[Section 1. Introduction to an oak woodland]

[Screen 1 - Introduction video]
David Streeter:
Oak woodland occupies a special place in British ecology because in the minds of many people it represents the natural vegetation cover of lowland Britain before man became a significant influence. However, as no examples of this wild wood survive, nobody really knows what it looked like.

Woods are typical ecosystems: a combination of biological communities occupying a physical environment. However, in many ways woods are difficult ecosystems to study because they are more complex than most. The size that they occupy is large, from the soil surface to the top of the tree canopy, and this space is occupied by trees, shrubs, herbaceous vegetation and the ground layer, producing a huge variety of habitats, generating an enormous diversity of organisms. The canopies of the oak themselves support a larger number of different invertebrate species than any other tree in Western Europe. And add to this the enormous numbers of species dependent upon dead and decaying wood and you have the most species- rich habitat in Britain.

[Section 2: A sparrowhawk's eye view

[Screen 4 - Introduction video]
David Streeter:
When trying to understand something as complex as the interrelationships between the different species in an ecosystem like a wood, it's helpful to focus on a single species in order to find out how it manages to maintain itself and survive as part of the community.

Narrator:
Many oak woods contain breeding pairs of sparrowhawks. They're the commonest woodland birds of prey. Like any species, sparrowhawks have a set of characteristic features.

[Screen 7 - video]
Providing food for a nest full of sparrowhawk chicks is a full-time job. What's likely to be the main item on the menu?

[Screen 7 - correct feedback video]
When the male's feeding the brood it's quite noticeable that, if they've got their timing right that they pretty well seem to specialise on tits. The trouble is the male tends to pluck them and pull the head off before they're brought in. And you don't have a field guide for plucked and headless birds so you're dependent on identifying them from the legs.

[Screen 8 - video]
The tits must find enough food to raise their young. What's likely to form the main part of their diet?

[Screen 9 - video]
Winter moth caterpillars have to find their own supply of food. What's likely to be their preferred choice?

[Screen 9 - correct feedback video]
After emerging from the eggs, the caterpillars start eating immediately. They move only as far is required to find food. Their body temperature is the same as their environment as they are cold blooded. The low protein diet of the leaves means they need to consume a high volume of leaves to grow rapidly. As a result they produce large quantities of faecal matter (known as frasse).

[Screen 10]
Well done. What you've just built up is a nutritional sequence known as a food chain.

[Section 3: How much food to raise a brood?]

[Screen 12 - Introduction video]
David Streeter:
Food chains aren't just about what's on the menu; the quantities consumed are equally important and there are important knock-on effects as you move along a food chain.

[Screen 13 - question video]
Narrator:
Our chain ends with a sparrowhawk. On average, how many tits will each nestling consume before it leaves the nest?

[Screen 13 - feedback]
Prof. Chris Perrins:
There's a lot of current worry about whether birds like sparrowhawks, the predatory birds, are taking very large numbers of their prey and reducing the numbers of the breeding pairs. It's difficult to quantify exactly what the total predation round the year is, but they're taking 25-30% of the fledgelings and obviously quite a lot more during the rest of the year.

[Screen 14 - question video]
Narrator:
What about the young tits. On average, how many caterpillars does each consume while it's in the nest?

[Screen 14 - feedback]
Prof. Chris Perrins:
The easiest time of year to see what's going on from the food point of view is to watch them in the nest because we can easily fit up a camera behind the nest that's designed to take a shot each time the tit comes in with a caterpillar in its beak. You have to realise that the tits have these very large broods and if they're to raise 10 or so young, they've got to be able to find food very easily, and the parents actually bring in 7 or 800 meals a day, 7 or 800 caterpillars during a day, and they can't waste time if they're to do that.

Dr Stephen Dury:
The numbers of fully fed caterpillars can be estimated by using funnel traps of known area, say half a square metre placed randomly beneath a tree canopy, and the numbers of caterpillars that fall into these funnel traps can give an indication of how many caterpillars are dropping down to pupate. It has been calculated that the number of winter moth on average is of the order of 100 per square metre of oak canopy, the square metre being a ground projection. But occasionally population outbreaks occur, epidemics of caterpillars occur, and the population can increase to about 1200 per square metre of oak canopy.

