In The Blink Of An Eye (20 page)

In the second half of the eighteenth century, before the declaration of evolution, the Reverend Gilbert White wrote many letters to Thomas Pennant and Daines Barrington, acquaintances who shared his interest in the natural history of Britain. White lived in the Hampshire village of Selborne, and used the wildlife of his parish to encourage the zoological curiosity of his fellows. In 1788, more than a hundred of his letters were gathered into a single volume.
The Natural History of Selborne
became the fourth most published book in the English language.

White, Pennant and Barrington described the wildlife of Selborne, and some of the nature encountered during their expeditions around Europe, as they saw it. They painted a vivid picture, one in existence only under daylight. But did they acknowledge life at night? And did Darwin observe the fields and woodland surrounding Down House as the sun went down? The answers are ‘no' and ‘no' again. The previous chapter begs the question ‘What about nocturnal animals?' Well, I had a reason for overlooking this subject. Night-time on terrestrial Earth is a grey area. It is neither bright nor completely dark.

Darwin, faced with a mountain to climb in any case, ventured only into the world he saw with clarity. Humans have adapted to the visual
world of daytime. But a letter from Thomas Pennant to Gilbert White indicates that there also exists a visual world at night. During a tour of Scotland, Pennant noted his sighting of an eagle owl.

I once spotted an eagle owl in the heart of England. Driving home in the dark, my headlights picked out the sign for my home village. All seemed perfectly normal, except that an eagle owl was perched on the sign. Wait a minute. An eagle owl, in England? I must have been mad or drunk. But I knew I hadn't been drinking. Maybe the eagle owl, over 2 feet tall, was a figment of my imagination. I was unaware of Thomas Pennant's sighting at the time, but I knew my owls. And eagle owls do not live in Britain.

I decided to forget about my apparition . . . until I turned on the radio the following morning. Concluding the regional news was a story about an Egyptian eagle owl - one that had escaped from the local wildlife park. Suddenly I chose to recall my apparition of the previous evening. And the most memorable part of that bird was its eyes - its
huge
eyes.

What Thomas Pennant saw in the eighteenth century is no longer relevant today. Although they once lived in Britain, eagle owls reside elsewhere now. But where they do exist, they are active at night. And to catch their prey they use sound . . . and light.

In the previous chapter we learnt that through larger eyes pigments would appear brighter, because big eyes sample a larger segment of the pigments' multidirectional reflectance. At night the Earth is lit by moonlight - the sun's rays reflected from the moon. Humans cannot efficiently detect these rays and often fall short of the visual frontier at night.

Now Darwin's exclusions and the eagle owl's eyes become interesting. What Darwin could not see, the eagle owl can. The theme of this chapter is darkness, and what happens to wildlife that is deprived of light. But on a journey into total darkness it is worth adjusting our eyes via intermediate cases, beginning with the first step.

Without the aid of night-vision equipment, it is not surprising that Victorian and earlier naturalists concentrated their efforts on daytime. But while they gazed into their perceived darkness, nocturnal rodents scurried in front of their eyes, and owls were watching them.

Mammals were never going to be champions of camouflaged shapes. Their highly sophisticated machinery, particularly their warm blood, calls for a generous volume compared to surface area - they must be roundish. Still, they try their best to be camouflaged, as with the lioness hiding itself in the grass. They have succeeded with background-matching colours, but sometimes that is not enough, in which case they are compelled to evolve in darkness.

It is interesting that on land the same physical environment exists at night as it does during the day. Trees and rocks continue to provide nooks and crannies . . . but no longer areas of brightness and shade. And the evolutionary outcome? There are considerably fewer species active at night compared with the day. There really are fewer niches - ‘ways of life' - available at night.

The reduction in niches caused by the lack of light is central to this outcome. And then comes the secondary factor - feeding. Ripples travel down the whole food pyramid. Fewer niches lead to fewer species near the base of the pyramid. This in turn narrows the whole pyramid, where at the top there are fewer predators. But the night-time pyramid occupies the same physical space as that of the day-time pyramid. So the food web becomes stretched and offers less opportunity for tangling, or for evolution to cross lines. Evolution maintains a comparably low diversity at night.

Heat is partly responsible for this. It is warmer during the day than at night, and many animals are adapted to warmth. But animals from most phyla
can
be adapted to the cold. This is not an evolutionary impossibility. So we can consider at least part of the day-night biodiversity difference as evidence towards the power of light as a stimulus affecting life on Earth. Begin to remove this stimulus and evolution becomes much less complicated. I say ‘
begin
' because night-time on land is only a step towards total darkness.

At night, other senses are employed. But this is where the big difference between light and the other major stimuli becomes clearly evident. I refer to the difference in presence. Light strikes the Earth and oozes through the canopies of trees, between rocks and blades of grass, and into the waters - it cannot be avoided. Light infiltrates an environment whereas the other major stimuli do not. This explains why owls, equipped with extremely sensitive hearing and the potential to further develop other senses, do not relinquish their use of light. In fact vision has evolved further in owls. A mouse that has detected the flight of an owl may freeze and become inaudible - the equivalent of invisible to light. But where invisibility demands great evolutionary effort, inaudibility requires only temporary stillness.

Up to this point I have considered the major senses - senses that are common in nature. These are smell and taste (which are quite similar), sight, hearing and touch. But at night, one of the minor stimuli becomes important. This stimulus carries the advantage of light in being unavoidable. As described in Chapter 3, bats hunt using radar.

