Sara Lil Middleton
With the world currently trying to learn the difference between ‘social distancing’ and ‘picnic in the park’, it seemed pertinent to evaluate what it actually means to be an individual. Intuitively we understand what an individual is — a separate physical entity: for example, you or me, a mug, or a magazine. In the everyday world we act on this intuitive understanding, often trying to avoid other people [individuals] on a crowded bus or train. However, as I quickly found out when I started my fieldwork last summer, in biology the boundaries of what constitutes an individual becomes a little blurred.
Super-organisms: the whole is more than the sum of its parts
The word individual derives from the Latin individuus meaning ’not divisible’1. In biology, the unit for individuality is the organism2, which is made up of interacting sub-units such as organs and cells. So clearly a human hand on it’s own is not an individual but what about bees? Several bee species (right image) are eusocial, where individuals are socially organised, living in colonies with clear group roles (e.g. worker bees maintaining the hive vs. the queen bee laying eggs). These bee roles are analogous to organs performing certain functions in the human body. Therefore, a colony made up of thousands of interacting bees with a common goal is a super-organism. Other examples of this across the tree of life include: ants, corals, jellyfish, and some fungi.
You might be surprised to know that the largest living individual organism on earth is actually a type of fungi aptly named Humongous Fungus (Armillaria ostoyae left image) and not the blue whale3! Found in Malheur National Forest, Oregon, this super super-organism covers an area three times the size of New York’s Central Park. This massive mushroom earns a living as a parasite, by sucking up nutrients from tree roots — eventually killing them. Through genetic testing, scientists discovered that all the different fungus ‘edges’ were in fact genetically related and individual cells were communicating.
Clonality: complicating the picture
In the plant kingdom things can get tricky too. Imagine a bunch of grass in your local park or back garden, it may appear to be an individual, but a bunch of grass three metres away may actually be part of the same plant, connected by underground roots (rhizomes). Grasses (right image) and some other plants such as strawberries send out extensions of themselves known as ramets. Confusingly, these extensions can be both a form of reproduction (i.e. making a brand-new individual) or growth (i.e. an individual getting bigger)4. Clonal plants can have many ramets which form a genet — a genetically identical individual. Sometimes there is a disconnection between the ramets, so now once a united ’individual’ appears to be separate bunches, which all have the same genetic make-up. Although physically separated, these bunches of grass are genetically identical, posing the question whether this is one individual or several?
Corals: two become one
Corals are an interesting case when it comes to individuality. Corals are immobile animals that have formed a clever cooperation with photosynthetic algae called zooxanthellae. Each individual coral with the algae living in its tissues is known as a polyp, and many polyps form a large group — a.k.a a colony. This animal–‘plant’ association has been around for 210 million years 5 . The algae provide nutrients for the coral from photosynthesis and, in return, the coral’s waste products are used by the algae to make food. This close partnership is so intertwined that corals can die (from starvation) if the algae disassociates. The disassociation happens during a coral bleaching event, when sea temperatures become too warm. Both parties, the coral polyps and the algae, are dependent on each other for survival, making it a clear case of an integrated individual.
Corals can clone themselves via budding, for example when a bit of a coral breaks off during a storm. The budding process creates a new colony that is genetically identical to the ’parent’ colony. Corals can also reproduce sexually (i.e. with eggs and sperm) which produces genetically different individuals. Other times the polyps grow outwards from the colony in an endless growth-death-regeneration cycle which can make it tricky to identify an individual coral through time.
So why does it matter?
A super massive mushroom and twin grass bunches seem to put into question the concept of individuality. But does it actually matter? Well, that depends on what questions you want to answer. In demography, the study of population trends of an individual species, information on survival, growth, and reproduction of individuals need to be clearly characterised in models. Defining an individual one way or another can result in very different outcomes! The same goes for evolutionary questions, where the unit of natural selection is the individual6.
When I started fieldwork in May 2019, I hadn’t given this much thought (right image). The first week, I was staring at different plants in my experimental plots for several hours trying to work out who was who. My research involves understanding how a grassland responds to an experimental drought treatment. This involves following the lives of 140 individuals of the grass species Brachypodium sylvaticum together with a few neighbouring species as well as measuring several plant traits. Luckily my study species is not highly clonal and forms distinct bunches.
On the face of it, nature appears to be simple, following the rules of what it means to be an individual. However, look around and dig a little deeper and you find super-duper fungi or cooperative corals that blur these boundaries.
- Pan, J.J. and Price, J.S., 2002. Fitness and evolution in clonal plants: the impact of clonal growth. In Ecology and Evolutionary Biology of Clonal Plants (pp. 361-378). Springer, Dordrecht.
“I am plant ecologist on the NERC Environmental Research DTP working across the Plant Sciences and Zoology Departments. I use biological characteristics of plants to measure how plant communities respond to different aspects of environmental change. I also have a keen interests in sustainable food production systems through my involvement with Bananageddon, a documentary film project on bananas (https://bananageddon.webflow.io and @banana_truth).”