Narrator:
There are so many caterpillars in the oaks that only a small percentage fall prey to woodland birds.

[Screen 15 - question video]
Narrator:
Oak leaves form the final link in our food chain. On average, how many leaves does a single caterpillar consume before dropping off the tree to pupate?

[Screen 15 - feedback]
There are over a hundred thousand leaves on a mature oak so one tree is not enough to raise a single sparrowhawk fledgeling.

[Section 4: Energy flow]

[Screen 18 - Introduction video]
David Streeter:
In many ways leaves and blue tits are no different from the food that humans eat. They provide energy and nutrients for species higher up the food chain, and important ideas emerge from looking at food chains from an energy point of view.

[Screen 22 - Photosynthesis video]
Narration
Like all green plants, oak trees use carbon dioxide and water to make vital organic compounds. This process is called photosynthesis. Photosynthesis takes place inside the oak leaves in tiny green structures called chloroplasts, which capture light energy from the Sun. What happens next is a complex chain of reactions that can be summarised fairly simply - water and carbon dioxide are converted using the Sun's energy into simple sugars called carbohydrates. The oxygen released in the reaction diffuses from the leaves into the surrounding air for use by other organisms.

[Screen 29 - Energy fates videos]
Sparrowhawks pluck their captured prey before bringing it to the nest. When the nestlings are very young, they move around very little. They gain weight rapidly but use some energy to keep warm. Later, they lose their downy feathers as their new flight feathers develop. Just before they leave the nest, they repeatedly exercise their wings.

Tit nestlings (here we can see blue tits) take their prey whole. The nestlings call persistently, gaping and agitating to attract the attention of their parents. The enclosed nest keeps away the chilling effects of the wind and rain, so less effort is needed to keep warm. During their time in the nest, the nestlings rapidly increase in body weight. The parents keep the nest clean by removing white faecal pellets.

After emerging from the eggs, the caterpillars start eating immediately. They move only as far is required to find food. Their body temperature is the same as their environment as they are cold blooded. The low protein diet of the leaves means they need to consume a high volume of leaves to grow rapidly. As a result they produce large quantities of faecal matter (known as frasse).

[Section 5: Food webs]

[Screen 33 - Introduction video]
Prof David Streeter:
Individual food chains tell only part of the story. Woods contain many species of animals and plants each with their own particular food chains and, considering the wood as a whole, reveals many important ecological patterns and ideas.

[Screen 34 - weasel feedback video]
We've had quite a lot of problems with weasel predation in the nest boxes over the years. We're not entirely sure that that's all natural in the sense that the weasel takes a lot of nests early on in the season when they're laying when the ground cover in the wood is open and the weasels can obviously see the boxes, a lot of them, and take all the nests. There were two interesting things about the story in that they really only take tits in large numbers in years when the mice and voles are scarce, so it looks almost as if it's a fall-back food for the weasels. They don't come up the trees and look for nests if there's plenty of small mammals for them to take. And the other thing is they really only switch into the tits when the tits are very abundant.

[Screen 34 - woodpecker feedback video]
Woodpeckers, perhaps surprisingly to many people, great spotted woodpeckers, do take quite a lot of small birds, and they raid the nest boxes primarily when the young are in the nest. And of course they can't get in through the small hole so they make their own hole in the side of the box and they make it at the same level as the chicks are calling inside so they can obviously hear where they are and they make a hole and just whip out the chicks one after another and fly off to their own nest with them. And obviously a few pairs learn to specialise in this because you may get quite a lot of nests lost in an area to woodpeckers.

[Screen 36 - video]
Analysing food webs into trophic levels - Professor David Streeter:
The organisms within an ecosystem can be grouped together according to the ways in which they obtain their energy, or the kinds of foods that they eat, thus plants obtain their energy through photosynthesis, herbivores feed on the plants, and carnivores feed on other animals. These groupings are called trophic levels.

Narrator:
Three items in the food web are not assigned to trophic levels. They are a decomposer, a detritivore and a parasite. We'll look at these shortly.

