In advance of Natural England publishing a conservation strategy for great crested newts in Kent I thought it would be timely to publish my own status assessment using records that are available up to March 2017.
The term 'Kent' describes an area that is something of a moving target - due to changes in political boundaries. For the purposes of this analysis I will be including all areas that are defined within the boundary of the two Vice Counties: East Kent and West Kent. This should be taken into consideration when making comparisons to any other data that may be published by others in the future.
The status assessment will use the broad categories identified by JNCC in their description of Favourable Conservation Status. These include:
Future Prospects is likely to include factors that may positively or negatively influence the future conservation status of gcn. One such factor is climate change. I will leave Future Prospects for a future discussion.
Range simply describes the maximum expected occurrence of a species within the area of interest. It can be defined in several different ways and many assessments simply use basic Distribution maps. Here is a distribution map for gcn in Kent.
It is worth noting that only a very small number of gcn populations occur in that area of West Kent that is now considered to form part of Greater London.
For the current assessment of range I have chosen to use a model that combines recorded distribution with Landscape Habitat Suitability.
No. of 1 km squares in Kent (i.e. total area) = 4456
No. of 1 km squares predicted to be occupied by gcn = 2613 km
Of course, not all areas offer the same suitability and the prediction therefore determines range based on three levels of predicted occupancy (low, medium & high). The figures quoted above include only 1 km squares with scores of medium or high.
The predicted range of gcn can be visualised as follows.
It is unrealistic to expect surveyors to count the actual number of gcn in Kent. At individual ponds, populations can be assessed by using simplistic methods that provide relative counts (eg. the methods currently required by NE for gcn licence applications), or more complicated statistical methods that control for the detectability of newts in a pond (eg. capture - recapture).
But even simple relative counts require significant effort and are impractical for estimating the overall Kent population. I have therefore chosen to represent population as the number of occupied ponds. GCN can occupy ditches and such waterbodies are not included within the current assessment.
Even so, there are a lot of ponds in Kent...
Ponds stored on the database are subject to constant review. New ponds are discovered and added, old ponds may be found to no longer exist. As of March 2017, there are 17637 ponds included within the Kent database...
Of these ponds, 17206 are found in 1 km squares where we predict gcn are likely to occur.
This is very clearly most of the ponds! But remember that the habitat that most influences gcn presence in Kent is pond density, so this should not really be a surprise.
Previous work has investigated the relationship between individual pond habitat suitability and rate of occupancy of those ponds by gcn. Using these approaches, we have predicted that gcn will be found in 47.1% of available ponds. We can therefore estimate the total number of ponds in Kent that are occupied by gcn:
0.471 x 17206 = 8104 occupied ponds (or populations)
Of course, not all of these ponds will be high quality and considered suitable for breeding. By restricting our assessment to only those ponds that are likely to score as good or excellent (26.8%), we can estimate the number of ponds in Kent that are considered suitable for breeding by gcn:
0.268 x 17206 = 4611 breeding ponds (or breeding populations)
GCN Habitat can be considered as either terrestrial or aquatic. For aquatic habitat, we may wish to simply consider the number of ponds (17206) or the area of those ponds. The relationship between gcn occupancy and pond area is unclear. GCN appear to favour medium sized ponds and tend to avoid very small and very large ponds (possibly because of pond permanence and likely presence of fish respectively). But there are many exceptions.
Since pond area is a value recorded on the database, we can sum the total area of ponds known to be present in the predicted range of gcn:
Aquatic Habitat = 7311780 square meters = 731.18 Ha = 7.31 square km
For terrestrial habitat, we can examine our range model and identify non-aquatic landscape features that are considered to positively influence the likely presence of gcn. These include:
arable (yes, that's correct - arable!) - see next post for some discussion
By calculating the total area of these landscape features, we can estimate the extent of suitable terrestrial habitat within the range of gcn:
Terrestrial Habitat = 1844 square km
We may consider the total area of suitable habitat available to gcn as the sum of aquatic and terrestrial habitat values:
Total Habitat = 1851 square km