[Illustration]
Best known of the "gardens" and "animal husbandry" of the lower animals
are those of the ants; the aphis kept by the ants for the sake of a
sweetish secretion, and the underground fungus garden of the ants. In
the vertebrates I know nothing comparable to this, but we do get a
number of cases where there is a definite relation between the animals
and the growth of vegetation.
It has been said that in the antarctic the nesting colonies of some
penguins are detrimental to the vegetation. The constant passing
and standing of the birds on the limited areas of soil preclude the
growing of vegetation over sufficiently large areas to be an important
factor in hindering plant growth. But the reverse is true of the
Johnny penguin in the Falklands, where it is sometimes known as the
best farmer in the country. The Falkland Islands, off southern South
America, are cold, wet, and windy. Sheep raising is one of the main
industries. And the Johnny penguin helps to provide better pasture
for the sheep. The birds nest in colonies and their droppings help
to enrich the land so that the grass grows taller and richer. Rather
than using the same area for their breeding colony each year the birds
select a new, clean area at the beginning of each breeding season, so
that they improve the ground over a larger area.
From the arctic comes another example of a relationship between bird
and plant. On the arctic barrens, here and there, are large boulders,
erratics left by the glacier that covered the land in times past. And
on these boulders, and here only, one finds patches of bright yellow
or reddish lichen known to scientists as _Xantheria_ or _Xanthoria_.
Apparently its presence is owed to the fact that these boulders are
the lookout places of snowy owls, hawks, and other birds. Their
droppings, falling on the rocks, provide the nutrient layer necessary
for the growth of the lichens. It is probable that these lichens are
transported from place to place by the birds carrying the soredia on
their feet. In recognition of the close relationship between these
lichens and birds an ecologist has coined the rather formidable term
"ornithocoprophilous" to express the relationship.
Also in the arctic are the arctic-fox gardens. The arctic fox often
makes its burrows in sandy places, and about the entrance to the
burrow accumulate remains of former meals, fox droppings, and suchlike
animal debris. This in time enriches the soil and the vegetation
there grows taller and more lush than elsewhere on the barrens. This
lush vegetation attracts the small, mouselike arctic rodents, the
lemmings, that feed on green, succulent vegetation. There is of course
one further step in this chain. One of the important foods of the
arctic fox is the lemming, which he thus brings to his door by the
richer vegetation he unwittingly causes to occur there. A charming
arrangement, one of the old naturalists called it.