Dartmoor tick indices

Tick index = distance between ticks

"Tick index" is the notional distance between ticks in a habitat - based on population density (see below).

It is derived from:

1. knowing the area sampled for ticks by dragging a blanket over the ground
   - using GPS or a 50-meter tape for distance travelled and knowing the width of the blanket (1 meter)

2. counting the number of ticks collected

3. calculating the area per tick (area/ticks)

4. calculating the square root of the area to give a notional side of the area as a square

5. using this as a guide to the distance between ticks on the ground
   - assuming an equally distributed population - on a square grid!

The distance is measured by the trip odometer on a GPS unit, not the saved track length because this is shortened by points-stripping during saving. In Yarner Wood, regular sampling routes are measured using a 50-meter tape.

Tick index
The calculated distance between ticks in a given habitat,
assuming an equally distributed population.
In this diagram, the area per tick is 9 square meters,
therefore the tick index = 3.
 

Diagonal of a square > sum the squares of 2 sides and derive the square root = diagonal

The numbers are derived by collecting ticks at roughly fortnightly intervals in three locations:

• open moor - represented by Ringmoor Down

  • grassland - mainly Purple Moor Grass (Molinia caerulea)

  • bracken - an area adjacent to Brisworthy Plantation

•  woods - represented by Yarner Wood National Nature Reserve

  • ground leaf litter, mainly sessile oak leaves

  • undergrowth, mainly whortleberry.

  15 Aug 2009
PRELIMINARY RESULTS: The results below indicate that the highest risk of encountering ticks (where the lower numbers indicate the distance between ticks on the ground) was:

Variation in summer numbers - due to the no. of larvae and the vagaries of hatchings?

The following survey was abandoned on 27 June for safety reasons .............

Some explanation of the Yarner Wood calculations .....

• Yarner Wood, using MemoryMap, is 140.44 hectares = 1,404,400 sq. meters. Dividing this area by the swept area (452 sq. meters) gives a factor of 3120 (no allowance is made for hard surfaces such as the road into the wood, car park, tracks, buildings, trees cumulative cross-sectional area and water). This factor is used to calculate the total tick population at the collection date.

  • Initially, only the ground was sampled because of the problems in trying to drag the undergrowth and the fact that it was impossible to keep the blanket "flat", so the area covered would actually be highly questionable. Also, there were no ticks on two occasions when this was tried in the winter.

  • However, on 22nd March, a 355-meter drag through the undergrowth produced 23 ticks (mainly nymphs but also males and females). This was more than the number found on the ground.

  • It may be that the fall in tick numbers on 8th March may reflect the fact that the ticks were then climbing up the vegetation?

  • Henceforth, it seems that both areas should be sampled.

  • On May 10th there was a huge disparity between the tick numbers for the ground cover and the taller vegetation. A large number of larvae were collected from the ground sweep, probably a new "hatching" period(?), in fact there were so many that removing them from the blanket was abandoned and I stopped counting after locating 30 still on the blanket - they were left there to be dealt with by very hot washing, even simmering in an old preserving pan.

  • On June 27th the undergrowth "drag" was abandoned because of the shoulder height bracken (being awkward to drag over) and the very high numbers of larvae in the area - the risk of personal exposure was finally considered to be too great.

NB - THIS CHART NEEDS UPDATING ......

Red datum points indicate damp/wet collecting conditions and may be replaced by later dry day results

While the figures above are for "questing" ticks, there are probably more that are not questing and these are not represented in these figures, which are therefore an underestimate of the population.

"Tick index" - I am sure someone must have proposed this idea in ecology a long time ago, I know there are books about mathematical ecology. There's little new under the sun!

Addition to "Tick index"-
Actually, this is more correctly called "population density" but I like the idea of citing a distance between ticks so that non-scientific types can have an easily understood idea of tick density in a habitat. Of course, the concept of square area between ticks is a simplification, they could be circular patches, with complications in describing a distance, or a figure may be given with square areas to describe the diagonal separation between individuals.

More scientific descriptions in the field can be found here:

• estimation of population - lecture course notes

• from Chapter 52, Campbell & Reece, 2002, by Stephen T Abedon

Viewers could also put "population density" into an internet search engine and spend a while deciding for themselves which they prefer .....

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