Permaculture Design Principle 10 – Edge Effect

The tenth Permaculture design principle is ‘Edge Effect’ – the use of edge and natural patterns for best effect.

This design principle is concerned with increasing diversity and productivity in our systems by emulating the ecological phenomenon known as the “edge effect”, and the patterns found in Nature.

To understand this design principle, first we will explore the edge effect as it relates to Permaculture design, and then look at how we can incorporate the patterns of Nature into our designs to make our systems more efficient and productive.

Edge Effect

The edge effect is an ecological concept that describes how there is a greater diversity of life in the region where the edges two adjacent ecosystems overlap, such as land/water, or forest/grassland. At the edge of two overlapping ecosystems, you can find species from both of these ecosystems, as well as unique species that aren’t found in either ecosystem but are specially adapted to the conditions of the transition zone between the two edges.

For the sake of clarity, we must first define some key ecological terms.

• An edge is the boundary or interface between two biological communities (e.g. forest and grassland) or between different landscape elements (e.g. land and water).
• An ecotone is the transition zone along the edges of two adjacent ecological communities, where one ecological community meets the other (e.g. the area between forest and grassland). The transition from one ecosystem to the other can be a very gradual or a very sharp one.

Edge environments occur naturally at many ecosystem boundaries, some examples of these are:

• along the perimeter of bodies of water, such as river, lakes and streams
• where forests verge on rock outcrops, riparian areas (i.e. river banks), grasslands
• along outcrops of exposed rock and cliffs
• where forested areas border clearings
• where sharp discontinuities in soil type or hydrology exist
• where estuaries meet the ocean

The following diagram illustrates how the edge effect operates:

In this example, each ecosystem, labelled A and B, contain only three species, coloured red, blue and yellow.

Ecosystem A contains 3 species represented by squares and ecosystem B has 3 represented by circles.

In the region where they overlap, called the ecotone, there are red, blue and yellow squares and circles.

The combination of squares and circles (which represent six species) produce unique conditions which can now support three new species, represented as red, blue and yellow triangles.

So, while ecosystems A and B each contains three species, the overlapping transition zone contains nine.

This increase of diversity that results from ecosystems overlapping is known as the edge effect.

The ‘edge effect’ – Where two ecosystems overlap, the overlapping area supports species from both, plus another species that is only found in the overlapping area.

These ecotones (the regions where the edges of two ecosystems overlap), contain a greater diversity of species than either of the two separate ecosystems, and have significantly greater productivity, for the following reasons:

• Resources from both ecosystems can be accessed in the one place.
• Conditions such as air temperature, humidity, soil moisture and light intensity levels all change at edges.
• Variations in the conditions at the edges can create favourable microclimates which can support unique species.
• Increased availability of light to plants along the edges allows more plants to be supported (greater diversity) and increases productivity.
• Increased plant diversity increases herbivorous insects, which increases birds, and ultimately predators.
• Ecosystem edges and borders act as ‘energy nets’ or sieve, capturing the massive movement of materials, nutrients and energy across their boundaries – leaves and soil are blown by the wind against barriers, shells wash up on the beach, etc.
• Adjacent ecosystems are connected via flows of energy, material (nutrients) and organisms across their boundaries, and these flows can exert strong influences on the fertility and productivity of ecosystems.

It is important to note that the environmental conditions at the edges of ecosystems usually different from those deep within the ecosystems themselves.

The increased productivity and diversity resulting from the edge effect is clearly observable in Nature. Mangrove ecologies (land/sea interface) and reef ecologies (coral/ocean interface) are some the most highly productive natural systems. Riparian areas (the banks of rivers and streams) are very rich in biodiversity. Traditional human settlements are usually located at the highly productive transition zones between ecosystems, such as alongside rivers, estuaries or ocean, between foothills and plains, the outskirts of forest, or any combinations of these.

