Use and value diversity

The old saying: don't put all your eggs in one basket

Holmgren pairs this principle with the proverb don't put all your eggs in one basket. Whoever carries everything in one basket loses everything when that basket falls. History knows the farming version of this. In the Irish famine of the 1840s, many of the rural poor lived largely on the potato, and much of the crop was a single variety, the Lumper. When late blight arrived, there was little to fall back on. The famine had social and economic causes as well, but the lack of diversity in the fields was one reason why the blow fell so hard.

Ecologists have given the proverb a scientific name. The insurance hypothesis, set out by Shigeo Yachi and Michel Loreau in 1999, describes how a community of many species keeps working in a fluctuating environment, because when conditions harm some species, others are favoured and take over their role. Diversity does not prevent bad years. It keeps a bad year from becoming a collapse.

How it looks in nature

A long experiment at Cedar Creek in Minnesota gave one of the clearest pictures. David Tilman and John Downing followed grassland plots with different numbers of plant species through a severe drought in the late 1980s. The more species a plot held, the less of its plant mass it lost in the drought, and the more fully it recovered afterwards. The species-poor plots were not simply smaller versions of the rich ones; they were more fragile.

Nature also shows that diversity lives from difference. In a classic study of North American spruce forests, Robert MacArthur found several warbler species living in the same trees, each feeding in a different part of the crown. They share a habitat because they use it differently. A mixed planting works the same way: deep and shallow roots draw water from different layers, tall and low plants catch light at different heights, early and late flowers feed pollinators across the season.

Fast-growing newcomers are part of this picture. Where people have disturbed land, robust pioneer species often arrive first, covering bare soil and building biomass. Terra Humana sees in them a solution strategy of nature and one of the greatest potentials for rapid renaturation, provided they are guided and later replaced step by step by the target species. Species on the EU list of invasive alien species of Union concern are never used by Terra Humana.

Where the principle works in the TerraWiki

These pages show diversity at work, each in its own way:

• Polyculture: many crops in the same space share light, water and soil and spread both yield and risk.
• Plant guild: a small community around a central tree, built from members that do different jobs for each other.
• Food forest: diversity in layers, from tall trees to ground cover, with harvests spread over the year.
• Syntropic agriculture: species of different life spans planted together, diversity arranged in time.
• Seed cycle: saving and sharing seed keeps local varieties alive and lets them adapt to a place.
• Climate-adapted trees: planting a mix instead of betting on one species is insurance against an uncertain climate.
• Soil food web and mycorrhiza: the diversity below ground that keeps the visible diversity fed.
• Pioneer plants and nitrogen-fixing plants: the first members of a diverse system, preparing soil for the ones that follow.
• Reconciliation ecology: diversity built into the land people use, not only into reserves.

At Sa Medada

Sa Medada is a slope in Jerzu, in Ogliastra on the east side of Sardinia, with mild winters and a dry season lasting roughly from May to September. Diversity was there before Project Eden: the old olive grove, and the macchia on the steeper slopes with its many hard-leaved shrubs.

Project Eden adds to it without betting on a single crop. Banana, mango, avocado, guava, cherimoya, papaya, passion fruit, lychee, monstera, moringa, tulsi, persimmon and citrus grow in the open ground. Black and white sapote, tree tomato, pawpaw, dragon fruit, fig, cherry, apple, apricot, Surinam cherry and feijoa are in pots or partly planted out, and the first tree, a service tree, was planted in 2024 on the very hot southern flank. Among the nitrogen fixers in the soil are Tephrosia, the coral tree, Inga edulis, pacay, Bambara groundnut, fava bean and Leucaena. We give no yield figures.

The previous principle, use small and slow solutions, keeps change at a scale people can follow. The next one, use edges and value the marginal, looks at the places where diversity is richest: the edges between one world and another.