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"Tick index" is the notional distance between ticks in a habitat  based on population density (see below).
It is derived from:
knowing the area sampled for ticks by dragging a
blanket over the ground
 using GPS or a 50meter tape for distance travelled and knowing the width of the blanket
(1 meter)
counting the number of ticks collected
calculating the area per tick (area/ticks)
calculating the square root of the area to give a notional side of the area as a square
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 pointsstripping during saving. In Yarner Wood, regular sampling routes are measured using a 50meter 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:
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 


Brisworthy bracken  2009 


Yarner  leaf litter  2009 
Date  20Jan  28Jan  15Feb  08Mar  22Mar  08Apr  21Apr  10May  29May  08Jun  27Jun  19Jul  25Jul  Aug  27Aug  10Sep  26Sep  10Oct  28Oct  16Nov  24Nov 
13Dec 
27Dec 
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  18°C  16°C  21°C  16°C  24°C  17°C  19°C  19°C  18°C  20°C  16°C  16°C  16°C  14°C  11°C  7°C  7°C 
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 


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 crosssectional 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 355meter 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 nonscientific 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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