RECENT DECLINES IN THE WORCESTERSHIRE FLORA (1970-2002) ASSESSED FROM CHANGES IN TETRAD DISTRIBUTION.
J.J.Day
March 2003
The results presented here are preliminary, the original data has not been fully verified. However, they were considered important enough for the preliminary findings to be reported, and conservation bodies to be alerted. The declines should not be taken as absolute.
During a verification check on the flora database, Bert Reid produced a list of species showing the largest apparent changes. The species with changes likely due to recorder error, nomenclatural change, etc. were weeded out. There remained an alarming number of species indicating a recent sharp decline in frequency, as measured by their tetrad distributions. This group seem to warrant further investigation. So a preliminary interrogation of the database was undertaken. The database holds approximately equal numbers of records ( 200,000 to 250,000) for each of the periods 1970-1986 and 1987-2002. Records for the two periods were compared.
The following percentage losses were calculated :-
Column 1
Gross percentage decline 1970-2002
(Total of non-re-found tetrads 1970-1986 / Total recorded tetrads 1969-2002).
This can be taken to represent a minimum decline.
Column 2
Absolute decline within known distribution.
The datasets for the two periods are of a similar size but do not come from
comparable surveys. The early set mainly derive from site based records whilst
the later set mainly from area surveys and cover more of the county. Therefore
to assess actual declines, the re-find percentage was calculated for populations
known in the period 1970-1986.
(Total of non-re-found tetrads 1970-1986/Total re-found tetrads 1987-2002)
Column 3
No two surveys will produce identical lists. Decline rates will be overestimated comparing two surveys. A number of assessments of the Worcestershire Flora Project data have been undertaken. The probability of re-find is for most species within the range 0.65-0.85. For the most realistic of assessment of actual decline the figure in column 2 has been adjusted by a factor of 0.7. This is considered the best estimate of decline, at this stage.
| 1 | 2 | 3 | |
| Elodea canadensis | 29% | 55% | 39% |
| Myriophyllum spicatum | 30% | 54% | 38% |
| Equisetum palustre | 14% | 51% | 34% |
| Campanula latifolia | 26% | 48% | 34% |
| Myosotis laxa | 13% | 46% | 32% |
| Listera ovata | 24% | 45% | 32% |
| Mentha arvensis | 13% | 46% | 31% |
| Eupatorium cannibinum | 16% | 33% | 23% |
| Genista tinctoria | 18% | 33% | 23% |
| Potamogeton pectinatus | 19% | 32% | 22% |
| Orchis mascula | 18% | 31% | 22% |
| Myosotis scorpioides | 13% | 25% | 20% |
| Rhinanthus minor | 12% | 27% | 19% |
| Scutellaria galericulata | 17% | 26% | 18% |
| Epilobium parviflorum | 7% | 24% | 17% |
| Paris quadrifolia | 11% | 24% | 17% |
| Galium odoratum | 7% | 21% | 15% |
| Dactylorhiza fuchsia | 11% | 21% | 15% |
| Hypericum tetrapterum | 8% | 22% | 15% |
| Myosoton aquaticum | 12% | 19% | 13% |
| Succisia pratensis | 10% | 18% | 13% |
| Ulex europaeus | 7% | 18% | 13% |
| Athyrium felix-femina | 11% | 17% | 12% |
| Phyllitis scolopendrium | 8% | 15% | 11% |
| Sparganium erectum | 7% | 14% | 9% |
A few species, which had not been identified as declining, were also assessed. These give credibility to the trends observed and give an indication of the apparent loss due to survey methodology.
|
Oxalis acetosella |
5% |
10% |
7% |
|
Anemone nemorosa |
4% |
10% |
7% |
|
Mentha aquatica |
2% |
4% |
3% |
For several species the resolution on the data is more precise. The decline rates were recalculated on the basis of monad distribution.
Comparison of tetrad to monad decline
First figure = % loss as measured by tetrad distribution.
Second figure = % loss as measured by monad distribution
|
|
1 |
2 |
3 |
|
Listera ovata |
26%->24% |
45% ->54% |
32% ->38% |
|
Genista tinctoria |
18% -> 22% |
33% -> 47% |
23% -> 33% |
|
Orchis mascula |
18% ->19% |
31% ->45% |
22% -> 32% |
|
Paris quadrifolia |
11% ->10% |
24% ->32% |
17% -> 23% |
For most uncommon species the monad distribution is a very close approximation to actual populations. The higher resolution on the data indicates a worse situation.
Summary
The suite of species represented come, in the main, from the widespread
element in the flora. The decline in many of these species was unexpected and
emphasises that substantive losses are still occurring in the countryside. The
trend towards uniformity and abundance of a narrow group of species within the
rural landscape is cause for concern.
| WBRC Home | Worcs Record Listing by Issue | Worcs Record Listing by Subject |