Radar is a minor stimulus/sense as a result of requiring considerable evolutionary expense and chemical and mechanical effort just to infuse the stimulus into an environment in the first place. Light, on the other hand, is a pre-infused stimulus. Only
then
, when radar has been launched into the air, can its detection be compared to vision. And even so, a bat's radar invites little evolutionary change in the animals not directly affected by this stimulus. Light, on the other hand, affects everything in the environment where it exists.

The owl is completely unaffected by the bat hunting moths around it. During the daytime, however, apparently isolated predator-prey relationships begin to interact with each other. The food web and animal behaviour become increasingly complex. So in addition to the direct reduction in niches at night, through the degeneration of light and shade partitions for instance, evolution is stimulated much less at night. Again, in this chapter I place emphasis on the predator-prey scenario because the first rule of survival is to avoid becoming a meal. So this interaction is as important as it gets.

On land, the transition from light to almost dark happens quickly, during sunset or at dusk. So few animals on land are adapted to anything
other than light or almost dark conditions. But in the sea there is another transition from light to dark - a transition in space. Marine animals can be compared from different depth ranges, living under different light levels.

The biggest clues towards solving the Cambrian enigma from night-time on land are the reductions in both biodiversity and complexity of behaviour that accompany a reduction in light. We will develop this understanding throughout this chapter, but further clues can be found in the deep sea, where evolution within a tiny branch of the animal tree can be tracked through time.

The Scavengers of East Australian Seas, or ‘SEAS', expedition was established to scientifically document the entire community of scavenging crustaceans - the group of arthropods that include the crabs, shrimps and lobsters - along the east coast of Australia. Before 1990, traps were set for these animals, but these were poorly designed and caught only individuals bigger than a few millimetres. In fact a twelfth-century fish/crayfish trap was recovered from the River Thames where it passes the Tower of London, and its design turned out to be superior to twentieth-century traps. Its overall form was that of a wickerwork cone, with a funnel-like entrance. Beyond the entrance lay an additional but narrower funnel-like entrance, creating two chambers inside the cone that could hold catches of different sizes. The victims would have been lured into the cone by bait in the smaller chamber. The whole trap was weighed down on the river-bed by two large flints, and connected to the surface by a rope.

Not only had scientific scavenger traps fallen below twelfth-century standards, but they had been set sporadically - on a random basis within small areas, and without the bigger picture in mind. Jim Lowry had been thinking about this lax approach for some time, and decided
he
would paint the bigger picture, and in turn lay the foundations for scavenging crustacean conservation.

Scavenging crustacean communities are exceptionally important
because they clean the sea floor of dead organic matter such as fish carcasses, which would otherwise consume valuable oxygen in the water as they decayed. And throughout the course of a normal day there is quite a fall of bodies to the sea floor. Also scavengers are a noteworthy part of the marine food web - they in turn provide food for other inhabitants of the sea, and complete the cycle of organic nutrients.

Figure 4.1
A twelfth-century fishing trap recovered from the River Thames.

Jim Lowry had moved from Virginia in the USA to the Australian Museum in Sydney via a lengthy spell in New Zealand. He chose his back garden as a study site - the east Australian coast, in fact, and no small undertaking.

Jim Lowry lives on a small island within a marine inlet to the north of Sydney. He travels to work by motorboat and motorbike. His bike is a beautiful, black and chrome 750cc machine. His boat is rather less impressive, but is affectionately known as ‘The Flying Scud'. Scud is the American slang, though not quite a household name, for amphipod - a type of crustacean. Amphipods are commonly encountered on beaches, near rock pools, in the form of ‘beach fleas'. Often they have shrimp-like bodies that are flattened from side to side. Jim Lowry studies amphipods. He produces (along with his co-worker, Helen Stoddart) some of the finest taxonomic work to be found anywhere.

Taxonomy is arguably the oldest scientific profession. It involves
documenting and describing new (to science) species using consistent methods, and is one of the most essential of all scientific disciplines. Scientific classification began with the Swedish botanist Carl Linnaeus in the eighteenth century. We still use his system, but Darwin and Wallace's theory of evolution has allowed scientists to see diversity as the result of a dynamic process rather than a static picture. Considering the extinction rate induced by humans, and that only about 10 per cent of the Earth's species have so far been described, we should really be in a hurry to get on with taxonomy. Taxonomy is also important from an evolutionary perspective. We must describe and collect nucleic acids from the species alive today in order to perform evolutionary and genetic diversity analyses. Better to collect DNA from species while they are alive rather than extinct. Remember the drama of collecting ancient DNA from just a single extinct species such as the mammoth? Unfortunately we have been a little slow off the blocks, to say the least. Today species are disappearing faster than they are being described.

Jim Lowry's interest in scavengers stems from the amphipod connection - amphipods are among the chief scavengers. The other principal scavenging group was thought to be isopods. Isopods are also shrimp-like animals but their bodies are typically flattened from top to bottom, rather than side to side. Isopods include woodlice - the only members of the group with any notoriety, although probably bad examples since most isopods are marine.

Jim Lowry designed a scavenger trap not too far removed from the twelfth-century model. Plastic drainpipes were sectioned into short tubes to form the frame of the trap and to provide a robust structure; his traps were destined for deeper waters. Plastic funnels were cut accordingly to provide two different apertures, and they were glued into the ‘drainpipe' tubes to form the two chambers. A mesh was fixed at the end of each tube to allow water to flow through the trap rather than sweep it away. The size of the mesh was important - holes half a millimetre in size were selected, allowing anything smaller to escape, but anything larger to be caught.