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Diversity and its role in resilience

Tim Sonder, October 17, 2020

Why is diversity—especially biodiversity—so critical in a world with climate change? 

How can we design and plan using diversity to improve our own gardens and landscapes and to make them resilient to both long-term and short-term changes and stresses? 

And can we have an impact on the global climate crisis at the same time?


Permaculture’s Design Principle 10 (in the Holmgren 2002 version of the Principles) is “Use and value diversity,” as we have previously discussed. This principle was originally Bill Mollison's #9, published in the 1991 Introduction to Permaculture book, and probably was not focused on climate change nor even developed due to the great concerns about loss of biodiversity we have now. It’s simply rooted in that systems are more resilient and dynamic if they are diverse. Here is Mollison’s idea:

9.Diversity: the sum of the yields in a mixed system will be larger than in a monoculture. Stability is produced when elements are cooperating. Not the number of elements is central, but their functional connections. It can be summed through the examination of the dynamic interplay of order and chaos, entropy, tidiness, control, and creativity.

In nature, a dynamic system slowly heads more and more towards something stable through what we call succession. However, external changes then push and prod at that stability and create change. The fact that it is diverse and dynamic allows the system to adjust to those disruptions again and to settle back to a new stability—one which may be nothing like its predecessor.


An example

Let’s look at a non-biological gardening system: water. How can we have systems that protect against drought and against flooding? What water sources might we have?

  • Rainfall

  • Plumbing/city water

  • A well

  • A spring

  • Rain barrel storage

  • Pond storage

  • Swale or rain garden

  • Dew-capture devices

  • Gray-water use

This way we have choices and various sources and are not dependent on any one thing.

The problem

Today’s agriculture has become one largely of monocrops. A more resilient system is a polyculture. Nature creates polycultures, although they may be dominated by a few pushy (invasive?) plants. Some traditional agriculture used polycultures. (The “three sisters” of corn, beans and squash is frequently mentioned.)

Today, we see monocultures not only in large-scale agriculture, but replicated in smaller farms with row crops and even in our cities and towns.

Everyone loved the American Elm as the ideal street tree—but when disease hit towns across America, they found they had no trees left. And our urban foresters did not learn this lesson quickly: Areas (like Crawford here in Skevanston) were planted with nothing but Ash trees, and we know how this went—a row of stumps down the middle of that street for many blocks.

Let’s look at the world’s food crops for a moment. The bananas we are familiar with in the store are all one variety—the Cavendish. It turns out this variety had replaced the Gros Michel after all those plants globally began to succumb to a pathogen. The Cavendish variety we now depend on for 99% of the commercial bananas worldwide is 100% dependent on us and is a clone—producing no seeds and having basically no genetic variation. It’s now threatened and disease could wipe out all the plants in the world and we have no diversity in the system. This is not a good plan!

Now, using another Permaculture Principle, Value the Marginal, take a look at cranberries. New England bog cranberries are not diverse. In terms of natives, one cranberry tends to dominate. Today, 4 varieties dominate in the industry and hybrids are being developed because of “false blossom disease” caused by leafhoppers. If something were to happen in the bogs of New England, we could lose at least a season if not multiple years of harvest. Some of this is due to humans planting and selecting only one type and creating hybrids from that, and part is due to the most vigorous plant taking over. In a more marginal environment for cranberries, the bogs of North Carolina, one finds fewer individuals but much more diversity as each individual tries to find a niche and no single type dominates. So North Carolina’s diversity of types makes it more likely some cranberries will produce fruit and thrive each year. (Today a variety native to Wisconsin is being planted widely in New England.)

Depending on what source you read, there are about 30,000 known human edible plant species, and humans use about 150 of them on a regular basis. But only 3— corn, wheat and rice—account for 60% of humans’ calorie and protein sources. And 90% of the individual crop varieties farmers planted traditionally have disappeared from farm fields. This leaves the entire global population at risk.

