The grizzlies are solitary animals, and at the start of spring hunger makes them even more dangerous.
So, when an adult male approaches a group of young bears,
they know it's time to stop their games and make a rapid getaway.
In times of hunger the adult bears are capable of killing and eating young of their own species,
so it's probably best to take no chances.
The climate has also determined the vegetation of Alaska.
Wherever temperatures permit, the land is covered with coniferous forest.
Their small leaves, the needles, are not shed in the winter,
which permits them to take advantage of the warmer months from the very first day.
And this they do so effectively they block out almost all the light.
Except for the moss that covers the forest floor,
few plants are capable of living in their shade
The trees with deciduous leaves, less well adapted to the cold,
have to take refuge along the shores of the rivers and in the more protected areas,
and that is where the large herbivores go in search of them.
The spring brings fresh vegetation, an irresistible feast for any moose.
Like the bears, they are solitary animals.
The only groups outside the mating season are mothers with their young,
and these are precisely the most dangerous because the mother will allow no one,
not even one of her own species,
to get close to her young.
An absent-minded moose has come to drink at the same pool, unaware of her presence.
The calf doesn't seem to mind the intruder, but its mother does not exactly welcome visitors
and it's not long before she approaches the stranger to make her position clear.
There have been cases of female moose killing people who got too close to her young.
In a land of bears and wolves, the most dangerous animals of all are, paradoxically, the moose.
The unwitting intruder knows this, and moves off in search of quieter pastures.
No one, not even the large males, would dare challenge a mother,
and even less for a bit of water or a few willow leaves.
The deciduous plants, in contrast to the conifers, shed their leaves during the winter,
and in a land as cold as this that creates real problems.
When warmer weather arrives they have to waste precious time growing new leaves,
which reduces their summer to the minimum.
And that without counting the herbivores or the most feared enemy of all,
the leaf-eating caterpillars.
Hidden by the leaf itself, the tiny caterpillars feed on the internal tissue,
robbing the plant of its food factories, the photosynthetic cells.
With luck, the birch will lose only part of its foliage,
but if the attack is widespread it will die.
The good weather does not last long enough to produce new leaves to replace the damaged ones.
The same climatic conditions that give the conifers a considerable advantage over the deciduous trees
are also a determining factor in Alaska's animal life
Many species, such as this bald eagle, have given birth to their young during the spring
and have to make sure they complete their development before the winter freeze returns.
To achieve this in so little time, the fauna needs a source of food the land is simply unable to give them.
But what Alaska doesn't have it receives every year, just at the right time, from the sea.
As the summer advances, the grizzly bears leave the pasture
to visit the shores where this manna from heaven is about to arrive.
With all the calm in the world they walk along the river bank, examining it for food.
But this year, the gift from the sea is a little late in coming.
For the time being the only food around is algae and the odd dead fish.
Nothing vaguely interesting for their empty stomachs.
After sniffing around in the sand for something to eat,
the male understands it's still early.
It might be a long wait,
so the best thing you can do is make yourself comfortable and dig yourself a good seat in the soft sand.
And now all that's required is a little patience.
He was the first to arrive, but he's not the only that comes to the coast.
Soon, he will be joined by another adult male with similar intentions,
and the disputes will begin to determine who gets the best spot along the shore.
The loser doesn't have to move far.
Just a few metres away, he goes back to padding around in search of food.
It won't be long before these two are accompanied by other bears who,
like them, have abandoned the grasslands, and now impatiently examine the water.
All of them are waiting for the salmon to arrive, their only hope in this cold, hostile land.
Each year thousands of salmon return to the rivers where they were born,
providing the local wildlife with the food they need to build up the reserves necessary
to survive the long, harsh winter.
They are manna from the sea, the reserves of protein and fat everyone is waiting for.
The salmon, quite simply, are life.
The role of the salmon is so important that even those species that don't eat them are affected by the phenomenon.
These are sea otters, one of the few sea mammals that remain along the coast of Alaska all year round.
They are relatives of the river otter, but unlike them, the sea otters spend 90% of their lives in the water.
There, they eat, sleep, reproduce, and even give birth to their young.
The air trapped in their fur helps them to float,
but it also provides insulation against the cold, essential this far north.
Thanks to this, they can float on their backs,
their favourite posture, while they eat, sleep or simply play.
They have been a protected species for many years,
but still their numbers are now declining, due in part to the salmon.
Salmon are not the basic diet of the otters,
but they do attract the seals and sea lions, which come to Alaska every year.
And they, in turn are the favourite food of the largest predator in the ocean: the orca.
So, when salmon are scarce, the seals and sea lions have fewer young,
and when there are not many seals, sea lions and salmon around,
the orcas have to find an alternative source of food:
the otters.