What is a species?

tuberaria guttata

Hello to all reading this.  My name is Tom and I am the lucky recipient of the 2014 Della & Fred Worms Scholarship from the Friends of JBG.  I hope to write a few articles on some of the most interesting elements of my time in Jerusalem.

As I write this I have just returned from a sunrise hike up the famous hill fort of Masada and a casual float in the Dead Sea. However, as Friends and followers of the Gardens in Jerusalem, I’m sure you know of all the popular weekend destinations for tourists. What I hope to be able to do through these blogs is give you a personal insight and a perspective into the work going on at JBG that you may not ordinarily get. We will start by discussing the curious case of one of Israel’s rarer plants, Tuberaria guttata.

I am cataloguing rare annuals Sara (one of the 2012-13 interns) has grown and creating image sheets and a complimentary database to aid future scholars in decision making. During this project I happened upon a couple of pots of Tuberaria guttata (L.) Fourr. It is a beautiful little annual Mediterranean herb from the family Cistaceae that is just coming to the end of its flowering window now. Whilst the striking features warrant attention in their own right, observing the wider distribution and reproductive behaviour of the species throws up some very interesting questions, the most important being what is a species?

This was not the first time I had encountered this species; in November 2013 I had been searching for this plant in…wait for it…Wales. In the UK there are colonies of this species in north-west Wales, south-west Ireland and a newly discovered colony in the Inner Hebrides.  In Israel, as in all of these locations, it is regarded as rare.  As a result, the species can be understood as having a wide distribution but being locally scarce. The reasons for this are complex and diverse, but it can be safely asserted that the UK represents the northern extreme of range whilst Israel probably represents the southern extreme. It occurs throughout the Mediterranean at other localities in between these two points.

It is a very variable taxon throughout its range, with morphological differences commonly observed. The UK form, for instance, is shorter and more compact than its Mediterranean counterparts, with stipules absent from the upper leaves, which are oblong rather than elongate. The basal leaves are also wider and the flowers possess bracts, unlike the plants in much of the rest of the range. The black markings at the base of the petals is also very variable, from well pronounced like the Israeli form to completely absent elsewhere. How do you attempt to classify something so variable based on morphology alone?  One method is to look at the genetics, which complicates the issue further with this little plant. Us humans, you may know, are diploid organisms, meaning we have two sets of chromosomes. All the UK material of T.guttata has 6 sets of chromosomes (hexaploid), whereas the Israeli form has four sets (tetraploid) and material is also known to exist with 8 sets (octoploid).  The species also readily hybridises with other Tuberaria species (e.g. commutata and inconspicua) in its southern european range. If hybridisation occurs within a population of T.guttata, then the offspring themselves can grow up and hybridise with T.guttata, and the offspring of that generation can hybridise and so on and so on. A situation can arise in which you have a plant which is morphologically identical to T.guttata but is actually hybrid in origin.

This may sound a touch pedantic, but it has very important consequences for conservation. Decisions have to be taken on the priorities for conservation, and where to allocate often scarce resources. The UK form used to have subspecies status (ssp. breweri), elevating the perceived rarity a degree, but this has now been dropped. Is the Israeli form different to its southern european counterparts or the UK form? How different? Different enough to warrant status as a new species? If you give it new species status, immediately it becomes a critically rare Israeli endemic that occurs nowhere else in the world. It instantly generates an issue for national biodiversity, necessitating resource input to assess the level of threat and determine the action necessary to ensure survival. In this case I suspect this is not necessary (although I am nowhere near qualified to make that judgement!) but the quirks of this little Israeli plant illustrate the inherent difficulty in classifying nature, and its far-reaching human implications. Rarity is a natural phenomenon, and nature by definition is dynamic and ever-changing. Sometimes even our best attempts to understand and conserve our natural world, whilst good-intentioned, fail to recognise this.

I hope you found this as interesting to read as I have found the subject. I will write some more soon.

Thanks to Nigel Brown at Treborth Botanic Garden and the University of Bangor for much of the scientific information included in this blog article.