Reappraisal of the Dilution and Amplification Effect Framework: A Case Study in Lyme Disease

Abstract

The role of biodiversity in regulating zoonotic disease in ecological communities has been broadly referred to as the biodiversity-disease relationship in disease ecology. Whether biodiversity decreases or increases disease risk, known as a dilution or amplification effect respectively, remains unclear. The literature has focused on the strength, generality, nature, and context dependencies that could explain contradictory evidence. We suggest that a continued focus on this approach to resolving the biodiversity-disease debate detracts from a more foundational problem with testing these dilution and amplification hypotheses, in that these hypotheses are not falsifiable as proposed. When tested and interpreted as net effects in a system, these hypotheses do not possess a true null outcome and they are vulnerable to ad hoc explanations. To remedy this problem, we propose that biodiversity and disease risk can be modelled as latent variables in multivariate causal models to test specific mechanistic pathways. We present a case study on Lyme disease through a systematic review and meta-analysis, concluding that there is little evidence for a net dilution or amplification effect. We demonstrate how testing these net effect hypotheses falls short of providing robust evidence for its underlying mechanisms. While these hypotheses have previously been helpful in conceptualizing this idea of biodiversity as a potentially protective factor for human health, they need further specificity and clarity to move forwards with greater unity.