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Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Saturday 16th August 2014

One of the ‘facts’ about Jacobaea vulgaris (syn. Senecio jacobaea), common ragwort, that gets spouted in almost any discussion about the plant is that it is increasing out of control. I know enough about the Internet to know that even a complete refutation of that claim would not prevent it being made but I did hope a new report into the frequency and abundance of the five weeds covered by the Weeds Act 1959 as well as Invasive Non-native Species (INNS) would provide a definitive answer to the point, one way or the other.

So, I was disappointed to find that a report from the Centre for Ecology & Hydrology entitled ‘Analysis of change in frequency and abundance of injurious weed and selected invasive non native species in England: Final report for Defra. Project WC1042.’ did not offer any level of certainty about the issue.

The report is an attempt to evaluate the prevalence of the five weeds covered by the Weeds Act 1959 and to look at the situation regarding invasive non-native species (INNS) such as Heracleum mantegazzianum, giant hogweed, Impatiens glandulifera, Himalayan balsam, and Fallopia japonica, Japanese knotweed. The main source of information is the Countryside Survey of Great Britain though the report says that the survey areas do not offer sufficient occurrences of the INNS to make a meaningful assessment.
So there’s the first problem. Survey areas that do not have enough of the INNS to allow an assessment are, clearly, not representative of the country as a whole. That has to be kept in mind when reading what it says about common ragwort, in particular.

There’s a further problem; what the report calls recorder effort. The authors have had to try and find ways to allow for the different amount of detail provided from area to area and the difference from one survey to the next in the same area. It is, of course, impossible to know how accurate the adjustments made for this are but, I suspect, they are not that accurate.

As well as using the countryside survey the authors have drawn on other work. This includes a detailed analysis of 40cm x 40cm ‘cells’ within 10m x 10m plots on 11 areas conducted by the Environmental Change Network (ECN). On page 19 of the report, Figure 3.2 shows the proportions of cells counted that were occupied by common ragwort at these ECN sites.

For the six sites shown on the graph, Porton Down is so different from all the others that it cannot be said that the figure as a whole offers a meaningful snapshot of the national situation.

With these limitations in mind, I think everyone should be careful about how these results are used. I’m not inclined to rely on the findings, summarised  in Figure 5.1(b) on page 42 of the report, that suggest that common ragwort increased in frequency from 1990 to 1998 before falling back so that, by 2007, it was less frequent than it had been in 1990. The furthest I would want to go is to say the survey finds there is no justification for claims that common ragwort is spreading out of control or that it increases year on year.
The report’s authors are well aware of the limits of the data available to them and are, therefore, wary of drawing too many conclusions. Against that background, it is important that they say that the frequency of common ragwort in the survey plots was positively related to the use of land by horses.

’m very comfortable with saying that there is some component of the common ragwort distribution story that results from over-grazing by horses. Of course, common ragwort occurs in places where there are no horses but, equally, those places are not a problem for horse-owners unless the land is being used to produce forage.

What the report is saying is that, if there is enough space in a paddock then the horse(s) in that paddock will not cause enough disturbance of the ground to provide opportunities for common ragwort to thrive.

A simplistic solution would seem to be to calculate what area of land each horse needs to avoid such disturbance and mandate that for all paddocks. Of course, it is not as easy as that because different ground types and different climates, even year to year in the same area, mean that what is adequate in one place at one time will be insufficient in a different place or year.

Online, there is some guidance for ‘reasonable’ conditions but I have to say I am disturbed to find that the British Horse Society (BHS) recommends around half the area recommended by others.

What needs to come from this report is the recognition that the answer to common ragwort in horse paddocks has to come from changes in the management of the horses not of the plant.

The BHS should be pressed to increase its recommended minimum pasture area and should focus its attention on better care from horse-owners. If it won’t campaign to bring about changes then legislation might be required to set, at least, the very minimum area per horse that on well-drained, good ground would prevent widespread disturbance.

Of course, any move to increase the paddock available would make keeping horses more expensive and that would lead to horses being abandoned. That’s why I think it is time for the proposal made by the Princess Royal to be taken seriously. She said, a few months ago, that there needed to be a UK market for horsemeat so that there was both a disposal route for unwanted horses and an incentive to take proper care.


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