[Screen 45 - Prof David Streeter on woodland decomposers]
The canopy of an oakwood produces something like 15,000 of tonnes of wood, root and leaves per hectare per year, and only about 1 per cent of this is utilized by the herbivores. The rest is either stored as wood or falls to the ground as leaf litter, and this leaf litter is decomposed by a vast army of bacteria, fungi and invertebrates, and this diverse array of invertebrates are called detritivores.

[Screen 45 - Parasites]
Every species in the wood carries a burden of parasites. Some are obvious. The blood-sucking ticks on the hedgehog are clearly visible between the spines. The oak tree itself is host to a large number of wasps. Their larval stages induce the tree to make a wide range of growths or galls. Many other parasite species live unseen within their hosts.

[Section 6: Getting the timing right]

[Screen 47 - Introduction video]
Prof. David Streeter:
One of the fascinating things about woodlands is the way that they change with time. They're dynamic not static systems, and the species in the biological communities respond to short- and long-term changes in the weather and the seasons, and in the regeneration cycles of the trees themselves.

[Screen 48 - lammas growth video]
Oak trees are very unusual in having recurrent flushes followed, or interspersed with dormant periods. Once the terminal bud opens the shoot starts elongating very rapidly. This will stop elongating around mid-June and become dormant. It may stay dormant during the following winter or it may, after a dormancy period of four to five weeks, start growing again. This is known as lammas growth and will produce a growth period of a further four to five weeks before becoming dormant again. The lammas growth enables it to reproduce its leaf area and continue producing resources for the growth of the tree.

[Screen 48 - onset of senescence video]
With the shortening days, the chloroplasts in the oak leaves break down and they lose their green colour. As the ageing process, or senescence, continues the leaves progressively change from yellow to brown.

[Screen 48 - end of leaf fall video]
The dead leaves are no longer functionally connected to the tree and the wind blows them down.

[Screen 58 Durations are not the full story video]
Prof. David Streeter:
There's another very interesting aspect to timing that needs examining in more detail, and that's the relationship between breeding and food supply. Successful breeding depends upon getting the timing just right and the ability to cope with variations in environmental conditions from year to year.

[Screen 59 - Adult emergence]
Throughout the range of the winter moth, adult emergence is always in early winter. This ensures that the eggs are laid on the tree and have the same - experience the same - climatic conditions as the buds and this helps to ensure synchronicity between the two.

[Screen 59 - Oak leaf tannins]
Immediately after bud burst the leaves are at their most nutritious - high in nitrogen content, no tannin content, very low in toughness, high in water content, very succulent. But as the leaves expand and develop they become less nutritious from the caterpillars' point of view. Also tannins increase. It's now believed that tannins may have a direct toxic effect and they also may contribute to leaf toughness and leaf unpalatability.

[Screen 60 - Early/late breeders]
The differences in success between early and late breeders are really quite dramatic. The earliest pairs to breed seem to be able to get their timing about right, so they have their babies in the nest and just fledged also when the caterpillars are abundant so they've got right through the nestling period into the fledgeling period, they've got plenty of food. But once that food supply starts to diminish then the late pairs are really in trouble and there is a very dramatic decline in survival right through the season from the beginning onwards.

[Screen 61 - Early/late breeders]
The earlier sparrowhawks at any rate tend to hatch their young just at the time the tits are fledging so there's the maximum number of baby birds there out in the wood at that time. But rather surprisingly in a way, if you look in detail at the tits that the sparrowhawk takes, they take most of them within a week of fledging, so although a week or two later many of those baby birds are still there they don't seem to be so vulnerable. And the result of that is that the later sparrowhawks aren't so successful. They've missed the boat so to speak, they've missed that window of real opportunity and they're not so successful at raising their chicks.

[Screen 61 - Breeding triggers]
We don't know exactly what triggers the breeding time, but there is quite a bit of variation between pairs of course. What seems to be quite likely is that the female obviously needs a lot of food to get into breeding condition and to form eggs and lay them and it looks as if what may happen is that it's when the first blackbirds and thrushes, the first baby fledgelings of those are out in the wood and are easy game, that that provides the trigger, she can get plenty of food at that moment but not before.

