The dilution effect describes an ecological phenomenon where increased biodiversity within a host community leads to a reduced prevalence of an infectious disease. Although this appears counterintuitive, the presence of many different species actually lowers the overall risk of infection within the community. This reduction in disease risk, or “dilution,” is a direct consequence of how pathogens cycle through a diverse ecosystem.
Understanding the Ecological Concept
The dilution effect occurs when the transmission of a pathogen is dampened by the variety of species present in a particular environment. This mechanism hinges on the fact that not all host species are equally effective at transmitting a disease. Scientists categorize hosts based on their ability to pass the pathogen back to the vector, a measure called ‘host competence’.
A highly competent species is referred to as a reservoir host because it readily becomes infected and efficiently transmits the pathogen to a feeding vector, such as a tick or mosquito. In contrast, species with low competence are termed non-competent hosts; they are poor transmitters, often clearing the infection. The core of the dilution effect is that as species richness increases, the pathogen’s concentration is spread across many non-competent hosts, reducing the proportion of infected vectors in the environment.
How Biodiversity Protects Against Disease
The primary process driving the dilution effect is known as “encounter reduction” or “encounter dilution.” This mechanism applies particularly to diseases transmitted by vectors, like insects or arachnids. When a vector takes a blood meal, it does so indiscriminately from the available host community. In a low-diversity system, the vector is highly likely to encounter and feed on the few reservoir hosts present, thus maintaining or amplifying the pathogen’s presence.
When a community is highly diverse, the vector’s chances of encountering a non-competent host increase. A bite taken from a non-competent host acts as a “dead end” for the pathogen. Since the non-competent host does not transmit the pathogen back to the vector, that feeding event fails to contribute to the infectiousness of the vector population.
This ecological filtering process lowers the infection prevalence in the vector population. The presence of numerous non-competent hosts acts as a buffer, reducing the frequency of successful transmission cycles.
Documented Examples in Nature
One of the most extensively studied examples of the dilution effect is the transmission cycle of Lyme disease in the northeastern United States. The bacterium that causes Lyme disease, Borrelia burgdorferi, is transmitted by the black-legged tick, Ixodes scapularis. A small mammal called the white-footed mouse, Peromyscus leucopus, is the single most competent reservoir host for the pathogen.
White-footed mice infect between 40% and 90% of the larval ticks that feed on them. In areas with low host diversity, the ticks are concentrated on these mice, leading to a high infection rate in the tick population. Conversely, in more diverse forest communities, the ticks feed on a wide variety of other hosts, such as opossums, squirrels, and birds.
These alternative hosts are generally poor reservoirs for the Lyme bacterium; for example, opossums efficiently kill the ticks that feed on them. These non-competent hosts intercept the feeding ticks, diluting the effect of the highly competent white-footed mouse. This protective biodiversity results in a measurable decline in the prevalence of infected nymphal ticks, which are the stage most likely to transmit the disease to humans.
Importance for Public Health and Conservation
Understanding the dilution effect provides a strong ecological argument for the conservation of natural habitats. The loss of biodiversity, often driven by human activities like habitat fragmentation, can inadvertently lead to an “amplification effect”. When diverse habitats are reduced to smaller fragments, the less resilient species—often the non-competent hosts—are the first to disappear.
This loss of diversity leaves behind communities dominated by the most resilient species, which frequently include the highly competent reservoir hosts. The remaining habitat fragments thus concentrate the pathogen in the most efficient transmitters, increasing the overall infection risk for humans and domestic animals. Conservation strategies focused on maintaining high species richness are increasingly recognized as a form of preventative public health policy.