In understanding edges, we need to keep in mind that they are the interfaces by which one ecosystem connects and interacts with another. Ecosystems themselves do not function in isolation, they are all interconnected in a web of life, like all things in Nature. The following extract expresses the idea clearly:

“…ecosystem ecologists recognized very early on that ecosystems are open to the flux of living and nonliving matter and organisms, and that ecosystem dynamics could not be understood unless ecosystems were treated as open systems subject to sometimes massive movement of materials across their boundaries. By tracking the exchange and storage of such “common currencies” as nitrogen and organic carbon among biotic and abiotic system components and their throughflow across system boundaries, ecosystem ecologists demonstrated how ecosystems functioned as highly interconnected networks.”

Source: Bart Johnson, Kristina Hill – “Ecology and Design, Frameworks for Learning”, Island Press, 2002

Using the Edge Effect in Design

As we have seen, edges serve as the interfaces of ecosystems, and these borders are much more productive and rich in life.

What this means in terms of Permaculture design is that:

• There is a greater number of mutually beneficial relationships between the elements at the edges.
• Edges serve as ‘energy traps’ since they are the points where materials, nutrients and organisms flow across ecosystems, and there is increased cycling of materials and nutrients at the edges.
• Edges create beneficial microclimates.
• The edges of ecosystems are very important in supporting biodiversity and in the production of biomass.

We can take advantage of the natural phenomenon of the ‘edge effect’ to increase the productivity and yields of the systems we design. We bring this about by increasing the available edge in our designs.

The way we increase edge is by looking to the patterns of Nature and emulating these patterns in our designs.

Nature has evolved to be as efficient as possible over hundreds of millions of years, and we curiously find that in Nature’s designs there are no straight lines in Nature, but a variety of patterns which we see repeated throughout.

So, let’s have a look at Nature’s patterns that allow us to arrange elements more efficiently!

Patterns

When we look to Nature, we find similar patterns repeated through all forms of life. These patterns are there not for aesthetic reasons, not just for looks, but because of the efficiencies they provide.

Nature has perfected packing as much as possible into small spaces and optimizing the organization of things. In many natural systems, the surface areas that serve as interfaces to the surroundings are maximised by increasing edge through patterns.

Lobular or Crenellated Patterns

A lobular (having small lobes) or crenellated (having square indentations) edge provides more edge than a straight line.

Rivers run winding courses through the landscape, which increases the water penetration into the land and creates a greater area riparian ecosystem than if they were running in a straight line.

Aerial photo of the Mississippi-River

Similarly, the macrocosm pattern are reflected in the microcosm, our own intestines wind the same way to maximise the length, and therefore surface area, to absorb nutrients from the food we digest.

Human intestines show same wavy (crenellated) pattern

We can still go further into the microcosm and find the same patterns. If we look inside the cells of living organisms, we find small structures called Mitochondria – oblong shaped organelles that are found in every eukaryotic (non-bacterial) cell. In the animal cell, they are the main power generators, converting oxygen and nutrients into energy. This process is called aerobic respiration and is the reason animals breathe oxygen.

Mitochondria, the ‘power generators’ inside living cells, showing wavy pattern in their inner structure

We can replicate this pattern in our designs to maximize the available edge. If we are building a pond for example, without changing the size of the pond, we can double the length of the edge (the earth/water interface), and therefore squeeze in twice as much productive plants around it. In the example below the mathematical calculations show how for a pond based on n 11.3m circle, we create 100 square metres for water surface, and by changing the edge from a straight one to a wavy, we can double the effective circumference.

We can use the same principle in garden bed design. A wavy path through a garden gives us more edge to plant along, and more space to access the garden. We can increase the accessible space and edges in a garden using ‘keyhole beds’. A keyhole bed allows greater access into the garden beds without having to step into the soil, this preventing soil compaction, which hinders plant growth.

The same concept can be applied at the next level down from garden beds, to the actual planting layout within the beds, to optimise the use of space and therefore increase yields.

The circles indicate the space allocated for each plant, so the plants remain the same distance apart in both cases. If a circle is 15cm (6”) wide, then the plants in both arrangements are always this distance apart from each other. When we change the planting arrangement from straight to ‘wavy’, we can increase the amount of plants in our garden bed in this example from 70 to 86.