[Screen 63 - Stephen Dury on bud burst video]
Within a single tree, bud burst tends to start at the top of the tree and work its way down but over a very rapid period of time, perhaps 1 to 2 days, but between individual trees there may be a difference of 2 to 3 weeks between bud burst. This seems to be genetically controlled because the order of flushing remains the same from year to year, although the time of flushing can vary according to the temperatures in early spring.

[Screen 63 - Prof. David Streeter on variation video]
The individuals of any one species differ from another in a number of different ways either in appearance or in physiology, and this is the reason why bud break in neighbouring individuals of oak trees in the same wood occurs at slightly different times within the same season. This means that from year to year the actual timing of bud break is unpredictable. The consequence of this may well be that it reduces the probability of the oaks being defoliated by moths in every consecutive year because it's very difficult for a species like a moth to evolve in response to an unpredictable event.

[Screen 64 - Dr Stephen Dury on winter moth synchronicity]
The eggs from a single female hatch over a period of time. This ensures that some eggs are more likely to be synchronised with bud burst and also the eggs from a number of females will hatch at different times, so there is always some synchronicity between egg hatch and bud burst.

[Field Guides Media]
Moss: Media File
Dr Sarah Watkinson:
There are hundreds of different species of moss. At first sight they tend to look rather similar to each other, but if you look closely at this stone, you can probably see that there are at least 2 different sorts. The leaves of a moss aren't really like the leaves of higher plants at all. They are just flat plates of green cells, only one cell thick, and they don't have a waterproof cuticle like higher plant leaves do. And that means that the cells can just soak up water directly from the surface, and with the water, nutrients go into the leaves directly. They're quite different from the leaves of flowering plants which have a nutrient supply and a water supply that come long internal conducting pipes. These are the capsules of the moss and you can see the little bags at the tops of stalks. The stalks move about a little bit in the wind, and then at the tip of the capsule there are little teeth which move about when the humidity changes and help to get the spores out into the atmosphere.

Fungus: Media file
Dr Sarah Watkinson:
Fruiting bodies and spore release - Toadstools often suddenly appear in the autumn and this usually follows a summer of vigorous underground microscopic growth and toadstool production gets triggered by things such as a downpour of rain, a cooling of the atmosphere and then as the mycelin hits the surface, the light stimulates production of toadstools. The mycelin they're linked up to is obtaining nutrients either from living plants or from dead plants, and the nutrients that it's supplying are being carried along to the toadstool of which the sole function is to disperse spores into the air.

Leaf rot fungus - What we're looking at here are the leaves that have fallen on the floor of a deciduous woodland and we're seeing some very important processes going on, because without these processes carried out by fungi these leaves would simply remain here and pile up indefinitely. Just here, attached to this leaf, there's what looks like a fairly inconspicuous little bit of cotton-wool, but these are cords of hyphi stuck together, and it's these cords which are able to grow all the time as a perennial mycelin in the leaf litter and from these branch out hyphi which are actually going to break down the leaf. And if I lift up this leaf a little bit you can see how the fungal mycelin has grown into the leaf and it's connecting this leaf to another little bit of leaf and if I turn the whole thing over, you'll see how it's enabling the fungus to grow from one lump of decomposing litter to another.

Currant gall: Media file
Dr G. McGavin:
Galls are a collective name for any growth, unusual growth, overgrowth of a plant or any area of a plant. It can be bud, a leaf or root or what have you. And it can be caused by a wide variety of organism. A gall is entirely made from the host plant. It can vary hugely from a simple blob, a swelling, inside which there may be an egg or an insect grub. It can assume a variety of shapes depending on the gall format. We still don't really know how they instruct the plant to make a gall and to make a gall of a particular shape. So there are obviously very precise instructions, saying 'I want a gall of this shape, this size and that's where I will live. And you will give me food as well.' That's a big area that we're only now just beginning to tease apart.

Sexton beetle: Media file
Sexton beetles play a vital role in the woodland ecosystem. They're detritivores, seeking out the corpses of dead animals and burying them. They lay their eggs in the burial chamber and once hatched the maggots use the corpse as a nutritous food supply, leaving just a mound of fur and bone.