Abstract
Pathogen transmission responds differently to host richness and abundance, two unique components of host diversity. However, the heated debate around whether biodiversity generally increases or decreases disease has not considered the relationships between host richness and abundance that may exist in natural systems. Here we use a multi-species model to study how the scaling of total host community abundance with species richness mediates diversity-disease relationships. For pathogens with density-dependent transmission, non-monotonic trends emerge between pathogen transmission and host richness when host community abundance saturates with richness. Further, host species identity drives high variability in pathogen transmission in depauperate communities, but this effect diminishes as host richness accumulates. Using simulation we show that high variability in low richness communities and the non-monotonic relationship observed with host community saturation may reduce the detectability of trends in empirical data. Our study emphasizes that understanding the patterns and predictability of host community composition and pathogen transmission mode will be crucial for predicting where and when specific diversity-disease relationships should occur in natural systems.
Highlights
Emerging field and laboratory data lend support to the dilution effect, where high plant and wildlife diversity often reduces disease severity or pathogen spread in a variety of multi-host pathogen systems [1,2,3,4,5]
We find that non-monotonic relationships between host richness and pathogen transmission can occur under certain conditions, but that high variability could lead to low detectability of such trends
Community Abundance-richness Relationships We found that a saturating abundance-richness relationship with density-dependent transmission led to a clear non-monotonic trend in which there was an initial increase in community R0, followed by a decrease in community R0 at a higher range of richness values
Summary
Emerging field and laboratory data lend support to the dilution effect, where high plant and wildlife diversity often reduces disease severity or pathogen spread in a variety of multi-host pathogen systems [1,2,3,4,5]. The specific mechanisms driving diversity-disease patterns have been debated extensively in the literature, because various ecological and epidemiological properties of host communities can influence the spread of pathogens. Both host richness and host abundance (or density) are expected to affect diversity-disease trends [8]. Mitchell and colleagues [10] found that, of 11 foliar fungal pathogens of plants, roughly half showed a dilution effect due to reduced focal host abundance, rather than a richness effect
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