A soccer-field sized patch of forest in frigid Alaska has about 40 different species of plants,
compared with about 70 in temperate England and 300 in the Amazonian rain forest.
These biodiversity differences hold true for entire countries, too: England has 1500 plant
species, while tropical Guyana.
The super-diversity of tropical rainforests is only equalled in one other type of ecosystem
on Earth: scrubby fire-prone shrublands that grow in western Australia and southern Africa.
These shrublands may not look as majestic as tropical rainforests, but in a given area,
they’re home to similarly stupendous numbers of species.
Which doesn’t necessarily mean that the rainforests and shrublands are easy places
for plants to live.
In fact, both ecosystems owe their enormous diversity, in part, to the fact that their
soils have critically low supplies of nitrogen and especially phosphorus, nutrients plants
need in order to grow.
The plant world’s leading biodiversity hotspots are, quite literally, dirt poor.
Logically, it seems like richer soils should support more species.
But in nature, as in human society, ‘plenty of resources’ doesn’t necessarily translate
into ‘everyone gets plenty’.
In meadows, forests, and wetlands around the world, we consistently find more or bigger
plants but fewer species where soil nutrients are highest.
The fastest-spreading species soak up most of the extra nutrients, which lets them keep
growing super fast, which lets their roots suck up so much water, and their leaves snatch
up so much sunlight, that other, slower species actually get LESS of those resources than
otherwise.
So in rich soils, slower species die out while the fast-growers win big.
On the other hand, poor soils don’t provide enough nutrient capital for fast-growing plants
to build their massive infrastructures and take all the resources.
So poor soils inhibit the greedy and allow everyone else to scrabble by.
We see this pattern in human society also - there’s a far greater number of businesses
– mostly small – in poor countries, while fewer bigger companies dominate in rich countries.
But crummy soil isn’t the only thing that helps super high diversity blossom; for example,
beaches, mountaintops, and other places frequently ravaged by harsh weather or catastrophic events
have poor soils AND few plant species.
The other major prerequisite for hyperdiversity is time.
On most of the planet, glaciers regularly bulldoze away ecosystems and grind up mineral-rich
rock, creating new soil perfect for growth but not diversity.
However, our high-diversity rainforests and shrublands have spent millions of years beyond
the reach of the ice sheets, leaving their residents plenty of undisturbed time to evolve
a wide variety of ingenious strategies for surviving nutrient poverty – strategies
that have allowed for the development of tall, diverse, rainforests in wet poor soils and
scrubby, diverse, shrublands in dry poor soils.
The human landscape also seems to follow a similar pattern, with the highest cultural
and linguistic diversity as well as the greatest number of businesses in climatically stable
places where humans have been the longest and where economic resources are scarce.
So in some ways, the poorest places on earth are actually (also) the richest.
This episode of MinuteEarth is supported by the The Kwongan Foundation at the University
of Western Australia, which promotes the conservation of Australia's amazing biodiversity.
To learn more about the ecology of the hyperdiverse Kwongan ecosystem, check out Plant Life on
the Sandplains in Southwest Australia, or visit the Kwongan Foundation online.
Thanks so much to the Kwongan Foundation - and the Kwongan - for making this MinuteEarth
video possible.