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Restore Nature, Issue #001 -- restoration benefits to you !
May 06, 2023


Choosing plants to boost biodiversity

The first newsletter listed 6 benefits of wildlife gardens to human beings.

The second letter listed 26 tips for creating biodiversity support in small urban spaces.

This, the third newsletter focusses on choosing plants to boost biodiversity. I can’t mention specifics as people are reading this from all over the world, so I’ll focus on general principles. Here is the content of the letter: choosing plants for biodiversity 1. Plant diversity is foundational 2. Native plants are crucial. 3. Create diverse plant habitats 4. Growing food and biodiversity 5. What you don’t do is important 6. Create a functional ecosystem Citizen science research is needed worldwide. Forest layers and edges. Observe nature. Takes time The neighbourhood

Plant diversity is foundational

A greater diversity of plants increases biodiversity. Firstly, because plants are part of biodiversity and diversity in the plants is creating biodiversity per se. Secondly, all plants have a different microbiome on their surfaces and roots and thus a greater variety of plants increases microbial diversity. Microbial diversity is the foundation of a resilient, disease resistant ecosystem with healthy soil and plants. Thirdly, a greater diversity of plants supports a greater diversity of insects. They do this by providing a greater variety of leaves for chewing and sucking insects, expanding pollen and nectar availability to insects with different morphology and seasonality, providing fruit, shelter and more. Why would this great diversity of insects be so important ? Plants, through photosynthesis, harness the sun’s energy and make it available as food to herbivores like caterpillars. In the northern hemisphere temperate forests the caterpillar is the greatest converter of plant calories to animal calories (Tallamy), and thus the supplies the bulk of energy to the rest of the food chain’s trophic levels.

In the Western Cape it is a little different. I didn’t find any sources on the conversion of plant to animal calories, but the most common insects in the wild in these parts are outlined in my article on stink bugs : being leaf beetles, weevils, parasitic wasps and stink bugs, in that order.

Wherever you are on the globe, the different insects feeding on different plants in different seasons ensures a longer period of food availability to birds, reptiles and amphibians. Their numbers are in decline everywhere, mostly due to loss of habitat and food sources.

Native plants are crucial

As plants are unable to flee from their ‘predators’ they develop structural and chemical means of defending themselves such as thorns and spikes, hard unpalatable leaves, poisons and aromatic insect repellents and bitterness. Their communication systems are mind boggling. If a herbivore starts to devour a plant, its neighbours can even be alerted and start to change chemically to make them unappetizing, or even make the herbivore sick. But these developments set an arms race in motion. Caterpillars have evolved immunity to the poisons of their hosts and are often host specific, and oak trees are hosts to an enormous number of them in the northern hemisphere.

The herbivore, such as a caterpillar or sucking insect, must develop immunity to plant poisons. As there is such a diversity of plants in the environment, it would be unlikely that insects became immune to all plant poisons. Instead they have become specialized. A caterpillar may only be immune to the poison of one plant, with which it co evolved by being in the same region. This plant becomes its host plant. For this reason biting, chewing and sucking insects often have a host-herbivore relationship that is unique. Without that plant, the insect will die out. In addition, insects which feed on nectar have evolved specialized mouth parts to be able to access the nectaries of specific flowers, and the plant and insect have become ‘co-dependent’, relying on each other to be able to survive at all. This is why for the greatest insect diversity, native plants are crucial. Not all insects are generalists and able to feed off a wide range of flowers.

How does the gardener find out which plants will be the best supporters of insect diversity ?

Doug Tallamy has put out a number of wonderful films on gardening for biodiversity. He also had some of his students organize a study on plant and insect relationships to determine some of the keystone plant species in his environment, temperate woodland. The study was based on records submitted by the public via cell phone photographs.

I asked myself why we don’t have something like this in the Cape floristic region, the most unique and diverse of biomes. In the meantime I started a Whatsapp group called Zoobot for this purpose. There you can submit a photograph of a animal-plant interactions such as feeding. You can have the plant and insect identified on platforms like iNat before submitting, or someone in the Zoobot group may be able to guide you to an ID. This will help with enriching available information on plant animal, attracting rain, maintaining plant and animal biodiversity and more. The most wonderful effect of restoring nature to cities is the psychological benefit to humans of being surrounded by natural things. It can even stop people having heart attacks or doing robberies ! This is why I've chosen the human benefit as the first topic I'll address in the Restore Nature newsletters

Create diverse plant habitats

Create as many different habitats for different types of plants and animals as possible. These could be forest, hedgerows, shrubland, wild-flower meadows, wetland, ponds, low-nutrient rocky areas, sandy areas, decaying wood and leaves. I will use my home town as an example here. In Cape Town, as is the story worldwide, we find a city is expanding right on top of a biodiversity hotspot. We once had a ‘patchwork’ of different vegetation types. There are currently 18 different recognized vegetation types in the Cape floristic region listed on Wikipedia.

