On Permaculture and Plant-Animal Interactions

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On Permaculture and Plant-Animal InteractionsA senior experience paper by Nathan Ryan

Winter 2013

Altamirano: The Garden of Eden!Gabriel: It's a trifle overgrown.

~ The Mission (1986)

IHRTLUHC

Nathan Ryan '13


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IntroductionThe need for alternative farming practices is becoming increasingly accepted in both lay and

scientific circles. Biological research has shown the weaknesses of industrial farming systems, but

more exciting is the large amount of literature supporting alternative agricultural traditions, both new

and old. An exhaustive discussion of the biology of farming practices is impossible here. However,

what more comprehensive discussions of agroecology tend to forget to provide are ways of thinking

about agriculture that promote healthy and sustainable farming practices. Permaculture attempts to

provide just such a paradigm. The movement embraces almost the whole scope of human experience,

however, farming practices are greatly emphasized.

In essence, permaculture is a movement about rethinking the way we humans interact with and

shape our environment. The wisdom of permaculture is distilled into twelve design principles (see

Appendix I). While each is firmly based in a scientific understanding of our world, there has been little

rigorous defense of permaculture in the scientific community. My goal here is show that the principles

of permaculture have strong support in the scientific literature, particularly as it relates to the biology of

food. Obviously, a detailed look at all twelve principles would be prohibitive so I will focus only on

one principle, number ten: Use and value diversity. 11 Hopefully, an in-depth analysis of just one of

the principles will convince the reader that the other eleven bear some consideration. Diversity in a

number of facets is explored: from the diversity of species to the diversity of crop-field boundaries.

First, however, I think it is useful to provide some background on the broad basics of permaculture.

Let me sketch two scenarios comparing permaculture to modern farming practices to demonstrate the

overarching differences:

Industrial farming is ultimately reductionist in nature. The farmer tries to eliminate all biotic

interactions in his/her crop of choice. The following is a stereotypical account of the industrialist's

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practice. A monoculture of genetically identical individuals is planted, perhaps with limited crop

succession (otherwise called crop cycling). Application of water and nutrients are carefully controlled;

pesticides and herbicides are applied to isolate the species being cultivated from the influence of wild

insects and weeds. The crop is harvested when ripe, and the next planting is prepared. Such a scenario

is a scientist's dream, nice and neat, no fuzzy edges or loose ends everything defined and in its right

place. Only its not and we know its not. All that effort to remove the plant from a community setting

has its own implications. The maintenance of an imposed order of simplified agriculture against the

natural tendency toward entropy, diversity, and stability demands energy and resources.2 It is the very

picture of Sisyphus rolling his boulder up a hill an unhealthy practice.

Permaculture, in contrast, embraces the biotic and abiotic interactions that factor into food

production. A particular species is viewed only in the context of its place in the larger agroecosystem.

In contrast to the primary reliance on quick-maturing annuals in industrial systems, forests are the

natural models for permaculture ideas of agriculture.11 The permaculturist farmer establishes food

forests. In tropical and subtropical climates (and to some degree temperate climates), these might take

the form of a heavily wooded forest with dense accompanying understory layers. However, even

where the environment is not conducive to woody plants, the metaphor of a food forest is still accurate.

The farmer designs and manages an ecosystem that is beneficial for the human as well as all the other

organisms that he or she shares the land with. Altieri et. al. (1983) have five key directives to restore

agroecological health which summarize the goals of permaculture farming systems: we need to (1)

diminish energy and resource use, (2) emphasize community stability, (3) maximize the recycling of

nutrients and organic matter, (4) promote multiple-use, (5) and engender efficient energy flow.2 In

essence, what both permaculture and these directives recognize are the interconnectedness of natural

systems and attempt to restore an ecological approach to farming.

Part of restoring an ecological approach to farming involves valuing and embracing diversity

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(design principle #10 if you will recall). There are a number of aspects that this could relate to on a

plot of land, but the most relevant to biological studies are crop diversity and landscape diversity.

While diversification of crop systems have many important biological consequences, I will limit the

discussion of this paper to the effect on plant-animal interactions, because I find the narrative contianed

within most persuasive. In each section, I provide evidence that the implementation of diversification

strategies is beneficial (Evidence) and then proceed to examine the mechanisms through which these

strategies impacts the biology of plant-animal interactions in agroecosystems (Theory).

