Nature 2016 June

A natural history treasure

Recently, I’ve been enjoying the celebrations and tributes across the TV channels to a 90 year-old ‘natural history treasure’. I refer of course to Sir David Attenborough, whose prodigious natural history documentaries span over 60 years and have been given a bumper airing recently. Lesser natural history documentary makers might fall into the trap of assigning motives to plants and animals, perhaps even describing them as cunning or imaginative. In contrast, Attenborough elegantly reminds us that the almost infinite variety in form and function is a consequence of environmental and competitive pressures selecting only the fittest of varieties to survive.

Amongst Attenborough’s back catalogue of the various ‘Life’ series, are two lesser known gems, neither is given the same limelight as ‘Life on Earth’ or ‘The Living Planet’.

The first of Attenborough’s Life series I have particularly enjoyed watching again is ‘The Life of Insects’.

One home-spun segment features the habits of flies and I am reminded of the appropriateness of the name of that most ubiquitous of indoor insects, the house fly or Musa domestica. Its name suggests not only it has permission to occupy our homes each summer but has carte blanche to wander from room to room in search of unguarded food and company of the opposite sex.

The most important evolutionary adaption is the fly’s association with animals and man. Flies have been our companions as far back as our earliest ancestors, travelling alongside early nomadic groups.

One of the commonest observations concerns the lesser house fly’s habit of flying round and round an overhead light, typically in the same direction and by way of regular 90-degree turns. Scientific evidence strongly suggests the behaviour is down to how the fly navigates using the sun, orientating so as to keep the light source in the same position. Outside it is capable of flying at up to 25cm per second and needs a suitably effective way to orientate itself. Indoors an overhead light only provides a singular point of reference from which a fly will experience the same photo-sensitive sensation. This flight habit is usually typical of the male fly and can be described as marking its territory, reserving the domain for a female which may be loitering on a nearby wall or feeding on discarded food.

The house fly’s success has not only been down to choosing the right partner in us, but also its fecundity (reproductive fertility) and ability to home in on food supplies. During its typical life-span of 25 days a female is capable of laying at least 900 eggs and though most may not hatch, and many of the maggots will die, a single pair of flies could be responsible for up to 1,000 descendants within the subsequent two month period. Luckily for us 99 per cent of all such flies fail to make it to reproduce, with predators and cold weather drastically reducing populations each year.

The second of these two less well-known series is ’The Private Life of Plants’. Tucked away in the undergrowth of this series is a sequence on evolutionary specialisation affecting seed dispersal which got me thinking about what different examples of dispersal could be found locally. There are five approaches to seed dispersal. The use of gravity or barochory was noted by Isaac Newton, who discovered to his cost that embedding the seeds in a heavy and spherical fruit structure increases the likelihood of reaching the ground and rolling away from the tree. Locally it is best exemplified by the many apples found on the Commons.

Of course it helps to have a tasty fleshy surface to attract birds and animals to aid secondary dispersal. Perhaps the most evolutionarily advanced and dramatic method is ballistic dispersal or ballistichory where the plant has engineered an explosive potential within the fruiting body through drying out or through increased water pressure within cells. What is more, the angle of projection maximises the distance. Thus seeds are distributed much further – up to 200 feet – from the host. Two good local examples are meadow cranesbill, which catapults its seeds and gorse, which uses pods which twist as they dry out and eventually explode, hurling the seeds in all directions.

Another example of seed dispersal is the use of wind, with dust, plumes and wings the three types. The ‘dust’ method is widespread, shaking out minute seeds (e.g. foxgloves) in vast numbers. The ‘plumes’ are best exemplified by certain members of the daisy family, such as dandelion and thistle, where the elongated seeds possess a parachute or a pappus that aids dispersal according to the prevailing air currents. The ‘wing’ types use wind currents but are aided by aerodynamic features and air sacs and are found locally with the pine, sycamore and eld maple trees. The fruit has one or two wings and are colloquially known as keys, helicopters or polynoses!

Next is the use of water or hydrochory. Where we live there may be minimal running water aside from the roadside ditches but even so we can find pond examples with water lilies whose fruit first float, then sink and become submerge in detritus before sprouting. In contrast, some buttercups use rainwater to splash their seeds out of the fruits. Many mosses and fungi also use water to dislodge or set off their explosive devices.

Animals offer a very efficient dispersal vector known as zoochory. Hair readily provides a medium onto which a fruit becomes attached by barbs or sticky substances and transported great distances. How often on return from a country walk have you found the burr of the burdock or the trailing stems of cleavers with seeds attached wrapped around your socks or trouser turn-ups? Many trees and plants rely on animals to disperse seeds by consuming, digesting and excreting the seeds. When not following their natural habit of catching small mammals and chickens, foxes also consume large quantities of soft fruit. Both jays and grey squirrels store hazelnuts for harvesting but will not remember where all are stored, thus providing an opportune start to an overlooked nut.

There are many other examples to be found on an everyday walk. Next time you are out and about, spot the plant, tree or its fruit or seed and guess how it is dispersed through woodland, common, hedgerows, pastures or water.

Our natural history and national treasure, Sir David Attenborough has some new ‘Life on Earth’ style documentaries appearing later this year.