Effects of host population size and isolation on parasite dynamics in a naturally fragmented host metapopulation

Lead Research Organisation: University of Glasgow
Department Name: Environmental and Evolutionary Biology

Abstract

Parasites, by definition, need hosts in order to survive. Many things may keep a parasite from successfully spreading: hosts may come into contact too rarely, some hosts may be resistant to infection, others may die before the infection is passed on. As a host population gets smaller or decreases in density, the number of opportunities for parasite transmission become increasingly more rare to the point where the parasite may become locally extinct. This, in essence, is the idea of a host population threshold: that there is a certain population size or density below which the parasite cannot persist. Extinction of a parasite in one host population may only be temporary if it can be reintroduced again from a neighboring host population and if by that time the number of hosts has increased again above the threshold number. If there is a whole group of such host populations, the parasite may go extinct at different places at different times but never disappear from the system as a whole. Ecologists would call such a system a metapopulation. Knowing where and when host population thresholds and metapopulations are found for parasites in nature would be of great interest, because it could explain why the risk of becoming infected varies between different places and could give us clues about how to potentially eradicate certain diseases. Yet at this point, we know almost no examples of host population thresholds and parasite metapopulations in wild species. This project will study two rodent species and their parasites (fleas, lice, a virus, and different types of gut parasites) on a group of natural islands, situated within a large lake. This system offers a unique opportunity to look for host population thresholds because the islands differ in size. This means that the number of rodent hosts living on them will also vary to the extent that larger islands may be able to support a particular parasite wheras smaller islands may not. Because islands are separated by considerable expanses of open water, it is also unlikely that rodents and their parasites will move between them very easily. Because of this limit to host movement, this system may also well constitute a metapopulation for certain parasites. I will monitor rodents (by live trapping) and their parasites (by collecting rodent faecal and blood samples) on these islands for two years. Based on these data I will determine whether this system could provide one of the first examples for host population thresholds and metapopulation structure influencing and potentially limiting the distribution of parasite species within a landscape.

Publications

10 25 50
 
Description Over the course of two years, this project generated data for over 650 individual wild rodents, comprising two species, across ten sites within a field study system in Scotland. In addition to host demographic information, data on infection status of each individual were recorded for a suite of fifteen types of parasite (including ticks, fleas, worms, unicellular parasites, bacteria, and viruses).

Key findings:

Despite island rodent populations being isolated and usually very small (<<100 individuals), the majority of them supported parasite communities very similar to those found in mainland populations

The only parasites partially or fully absent from island populations were ectoparasites (rodent tick, fleas) and a virus causing short-lived infections (cowpox)

Being dependent on vector species or intermediate hosts for transmission did not measurably elevate a parasite's risk of local extinction

At an individual level, rodents on islands were more frequently infected with gastro-intestinal parasites (worms, protozoa) and experienced higher parasite burdens for these groups than their mainland counterparts
Exploitation Route Our finding that helminth parasites were unaffected by host population size and isolation suggests that the concept of a critical community size (a host density or size threshold below which parasite is likely to go extinct) may not be applicable to macroparasites. This has implications for helminth control, for example in livestock, since our results suggest that that smaller host populations will offer no higher likelihood of eradication success.
Sectors Agriculture, Food and Drink

 
Description Blodwen Lloyd Binns Bequest Fund
Amount £1,965 (GBP)
Organisation Glasgow Natural History Society 
Sector Learned Society
Country United Kingdom
Start 08/2016 
End 12/2016
 
Description Funded PhD studentship
Amount £54,000 (GBP)
Organisation Free University of Amsterdam 
Sector Academic/University
Country Netherlands
Start 09/2008 
End 09/2011
 
Description Shared PhD studentship co-funded by partner (James Hutton) and Glasgow University
Amount £73,500 (GBP)
Organisation James Hutton Institute 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2016 
End 03/2020
 
Description Ticks and Lyme disease w C Millins and L Gilbert 
Organisation James Hutton Institute
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution The data collected during the award regarding small mammal and tick distribution formed the basis for a follow up study examing Lyme disease risk in this system.
Collaborator Contribution PhD supervision, drafting of co-authored manuscripts
Impact PhD student has submitted, manuscript in preparation.
Start Year 2013
 
Description bank vole immunogenetics - B Tschirren 
Organisation University of Zurich
Country Switzerland 
Sector Academic/University 
PI Contribution Provided DNA samples collected as part of this research project for larger European-wide study on immunogenetics in bank voles. Collaborated on joint publication.
Collaborator Contribution Partner undertook all genetic analysis of provided samples and took lead on publication.
Impact Paper in review with PLos One
Start Year 2012