What on Earth …? Thinking about the biosphere.

Picture credit: NASA.

John writes …
 
This blog has been inspired (if that’s the right word) by several intersecting lines of thought. Firstly, I started writing this on Earth Day, a ‘commemoration’ established in 1970 to mark the ‘birth of the modern environmental movement’ (www.earthday.org). That movement can probably be traced back to the publication of Rachel Carson’s influential book Silent Spring (Houghton Mifflin, Boston /Hamish-Hamilton, London, 1962). In the book, the author demonstrated the effects on the biosphere of the routine use/over-use of pesticides, especially DDT, showing that their damage went far beyond their original targets, partly because of the networks of interactions of different groups of living organisms. This topic was explored further in another book, Pesticides and Pollution by Kenneth Mellanby (Collins, London, 1967). 

Since those days, use of the most damaging pesticides has been discontinued but human activity continues to damage the environment in different ways, of which climate change is currently the most pressing. Earth Day, with its focus on the thin layer of the planet’s surface in which life occurs (the biosphere), reminds us that Earth is the only ‘home’ that humankind has known and that although apparently independent of nature, we are still actually dependent on the way it functions. The changes in climate are already having effects on the ‘liveability’ for humans in some parts of the world, including low-lying islands. There are also clear changes in the global distribution of many living organisms, changing the ‘balance of nature’ in many locations. Further, as I indicated in an earlier post, if we continue as we are, parts of Africa, South America, Asia and Australia will become uninhabitable. Earth Day is a good opportunity to remind ourselves of this.

The Internet of Living Organisms
 
I have already hinted at the second train of thought in the previous two paragraphs. Affecting one part of an ecosystem affects the other parts because of the linkages between organisms, which function at several different levels. One example, which is very relevant to current concerns, is that a decrease in the abundance of pollinating insects, may lead to a decrease in seed production. This not only affects the reproductive success of the relevant plants but also affects those animals which use the seeds for food. This in turn may affect the reproductive success of those animals. Inter-species balances in the ecosystem will have been altered. Nor are humans immune to this. Although all our ‘staple’ cereal crops are wind-pollinated, many others, including for example, fruit crops, rely on insect pollination.

This straightforward example, involving relatively short chains of interaction is actually just an indicator of a much more complex situation. I am writing this in our study/exercise room (yes, really – there is an exercise bike behind me and Swiss ball beside the desk) looking out at a beautiful group of mature trees beyond the bottom of the garden, between our row of houses and Topsham quay. Each of those is home to thousands of insects and other invertebrates, a food source for many local birds. Birds use the trees as lookout points, song-posts and nest sites. In the autumn, the hawthorns will provide berries for birds and small mammals, followed later by ivy berries: ivy is growing up several of the larger trees. And we should not forget that the dry seeds/fruit of other trees, including ash and pine, also provide food for squirrels and other small mammals such as wood mice. The trees themselves of course fix carbon through the process of photosynthesis, thus contributing positively to the local carbon budget. Further, the trees have stabilised the ground around them, providing a habitat for a particular group of non-woody plants that thrive in clearings and woodland edges.

If we go below ground, we will discover that most of the trees are in a close relationship with fungi, in the form of mycorrhizas (‘fungus-roots’). Depending on the species of tree and of fungus, these take different forms but the key feature is that the relationships are mutualistic, benefitting both tree and fungus. Further, the fungal mycelia – threads of growing fungus – extend from tree to tree providing an underground network for chemical signalling between the trees. The size of such networks can be astonishing. In a well-established forest in Sweden between 60,000 and 1.2 million ectomycorrhizas were present in one square metre of soil, with the mycelia of each fungal individual extending up to 100 square metres (Mycorrhizas - the wood wide web | Trees for Life). If anyone is interested in finding out more about this world beneath our feet - the ‘Wood-wide Web’ - I recommend Merlin Sheldrake’s excellent book Entangled Life (Random House, London and New York, 2020). 

Fly Agaric toadstool in a small wood in North Yorkshire. The toadstool is the fruiting body produced by an extensive underground fungal mycelium.

I hope that by now, readers will appreciate why I often refer to the biosphere as the internet of living things. With that in mind, I want to return to a topic mentioned briefly above, namely pollination but I will re-introduce it by a slightly circuitous route. Ten years ago, the community of Topsham was fortunate in being able to purchase a parcel of land (2.4 hectares/ 6 acres) beside the river at the south end of the little town. The land, which includes a small copse, is managed as a semi-wild open space, completely free to access. One of my roles on the management committee is to survey ‘biodiversity’, keeping a monthly record of the main features. Thus, this month I have been very pleased to see so many Arum lilies (Arum maculatum) in flower in the copse. This plant is famous for temporarily trapping flies within its flower. Flies are attracted to the plant by the warmth of a part of the flower called the spadix, crawl down into the flower and are trapped therein by the springy hairs (the biochemistry of heat generation in plants is an interesting topic in its own right, but lies outside the scope of this post). While feeding there, the flies become dusted with pollen, the springy hairs wilt, releasing the flies which then repeat the process with another Arum plant. Pollen is thus transferred from plant to plant.

Arum flower.

Cartoon of the Arum pollination mechanism. Redrawn from Lords and Ladies (C.T. Prime, Collins, 1960) and published in Functional Biology of Plants (M.J. Hodon and J.A. Bryant, Wiley-Blackwell, 2012).