Before colonization there would have been small pockets of forest along rivers, the edges of swamps, in mountain ravines and along the high rainfall lower slopes of the mountains too. An example is the extensive forest on the eastern slopes of Table Mountain which was destroyed by the Dutch in the first few years after van Riebeeck’s arrival for firewood and building material.

Depending on the underlying soil, in addition to forest we would have found different types of Shrubland such as Fynbos (on sand and degraded sandstone), Renosterveld (clays, shales and granite) and Strandveld (coastal sand dunes), and the vegetation you find on rocky outcrops, cliffs and mountaintops, alpine vegetation in the seasonal snow fields, and the water tolerant plants found in small seeps and large wetlands, as well as estuaries with tidal flats. Of these different types of vegetation, some have been nearly destroyed, and some maintained, through being in inaccessible places where it wasn’t convenient to build houses or plough the soil.

The survival rate of the different vegetation types in your region could help you decide what artificial habitats to create. However, in general, globally, it is good to find out what the potential natural vegetation in your area was before urban development. The website of the City of Cape Town offers a detailed map down to the level of suburbs, on which you can see this. I hope there will be a similar resource in your region. If not, just imitate the wilderness closest to your urban environment.

I am a great admirer of Markus Gastl who creates insect gardens in Germany. He realized that a greater diversity of native flowering plants thrive on poorer soil. This is true almost worldwide, for meadow gardens, prairie gardens, and particularly in our low nutrient adapted Fynbos. Usually it is because the vigorous plants like robust grasses dominate in ‘fat’ fertile soils and out-compete most of the flowering plants. Gastl stripped his plot’s topsoil and trucked in loads of builder’s rubble and gravel to cover his land about a meter thick. He planted a very beautiful wild looking flower garden, and developed various types of habitat for insects using stone piles, sand pits, wood piles, forest and flower meadows, rockeries and ponds. If you are able to do all this, you can register your garden with his ‘Hortus’ network of gardens and become a tourist attraction.

Growing food and biodiversity

If you want to grow useful or exotic plants like vegetables you can always mix the native and exotic, the wild and the well tended. There are so many ways to do this that my suggestions are only an impoverished form of the creativity possible: You can create a food forest with a backbone of hardy native trees. This old food forest in New Zealand was built on this principle of a native backbone protecting the more tender exotic food bearing trees, including apples, grapes and many other food plants foreign to New Zealand.

You can allow native ‘weeds’ space outside of your vegetable patch.

You can mix native perennials like herbs into your food garden borders, or vegetables into your native plantings. For example mustard thrives between my perennial succulents in the rainy season. Many local gardeners plant hedges of native aromatic plants around their vegetable patches. They act as windbreaks, soil builders and cabbage butterfly repellent !

I haven't mentioned the genetic diversity of food plants themselves. That is worthy of another article.

You could use Markus Gastl’s wonderfully biodiversity sensitive 3 zone permaculture system.

You can imitate the traditional custom that maintained some diversity in the British isles for hundreds of years, create a hedgerow around your field or garden. Plant a mixed species native hedge with some bushes, small trees and climbers around the border of your property. You will be surprised how much area just 1m, or a yard of hedge takes up without intruding on your growing space for veggies and flowers in the middle of the garden. At the same time it will provide a windbreak that makes your garden a sheltered oasis for the exotics. The garden boundary is also the longest stretch in your garden, allowing a great diversity of hedge plants choices.

I’ve started a series of articles on native hedge plants for the Cape Town area. Wherever you live, you can observe what is growing on the rural margins of your town and take some cuttings and seed and create your own mixed hedge. In South Africa the choice of plants in the nurseries is generally restricted to well known, fast growing and boring exotic hedge plants, with dense, small leaved, shiny dark green foliage, and the idea behind them is a monocultural hedge, to be kept neatly clipped like topiary. Such a waste of an opportunity to do good for mother nature, but it feeds a whole industry. When you buy a fast growing hedge, you will be clipping in all your free time. But the mixed hedge can include flowering plants, spice plants, fruit and berry trees, so it can add to your harvest. The hedge is also edge, a transition from a miniature forested environment to the open sunlit spaces, and edge creates biodiversity, as any permaculturist and ecologist will tell you.

