‘Ecosystem services’ is a phrase readily used when discussing conservation measures. It refers to the benefits that we receive from the natural world such as carbon sequestration, nutrient cycling, pollination and drought prevention. In 2010 the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) was established to assess the effects the deterioration of ecosystems has on human health, focussing on ecosystem services. Biodiversity is vital for ecosystems to maintain fundamental structures and processes, enabling the ecosystem to withstand disturbances. One service that has so far been poorly understood is the role of ecosystems in regulating diseases.
The human population is expanding at an uncontrollable rate, meaning we require increasingly more land to live on and produce food. As such, our encroachment into the remaining natural world is bringing many people into contact with wild animals increasingly frequently. This is particularly a problem in the tropics and sub-tropics where deforestation is at its most ravenous. Biodiversity is highest at these central latitudes, including human pathogen species richness .Emerging Infectious Diseases
Emerging infectious diseases (EIDs) are a huge burden on economies and public health [1,2]. They include diseases where antibiotic resistance has produced new strains (for example, chloroquine resistant malaria). Since the 1940s about 60% of EIDs have been caused by zoonotic pathogens [1,2] (Figure 2). The World Health Organisation (WHO) defines zoonotic diseases as, “any disease or vector that is naturally transmissible from vertebrate animals to humans and vice-versa” . Prominent examples include rabies, HIV, avian influenza and, recently, Ebola. For a disease to become established in a new host species there a several steps; essentially, it has to first be transmitted to a new host (‘spill-over’), then produce the transmission stages of its lifecycle and spread through the whole new host population .A study found that for zoonotic EIDs with a wildlife origin, wildlife species richness was a good predictor for the emergence of a novel pathogen in humans, or, more accurately, a reduction in wildlife species richness . There are several possible explanations as to why a reduction in the biodiversity increases disease transmission. If the transmission is density dependent, and losses in biodiversity result in an increase of the optimum host, disease transmission is increased. In more diverse ecosystems the host is less likely to come into contact with another member of its species. A further cause is that the species that are more resilient to biodiversity loss also tend to be good hosts of disease. They often have traits such as a fast growth rate, and invest less in immunity, traits that increase pathogen prevalence .
Biodiversity loss and outbreaks
Almost half zoonotic diseases have emerged due to land use changes, bringing humans into contact more frequently with wildlife . The recent outbreak of Ebola in West Africa is a prime example of how biodiversity loss affects disease emergence. It is thought that the outbreak is due to increased contact with fruit bats. These are carriers of the disease , but rarely die from the infection. Due to extensive deforestation in West Africa for agricultural practises and extractive practises, such as logging and mining, these bats have lost virtually their entire natural habitat. They become concentrated in the remaining patches, which border an increasing amount of farmland and populated areas. A random encounter with an infected fruit bat is believed to have triggered the most recent epidemic.
Another zoonotic disease outbreak for which, again, fruit bats are the unintentional culprits, is the Nipah virus . Agricultural practices of keeping farm animals in high densities provide beyond perfect breeding grounds for pathogens. In Malaysia the Nipah virus was introduced to farmed pigs from wild fruit bats. The virus flourished in the sheds where the pigs were kept, tightly packed together, allowing for rapid transmission. The virus then naturally spilled over into humans. Farms predominantly focus on one or two domesticated species or cultivated crops, so biodiversity is extremely low. As such, we are creating a world of monocultures. Regional scale variation helps maintain broader patterns of native biodiversity. Just as small populations of a species with poor genetic diversity are at higher risk from disease, so are small ecosystems with poor biodiversity.
The reaction required
As a result of outbreaks like Ebola and the Nipah virus, bats and other wildlife have received bad press. But, it is important to remember that they are not actively attacking the human population. It’s time to start connecting the dots. Every time any part of an ecosystem is changed it has consequences. Unlike the economic change that comes from increasing crop production, which is felt almost instantly, ecological changes lag behind and can often not be apparent for decades. Moreover, biodiversity loss is not acting alone in the emergence of infectious diseases. Climate change, invasive alien species, pollution and other anthropogenic disturbances, such as conflicts, are all involved. Unraveling these complicated interactions will be vital in understanding the emergence of infectious diseases. But, the primary focus should be on preserving ecosystems and their endemic biodiversity .
1. Jones KE, Patel NG, Levy MA, Storeygard A, Balk D, et al. (2008) Global trends in emerging infectious diseases. Nature 451: 990-993.
2. Murray KA, Daszak P (2013) Human ecology in pathogenic landscapes: two hypotheses on how land use change drives viral emergence. Current Opinion in Virology 3: 79-83.
3. WHO (2014) World Health Organization – Zoonoses. http://www.who.int/zoonoses/en/ (Accessed: 11/12/2014)
4. Keesing F, Belden LK, Daszak P, Dobson A, Harvell CD, et al. (2010) Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468: 647-652.
5. Daszak P, Cunningham AA, Hyatt AD (2000) Emerging Infectious Diseases of Wildlife– Threats to Biodiversity and Human Health. Science 287: 443-449.