While most investigations that identify factors that control population dynamics tend to focus on predation and competition, there is little empirical evidence of the role that parasites play on host populations. This study aims to assess the role of parasitism of red-backed vole by bot flies in boreal forests in Quebec, Canada.
The red-backed vole was chosen for this study as it is the most abundant small mammal and primary host of bot flies, in eastern Canada. Density-dependent impacts of bot fly parasitism on host survival and reproduction would ultimately be needed if this parasitism regulated vole populations.
In order to determine the potential impact that bot flies have on regulating populations of red-backed voles, this study:
- Assessed the impact of the parasites on:
- Probability of vole survival under stressful conditions, and whether this was density dependent.
- Reproductive activity in females
- Indentified main factors driving the relationship between:
- individual risk of infection and
- abundance of bot flies in red-backed voles, host abundance, two host life history traits (sex and body conditions) and combination of three habitats variables
- Impact of bot fly prevalence on growth rate of red-backed vole population between mid-July and mid-August
The probability of vole survival in live traps decreased with bot fly infection. The authors state that this indicates that infection by bot flies on red-backed voles is a recent coevolution since strong host-parasite coevolution should not reduce host survival (as this would impact parasite survival). However some authors argue that the reduced host survival is not so much due to coevolution hypothesis as it is an artifact of mark-recapture studies. Resident voles would be more highly infested (due to bot egg placement near burrow openings) and more resident voles would be recaptured than transient voles which would inflate survival estimates of highly infected resident voles.
Probability of infection and abundance of bot flies in red-backed voles, were both negatively correlated with vole abundance. The authors concluded three possible reasons for this.
1. A one year time lag between population cycles of host and parasite since the bot fly requires two years from egg deposition to adult emergence.
2. When vole populations are at capacity, a larger proportion is transient and these transient voles have lower probability of infection.
3. Dilution effect of all hosts could occur when vole population is high.
Sex and body condition of the host and habitat variability were not significant predictors of infection probability and abundance of bot flies.
Bot fly infection prevalence was linked to a decrease in short term growth rate of vole pop over summer. This study indicates that bot flies can impact host population growth, during peak reproductive season of the host.
Bot flies have the potential to reduce survival of red-backed voles, an effect that may apply to large portions of populations. This study supports the idea that parasites may play a role in regulating population dynamics of hosts, although bot flies may be more of a limiting factor than a regulating one.
Another key point from discussion in class: since most mortality in live traps was due to hypothermia, the conclusion from this would be; don’t freeze the voles in your experiment, it might screw with your data.
Wasn't this a fun paper? And a great summary! A few questions, however, linger in this little head of mine. To pick up where the summary left off, in general, I would agree that freezing voles (well, letting them die of hypothermia) is a bad idea and would screw with data, but here, it was an integral part of the study. The authors state: "To clarify the direct relationship between bot fly infection and host survival, one would have to use a metric of host survival that is unrelated to the probability of recapture and to control for predation and direct competition." My take was that they left the voles in the traps for the whole night to intentionally stress them a bit (despite the cotton ball pillows) and then use the percent dead as a correlate of decreased fitness caused by bot fly infections. As much as I hate to think of any furry critters dying in a little aluminum box, would be quite concerned about bycatch of less abundant species, and would hate to see the CCAC and IACUC paperwork for the study, only 137 voles died in 3,196 trap-nights. How many Red-backed Voles would a single Northern Hawk-owl take out in a similar time frame and on a much smaller spatial scale?
ReplyDeleteI actually had a few questions about the methods, one of which is, why not kill more voles? Again, don't get me wrong, I love cute fuzzy things and probably could not have done this myself, but, in the interests of science, a majority of the critters that died were infected with bot fly babies. By eliminating so many infected hosts in the July trapping session, and not taking a proportional number of uninfected individuals out of the population, were the authors manipulating the dynamic of the bot flies' infection rates for the August trapping session?
I also fail to wrap my head around the modeling of population growth rate. Was this based on only the difference in trapping rates between the July session and the August session? If so, would there be anything about the voles' natural history that would make them more susceptible to being trapped in one month or the other. I know the authors also identified females with open vaginas, those lactating, and those with babies in the womb as reproductively active. Reproductive activity (in the summer) however does not equal population growth rate. I would think these to be critters that make a lot of babies with the intent of not all of them making it. What if it was a rough winter? Basically, while respecting possible limiting factors on the study, I would like to see a population monitored a bit longer before I started seeing conclusions made about population growth rates.