Citizen science – the public participation in scientific research – has a long tradition in the study of plants in Britain and Ireland, particularly in botanical surveys. Plant recording by an army of amateur botanists led to the publication of the first atlas of the British and Irish flora in 1962, a second version in 2002, with a third version coming out later this year. Apart from telling us where each species is found, these atlases provide invaluable information on how the flora has changed over recent time, due, for example, to the spread of introduced plants (aliens) and climate change.

Citizen-science can also play an important role in examining the distribution of particular variants of species, and the factors that may affect their presence and absence at different locations. There are, however, very few examples where citizen-science has contributed to this. One notable exception concerns the study of petal colour variation in Birdsfoot trefoil, Lotus corniculatus, a species found in a range of habitats – sand-dune, machair, sea cliff, pasture, meadow and roadside – and which flowers profusely during the summer months.

Flowers of Birdsfoot trefoil with light (left) or dark-tipped keel petals (right) at East Sands, St Andrews, June 2020. (Photos R J Abbott).

The keel petals in Lotus corniculatus can be entirely yellow or dark-tipped, with degree of pigmentation of dark keels varying between plants. Presence or absence of dark-tipped keel petals is controlled by a single gene for which there are two alleles (variant forms of the same gene). One allele causes production of dark-tipped petals, while the other does not, with dark being dominant to yellow. For convenience, individuals are grouped into two types: those having entirely yellow keels are termed ‘light’, and those with dark-tipped keels termed ‘dark’, irrespective of degree of pigmentation.

Between the 1970s and mid-1990s, the occurrence and distribution of the two keel colour variants, light and dark, attracted the attention of a number of botanists in Britain. Early survey work indicated a geographical pattern to the distribution of the two types, with light-keeled plants more common in the south and west of Britain, and dark-keeled ones more frequent in the northeast. However, this was based on observations at relatively few sites, so a more detailed survey was required to confirm the pattern. Terry Crawford and David Jones, then at York and Hull Universities, respectively, decided to undertake such a survey and recruited the help of other botanists and citizen-scientists in the process (Crawford & Jones 1988).

Volunteers from the general public were recruited via an article appearing in the colour supplement of the Sunday Times in the mid-1980s. This was a time before most people had access to a personal computer and long before smart phones were invented! Readers were asked to provide a record of the numbers of individuals of L. corniculatus possessing light or dark-keeled flowers at different locations in Britain. From these records, a map was constructed of the frequency of light and dark-keeled types in 10 x 10 km squares across Britain. A total of 125,503 plants were scored for 1,348 sites. Records for 566 sites were supplied by volunteers, with the remainder obtained by the authors and fellow botanists.

The survey showed conclusively that the light-keeled variant predominated in the warmer south and west, while the dark-keeled variant was more common in the colder, northeast of Britain. It also showed that substantial changes in frequency of the dark-keeled variant could occur over relatively short distances. This was made further evident by a survey conducted along the north-coast of Scotland where the frequency of the dark-keeled variant increased progressively from 23% at Sheigra (north of Kinlochbervie) in the west to 87% at Duncansby Head in the east.

Left: Frequency of light-keeled (white sector of pies) and dark-keeled petal variants (black sectors) of L. corniculatus at 18 locations along the north coast of Scotland. Right: Relationship between frequency of dark-keeled variant and National Grid Easting. (From Abbott 1991).

What may account for this geographical pattern? One possibility is that it is related to temperature with the dark-keeled morph favoured in colder areas. The keel petal tip houses the stigma and anthers of the flower, and it has been shown that temperatures on sunny days can be almost 6oC higher in dark-keeled petals relative to within light ones (Jewell et al. 1994). It is possible that increased temperature might increase successful fertilisation, seed formation and, therefore, the relative fitness of the dark-keeled variant in colder areas. However, this remains to be demonstrated.

The Lotus study clearly demonstrates the value of citizen science in research on plant variation, particularly in surveys of the distribution of distinctive variants within species. It is estimated that 217 species exhibit flower colour variation in the British flora (Warren & Mackenzie 2001), and there are many examples of variants occurring for other traits, e.g. flower structure, leaf shape, seed coat and gender. There is plenty of scope, therefore, for citizen science to play an important role in future research aimed at improving our understanding of variation within species and the factors maintaining it.


Abbott RJ. (1991). A morph-ratio cline for keel petal colour in Lotus corniculatus L. along the north coast of Scotland. Botanical Journal of Scotland 46: 131-135.

Crawford TJ, Jones DA. (1988). Variation in the colour of the keel petals in Lotus corniculatus L. 4. Morph distribution in the British Isles. Heredity 61: 175-188.

Jewell J, McKee J, Richards AJ. (1994) The keel colour polymorphism in Lotus corniculatus L: differences in internal flower temperatures. New Phytologist 128: 363-368.

Warren J, Mackenzie S. (2001) Why are all colour combinations not equally represented as flower-colour polymorphisms? New Phytologist 151: 237-241.

Richard Abbott
University of St Andrews


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