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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:

 

  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:

Ringmoor Down - 2009 June - this area is higher, more exposed, therefore cooler and later developing?
Brisworthy bracken - 2009 April & May
Yarner - leaf litter - 2009 May & June

 

Ringmoor Down - 2009

Date:   Jan-20 Jan-28 Feb-20 Mar-18 Apr-6 Apr-20 21-May 22-Jun 20-Jul 28-Jul 25-Aug 27-Aug 10-Sep 26-Sep 13-Oct 31-Oct 23-Nov 24-Nov 16-Dec 22-Dec
Number of ticks: 0 0 0 0 0 19 57 64 9 4 4 0 0 5 11 8 0 0 0 0
Drag length (m): --- --- 860 930 1,000 1.000 1,000 1,000 1000 1000 1000 1000 -- 1000 1000 1000 1000 1000 1000 1000
Area per tick (m): --- --- 0 0 0 52.6 17.5

15.6

111 250 250 0 -- 250 91 125 --- --- --- ---
area = index --- --- 0 0 0 7.3 4.2 3.9 10.5 15.8 15.8 0 0 15.8 9.5 11.1 0 0 0 0

 

Brisworthy bracken - 2009

Date:   Jan-20 Jan-28 Feb-20 Mar-18 Apr-6 Apr-20 21-May 22-Jun 20-Jul 28-Jul 25-Aug 27-Aug 10-Sep 26-Sep 13-Oct 31-Oct 23-Nov 24-Nov 16-Dec 22-Dec
Number of ticks: 0 0 5 30 29 29 34 8 3 13 0 5 0 13 6 3 0 0 0 0
Drag length (m): --- --- 861 648 686 500 513 500 753 501 500 500 -- 500 500 500 500 500 500 1000
Area per tick (m): --- --- 172.2 21.6 23.6 17.2 15.1 62.5 251 38.5 0 100 -- 38.5 83 166 --- --- --- ---
area = index --- --- 13.1 4.64 4.86 4.2 3.9 7.9 15.8 6.20 0 10 0 6.2 9.1 12.9 0 0 0 0

 

Yarner - leaf litter - 2009

Date 20-Jan 28-Jan 15-Feb 08-Mar 22-Mar 08-Apr 21-Apr 10-May 29-May 08-Jun 27-Jun 19-Jul 25-Jul Aug 27-Aug 10-Sep 26-Sep 10-Oct 28-Oct 16-Nov 24-Nov

13-Dec

27-Dec
Ticks 8 16 22 7 10 25 38 120 222 43 282 99 84 264 162 14 265 44 57 0 10 21 0
Tick population: 24960 49920 68640 21840 31200 78000 118560 374400 692640 134160 879840 308880 262080 823680 505440 43680 826800 137280 177840 0 31200 65520 0
Area 56 28 20 64 45 18 12 4 1.1 10.5 1.6 4.5 5.4 1.7 2.8 32.3 1.7 10.2 7.9 0 45.2 21.5 0
Length = Index 7.5 5.3 4.5 8.0 6.7 4.2 3.4 1.9 1.0 3.2 1.2 2.1 2.3 1.3 1.6 5.6 1.3 3.2 2.8 0 6.7 4.6 0
Temp. 6 11 10 7 16 14 18C 16C 21C 16C 24C 17C 19C 19C 18C 20C 16C 16C 16C 14C 11C 7C 7C

 

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 .............

Yarner - undergrowth - 2009   

Date:   Jan-20 Jan-28 Feb-15 Mar-22 Apr-8 Apr-21 May-10 May-29 Jun-8
Wet
Jun-27 Oct Nov Dec
Number of ticks: - - - 23 15 20 14 -- 1 0 0 0 0
Drag length (m): - - - 341 341 341 341 -- 341
Tick population: - - - 94,783 61,815 84,240 57,694 -- 4,121 0 0 0 0
Area per tick (m): - - - 15 23 17 17 -- 341
area = index - - - 3.9 4.8 4.1 4.1 -- 18.4

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

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:

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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