Diversity of SpeciesEmbracing species diversity in agricultural systems is perhaps the oldest recognized aspect of

healthy agricultural practices in human history. The classic example are the Three Sisters planted by

a number of Native American cultures. Using a technique known as companion planting, beans, corn,

and squash are planted together. Each benefits the others in a way that ultimately profits the farmer.

The Native American's who used this technique recognized some of the positive impacts of

polycultures; more recent investigation has demonstrated additional benefits besides. Among these

higher yields are perhaps most striking and most relevant for arguing the importance of intercropping

in agricultural systems. Risch and Hansen (1982) outline six likely reasons for the higher productivity

of diverse plots: (1) enhanced nitrogen availability, (2) more efficient capture of solar radiation, (3)

moisture and nutrients are utilized more efficiently (4) lower erosion and less weed biomass, (5) greater

stability of field output i.e. if one crop suffers another compensates, and (6) mitigation of insect and

disease damage.19 Here I will provide evidence for these mechanisms impacting yields in polycultures

as well as delve into the theoretical basis for some of the more interesting of these characteristics of

diverse plots. The yield benefits of intercropping are generally only seen when Land Equivalent Ratios

(LERs) are calculated.19 LERs are a technique to compare the yield of intercropped and monoculture

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systems. LERs control for the reduction in the density of each member in intercropped systems, and

comparisons of yields without such considerations is meaningless.

Evidence

Temporal bean-corn polycultures have been embraced by industrial farming culture because of

the well known nitrogen-fixing properties of legumes. Spatial intercropping has even greater

ramifications for the two species. Systems in Brazil have shown that nutrient cycling occurs differently

in monoculture plots than it does in intercropped plots.5 When planted together, corn by creating

microclimates amenable to rhizobia increases the nodulation in the common bean. Additionally, corn

stimulates nitrogen-fixation by absorbing inorganic nitrogen from the soil. Nitrogenase activity

(responsible for converting atmospheric nitrogento ammonia) is inhibited by the presence of inorganic

nitrogen, so by removing inorganic nitrogen from the soil, corn facilitates nitrogenase activity and thus

nitrogen-fixation in legumes.30 The bean partner, besides increasing the inorganic nitrogen available to

the corn, changes the composition of the surface residue and in doing so, alters the carbon to nitrogen

ratios in the compost that results. The altered ratio then favors recycling of nutrients. Not only is this

polyculture more sustainable, the equivalent yield of corn was 30% higher in intercropped systems

compared to corn monocultures.5 An increase in yield in intercropping systems is well documented,

20% to 60% is common in Latin America and is quite conservative for some comparisons. 1

Agroforestry is another practice that increases the diversity of species and has also been shown

to impact microclimate and insect herbivore associations. In certain areas of Mexico, traditional

planting practices make good use of these properties of polycultures.3 Many weeds which are called

wild plants by the local farmers are intentionally left after the harvest of the hand-planted plant; not

only that, some species (of genus Solanum, Jaltomata, andPhysalis) have fruiting phylogeny seems to

have adapted to be in sync with species harvesting timing. These wild plants are used to reduce

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erosion, produce organic litter to fertilize crops, and influence pest dynamics. Particularly in areas with

soils that are general sandy, well drained, and largely infertile, farmers value organic fertilizers over

chemical fertilizers observing they burn the soil and create salinity problems. Chemical fertilizers

do not create the humus that organic fertilizers do; this lack or addition of humus has a large impact on

soil properties. Manure, supplemented by leaf litter especially from the wild maguey plant, once

applied will provide 3-4 years of nutrition for a soil. The farmer then rotates the manure application to

low-fertility areas of their land. Pest control is largely done with cultural control methods,

intercropping among these. Their management techniques of using diverse planting strategies

influence the arthropod abundances and distribution.3

Alfalfa, planted in strips with corn, was observed to positively correlate to the number of

predators of potential insect herbivores and negatively correlate to some of these insects. The timing of

alfalfa cutting also seems to impact insect distributions in intercropped areas. Tree crops also were

observed to influence microclimates and soil characteristics. Soil organic matter, nutrient content, and

soil adsorption were all greater around intercropped trees. Under the canopy, solar radiation

interception and moisture interception created moister soils and general combated the general aridity of

the study. Some species of plants are also used in these traditional farming systems as trap crops.3

