Our habits lead to environmental degradation that degrades ecosystems and reduces their complexity. Simpler ecological systems, such as forest plantations, usually have fewer species than the natural forest ecosystems.

Foto: Håkan Tunón

Lyme disease, tick-borne encephalitis (TBE) and West Nile virus are all examples of diseases spread between animals and humans. Current research shows that zoonotic diseases are increasing in scope globally. There are undoubtedly many reasons for this but among the conceivable explanations are the increase in contact zones between animals and humans and the environmental impact of our lifestyles, leading to loss of biodiversity and less complex ecosystems. As an ecosystem becomes less complex, it supports fewer and more adaptable species; i.e., there is a loss of biodiversity compared to the original conditions. Those adaptable species that survive such changes to the ecosystem spread diseases to humans and there is evidence suggesting that these species are often the ideal animal hosts for various pathogens.

The impact of the dilution effect

The relationship between increased biodiversity and decreased disease risk, something known as the dilution effect, was proposed in the early 2000s. Some of the earliest studies were conducted on Lyme disease in the eastern United States, where the disease is widespread. While this was initially a controversial issue, as more studies covering a range of diseases have been published so have many researchers begun to accept that, at least at a local level, a general relationship exists. The dilution effect can take a number of different forms but the common denominator is that a species-rich ecosystem often reduce disease risk. In simple terms, a sudden change to the ecosystem may decrease several species that competes with the host animal of a harmful virus, leading to more contacts between individuals in the host population. This in turn leads to a greater spread of the virus within the host population both over time and geographically. In such a scenario, the change to the environment leads to an increased risk of humans falling ill.

General relationship

Together with colleagues from the Swedish University of Agricultural Sciences (SLU) in Umeå and researchers at the Cary Institute of Ecosystem Studies in the United States, I have performed a scientific literature review and subsequent meta-analysis of studies of whether biodiversity loss generally leads to increased disease risk. Our intention was to understand whether this is purely a localised phenomenon or if the relationship exists on larger geographical scales and differs depending on latitude. While our results suggest a general relationship regardless of geographical level and latitude, the link does appear to be stronger in temperate regions than in tropical and subtropical regions. There remains a level of uncertainty at the highest geographical levels, continental and global, and it is therefore important that further studies are conducted to clarify the relationship there. We also see a need to develop the field of research globally, given that the majority of studies have been conducted in the temperate regions of the United States. A number of pathogens (infectious agents) of importance from a human health perspective are not represented in the analysis, often those originating in other parts of the world, such as the new coronavirus (SARS-CoV-2) and the Ebola virus.

The new field of planetary health is focused on the links between human health and the health of Earth’s ecosystems. The idea that natural biodiverse ecosystems present some kind of risk to humans has taken root in society; indeed, it is true that many potentially harmful pathogens do circulate among wildlife. There is however mounting evidence that it is when we humans interfere with natural environments that the risk of new animal-borne diseases jumping to humans increases. This is likely to occur as an effect of both an increase or change in contact zones between people and animals and the increased spread of pathogens within wild host-animal populations – possibly due to loss of biodiversity and thus the dilution effect described above.

Further studies needed

It is clearly difficult to study the spread of pathogens among wild animals; however, in order to improve risk assessments of potential zoonotic outbreaks, I propose the expansion of the monitoring of pathogens in wildlife populations. Increased sampling for harmful viruses, bacteria and other pathogens in wild animals may prove just as important as maintaining good statistics on human-to-human transmission. The aim is to obtain greater knowledge of the entire system, leading to concrete proposals for and the implementation of preventative measures.