Could forest fragmentation be a factor in the high incidence of blacklegged tick/lyme in Nova Scotia?

tickThe evidence is sufficient that I breathe more easily when traversing larger patches of older forest, and I am more on guard for blacklegged ticks/lyme when I am in smaller patches or in and out of clearcuts.

UPDATE July 10, 2018 More evidence that forest fragmentation a factor in ticks/lyme increase:
Forest ecology shapes Lyme disease risk in the eastern US
Predators, acorns, & fragmentation regulate numbers of infected ticks
Science Daily, July 9, 2018. Related scientific paper:
Tick‐borne disease risk in a forest food web
Richard S. Ostfeld et al., Ecology, 99(7), 2018, pp. 1562–1573 “…Given the notorious challenges with diagnosis and treatment of tick-borne illnesses (Sanchez et al. 2016), and the high costs to patients and society of these reactive approaches, prevention of exposure based on ecological indicators of heightened risk should help protect public health.”


The South Shore of Nova Scotia, according to Mount Allison University biologist Vett Lloyd cited in the Chronicle Herald on May 19, 2018, is “probably the worst place in Canada” for lyme disease-carrying black legged ticks. The worst that is for humans but the best place from the ticks’ perspective.*

Prof. Lloyd goes on to say:

“The South Shore has the perfect climate for ticks…The climate is mild, it’s moist, lots of fog, you have plenty of boggy areas that keep everything moist. They were introduced primarily on migratory birds. Once they came, they decided they liked it…”

Out of a practical interest in avoiding being bitten by lyme-carrying ticks, I have been following literature on factors affecting abundance of blacklegged ticks and on preventative measures for a while (view Forestry>Pests>Ticks)
*View also Lyme disease cases per 100,000 people in 2016 (Canada)

I  had noted  that there was some evidence that the abundance of blacklegged ticks/lyme may be affected more by the abundance of small rodents than deer (the rodents are hosts to one stage of the tick life cycle, deer to another) and that forest fragmentation is contributing to increase of these rodents.

Could the extreme fragmentation of mature forest and increased early successional forest and increased forest edge associated with clearcutting in NS be a factor in the high incidence of lyme?

Low incidence of blacklegged ticks is one more reason we can be at ease in larger patches of older forest in Nova Scotia and support efforts to conserve more older forest on the Nova Scotian landscape.


There are no published studies on this topic for NS, but there is  evidence from studies elsewhere that forest fragmentation results in higher incidence of ticks/lyme, for example in NY state:

Effect of Forest Fragmentation on Lyme Disease Risk
Brian F. Allan et al. 2003. Conservation Biology 17: 267-272
Abstract: Forest destruction and fragmentation in the United States recently have been shown to reduce mammalian species diversity and to elevate population densities of white‐footed mice (
Peromyscus leucopus). One potential consequence of reduced species diversity and high mouse density in small fragments is an increase in human exposure to Lyme disease. Increased risk of exposure to this disease is expected because of the role of the white‐footed mouse as the principal natural reservoir of the Lyme bacterium, Borrelia burgdorferi. Blacklegged ticks (Ixodes scapularis) feeding on mice have a higher probability of becoming infected with the bacterium than do ticks feeding on any other host species.  We hypothesized that small forest patches (<2 ha) have a higher density of infected nymphal blacklegged ticks, which is the primary risk factor for Lyme disease, than larger patches (2–8 ha).

…We found a significant linear decline in nymphal infection prevalence with increasing patch area and a significant exponential decline in nymphal density with increasing patch area. The consequence was a dramatic increase in the density of infected nymphs, and therefore in Lyme disease risk, with decreasing forest patch size. We did not observe a similar relationship between the density of larval ticks and patch size. These results suggest that by influencing the community composition of vertebrate hosts for disease‐bearing vectors, habitat fragmentation can influence human health.

A recent article by investigative reporter Mary Beth Pfeiffer, Ticks Rising, provides a good overview of factors that may be contributing to increased abundance of ticks/lyme. About forest fragmentation, she writes (bolding mine):

In states from Maine to Florida and New York to California, across the breadth of southern Canada and in many parts of Europe, once-sweeping woodlands have been reduced and divided, often into idealised forest fragments at the periphery of residential tracts  places where people can be close to, support and observe wildlife. Multitudes live, work and play in or near these green spaces in a new epoch tentatively called the Anthropocene, the era marked by the hand of humanity. The irony is that these adulterated slices of nature and de facto nature preserves are incubators, in many of these places, of Lyme disease. The smaller the patch, in fact, the higher the proportion of diseased ticks, as documented in a study in Dutchess County, New York, where the per-capita rate of Lyme disease is among the world s highest.

In these fragments, small mammals, such as white-footed mice in North America and garden dormice in Europe, have found havens, thriving in the absence of predators such as foxes. In the language of tick-borne disease, the mouse is quaintly called a  host for ticks and a  reservoir of Lyme disease, the place where baby ticks, almost too small to be seen, get their first sip of infection. In city parks, suburban tracts and exurban preserves, people come skin-to-skin with these ticks. In scores of studies, other environmental factors besides climate change, many controlled by human beings, are pointed to as drivers of this epidemic. The slicing and dicing of forests, and the loss of biodiversity that followed, is surely high on a complex and evolving list. But while there is no single explanation for the 20th-century emergence of Lyme disease, there is ample evidence that climate change has played no small part. …

Tree cover loss on the South Shore 2001-2016 (pink), and tree cover gain (blue) – most of the gain is in clearcuts conducted prior to 2001. From Global Forest Watch Map, accessed June 16, 2018.
Click on image for larger version

So… could such forest fragmentation be a factor in the high incidence of BLT/lyme on Nova Scotia’s South Shore?

The map at right shows the pattern of clearcutting, with pretty intense clearcutting/fragmentation-of-older forest from Port Mouton to Halifax, and relatively little towards southwest extremity of the South Shore. I will have to leave it to the experts with the data on tick/lyme incidence to look for any correlation (and I hope they do investigate).

For myself, the evidence that bigger patches of older forest are likely to have fewer blacklegged ticks than smaller patches, is sufficient that I can breathe a little easier when I am traversing  larger forest patches, and I am a little more on guard for ticks when I am in the smaller patches and in and out of clearcuts.

I do have a little related anecdotal evidence of my own. Last summer/fall I spent upwards of 25 days walking the mostly forested landscape in the area of Sandy Lake, Bedford. A friend who is particularly concerned about ticks accompanied me on about a dozen days, and we regularly checked our clothing for ticks.

We encountered the blacklegged ticks on only one day which was the one day we were checking out an area clearcut in 2013. (On other days we were in intact, mostly older forest.) He found 5 blacklegged ticks on his pant legs, I had none. Besides spraying exposed skin with an Icaridin based personal insect repellant, which we both did, I wore tick-repellant clothing.

For more about tick-repellant clothing (unfortunately, not available in Canada), and links to more information about the blacklegged tick,  view Forestry>Pests>Ticks  on this website, and  on NS Public Health>Lyme Disease.

Better old forest than clearcuts… 


* Lyme disease cases per 100,000 people in 2016
NL 0.2
PEI: 2.7
NS: 34.4
NB: 1.5
Que: 2.1
Ont: 2.7
Man 3.9
Sask: 0.1
Alta: 0.2
BC: 0.8
The territories: 0

The figures are presented on a map of Canada, page 17 in the July 2018 print issue of MacLeans Magazine.

Thx to DW for highlighting the MacLeans map in a post on the NatureNS listerv.

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