Tropical agroforestry systems cycles are tight and closed. Rather than losing large amounts

of essential nutrients to harvest and leeching which then must be continually supplied, agroforestry

systems prevent such environmental loss. One striking example comes from a tropical system:

nitrogen inputs to coffee grown in agroforestry systems by shade trees exceeded those removed by the

harvest ten times.1

Theory Pest ControlIn studying diverse culture systems, Root (1973) developed the resource concentration

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hypothesis. Evaluating herbivore loads (biomass of herbivores per biomass of prey) on Collards,

higher loads were discovered in monocultures than in polycultures. Moreover, the herbivorous insect

assemblage was impacted by planting schemes. These differences, both in herbivore loads and

assemblages, between monocultures and polycultures were not attributable to changes in predator

populations, though higher predator:herbivore ratios were discovered in monocultures. Instead Root

proposed that the habitat composition influenced the distribution of herbivores and in doing so changed

the composition of the insect communities. Specialists of crucifers (of which Collards are a member)

were noticed to be the only insect population that has higher densities in monoculture plots and the

culprit of increasing herbivore loads in the simpler habitats.20

As a result of the homogeneous environment in monocultures, herbivores who are best adapted

to these plants tend to dominate the herbivore communities. Stated simply, the resource concentration

hypothesis posits that stand size and purity will exert a differential influence on the rate of

accumulation, tenure, and reproductive success of the herbivores that can feed on the host.20 Risch

(1981) performed detailed analysis of the impact that polycultures exert on herbivore foraging

behavior. He found that non-host plants modified subsequent foraging behavior, decreasing the tenure

time and thus increasing the rate of movement through a plot. This was found to influence the

emigration rates of plots.18 Differential rates of movement in polyculture plots and monoculture plots

based on herbivore perception of host and non-host plants leads to the increase of a few specialized

species in monocultures. By diversifying species, this trend is counter-acted, and high abundances of a

few specialized pests are avoided.20 Logically generalist herbivores are not dissuaded by polycultures,

and this has been demonstrated.10 Feeny (1976) theorized one reason why the resource concentration

hypothesis might be so effective in describing agricultural systems and their pests; he proposes that

because many of the crops that we grow in agricultural settings and particularly in dense

monocultures were domesticated from early successional plants, these species poses traits that make

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them inherently vulnerable to specialized pests especially when grown in large homogeneous patches.

Early successional plants have a number of qualities that make their domestication very useful, but also

some weaknesses as agricultural staples. Species of this sort have life histories which greatly impact

other aspects of their biology. They emphasize fast growth, large reproductive efforts, and swift

dispersion and colonization capabilities. In turn, they do not invest heavily in secondary metabolites,

relying instead on refuges in space and in time. Their evolutionary dependence on being hard to find is

a defense mechanism that humans have directly undermined in industrial management of agriculture.18

These conclusions have been supported by further research in the impact of intercropping of

corn-beans-squash. Beetle herbivore abundance differences between mono and polycultures were

observed to occur primarily because of the impact of intercropping on beetle movements. Dispersion

rates, feeding habits, and tenure on plants were all influenced by the presence of another plant species. 17

Studying the immigration (or diffusion) into both simple and diverse habitats, Wetzler and Risch (1984)

provided evidence that the faster diffusion ofCoccinellid beetles through polycultures likely accounts

for the greater abundances of the beetle in monocultures.29 It is important to note, however, that a

beneficial reduction in beetle populations was only achieved by intercropping a host plant with a non-

host species. Two host plants increased the abundance of beetles that found both hosts tasty.17

The impacts of the number of species planted together are not limited to herbaceous insects.

