What actually lives in Heene Cemetery – species?
The Heene Cemetery conservation team is proud of its list of wildlife species, a list that grows in length each year, but what exactly is a species? There is currently an unprecedented vocabulary crisis in Biology whereby the word ‘species’ is used in so many different contexts that it is questionable whether it is worth retaining. If we abandon it as no longer clearly defined, what do we then call the various microorganisms, plants and animals that we have in the cemetery? As usual, scientific developments proceed so quickly that the English language cannot keep up with them, and even if the latter did keep pace with the former, who is to be the arbiter of any revised vocabulary? These two questions do not have universally approved answers at present.
What is the traditional definition of a species?
Species were traditionally defined in terms of the mechanism of sexual reproduction, but that was in the days of the Ladybird Books, when Jane helped Mummy in the kitchen and Peter helped Daddy in the garden, or was it the garage? We would have been better prepared for the complications surrounding reproduction that biologists were to discover if Peter had sometimes helped Mummy and Jane had helped Daddy, but such role reversals at the time were unthinkable.
If there had been a Ladybird Book of sexual reproduction a species would have been defined as a group of individuals sharing common characteristics and capable of interbreeding sexually and exchanging genes as a consequence. The breeding individuals produce fertile offspring and collectively share a common gene pool. This mechanism means there will always be a degree of variation within the members of a species. Advantageous variations mark the success of the species in its natural environment, and the process by which success is achieved is called natural selection. Evolution into different forms thus takes place at the species level.
However, it became clear that hybridisation between species, and the existence of asexual reproduction, complicated this straightforward definition. A further complication arose when it became clear that no species existed on its own.
Species are really colonies
Apart perhaps from the simplest bacteria it is not accurate to describe living organisms as single species although we regularly do so in conversation. In reality they are all colonies of a number of species coexisting in various forms of biological partnership. Each has something to give to the other partners, and each takes from those partners what it needs to make up for what it lacks from food sources or cannot manufacture for itself. Nearly all known species exist in such co-operative relationships because they cannot perform on their own all essential survival processes. Such mutual dependency is how organisms happen to have evolved on our planet, and we must take this into account when planning and executing nature conservation tasks. Whatever we do to one species will inevitably affect its partner species. This fact is commonly neglected in traditional horticulture and agriculture because those who practise these unnatural processes rely on handed-down techniques that are unscientific and often developed for foreign species and cultivated varieties that are not in their natural environment. These will usually require artificial, trial-and-error, or laboratory devised approaches rather than natural tending based on ecological principles.
Heene Cemetery, June 2022
Heene Cemetery is managed as a native ecosystem in order to encourage colonisation by the native species indigenous to urban Worthing, thereby consolidating the site as a reservoir from which organisms can radiate into neighbouring properties. The fact that over the years the cemetery has had foreign and cultivated plants put in it does not alter the validity of the natural management approach because all will benefit to the extent that they will become part of the many food webs that exist in the cemetery regardless of the origins of their components. Clearly, the native species will provide the largest contributions to active food chains and webs especially as native species are in reality complex colonies of vital importance to the cemetery ecosystem. The most visually obvious colonies to be encountered in Heene Cemetery are the fungal partnerships known as lichens, and they will be noticed particularly on tree branches and headstones.
The modern concept of microspecies
In reality the inhabitants of the natural world do not conform to the simple definition of a species. The amount of variation in populations is so great, even though it is often not distinguishable other than by detailed, microscopic inspection, that we need a different vocabulary to describe it. We now talk about aggregate species (agg.), which is a range of closely related organisms sometimes called segregates or microspecies. Such a range is often called a species complex, and usually the members of the complex will have a common ancestor. The microspecies can hybridise or co-exist without developing morphological differences (visible structural changes). Hybridisation blurs the distinctiveness of species complex boundaries, and a complex becomes distinct when there are clear genetic differences (different DNA sequences) within, and reproductive isolation of, the population.
The Heene Cemetery Brambles
There is no better example of microspeciation than our widely studied native bramble, which is abundant in the cemetery. Dandelions also exhibit microspeciation, and you can investigate this in your own garden if you have a lawn with plenty of them. You should be able to find several specimens that are clearly different in their structural characteristics.
Bramble (Rubus fruticosus agg.), Heene Cemetery, October 2020
Each Spring new bramble shoots develop from the underground, perennial rootstock, but these shoots are biennial, having a two-year growth cycle. During the first year of the cycle the growth is vegetative (asexual), and if the growing tip comes into contact with the soil it will develop roots and produce a clone plant with the same genetic constitution as the parent plant. The compound, palmate leaves of these first year shoots have 5 – 7 leaflets, and if you look at a clump of brambles you should be able to find examples. In the second year of growth lateral shoots develop, and these have 3 – 5 leaflets, and these too are easily found. Flowers, and then fruit, develop on these lateral shoots. The fruit and seeds may be produced sexually, or by a process that is neither sexual nor asexual that is called apomixis, in which reproduction without fertilisation takes place. This happens when the bramble’s own pollen falls on its flowers’ own stigmas, stimulating fruit formation without fertilisation actually occurring. The seed produced is genetically identical to that produced by the parent.
A further variation takes place in brambles in which their chromosome number doubles or trebles. This is called polyploidy. Hybridisation between all these different forms can also occur, with the result that we have a bewildering variety of microspecies, distinguished by minute differences in the shoots, leaves, flowers, prickles, and fruits. This is why the plant is correctly listed as Rubus fruticosus agg. and not as Rubus fruticosus.
The bramble ‘fruit’, or blackberry, is no more simple than the plant that bears it. The fruit as such is actually just one juicy sphere, or drupelet, and what we call a blackberry is really an aggregate fruit, or drupe. The words ‘fruit’ and ‘berry’ are two of the most misused words in the English language. To be a fruit a flower part must contain seeds, so a tomato is a fruit. A berry has its seeds enclosed. So, a strawberry is not a berry, as the seeds are external, but a tomato is a berry, and so is a banana!
The next time you host a dinner party it would be quite a novelty to offer two different blackberry and apple pie desserts, one made from blackberries picked from stems that have borne fruits created by sexual reproduction and another from blackberries picked from vegetatively reproduced stems. When picking these you will be guided by the number of leaflets on the stems. The two pies should taste slightly differently, and this will be an impressive talking point with your guests.
Simplicity is no more
What has been described for brambles is over and above the normal variation that is found in all groups of reproducing individuals (what we used to call ‘species’), in which offspring have similar but not identical features to their parents. The natural world is thus shown to be one of considerable complexity, in which something so commonplace and essential as reproduction can occur by a number of different mechanisms. When the Peter and Jane Ladybird books went out of print, without unfortunately an edition with the title Peter and Jane go to the Cemetery, it saved the authors the nightmare of having to grapple with the realities of human reproduction, and the extra human variation that results. A green space like the cemetery is a microcosm of reproduction where a great deal may be learned by the observant. The bramble, with over 2000 complex variants discovered so far in the British Isles, may be an exceptional case, but if you inspect all individuals of what you have hitherto called a ‘species’ in your patch you will doubtless find that complex variation is the norm, just as it is in the human population. We might think of ourselves as special, but really we are just Homo sapiens agg. with no more claim to exclusiveness than brambles or dandelions.
Written by Brian Day