Stephen P. Ellner, Wee Hao Ng, and Christopher R. Myers (May 2020)
Disease outbreaks can be greatly facilitated by within-species variation in habitat or diet preferences of hosts
“Individual specialization and multi-host epidemics: Disease spread in plant-pollinator networks”
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When you’ve gazed at a patch of flowers, perhaps you’ve been mesmerized by the bees moving from flower to flower, collecting nectar and pollen. Maybe you know enough about bees to recognize bumble bees, honey bees and a few other common species. Much of what we know about insect and other animals’ behavior is based on observations that distinguish between different species, but assume that all individuals in a species behave similarly. It would be as if aliens visiting Earth conclude that every human roots for both the Yankees and the Red Sox, based on the aggregate behavior of humans in the northeast US. But like humans with narrow sports-team allegiances, many individual bees (and other animals) have much narrower feeding preferences than their species as a whole, with individual preferences changing over time.
The authors of this paper are part of a multi-university study of disease spread in communities with dozens of bee and flower species. Many bee diseases are spread by infected bees depositing pathogens on flowers, which can infect other bees visiting those flowers. The authors develop predictive models for forecasting and control of bee diseases. In this paper, they asked how disease spread is impacted by the narrow preferences of individual bees. Adding this feature to their previous model, they discovered that if bees stick to their preferences for a sufficiently long time before switching, this can have an enormous impact on whether a disease can persist. A disease predicted to die out if all bees are assumed to behave the same could actually be very strongly persistent due to particular bee-flower subnetworks that maintain infection. The overall prevalence of a disease in a multi-host community, in contrast, can either increase or decrease due to among-bee variability, depending on the details of the foraging network.
Abstract
Many parasites infect multiple species, and persist through a combination of within- and between-species transmission. Multispecies transmission networks are typically constructed at the species level, linking two species if any individuals of those species interact. However, generalist species often consist of specialized individuals that prefer different subsets of available resources, so individual- and species-level contact networks can differ systematically. To explore the epidemiological impacts of host specialization, we build and study a model for pollinator pathogens on plant-pollinator networks, in which individual pollinators have dynamic preferences for different flower species. We find that modeling and analysis ignoring individual host specialization can predict die-off of a disease that actually is strongly persistent, and can badly over- or under-predict steady-state disease prevalence. Effects of individual preferences remain substantial whenever mean preference duration exceeds half the mean time from infection to recovery or death. Similar results hold in a model where hosts foraging in different habitats have different frequencies of contact with an environmental reservoir for the pathogen. Thus, even if all hosts have the same long-run average behavior, dynamic individual differences can profoundly affect disease persistence and prevalence.