Predators of these herbaceous insects are also influence by planting scheme, though the relationship is

less clear. Generally, the expectation is that predators will be more abundant in polycultures; research

supports this, with notable exceptions though.4 Additionally, the distinction between predator

abundance and actual predation has often been neglected. Speight and Lawton (1976), however, found

both predator abundances and predation rates higher in diverse plots.4 As an exception that proves the

rule, Andow and Risch's analysis (1985) of Coccinellid predators in simple and diverse planting

schemes is informative in elucidating the complex relationship between habitat and predators. They

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find four theoretical explanations for the general trend of increased predator abundance in diverse

habitats: (1) local extinction of predators is inhibited by greater evenness in temporal and spatial

distribution of prey, (2) polycultures provide more diverse alternative food sources possibly in greater

density for predators such as pollen, which allows predators to maintain higher abundances, (3)

diverse habitats support higher total abundances of prey, (4) predator populations experience more

predation by their own predators in monocultures.4 In the specific system studied by Andow and Risch,

the distinction of roles between predator and herbivore of the Coccinellid beetle are murky. Several of

these conditions were reversed in their study.

Theory Nutrient Cycling

Nutrient management is one of the most crucial aspects to developing sustainable

agroecosystems, and in non-industrial farming systems the role of soil organisms in nutrient cycling

processes is key.30 Agricultural practices impact soil organisms and their ability to produce forms of

nutrients that can then be used by plants. The manner that plants may effect soil communities is varied.

Nutrient availability to crops depends upon the size of the soil communities which is in turn dependent

upon the organic matter composition of the soil. In soil there are three pools of organic matter each of

which has very different decomposition rates. The first is crop residue, most of which is decomposed

within days if not hours. The residual degradation products left over form the second pool and takes

anywhere from 10-100 of years. Polymerization of the most stubborn organic molecules forms humus

which, in terms of carbon content, makes up the bulk of soil mass. Material here can take hundreds of

years to decompose. From theprima materia, each of these successive transformations is the result of

different soil populations, and through them plant species have a hand in shaping a great deal of the

metamorphoses of the soil.9

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I think it is helpful to first outline the ways in which a plant influences what goes on below

ground. The most explicit way that plants manipulate soil communities is by the deposition of plant

material both when the plant is dead and when the plant is living. Crop residue is the more familiar of

the two. The composition of the litter that is transformed by soil organisms has important implications

not only for the soil communities but also for the nutrient of subsequent crops. The mineralization rate

of nitrogen and sulfur depends on C:N, C:S ratios of organic matter in the soil being decomposed. The

soil organisms' need for organic carbon (whose oxidation provides the decomposer metabolic energy)

drives the mineralization of these important agricultural nutrients Proportionally too much carbon and

the decomposition of the organic matter leads to no net mineralization of N or S. 9

Less familiar is the impact that plants exert on the rhizosphere a region which has far reaching

consequences for the soil biota. Wang and Dick (2004) define the rhizosphere as the volume of soil

adjacent to and influenced by the plant root and is directly impacted by the type of crop grown. 28

Microorganisms occupying this space have important influences on both the plant they associate with

(among these, enhancing plant nutrition and growth as well as inhibiting soil-borne diseases) and on

surrounding soil communities. The rhizosphere is formed by sloughed-off root matter and root

exudates which are highly dependent on the plant species.28 In addition to large amounts of a plant's

photosythate carbon (10-30% often, sometimes as much as 50%), crops secrete organic acid to increase

the solubility of some nutrients. The uptake of phosphorous is particularly challenging for plants

because of the large amount required and the insolubility of most inorganic forms. 9 Root longevity and

growth not only varies substantially among different plant species; microbial and fungi (esp.

mycorrhizal) activity hot spots are not fixed in time or space, dependent largely on both plant species

and plant development.9 This in turn influences the microorganism communities in the rhizosphere.

Soil in fact has a limited role in determining the community composition in the rhizosphere; plant

species is far and away the most important determinant.28

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Plant identity has impacts other than just on the composition of the soil communities closely

associated with their root systems. At least two manners of transference have been discovered

whereby the rhizosphere (hence also the crop) influences other soil communities. Mycorrhizal fungi

can act as a mediator between plant and soil communities, acting as an extension of the plant root

system to some degree. The abundance of mycorrhizal fungi is the most basic influence plants can

have on their symbionts, but the activities of the fungi are also manipulated by the plant partner

Additionally, the composition of the rhizosphere could influence the trophic dynamics of other soil

organisms. However it occurs, plant species do influence both the structure and function of bulk soil,

though to a lesser extent than in the rhizosphere.13 These trophic interactions to some extent dictate

decomposition rates and nutrient cycling. Predation by nematodes has a particularly large effect on

nitrogen mineralization because the C:N ratios of predators are larger than those of the microbial prey.