Today’s wheat farmers plant a monocrop of seed types and plant them in a monocrop-style: huge fields with just one type of wheat covering acres. When conditions are ideal for this one type, you get a bountiful harvest. But when they are poor, you get crop failure. 

Where we were, and where the new regenerative movement may take us

In more traditional agriculture, there is diversity in the wheat field. Something called a landrace is planted—which is the slightly different varieties of wheat, einkorn, barley or rye, for example— collected from past harvests and planted again, adapted to local environmental conditions.

A landrace is a dynamic population of a cultivated plant that has historical origin, distinct identity and lacks formal crop improvement, as well as often being genetically diverse, locally adapted and associated with traditional farming systems.

Unlike heterogeneous seeds you might get from a seed supplier or breeder, a landrace maintains a bit of genetic diversity between individuals, and one makes sure to save a variety of the seed each each. Being a variety of plants, they are more resilient to the different things nature might throw their way each year. Some years some types contribute a higher percentage of the yield, and some years others. The genetic diversity allows the crop to not only adjust to annual variations but also to slowly adjust to climatic changes over time.

Keystone species

Where climate change makes the biggest threat is in the interrelationships between species: Those pollinators who are dependent on only one type of plant; the plants which are dependent on only one pollinator. Keystone species are those on which the entire ecosystem depends. When these keystone species are threatened, the entire web of natural systems which relate to them at the very least shift and change, if not collapse.

In addition to typical ones like predators, prey, and engineers (like beavers), for gardeners we are very aware of these two types:

Keystone Mutualists

Keystone mutualists are two or more species that engage in reciprocally vital interactions. The disruption of one species impacts the other and, ultimately, the ecosystem as a whole. These pairs are often pollinators, like hummingbirds, that rely on specific plants for sustenance, and plants that rely on those pollinators to reproduce.


Keystone Plants

Keystone plants, like the Sonoran Desert’s saguaro cactus, are those that provide a critical source of food and/or shelter for other species.

As humans, we should be very concerned about causing that ecosystem collapse. As gardeners, we may wish to select a large number of the plants we grow by choosing those which will work with generalists. It’s a conundrum. 

Squash bees gather pollen exclusively from plants in the genus Cucurbita. Squash are really efficiently pollinated by squash bees, who, in turn, depend on the squash family. But bumblebees and honey bees will also pollinate squash if there are no squash bees around. 

Note: Because squash bees nest in the ground, often right under the squash plants, they may be sensitive to soil disturbance. The female bees usually build their nest cells 6 to 12 inches underground, and the next generation of bees spends most of the year sealed inside those cells. Tilling a squash bed at any time of year has the potential to destroy squash bees.

The problem in our own gardens

We all know the story of potato blight in Ireland, and how dependence on one type of one vegetable led to famine. Let’s look at how we might unknowingly do something similar in our own garden. 

So many of the plants we grow for food are in the brassica family. If we are not careful, we can set ourselves up for failure even if we appear to be planting a huge variety of vegetables. Let’s plan a garden with kale, collards, cabbages, brussels sprouts, and cauliflower. Hummm? Let’s add arugula and cress for greens. Some turnips for a root. Some spicy mustard and a bit of Asian green with pak choi. These are ALL in the brassica family and might all be targets of the same pests and diseases: white cabbage moth loopers, black leg disease and bacterial rots, flea beetles, aphids. 

So adding Chenopodiaceae family plants can help a bit: spinach, chard, beets, lambs quarters, vegetable amaranth; but those are all in one family, so still an issue.

The hidden situation

And those two plant families together have a hidden issue: they do not develop mycorrhizal relationships, so only planting all of those crops can have a devastating effect on the soil microbiology—decreasing the beneficial fungal community. By creating diversity above ground and planting a polyculture, one creates diversity below-ground. The interrelationship between plants on the surface and the biology in the soil is intense. A diversity on top means a diversity below—with different plants producing different root exudates and interacting with different bacteria, fungi, nematodes, etc. This both creates a system that is more resilient to external problems as well as a system much less likely to be dominated by any one pest or disease. Diversity can protect against the problems of a monocrop below ground, just like it can avoid creating a feast for an insect or fungal pathogen above ground.

