Sunday, 26 January 2014

Greater and Lesser Butterfly Orchids: Survey Results

There is very little to comment on at the moment;  the otter family seems to have disappeared and I have picked up nothing on the trail camera.  There are though some spraints by the slipway.   There are a few mergansers and great northern divers about, common seals and the occasional curlew and I saw a couple of bullfinches in the garden last week (new to us), but it is quiet, so much so that I have been photographing mosses, which I know virtually nothing about.

This post is therefore a look back to last summer and beyond.   Over the last 2 months I have brought together and analysed 6 years of data that I have collected on the two butterfly orchid species that grow on the croft.   Lesser butterfly orchid (Platanthera bifolia) and Greater butterfly orchid (P. chlorantha) are at a molecular level almost indistinguishable, yet they are still considered separate species and they certainly look different. The former is whiter and less creamy than the latter but position of the pollinia which carry pollen is the most telling way to discriminate between the two:- wide apart in greater, parallel and close together in lesser.
                                              Lesser butterfly orchid                      

                Greater butterfly orchid

On the croft the two species grow close together and in some patches they intermingle, which is unusual and gives scope for comparison with some of the environmental variables such as climate, nullified.

There is a degree of introgression, making the pollinia test to discriminate imprecise.

Lesser and Greater butterfly orchid plants growing less than 1 metre apart.

The data that I have gathered show statistically significant differences as follows:

a) Lesser butterfly orchids flower 7 - 9 days earlier than Greater butterfly orchids.
b) Height of the flower spike: Lesser average 17.2 cm, Greater average 25.3 cm
c) Width of the lowest of the 2 basal leaves: Lesser average 2.4 cm, Greater average 3.2 cm
d) Spur length: Lesser average 1.6 cm, Greater average 2.5 cm

Summarising, Greater butterfly orchids are more robust than Lesser butterfly orchids, a finding that is not always recognised in the literature.

The seed set efficiency, the number of flowers developing into a seed pod, appeared to be governed by environmental factors especially climate, and slug damage (a significant problem in 2013) rather than any interspecies difference.  Seed set in lesser butterfly orchids was fairly consistent year on year with on average 26%, whereas for greater butterfly orchids the average was 31%, but there was much more yearly variability. In some years the seed set efficiency was statistically significantly different, but in other years it was not and environmental factors, such as slug damage in 2013 seem to mask any difference.

Lesser butterfly orchid seed pods

                                Greater butterfly orchid seed pods

I found no difference in the number of flowers on the spike (Lesser average 12.3, Greater average 10.6, the average reduced by unusually low numbers in 2013 because of late flowering and slug damage)

I looked at the growing conditions.   Soil pH, ground slope, soil temperature and moisture content were not significantly different though equipment limitations might have masked differences.  There was though a difference in the sward height - the height of surrounding plants.   Greater butterfly orchids were commonly to be found where the sward was higher than the plant, whereas lesser butterfly orchids seem happiest in areas with less competition, though there is overlap.

So are they different species?   Morphology would suggest they are, and my data shows that there are differences in the physical size of the two orchids as well as the pollinia positioning differences.   But I am not so sure, and it may be that one is just a bigger version of the other, adapting to different habitat conditions.

The number of flowering plants has grown year on year.   The orchid areas are either mowed or strimmed once a year in early September, after seed set and dispersal, perhaps with some further strimming in the winter if the weather has been mild.

P. bifolia
Lesser butterfly flowering
P. chlorantha
Greater butterfly flowering plants

A few plants have flowered every year for 6 years; by contrast some flower in one year but do not reappear the next (though occasionally they may flower in a subsequent year).

I looked closely at the plants which flowered for the first time in 2012 comparing those that only flowered in that year with plants that flowered again in 2013.   17 new Lesser butterfly orchids flowered in 2012 of which 11 flowered again in 2013, and 6 either produced only a leaf or nothing at all.   I could find no statistically significant differences between the two groups (height, leaf width, number of flowers, seed pod development) though of the plants that flowered in both years, the leaf width was greater in the second year than in the first.

Similarly with Greater butterfly orchids.    There were 59 new flowering plants in 2012, of which 22 went on to flower again in 2013, and the remaining 37 produced only a leaf or nothing.   There were no statistically significant differences in the average height or number of flowers of the two groups, but the average leaf width in 2012 of the plants that went on to flower again was wider than for those that did not.

If anyone is interested in a more detailed account, please get in touch.

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