De Deyn et. al. (2004) discovered strong effects of plant identity and non-negligible effects of diversity

on both primary consumer and secondary decomposer nematode populations.6 As a rule, the greater

amount of trophic levels in a soil community the faster decomposition of organic matter occurs.9

This is not an exhaustive description of how crops influence soil dynamics. At a courser-

grained level, plants have a remarkable impact on the soil moisture, oxygen content, temperature,

erosion, leeching, the list goes on and on. These, however, are biologically less interesting than the

biotic interactions that shape soil communities and nutrient cycling. I haven't yet addressed directly

why polycultures and monocultures in agroecosystems should have differential impacts on the soil

communities, though I have hinted at them.

Although not studying an agricultural system, Zak et. al. (2003) discovered that plant diversity

significantly increased microbial community biomass and modified the structure of soil communities.

The observed change was attributable to greater productivity in more diverse plant systems, not the

increased species richnessper se. Not only was the community altered, rates of microbial activity

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which are responsible for carbon and nitrogen cycling were also increased again due to the enhanced

productivity of diverse plots.32 It seems apparent from the work of Kowalchuk et. al. (2002) that time

scales are important for the impact of plant identity on bulk soil communities. The authors found very

distinct profiles in the rhizospheres of two different plant species within a year but no detectable

differences in bulk soil samples. They suggest that the rhizosphere acts primarily as a major selective

force on the microorganisms living there, creating an environment that promotes the growth of some

community members at the expense of others. Additionally reviewing other relevant work, the

researchers concluded that the soil properties were the major determinate of bulk soil community

composition and this is the manner that plants mediate changes in non-rhizosphere soil.14

If plants influence soil communities by exerting a selective effort in the rhizosphere, the

selective forces are not limited to just the volume of soil surrounding plant roots.6 However, the link

between the plant and the below ground organisms becomes more vague the less intimately connected

with plant tissue the soil organisms are. Intermediaries and their inherent idiosyncrasies are bound to

complicate the issue, just as we saw with the impact of plant diversity on crop pests. The impact of

species-specific rhizosphere effects on surrounding soil communities are not insignificant though. The

picture that develops is a continuum between the rhizosphere and bulk soil of overlapping influences of

plant identity and soil qualities. These two are not independent of each other, but it is convenient to

treat them as such over short time spans. The organisms inhabiting the soil form the species pool

which plants can act upon. This is inescapable; a plant does not manufacture its own microorganisms

to inhabit the soil it grows in. The plants influence is strongest in the most immediate soil regions and

tapers with distance from the roots.13

The remaining question is whether these changes in soil community structure then changes the

nutrient cycling dynamics of the habitat. Hooper and Vitousek (1998) find such a relationship to exist.

Specifically, they point to two impacts of plant composition on nutrient relations. First, more efficient

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and total resource use occurs is more diverse systems. Second, soil community mediated plant

composition effects were as or more important in determining nutrient detention.12

Diversity of Crop-Field BoundariesDiversity can come at both larger and smaller scopes than the individual. This section discusses

the value of landscape diversity in agroecosystems. In permaculture design, food forests are arranged

into concentric circles around the home (see Fig. 1). Intensity of human use and cultivation is inversely

proportional to the radial distance from the center. As distance increases, self-maintaining elements are

depended upon; little human input is required but generally less yield is produced. The zones in

actuality do not form geometric circles nor are necessarily concentric and are designed to fit the

landscape. Still these zones are useful in describing the pattern that permaculturists aspire to create.

Starting from zone 0, the homestead, use begins as a garden around the house (zone 1); moves into

orchards with small livestock (zone 2); then to commercial crops, pastures, and large livestock (zone

3); to managed forests, wetlands, and rangeland (zone 4); and finally to surrounding wilderness (zone

5).

