Insect Ecology: Paper Discussion

A complete life cycle of Ixodes scapularis comprises of four phases namely eggs, larva, nymph, and adult. Host’s blood is required at every stage for growth into maturity, and a tick can go up to two years to complete the life cycle. The larval stage is the most vulnerable to the Lyme disease-causing vectors named Borrelia burgdorferi (Centers for Disease Control, 2012). Once infected, the tick remains infectious for the rest of its life.
The larval stage feeds on small rodents and birds before hibernating until spring when it changes into a nymph. Nymph feeds on large animals such as deer. They can also feed on humans but this happens mostly in its adulthood, and by female ticks (Centers for Disease Control, 2012) When they feed on humans, they infect them with the Borrelia burgdorferi thereby causing Lyme disease. The image below shows the four developmental stage of a tick.

Figure1 Adapted from Lyme Disease: A What You Need to Know, Centers for Disease Control, 2012, p.8.

Question 4

Deer’s contribution to Lyme disease varies significantly due to a couple of reasons. One reason for variation occurs is due to changes in the stages of ticks and particularly on feeding opportunities. For example, the larva feeds mostly on the small animals, unlike nymphs that feed on large animals such as deer. Therefore, increased infections of deer are likely to be more after the nymph stage and later. The other reason for variation depends mostly on the predation densities. Levi, Kilpatrick, Mangel, and Wilmers (2012) explain that deer densities near carrying capacity are likely to increase the incidences of Lyme disease since they are unproductive unlike at intermediate stages where they multiply significantly.

This multiplication reduces infections since hosts like deer are born uninfected.
Although ticks at their different stages feed on both small mammals and deer, studies indicate that deer’s contribution to Lyme disease is different from that of the small mammals such as mice and chipmunks. The reason for this is that unlike small mammals, deer do not infect ticks. Also, deer since hosts are born uninfected (Levi, Kilpatrick, Mangel, & Wilmers, 2012). This explains why the increased density of deer may have a non-linear relationship with incidences of Lyme disease.

Question 5

In ecological contexts, Mesopredator release occurs when the smaller predators increase in number after decline or removal of larger one or top carnivores leading to changes in ecosystem structure, functions and niches. For example, the replacement of fox by a coyote. Even though it may look like density will remain the same upon replacement, this is not the case. The reason for this is that fox predatory behavior involves caching prey. Therefore, fox kills more than they can eat at the moment to ensure stock for later days unlike coyote (Levi, Kilpatrick, Mangel, & Wilmers, 2012). Also, small mammals make the most significant percentage of Fox’s diet unlike in coyote.

Finally, foxes exist in larger densities. As such, small mammals are likely to increase upon replacement of fox by coyotes. Ostfeld, Canham, Oggenfuss, Winchcombe, and Keesing (2006) explains that coyote’s diet also changes with seasonality, environment (such as rural or urban areas). In a study by Dowd and Gese (2012) the coyote’s prey items were mule deer, elk, montane vole, and snowshoe hare in the occurrence of 20.1%, 12.5%, 12%, and 8.0% respectively (Dowd & Gese, 2012). This finding agrees with paper’s prediction shift in food web upon invasion by coyotes.

Question 6

According to Acorns and Lyme disease (2018), acorn mast refers to the years when there is an overproduction of tree-nuts. Since tree nuts are food for small mammals such as mice, their increase triggers a boom of these mammals that act as hosts for the Lyme causing bacteria (“Acorns and Lyme disease,” 2018). Consequently, Lyme disease incidences increases. This mechanism corresponds with the asymptotic mathematical concept that can be used to determine predation rate.

Question 7

In figures 1B and 1C, predation seems to influence both the density of infected nymphs and the Nymphal Infection Prevalence (NIP). These two, however, are differently affected as represented by solid and dashed lines depending on the densities of the predators. As aforementioned, hosts at or near carrying capacity are unproductive and predation is reduced (Levi, Kilpatrick, Mangel, & Wilmers, 2012). Therefore, any reduction in some hosts increases productivity and predation thereby increase the number of infected lymph. However, increase in density of hosts does not mean an increase in some infected hosts since host are born uninfected. In this case, NIP is more critical to Lyme disease transmission to humans and any intervention to control it can have desirable results.

Question 8

The authors used three sources of publicly available data, hunting records, landowner surveys and reported cases of Lyme disease in their analysis. Although useful, such accuracy of such data can be affected by some factors. Hunting records covers only recorded activities based records on licensed hunters. Therefore, those unlicensed activities are not captured. Also, data on hunting is influenced by regulations governing particular species that are considered endangered. Last, any changes occurring in hunting behavior could be biased and favor certain species over another. Landowner surveys may be insufficient in describing the deer population as they are based on observation. Also, the models used such as Pennsylvania’s sex-age-kill are usually based on estimates. As such, they are subject to local biases leading to improper utilization of factors such as time, season, and sex-structure.

Finally, records of reported cases may fail to capture the unreported cases. Another problem with using prevalence data is that it fails to consider the seasonality factors. Seasons are essential as they have influence development stages of ticks and infections (Centers for Disease Control, 2012). The authors used Wisconsin, Minnesota, Pennsylvania, and Virginia as their four focal states since the study animals, hunting culture and cases of Lyme disease have been present in these places abundantly. In fact, these places have the highest prevalence of Lyme disease in the US.

References

Acorns and Lyme disease. (2018). Cary Institute of Ecosystem Studies. Retrieved 17 February 2018, from http://www.caryinstitute.org/discover-ecology/podcasts/acorns-and-lyme-disease
Centers for Disease Control. (2012). LYME DISEASE: What you need to know. Atlanta: Centers for Disease Control and Prevention. Retrieved from http://www.cdc.gov/Lyme
Dowd, J., & Gese, E. (2012). Seasonal Variation of Coyote Diet in Northwestern Wyoming: Implications for Dietary Overlap with Canada Lynx?
Levi, T., Kilpatrick, A., Mangel, M., & Wilmers, C. (2012). Deer, predators, and the emergence of Lyme disease. Proceedings of the National Academy of Sciences, 109(27), 10942-10947. http://dx.doi.org/10.1073/pnas.1204536109
Ostfeld, R., Canham, C., Oggenfuss, K., Winchcombe, R., & Keesing, F. (2006). Climate, Deer, Rodents, and Acorns as Determinants of Variation in Lyme-Disease Risk. Plos Biology, 4(6), e145. http://dx.doi.org/10.1371/journal.pbio.0040145