What you don't do is important

Although planting natives is integral to biodiversity support, exotic weeds can also support insect and other animal life in a variety of ways. Therefore leaving a patch of weedy garden does much for biodiversity. Reducing mowing, or letting the lawn weeds flower has also proved to be as effective a wildlife support as planting labour intensive meadow gardens in the UK. If you are fed-up with gardening and don’t have time, but love the idea of biodiversity, just let your plot go wild with neglect. Bricking, tarring, or plastering it over will kill all chances of biodiversity thriving there, and it harms the hydrology of the whole city, increasing floods and the heat island effect. The messiest wild garden will keep you cooler and the open soil will help recharge aquifers and such indirect benefits. Of course the other thing it is important not to do is use biocides in the garden. Even N based fertilizers and weed killers can damage your soil’s microbiome and lead to knock on effects, a downspiral in soil health, and insecticides, even organic ones such as Neem, are generalist poisons and kill all insects they come into contact with. I grow veggies and snails are the biggest problem. I find that creating a barrier is enough to leave me some food. I’ve worked on my soil health for years, so that insects like aphids aren’t a problem. According to this article research on brix values, insects only attack unhealthy plants Netting, raised beds

Create a functional ecosystem

The alternative to careful plant choice focussed on keystone species is to create a functioning ecosystem with your local flora and hope for the best. In some parts of the world much more is known about the chemical capabilities and synergies between plants, as well as their relationships with insects. In those parts of the world you can practically download blow by blow instructions of what to plant, to kick-start garden biodiversity, whereas in the Cape Fynbos we still have so much ground to cover (forgive the pun). Our urban rehabilitation groups like Communitree are making grand strides. The emphasis is on rehabilitating soil with a three step succession process, and then replanting a specific vegetation type for your area and some of its highly local plant species. Thus one could recreate plant based ecosystems. The proliferation of insects and other animals is hoped for but the specific plants needed are not laid down. Only in the case of the sunbird project are specific plants required. If we replant natives it will go far towards supporting biodiversity, but knowledge of the levers in the system could make gardeners even more effective. We really need a lot of research on the nuts and bolts of our ecosystems and the functions of particular plants, insects and the like. The data collection could be done by citizen scientists on their cell phones, and the sense making by students at some of our local universities. One way of creating an ecosystem is to pay attention to the storeys of a forest. Many ecosystems are on their way to forest, as the climax form. Even in the Fynbos which is fire dependent for its diversity, old shrubland is also like a stunted forest, perhaps with fewer layers. My mother claimed the dune bush clumps of the Strandveld were like miniature forests, with closed canopies and small lianas, and sometimes even had ferns in the floor layer. When you try to have all the forest layers in the forest part of your garden, canopy, understory, shrub, herb, groundcover, roots and climbers, you will maximize plant diversity, diversity of the soil microbiome and hopefully the rest of the system. You can also boost diversity by increasing and or utilizing the edges. Edge is the border between two ecosystems like forest and meadow, where diversity is highest. So in a garden it could be the edge between an open area and a tall plant border, or between a dense orchard and a flowering lawn, or the edge of a wetland between soil and open water. The possibilities are numerous. Many more plants will survive in these edge zones than in dense stands of trees or open areas exposed to beating sun in summer.

Observing nature is very useful. Often one doesn’t know exactly what one is looking for. I collected seeds of a wild edible asparagus and it germinated very successfully. I planted the seedlings in my garden against the back fence where its warm and windless and choked with creeping weeds. They malingered on for a year or more. Some died. Observation of the plant in its wild habitat showed that it could grow in shade or scorching sun, and in both sand and hard baked clay, so it confused me. But it was paired with trees in 98 percent of cases. My second batch of seedlings was planted under bushes and trees close to where the roots radiate out from the stem. The shade was light to heavy, and I made sure they all had a broad collar of weed free soil mulched with wood chips. They took off magnificently within weeks of planting. Linking the observations in nature and my practice in the case of just one species took me over two years. Nonetheless I learned a powerful lesson about wild asparagus which cannot be found in any text book on gardening.

In my experience creating ecosystems takes time, and the neighbourhood needs to be friendly too. My own garden was largely native and wild from the start, with some food plants. It just keeps on changing. Every year its perfection as an ecosystem seems to increase. Plant diseases come and mostly go, the soil gets healthier, the plants get healthier. Canopies want to close and need to be opened here and there, and the vegetation becomes more lush. My slow growing fruit trees and indigenous food plants begin to take hold.

But I’ve seen a fall in insect and bird numbers over the last 25 years. The reason, I think, is that my garden is an island in a sea of tarred streets and concrete paved yards. There are hardly any plants in our suburb, let alone native plants. I’ve realized our garden’s biodiversity is going nowhere without bringing the whole neighbourhood on board, but the neighbours are not interested, and many of them are hostile to wild gardens. We need massive public participation to prevent the decline of our urban zoology. With a co-operative neighbourhood, biodiversity can go beyond the levels an individual gardener could ever achieve, and different types of habitat could be distributed across the area depending on people’s inclinations. The difficulty of getting there made me realize persuasion and leadership is even more important than gardening skill for biodiversity in urban areas.

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