Fig 1: Permaculture zones (adapted from Holmgren, 2002)11

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0: Homestead0: Homestead

1: Garden1: Garden

2: Orchard2: Orchard

3: Crops/Pasture3: Crops/Pasture

4: Managed Forest4: Managed Forest

5: Wilderness5: Wilderness


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EvidenceIn my treatment of the effect of landscape diversity in agroecosystems, I will primarily focus

only on the biotic interaction occurring on the farmer's plot of land. Conservation implications and

other environmental factors I ignore, though undoubtedly they have their own story to tell. Insect

herbivores and their predators are a major concern for the farmer as are pollination vectors and weed

species. Landscapes have been shown to shape all of these interactions.

Due to the intensity and frequency of disturbance, landscapes are particularly important for

cultivation areas such as gardens or fields. Field surroundings act as refuges and recolonization sources

for both herbivorous insects and their predators. Provocatively, simple agricultural landscapes have

exceptionally high levels of pesticide use compared to more complex landscapes, suggesting that

dynamics in complex landscapes may act as biocontrol on insects that feed on crops. In German wheat

fields, aphids are the primary herbivores. Important predators of the cereal aphids are carabid beetles,

spiders, ladybird beetles, hoverflies, gall midges, lacewings, and parasitoid wasps. Both the aphids and

their natural enemies live in wheat fields only seasonally and use other habitats for overwintering and

reproduction. Non-crop habitat increased the diversity of carabid beetles and spiders.

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Aphid

parasitism and mortality was greater in diverse landscapes but aphid colonization was also higher,

leading to similar aphid abundances in crops both in simple and diverse landscapes despite the large

amount of pesticides used in wheat fields in simple landscapes.21

The flow of populations across habitat boundaries in agroecosystems, crop-noncrop

interfaces, is a particularly important aspect of how landscapes influence biotic interactions in farming

systems. Increasing documentation shows that population distribution and abundance as well as

trophic interactions depend on processes that act at larger spatial scales than just one habitat. These

processes have important implications for biocontrol in agricultural settings.27 Studying the effect of

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landscape on the biocontrol of an invasive soybean aphid in the Midwest, Gardiner et. al. (2009) found

that landscape diversity increased aphid population suppression by natural enemies in soybean fields.

The authors were also able to determine the scale at which the landscape most influenced biocontrol

and found that the region 1.5 km around the study site was most influential. Aphids can have rapid

population recruitment and so cultural management of these crop pests must act early in the growing

season, before aphid populations reach a critical mass. The authors theorize that diverse landscapes

with surrounding forests and grasslands are able to support higher populations of generalist natural

enemies and/or enhance the ability of these predators to move through the landscape.7 Study of oilseed

rape cultivation in Germany has shown that older field margins (>6 years) that supported tree growth

allowed important parasitoid biocontrol species populations to grow larger and in turn enhance their

dispersion into crop fields. Increased penetration of cultivated habitats led to greater rates of

parasitism.25 Landscape complexity does not unfailingly reduce pest pressure, but overwhelmingly it

does.7

Herbivory is not the only challenge that human cultivated crops face. Competition from wild

plants are also a major concern for farmers. Landscape complexity can inhibit wild plant recruitment

by supporting animals that eat wild plant seeds. Most post-dispersal seed predators are found in

noncrop habitats; one would predict that fields in complex landscapes might harbor more seed

predators and as a consequence wild plants would experience higher seed predation. Menalled et. al.

(2000) found this to be the case in Michigan corn fields, finding higher seed predation by both

vertebrate and invertebrate animals.15 Whether seed predation of cultural species increases in complex

landscapes is another question that deserves inquiry.

Another way that surrounding habitat can impact crops is by altering the biotic interactions of

pollination. Of the leading global crops, 87 of 115 significantly benefit from the services of animal

pollination vectors.16 The insects (bees in particular) which largely make up the guild of species

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responsible for pollination in agricultural plants are an essential part of agroecology and landscape

effects on these critters then can have impacts on the crops which are dependent upon them. Thus

successful management of agroecosystems must include the cultivation of pollinators; implying the

conservation or creation of habitats supporting these animals. The impact of landscape dynamics seems

to be quite variable depending upon plant-breeding biology, land use patterns, and pollinator

communities; however, some general patterns are evident. Strong evidence suggests that isolation of

crop species from the natural habitat of wild pollinators (defined either in terms of linear distance or the

proportion of area of natural habitats in agroecosystems) decreases pollinator community richness,

visitation rates, and fruit and seed set.16

Theory

Understanding the interrelationship between animal-plant interactions and landscape is fraught

with difficulty. Predictive models developed by island biogeography and metapopulation analysis

make assumptions which may not be true in most agroecosystems. Often when analyzing animal

populations in a landscape, habitats are either characterized into suitable habitat or non-suitable habitat

(habitat or nonhabitat); nonhabitat becomes background fill and insignificant to population analysis.