There is increasing evidence that the idea of crop rotation being a good or necessary thing is based on a false premise. The supposed need for this is based on a crop “pulling too much of some nutrients” from the soil and building up pathogens in the soil. The solution may not be rotation, but diversity. A hugely diverse cover crop builds fabulous soil for all plants to come. Cutting-edge large-scale farmers I follow on Twitter are using “relay cropping,” where they still use rows and tractors, but the fields are more like stripes, with three different things growing—a low cover crop, a legume like soy, and a grass grain like wheat. With different timing for harvest and growth cycles, they can harvest two crops, get good yields, eliminate chemicals, and have roots growing in the soil at all times—roots of different plant families side by side in the row.

We can easily interplant in our garden to get these same benefits. And frequently get more yield. I have experienced much less disease pressure doing this. (See the Powerpoint from March 7, 2020 in the Resources section at It’s a learning curve, because spacing, timing, shading heights can be a challenge. I had a tomatillo plant that overwhelmed peppers and even zucchini. But got good zucchini yield despite this.

Another suggestion: Where possible include perennials in or around your vegetable garden as a way to both increase diversity and to maintain active roots in the soil all year round.

How agriculture and gardening could save the planet

Bringing our discussion around to climate change—and our efforts to make our environment more resilient while actively fighting climate change—increased plant diversity results in increased carbon storage in the soil. This increased carbon sequestration works to directly reverse climate change—but it also protects plants, soil, and water quality by both increasing water storage in the soil and increasing water infiltration rates to avoid runoff and flooding: carbon sequestration, drought and floods resilience by planting a diversity of plants! Three additional benefits beyond resilience!

Changing our practices can make a difference!  and 

A very recent paper in Nature suggests “Returning a strategic 30 percent of the world’s farmlands to nature,” and the diversity that means, could basically save the planet. The New York Times this week, said:


“It could be done, the researchers found, while preserving an abundant food supply for people and while also staying within the time scale to keep global temperatures from rising past 2 degrees Celsius, the upper target of the Paris Agreement.

Relinquishing 15 percent of strategic farmlands, for example, could spare 60 percent of extinctions and sequester about 30 percent of the built up carbon in the atmosphere. The authors estimate that at the global level, 55 percent of farmland could be returned to nature while maintaining current levels of food production by using existing agricultural land more effectively and sustainably.”


Annual weather variations

The other thing most gardeners know is that some years some things thrive and have a wonderful yield, and the next year can be a bust for that crop and a boon for others. By harnessing diversity and polyculture, you are setting yourself up for success when something fails. The same is true of varieties—maybe one variety you adore doesn’t do so well some years, so it’s good to have diversity in the varieties you plant in your garden: each responds to weather and disease and pests a little differently.

What does weather typically throw at us? Droughts, floods, cold and heat. And climate change is making the length, severity, and the timing of these weather events vary. That adds huge stresses in terms of “selection” of species and varieties. It impacts degree days and temperature in relation to the hours of darkness and daylight that trigger timing in both the plants we grow and the pests and diseases which afflict them. So having plants with a wide range in diversity of bloom timing and frost tolerance, of drought tolerance and ability to thrive in standing water, of resistance to wind and hail, all can ensure some plants thrive each year.

Let’s look at fruit trees, since they are all around us. Each tree variety might have a little different bloom time triggered by degree days, and need a different number of chill days to set fruit. Each variety is more or less susceptible to some fungal disease. Having diverse varieties protects against catastrophic crop failure in the face of both climate change and normal weather and pest variation. When that late frost comes in, the varieties still in an earlier bud stage will survive. Globally, fruit is highly threatened by a warming climate, since all apples, peaches, pears, etc., are dependent on chill days. Research is being done into varieties with less dependence, and this requires … biodiversity to have a wide-ranging genetic pool from which to plumb the depth of variety.