This article very pragmatically addresses the question of whether biodiversity conservation could be an effective public health tool against infectious disease emergence and transmission.
Determining whether biodiversity conservation is an effective public health strategy requires answering four questions: (1) Is there a general, causal relationship between host biodiversity and disease risk? (2) If the link is causal and negative for most pathogens, does the increased diversity of pathogens with more diverse host communities result in net total increase or decrease in infectious disease burden? (3) Is the net benefit of biodiversity conservation greater than the net benefit of diversity-degrading processes (agricultural land-use change and wild animal harvesting)? (4) Are conservation interventions feasible and cost-effective compared to standard public health approaches (vaccines and treatments)?
Regarding the first question, experimental and observational research shows that increased biodiversity is associated with reduced disease burden.
Overall, the available data suggests that there is some correlational support in many zoonotic systems for a dilution effect, and that some species or species groups are more important than others in transmission [Kilpatrick 2017: 4].
The dilution effect hypothesis originated to explain the Lyme disease system. Greater numbers of hosts that are less “competent” (at spreading Lyme disease) – opossums, birds, raccoons and skunks – dilutes the transmission of Lyme bacteria to larval ticks by more competent hosts – white-footed mice, eastern chipmunks and shrews. Changes in community diversity affect, for example, host-vector encounter rates and host and vector abundances.
much more research is needed to show that observed correlations are causal and to identify the mechanisms by which diversity is influencing disease risk [Kilpatrick 2017: 4].
The possibility of confounding factors in observational field studies is high because the same disturbances that change host diversity alters other aspects of transmission as well. For example, an ecosystem disturbance may, in addition to decreasing host diversity, also increase vector abundance, making it difficult to discern the proximate cause of increased disease rates. The authors note that the dilution effect may well cause decreased disease rates – more research is needed to determine this. But they caution that if the dilution effect turns out not to be the direct cause of decreased disease rates in any given pathogen system, then interventions to increase host diversity could be in vain with respect to that desired outcome.
Examples of potential conservation interventions to improve public health include preserving or restoring forest land, reintroducing top predators to control host populations, installing bat or owl boxes to increase predation of mosquitos (vectors) or rodents (hosts). Our still limited understanding of the mechanisms driving disease incidence patterns, however, make it difficult to predict outcomes for broad-scale land-use interventions, according to the authors. They argue instead that more targeted interventions aiming to reduce populations of key hosts in transmission may be more feasible public health tools than general land preservation. Even this, however, requires “deep understanding of both disease and population ecology.”
Further research to address this knowledge gap may be worth the investment, both for human wellbeing and for the planet. Exposure to nature has been shown to improve human mental and physical health and wellbeing, the authors note, regardless of biodiversity’s potential to reduce infectious disease. Furthermore,
If diverse communities can be shown to provide net benefits to human wellbeing, this could provide a powerful motivation for preserving Earth’s remaining biodiversity [Kilpatrick 2017: 7].
Kilpatrick, A. Marm, et al., 2017, Conservation of biodiversity as a strategy for improving human health and well-being, Philosophical Transactions Royal Society B 372, https://royalsocietypublishing.org/doi/full/10.1098/rstb.2016.0131.