opinion, the best way to prevent this effect is to protect natural habitats. "The beneficial effect of biodiversity is clear enough that we now need to take action to preserve it, " Keesing said.

However, in addition to biodiversity, other factors such as changes in land use and population growth also play a role in the increased spread of animal-borne diseases. Half of all animal-transmitted diseases that have occurred since 1940 have made it possible for humans to make the leap through land-use changes such as deforestation, changes in agricultural food production or hunting Eating game.

"When biodiversity decreases and contact with humans becomes more prevalent, a perfect recipe for new outbreaks of infectious disease, " explains Andrew Dobson of Princeton University t. Accurately identifying the variables involved in the incidence of infectious disease is difficult but crucial, says the researcher.

Stronger monitoring of livestock farming

Once the pathogen's jump to a new host has been achieved, the population density of the new host plays a crucial role. In Malaysia, for example, the Nirah virus first jumped from wild fruit bat to domestic pig. The high density of pigs in the farms, in turn, facilitated the transfer of pigs to pigs and ultimately the infection of humans.

The scientists therefore call, among other things, to monitor particularly closely areas in which large numbers of domestic and farm animals are kept. "This could reduce the likelihood that an infectious disease will pass from wildlife to farm animals and then from humans to humans, " Keesing said.

(Cary Institute of Ecosystem Studies, 02.12.2010 - NPO)