If a psychologist had studied me as a child, they would have classified me as an odd, geeky kid. If they had studied my parents, they would have observed that my science fixation, coupled with a poor performance at school, sometimes gave my parents cause for concern. I didn’t like school, not only because most of my cohort thought I was a bit of a weirdo – dressing in golfing trousers, pointed leather shoes, a trench coat and a green trilby for much of my teenage years didn’t help my cause – but also because we were not taught how the world worked but were rather given lists of facts to learn. I was never taught the scientific method at school. I researched it myself at home.
After making observations, the next step in the scientific method is to pose a hypothesis. A hypothesis is a plausible explanation of a certain observation. A psychologist observing me as a kid might hypothesize that I was genetically predetermined to be a science geek, but back in the early 1980s they could not have easily tested this. Hypotheses are only useful when they can be tested with further observations, with experiments, or with both. An untestable hypothesis is equivalent to a myth or a story: it may sound compelling, but knowledge cannot extend beyond what we can observe.
To demonstrate how to pose a hypothesis I will focus on the first of the questions I asked above: ‘why is that tree there?’ I chose this question for two reasons. First, it is something we can all relate to, but it may be a question you may have never asked while gazing at a tree. Second, just after I started studying for my Ph.D., I sat gazing at a silver birch in the middle of a field for an hour or so, pondering this exact question. It motivated the topic of my doctoral thesis: how do animals like squirrels and deer contribute to the distribution of trees across a landscape?
The immediate answer to the question of why a tree is in a particular location is that a viable seed of a tree species arrived at that location, that the conditions were right for the seed to germinate, for the seedling to flourish and survive to become a sapling, and for the sapling to grow to become an adult tree. To do this it needed to survive the threat of death from herbivores devouring it, or from pathogens infecting it with a fatal illness, while acquiring enough water, light and nutrients to grow. Although this answer is obvious, what hypotheses need to have been posed and tested to acquire this knowledge? It turns out, quite a few.
It must first be hypothesized and tested that trees develop from seeds, something that is now obvious to you and me. The hypothesis can be tested via an experiment that would involve collecting seeds produced by several species of tree, planting them in known locations and seeing what they develop into. Given our knowledge today, you might think this hypothesis is ridiculously trivial. But as recently as the mid nineteenth century many people still believed in the theory of spontaneous generation, where living beings could arise from non-living objects such as rocks and water. They held this belief jointly with the knowledge that crops grew from seeds, but the necessity of a seed to produce a new seedling was not universally accepted wisdom as life forms were thought to spontaneously arise under some circumstances. A few writers in the seventeenth century, for example, argued that wheat and rags would spontaneously generate mice.
When seeds are collected and planted in an experiment, not all of them will germinate and grow into seedlings, but if enough do, we will have evidence that seeds can develop into young trees. Yet this insight naturally leads to a new question: why do some seeds fail to grow into trees? We might hypothesize that local conditions determine whether seeds germinate and survive to seedlings. An experiment could then be devised to plant seeds in different soil types, and to provision them with different amounts of light, water and nutrients to see which thrive. If seeds germinate in some conditions, but not in others, this provides evidence that environmental conditions can help explain the observation that not all seeds succeed in germinating into seedlings.
Even when conditions are suitable for seeds to germinate and thrive, not all seedlings will survive to become saplings. The next question might be ‘why?’ Some seedlings may appear to have suffered herbivory from animals like insects, snails, rabbits or deer. Others may look diseased, perhaps having been attacked by viruses, bacteria or fungi. These observations naturally lead to new hypotheses that disease and herbivory are important sources of mortality for seedlings. The next experiment is to compare the performance of seeds, seedlings and saplings grown in favourable light, water and nutrient conditions, but with some exposed to herbivores and pathogens, while others are protected from them. If death rates differ between these two groups, then this provides evidence that herbivores and pathogens play a role in determining whether seeds survive to grow into adult trees.
Even when water, nutrients and light availability are provided in just the right amounts, and pathogens and herbivores are absent, there may still be some seeds that fail to germinate. We could now hypothesize that there might be something wrong with some seeds. Perhaps they have not developed correctly, have gone off, or there is something amiss with their genes, which means they are unable to germinate. To test these hypotheses, we would need to move from the field and greenhouse to the laboratory. We might use microscopes, weighing scales, mass spectrometers, gene-sequencing machines and even genetic engineering to compare the morphology, metabolism and genetic code of seeds before planting them in ideal conditions and then monitoring which germinate and which do not.
For part of my Ph.D. I conducted experiments where I excluded deer from parts of an English woodland to examine how they impacted seed and seedling survival. I did this by fencing off areas that I termed ‘deer-exclusion treatments’. I then chose unfenced areas of the same size that deer could access, and these were called controls. I went on to plant seeds and seedlings in each plot. I wanted the deer-exclusion treatment and control regions to be as similar as possible in all aspects except for whether deer could access them or not. I cut back the forest understorey in each treatment and control area with a scythe. At one point, I got a little bored, and to relieve the boredom I waved the scythe around while stating, ‘I am the Grim Reaper.’ At that exact moment a woman walking her dog came around the corner, saw me mucking around and quickly walked away. My field site was close to a secure hospital for the criminally insane, so I rapidly left in case the dog walker reported to the police that a patient may have escaped. At least by then I had ditched the trench coat and green trilby. Despite the hiccough, the experiment worked, and I went on to estimate how deer significantly impacted seed and seedling survival of several species in my study site.
At the end of my field experiments, I had helped understand where and why trees might successfully establish themselves. That was a useful advance, but it did not address the second of the questions asked earlier in the chapter: why is the tree upon which you are gazing of one species and not of another? Why was the tree I gazed at all those years ago a silver birch and not a coco-de-mer palm? To answer this question, we need to generate a new set of hypotheses.
These hypotheses might revolve around whether particular conditions favour seeds from some species of tree over others, or whether the proximity of potential parent trees, and luck, result in a seed of a particular species arriving in a particular location at a time that happened to be appropriate for it to germinate and thrive. Perhaps the silver birch seed needs different amounts of light, water and nutrients to germinate and to develop into a sapling than the coco-de-mer palm nut. This might get you thinking: why do different species of tree have such different-sized and -shaped seeds, and do seed size and shape enable some species to thrive in some environments but not in others? Scientists have addressed these questions, and by applying the scientific method they have been able to explain why one species has particularly giant seeds.