Because plants are static within the environment, individuals cannot move around  to find mates in order to bring the male gametes (pollen) into contact with the female gametes (ova/eggs). So, unless self-fertilisation becomes the norm, an agent is needed, as exemplified by the flies in the previous paragraph. Our current understanding is that the very earliest flowering plants were pollinated by insects, probably beetles. We do not know what attraction mechanisms were ‘employed’ by these early flowering plants – perhaps it was just the flower itself. However, over the subsequent 130 million years, flowering plants have multiplied, evolved and diversified. There are now about 300,000 species ranging hugely in shape and size. The majority (85%) are pollinated by animals, mainly insects (although some are pollinated by vertebrates, including birds, bats, small mammals and even lizards). The other 15%, which includes many trees, and all grasses and cereals, are pollinated by wind or in rare instances, water.  

For insect pollination, we can see that a variety of attraction mechanisms have evolved over the history of flowering plants. The plant provides a source of protein (the insects consume some of the pollen) and many cases also a source of sugar in the form of nectar. As mentioned already, the flower itself may be part of the attraction mechanism while many plants also produce floral scents. In some species which are pollinated by night-flying insects such as moths, those scents are produced in the hours of dusk and darkness. We can thus see that many flowering plants produce chemicals that are not needed for the general life of the plants themselves, namely nectar and scented molecules, but are needed to attract pollinators.
 
But it is not all a one-way street: many, perhaps even a majority of the insects which pollinate flowering plants are dependent on them for sources of sugar and protein. Just as the mycorrhizal fungi and their hosts are co-dependent, so are these flowering plants and their pollinators.

This introduces us to my third line of thought, concerning co-evolution, where natural selection favours / has favoured those variants in which relationships between different organisms are strengthened. The general inter-dependence of many plant species with many insect species is an example of this: flowering plant and insect evolution have been tightly linked over millions of years. However, there are some examples in which the linkage is even tighter, where the interactions are very specific. For the sake of space, I will give just one example, that of plants in the Yucca family, the most well-known of which is the Joshua Tree. I wrote about this in the book itself (pp. 183-184) but the story is so remarkable that I am happy to re-tell it here. Yucca plants are pollinated by Yucca moths, with each species of moth having a very strong preference for just one species of Yucca. The inter-dependence between plant and moth is so strong that one cannot live without the other (see Yucca Moths). I will start the story with the emergence from their pupae of the adult moths. This occurs in Spring and is synchronised with the flowering of the relevant Yucca species. Male and female moths encounter each other on the flowers and then mate. The male’s job is done and he soon dies. However, the female collects pollen from the anthers of the flower and gathers it into a ball under her ‘chin’. She then visits another plant and lays a few eggs in its ovaries. The emphasis here is on ‘few’ – too many would cause the flower to abort. She then does something which, even though I know the facts, I still find incredible. The female does not consume any of the ball of pollen she collected from the first plant. She does not need to eat because she will shortly die. Instead, she removes the pollen and scrapes it onto the stigma of the flower in which she has laid eggs. In effect, the female moth acts as a pollen carrier for the plant, with the reward of a safe place to lay her eggs. The plant’s ova are thus fertilised, ensuring that seeds will be formed. Some of these, as they develop, will provide food for the moth’s growing caterpillars but since there are only a few caterpillars (remember that only a few eggs were laid) several seeds survive to maturity and ripen. Alongside them, the caterpillars also reach maturity, leave the flower and go down to the ground to pupate. The pupae will remain in the ground until the following Spring when the new generation of adult moths emerges to start the sequence all over again. This is an amazing example (and there are others) of a rigid inter-dependence of two totally unrelated organisms.

Joshua tree.

In addition to the range of strong interdependencies in nature there are also examples where one side or the other has evolved in such a way that it ‘games the system’. One of my favourite examples of this is the Bee Orchid, the flowers of which somewhat resemble female bees. These attract male bees which attempt to mate with the flowers. In doing so, they pick up pollen but leave without the apparently promised ‘reward’ of a successful mating. When the male attempts to mate with another flower, some of that pollen will be transferred, thus ensuring a successful pollination of the second flower. In this instance, the plant is dependent on the bee but the bees need to mate is exploited without being met.

Bee orchid.

The interactions and inter-dependencies I have described here are snapshots from a highly interconnected biosphere. In respect of any of the damaging aspects of human activity, one question that I want to highlight concerns selective effects on particular components of networks. What happens if one member of a partnership is more affected by pollution or climate change than others? We can see straightaway that the well-established ‘balance of nature’ can all too readily become unbalanced. This should concern us.
 
Final thought
 
I started this post by discussing Earth Day and I want to say a little more about this. Christians – and indeed, Jews and Muslims - believe that our beautiful planet is the work of God the creator. Psalm 24, verse 1 puts it like this ‘The earth is the Lord’s, and everything in it, the world, and all who live in it’  In effect we live on Earth as tenants or stewards with a responsibility to care for it. I was therefore very taken with the words of Professor Katherine Hayhoe, an American Christian climate scientist, when she wrote on her website, ‘Isn’t every day Earth Day?’ (https://www.talkingclimate.ca). Let’s act on that.
 
As always, if you have any thoughts on what I have said in this post, please leave a comment below. Thank you.
 
John Bryant
 
Topsham, Devon
April 2025
 
Picture credits: the copyright of all images is owned by the author, unless otherwise stated.