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The species diversity of verges by comparison with hedgerow diversity, is brought about with considerable differences. Whereas hedgerows age and species composition have a strong correlation, other factors such as maintenance, geology (e.g. chalk or peat subsoils) and climate have very little bearing.
With verges, however, this is very different, the most dramatic in terms of effect are obviously mowing and herbicides, both of which cause drastic changes of the flora in question. Other factors concerning the diversity of the plants and animals consist mainly of:
1) Aspect; South facing verges tend to have a greater diversity than North facing ones due to the better illumination levels as well as a longer growing season; ie. Quicker to start in spring and still growing after the north aspect verge is in shade during the autumn.
2) Pollution; such as run off and splashing or over spreading of grit and salt during the winter months. Although some plants are actually adapted to this environment and flourish very happily with reduced competition from other species. Another pollutant is fuel and oil spillage as well as litter.
3) Disturbance; of passing traffic aiding seed dispersal, crushing by vehicle tyres parking on verges and trampling by pedestrians walking along the verge.
4) Acid/alkaline balance; when the soils are alkaline (ie. On chalkstone areas from leaching from the chalkstone road base.) a particularly efficient coloniser is the greater plantain(Plantago major).
Estimating Plant Diversity in Verges
About the simplest method of estimating the diversity of the verge flora is to create a transect across the verge at right angles to the road, 1 metre apart, laid parallel. This can be done with lengths of timber or pieces of string which are then bisected at regular intervals along the transact forming quadrats usually 1m² , and the numbers of each species noted.
A slightly more comprehensive method is to estimate the relationship between the total number of plants and the number of individuals per species that fill a quadrat.
Various correlation equations are available but probably the simplest is this one that follows that will discover the diversity index. One of the biggest advantages with this system is that when compared to the previous one, the individual species do not need to be identified. Merely being able to distinguish one from another is sufficient. So thus said the survey can be carried out by less skilled botanists which can be a time saver once the original species listing has been completed. These can then be identified by an index of letters or numbers.
Total = 45
D= N (N-1) D= Diversity index
å n(n-1) N= total # of individual plants
n= # of individuals/species
å = summation
So using the example in the equation would be worked out thus;
D=45 x (45-1) = 1980
å = 10x(10-1)+9x(9-1)+8x(8-1)+7x(7-1)+6x(6-1)+5x(5-1) = 310 = 6.387 or 6.4
This diversity index can provide us with a means of monitoring management strategies or pollution problems. If we found that the index had dropped from 6.4 to 3.2 for instance, then it would be possible to flag a problem area in need of attention.
It is likely, however to be a great deal of help in measuring the rate of colonization due to the drastic management methods of verges and from traffic and salt pollution.