When studying terrestrial animals on actual islands, this categorization largely makes sense. Most

terrestrial environments are not so tidy however. Nonhabitat can impede or promote dispersal of

animals; the line between habitat and nonhabitat is blurred by edge effects, spillover effects, and a

continuum of habitat quality rather than a binary distinction; most animals utilize a number of different

habitats and the manner in which they do so is largely identity dependent. As if the impact of

landscape composition is not murky enough, the scale at which animals experience landscape effects

(the functional scale) is also variable. A bee might be influenced by smaller scale changes in the

environment than might a bear.27

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Of particular relevance to more highly disturbed agroecosystems (e.g. industrial farms, to some

extent zones 1 and 3 in permaculture designs) is the patterns of flux across crop-noncrop interfaces.

These patterns are very dependent upon the animal species, but five general types of animals based on

their distribution across these interfaces have been described (see Fig. 2). These habits of dispersal are

particularly important in analyzing the landscape dynamics on insects, both for crop herbivores and

their natural enemies. Parasitoid species, because of their flight characteristics and dependence upon

noncrop habitat resources typically exhibit ecotone or disperser distributions. The precipitous drop in

abundance with further crop penetration means that landscapes with large interface perimeter lengths

compared to field area would greatly increase the capacity for parasitoid insects to act as controls on

herbivorous pests. Ballooning spiders, who have more of an ubiquitous distribution, instead would

benefit from noncrop habitats that allowed for the long-term build up of populations, the progeny of

who could annually invade crop habitats. Interfaces would be much less significant to these animals,

but noncrop habitat quality might be much more important for effective pest control.27

Fig. 2: Distribution patterns of five different types of animals. The graph shows a transecttaken at the crop-noncrop interface. (Adapted from Tscharntke et. al., 2005)27

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Abundance

Abundance

Noncrop

Noncrop

Crop

Crop

Cultural

Ubiquitist

Disperser

Ecotone

Stenotopic

Habitat typeHabitat type


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Interestingly, populations seem to show different distribution patterns based on their abundance

in noncrop habitat. Thies and Tscharntke (2003) found that in oilseed rape fields the distribution

pattern of three parasitoid wasps was highly dependent upon field margins. In fields surrounded by

relatively new field margins, the wasps demonstrated disperser distributions. When fields were

surrounded by older field margins (6+ yrs), the distribution patterns were ubiquitous in nature. The

authors also found that in their study system only structurally simple landscapes demonstrated

parasitoid disperser distributions which disappeared in more complex landscapes.24 These findings

suggest the influence of landscape goes beyond simply altering the spatial location of distribution

patterns of animals; landscape also fundamentally changes the distribution patterns themselves.

Besides changing animal interactions in crop fields, landscape diversity has implications that go

beyond facilitating specific repression by natural enemies. Beta diversity a measure of the

compositional diversity of several communities is one of the ways that complex landscapes build

faunal diversity. The beta diversity of communities among landscapes has been predicted to have

important impacts on biocontrol of agricultural pests. Providing spatial insurance, some natural

enemies though unimportant in one habitat may become important in another. Exploitation of pest

populations is thought to be more efficient with higher beta diversity. These ideas have led to the

formation of the insurance hypothesis which states that species richness can buffer against fluctuations

in ecosystem functioning, thereby insuring function in fluctuating environments .26 As highly variable

as agroecosystems often are, species richness might be essential for biological control to be

meaningful. Phenology becomes especially important. Annual crops are transient in nature and thus in

agroecosystems relying primarily on annuals, resources of crop habitats are also ephemeral. If predator

populations take a large amount of time to build up population levels in response to a booming pest

population, early outbreaks are unavoidable. High intercommunity compositional diversity allows for

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natural enemies with spatiotemporal complementary in resource usage to coexist. Seemingly

redundant predators quickly become essential. Definitive evidence supporting the insurance

hypothesis has not been found as of yet, but there is research that hints to the importance of beta

diversity in biocontrol.

