Dilution Effect Ecology: How Biodiversity Reduces Disease

The connection between the variety of life in an ecosystem and the prevalence of disease is a focus of ecological study. The dilution effect is a phenomenon where greater biodiversity can reduce the transmission rates of certain infectious diseases. This concept suggests a rich community of species provides a protective service against pathogens, and when an ecosystem loses species, this natural buffer can weaken, potentially increasing the circulation of diseases that affect wildlife and humans.

The Mechanism of Dilution

Many infectious diseases involve a pathogen, host species that carry it, and a vector, such as a tick or mosquito, that moves the pathogen between hosts. Vectors acquire pathogens by feeding on an infected animal. Biodiversity influences this cycle by affecting the probability of a vector feeding on an infectious host.

In an environment with low species diversity, a vector has limited animal species to feed on. If the dominant species is effective at transmitting a pathogen, the vector has a high chance of acquiring it, maintaining a high level of the pathogen in the local vector population.

In a highly biodiverse ecosystem, a vector has a wider menu of potential hosts. Many of these additional species are poor hosts, meaning they do not get infected easily or cannot pass the pathogen back to a feeding vector. When a vector feeds on one of these unsuitable hosts, the transmission cycle is broken, diluting the frequency of pathogen transmission and lowering the overall infection rate.

Biodiversity and Host Competence

The effectiveness of the dilution effect is rooted in a species’ “host competence,” which is its ability to become infected by a pathogen and transmit it to a vector. An animal that is easily infected and efficiently passes the pathogen to feeding vectors is a highly competent host, or a reservoir host. Conversely, species that are resistant to infection or are inefficient at transmitting the pathogen are called incompetent or “dilution hosts.” These animals absorb vector bites without furthering the disease cycle, as a vector that feeds on them is unlikely to become infectious.

The most competent hosts for many zoonotic diseases—those that can pass from animals to humans—are often resilient, generalist species that thrive in disturbed, human-altered landscapes where biodiversity has declined. As habitats are fragmented, more sensitive species that serve as dilution hosts tend to disappear, leaving a community dominated by highly competent hosts and increasing disease risk.

Real-World Examples and Consequences

Lyme disease is a clear example of the dilution effect. The disease is caused by the bacterium Borrelia burgdorferi and is transmitted by black-legged ticks. While these ticks feed on many species, the white-footed mouse is the most competent reservoir host, excelling at carrying and passing the bacteria to ticks.

In diverse forests, ticks also feed on a wide array of other animals which are far less competent hosts, including:

• Squirrels
• Raccoons
• Lizards
• Virginia opossums

Virginia opossums are particularly effective dilution hosts because they are meticulous groomers and kill the vast majority of ticks that attempt to feed on them. The presence of these other species dilutes the impact of the white-footed mice.

Deforestation and suburban development create fragmented forests that favor the proliferation of white-footed mice while pushing out larger species that act as dilution hosts. This ecological shift concentrates tick meals on the most competent host, increasing the percentage of infected ticks and elevating Lyme disease risk for people nearby.

A similar dynamic occurs with West Nile virus, which is transmitted by mosquitoes. Bird species common in urban and suburban areas, such as American robins and house sparrows, are highly competent reservoirs. In areas with high avian biodiversity, mosquitoes also feed on many other bird species that are poor hosts, which reduces the overall transmission and prevalence of the virus.

The Amplification Effect

Biodiversity does not always reduce disease risk. An increase in species diversity can lead to a rise in disease transmission, a phenomenon known as the amplification effect. This occurs when additional species introduced into an ecosystem are also competent hosts for a particular pathogen.

Instead of diluting the pathogen’s presence, these new species provide more opportunities for it to spread and persist. If a new animal species that is an effective reservoir colonizes an area, it could increase the total number of available hosts, raising the overall disease risk.

The outcome of biodiversity changes depends on the specific ecological context and the host competence of the species being added or removed from the community. Understanding which species are competent reservoirs versus dilution hosts is fundamental to predicting how biodiversity changes will affect disease.