0:00:24 > 0:00:28Just a stone's throw from the River Thames, Burnham Beeches -
0:00:28 > 0:00:31a woodland bought by the City of London in 1880
0:00:31 > 0:00:35as a green lung, a place for city folk to escape the grime and smoke.
0:00:41 > 0:00:45This is a pretty ideal day for me - blue skies, sunshine,
0:00:45 > 0:00:48wandering through a forest surrounded by beautiful trees.
0:00:48 > 0:00:51Some of these are more than 450 years old.
0:00:51 > 0:00:54But it's not easy keeping these old girls alive, you know.
0:00:59 > 0:01:01For hundreds of years, a combination of livestock grazing
0:01:01 > 0:01:06and pollarding was used to keep them under control and safe.
0:01:06 > 0:01:08Pollarding is basically coppicing or pruning,
0:01:08 > 0:01:11but it's done at a high level to promote tree growth.
0:01:11 > 0:01:15And it's done up there because if it was done at a low level,
0:01:15 > 0:01:18the animals would have easy access to all the lovely shoots.
0:01:20 > 0:01:24If that's not done, the trees can get top-heavy and topple over,
0:01:24 > 0:01:25just like this one.
0:01:25 > 0:01:29100 years of neglect here has taken its toll.
0:01:29 > 0:01:33But 20 years ago, they brought back the cattle and resumed pollarding
0:01:33 > 0:01:36to help save the trees for future generations.
0:01:36 > 0:01:39I'm meeting head ranger Martin Hartup.
0:01:39 > 0:01:41And I'm going to meet him by the invisible fence.
0:01:41 > 0:01:44"Which invisible fence?" I hear you say.
0:01:44 > 0:01:47That invisible fence. Hear it? CHIRRUPING
0:01:47 > 0:01:49- Hello, Martin.- Hello, Julia. - Hi there.
0:01:49 > 0:01:51So, what is all this jiggery-pokery, then?
0:01:51 > 0:01:55Well, what you carried over that invisible fence there was a collar,
0:01:55 > 0:01:58and this collar is worn by our cows when they're grazing this area.
0:01:58 > 0:02:01Now, the fence itself is a cable that's buried under the ground,
0:02:01 > 0:02:03only about four inches or so,
0:02:03 > 0:02:06and it emits a radio signal which is picked up by the collar.
0:02:06 > 0:02:09And when the cows get close to that buried cable, they hear that noise,
0:02:09 > 0:02:11- which we heard as you walked over it.- Yeah.
0:02:11 > 0:02:13That tells them they're getting close to it.
0:02:13 > 0:02:15Once they've learnt how the collars work,
0:02:15 > 0:02:16they know that if they get any closer
0:02:16 > 0:02:19or take another step onwards, they'll get an electric shock.
0:02:19 > 0:02:21And you can see where we've used it in trial areas,
0:02:21 > 0:02:23they graze it within about two metres of the line
0:02:23 > 0:02:25without any problems at all.
0:02:25 > 0:02:29- The advantage, of course, is no fences for us.- No fences.
0:02:29 > 0:02:32Nobody knows it's there, it doesn't stop anybody walking anywhere,
0:02:32 > 0:02:34there's no interaction with anybody,
0:02:34 > 0:02:36you can't touch like you would do a normal electric fence
0:02:36 > 0:02:38and get a shock off it - it doesn't work like that at all.
0:02:38 > 0:02:40It's not just the cows that need managing -
0:02:40 > 0:02:44some of the other creatures here do, too.
0:02:44 > 0:02:47Apparently, there are giant ants in this wood.
0:02:47 > 0:02:49I'll believe that when I see it!
0:02:49 > 0:02:51SHE SCREAMS
0:02:53 > 0:02:56These are Formica rufa - wood ants to you and me -
0:02:56 > 0:03:00and they're three times the size of normal garden ants.
0:03:00 > 0:03:03They can be found at half a dozen locations in the UK,
0:03:03 > 0:03:05but here they're in huge numbers.
0:03:05 > 0:03:09Conservationist Dr Helen Reid has been keeping an eye on them
0:03:09 > 0:03:11for more than 20 years.
0:03:11 > 0:03:14OK, so, we've got a big nest here at the base of this dead tree.
0:03:14 > 0:03:18- Uh-huh.- And they're building their nests like this,
0:03:18 > 0:03:20where there's bits of dead wood.
0:03:20 > 0:03:22So they're building the nest over the dead wood
0:03:22 > 0:03:25- and on the edge of the clearing. - Just thousands and thousands!
0:03:25 > 0:03:27How many ants do you think are here, Helen?
0:03:27 > 0:03:30Well, it's been estimated that the biggest nests might have
0:03:30 > 0:03:32a million ants in them, but I think probably most of them
0:03:32 > 0:03:33are rather less than that -
0:03:33 > 0:03:35probably up to half a million is more realistic.
0:03:35 > 0:03:38- Oh, look, they're everywhere! - Incredible, isn't it?
0:03:38 > 0:03:40- They're very, very efficient workers, aren't they?- Yes, yes.
0:03:40 > 0:03:43I mean, they're spending a lot of time going up the trees,
0:03:43 > 0:03:45a lot of time on the ground. So early in the year,
0:03:45 > 0:03:49they're picking up nesting material to bring back to bolster the nest
0:03:49 > 0:03:52and then they start feeding on aphids and honeydew
0:03:52 > 0:03:54from the aphids up in the trees.
0:03:54 > 0:03:57They're taking a sugar solution, and one nest like this might
0:03:57 > 0:04:02bring back 46 kilos of sugar in a season from the aphids.
0:04:02 > 0:04:06The ants are such top predators, they're eating other insects,
0:04:06 > 0:04:08and even depriving birds of food.
0:04:09 > 0:04:13The Rangers are well aware they could one day overrun the woodland.
0:04:15 > 0:04:17So, Helen, how are you going to manage them?
0:04:17 > 0:04:20Well, at the moment, the management we're doing on the trees,
0:04:20 > 0:04:22some of the things you've seen this morning,
0:04:22 > 0:04:24is really favouring wood ants,
0:04:24 > 0:04:26because they like little sunny clearings around the trees
0:04:26 > 0:04:29and they like it when we leave deadwood on the ground.
0:04:29 > 0:04:30So there may be things that we could do in
0:04:30 > 0:04:33our management to try and encourage the ants in certain areas
0:04:33 > 0:04:35- but discourage them in other areas. - Right.
0:04:35 > 0:04:37Look at my foot.
0:04:37 > 0:04:39Oh, I'm so itchy now.
0:04:43 > 0:04:44They're all over us.
0:04:44 > 0:04:47This is when you actually manage to get a hold of them.
0:04:47 > 0:04:48Now they're all over us.
0:04:48 > 0:04:50HELEN LAUGHS