This is the basic principle behind the system of Edge Cropping, where two crops are planted in alternating strips, i.e. rows of wheat with rows of lucerne between them, or corn with soybeans. The strips can be planted in ‘wavy’ lines to maximise the use of space and put more plants into a given area.

Such a system is also more commonly referred to as Strip Intercropping, where multiple crops are grown in narrow, adjacent strips, that allow interaction between the different species, but also allow management with modern equipment. This is adaptation of the basic system of intercropping to contemporary, mechanized agricultural practices.

Intercropping is the practice of producing multiple crops in a given space. Throughout time and around the world, intercrops have been used to better match crop demands to available sunlight, water, nutrients, and labour. The advantage of intercropping over sole cropping (growing a single crop in a field) is that competition for resources between species is less than exists within the same species.

Source: Strip Intercropping (Pm1763) January 1999 – Iowa State University, University Extension

Edges can take many more shapes:

• A zigzag pattern for a fence makes it more wind resistant and less likely to be blown over.
• Pitted edges, similar to a waffle iron, can be used in dry climates to trap wind-blown debris, organic matter, water and seeds.
• Gently curing paths running along the contour of a hillside provide access to maintain growing areas
• A ‘sun-trap’ can be made using sharply curved boundaries to protect plants from the wind and maximise heat.

Spiral Patterns

A spiral is another pattern that occurs frequently in Nature, and this shape can also be used to increase the amount of productive edge we have to work with.

Spiral pattern in a flower

Spiral pattern in a nautilus shell

When we utilise the pattern of a spiral in our designs, we use the pattern in three dimensions, our spiral pattern can rise into the air rather than just sit flat on the ground.

The most common application of this design technique is a Herb Spiral, as pictured below. The typical width of a herb spiral is approximately 1.6m (just over 5’) in diameter.

Using this size, we can see that a simple circular bed has an area of 2.0 square metres, but if we create a mound of soil 0.5m high, our area that we now have available increases to 2.4m. This represents a 20% gain in area. The higher the spiral (within reason), the more extra area we gain.

The other advantage we gain with a herb spiral are the multiple microclimates that are created.

• The side facing the sun is warmer and the mound acts as a thermal mass, favouring sun-loving herbs and ones which need more warmth.
• The side opposite the sun is shadier, favouring shade-loving herbs
• The top of the herb spiral is drier, as water drains away easier, favouring herbs that prefer dry conditions
• The base of the herb spiral is wetter, favouring herbs that enjoy more moisture

The elevated design makes it possible to grow plants that dislike excessive soil moisture in areas that can become waterlogged.

Through a single structure, we are able to garden vertically to increase the available edge, create multiple microclimates, increase yields and productivity, and add visual interest to the garden space.

Conclusion

By increasing the edges in our designs, we extend the interfaces to the surrounding ecosystems, trap more energy and materials that move through our systems, and ultimately increase yields and productivity.

Edge patterns can take various shapes – they can be wavy, lobular or crenellated, zigzag or spiral. Elevated mounds as edges increase growing area, provide wind protection, improved drainage and create multiple microclimates.

it is important that we select the most appropriate kind of edge patterns for our environment. Different systems will require different approaches, and the factors that we need to take into account when selecting the edge pattern are landscape, scale, climate and plant species.

Small scale systems can support greater pattern complexity, while for large scale systems, it is best to keep the patterns simple to minimise the work required to build and maintain them.

Now that we can emulate Nature’s patterns to optimise the efficiency of our gardens, we can have gardens that are more natural looking and aesthetically pleasing, and more productive too!

References:

1. Bill Mollison & Reny Mia Slay “Introduction To Permaculture”
2. Bart Johnson, Kristina Hill – “Ecology and Design, Frameworks for Learning”, Island Press, 2002
3. Banks-Leite, C. and Ewers, R. M. 2009. Ecosystem Boundaries. eLS.
4. Strip Intercropping (Pm1763) January 1999 – Iowa State University, University Extension