26

The rise of specialist pests and the preoccupation of industrial farmers with the damage they

cause crop species is unsurprising considering the link between trophic levels and the spatial range of

resource exploitation. Generally, the higher one is on the food chain the more space one uses. The

animals that control the pests of agricultural crops are predicted to be the most impacted by landscape

changes.26 For effective biocontrol measures, the farmer must look beyond the boundaries of his/her

fields.

Application

Permaculture is far from being accepted among the agronomic circles; if one searches, however,

its principles have strong support in the literature. In so much as permaculture is an attempt to

incorporate natural systems into agricultural practices, the burden of proof should really be on those

who defend industrial farming practices. If it was possible to demonstrate that these vastly reductionist

agroecosystems were healthy and sustainable (a point that I very much doubt could be successfully

argued), only then should such practices be used. Obviously one cannot write a paper saying nothing at

all except that someone else ought to write another paper, so I hope that I have persuaded even the

staunchest critic that the science of embracing diversity in permaculture is sound. For those more open

minded, I hope to suggest that the remaining eleven design principles deserve similar inquiry and I

believe that they will also emerge well founded in our scientific understanding of growing food.

Among the permaculture community, there is a perception that too much of its adherents

become hung up on the design phase and fail to put their ideas into action. Riverview Gardens is

among one of the leading farms in applying permaculture principles to create a operational business

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model. My work, in addition to the labor of growing the produce, has been to design one aspect of the

our food production focused on the CSA element of the Gardens. We are in the process of expanding

from 10 shares last year to 100 shares this year. Converting a golf course into a agriculturally

productive space has been challenging. A clay-sand soil, very little humus is in the ground and

decomposers are similarly bereft. Early on, the project managers decided to base their production on

hugelkultur beds (see Appendix II). These beds incorporate large amounts of organic matter into the

soil which then decomposed over a period of 10+ years. Another big push for Riverview is the

management of the whole 72 acre property. Food forests are in the process of being built and already

standing forests are being conserved. Other landscape features such as swales (see Appendix II) and

perennial gardens are also being incorporated.

My role in design has focused on the other aspect of diversity discussed in this paper. In our

annual produce, we are moving away from monocultures and instead using companion planting as our

source of biocontrol. We do not spray insecticides or pesticides. Instead we rely on the diversification

of species composition of crops to perform this role for us. Companion planting involves the formation

of guilds which are typically composed of important commercial species, herbs, flowers, and wild

plants. While we have not yet finalized our plans, we are convinced that we will be able to grow food

sustainably and more naturally.

AppendicesAppendix I: The Twelve Design Principles

These twelve principles in theory guide all permaculture practice, however most emphasize

some more than others. For a more detailed explanation of each, David Holmgren's bookPermaculture: Principles & Pathways Beyond Sustainability is a good reference (see referencessection for citation).

1. Observe and Interact

2. Catch and Store Energy

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3. Obtain a Yield

4. Apply Self-Regulation and Accept Feedback

5. Use and Value Renewable Resources and Services

6. Produce No Waste

7. Design from Patterns to Details

8. Integrate Rather than Segregate

9. Use Small and Slow Solutions

10. Use and Value Diversity

11. Use Edges and Value the Marginal

12. Creatively Use and Respond to Change

Appendix II: Riverview Landscape

Hugelkultur beds

Hugelkultur beds (or hugel beds) are a type of raised bed. The distinguishing feature of hugel

beds is the incorporation of large, bulky organic matter and debris. Our version involves first removing

the sod from a 3x30' area and then digging a relatively shallow trench perhaps a 1' deep. Logs and

branches are placed into the trench, followed by finer organic particulate (mulched wood, leaf litter,

pine neetles, etc.). The sod is overturned and placed over the now filled trench and then the soil from

the digging of the trench is piled over the whole thing (Fig. 3). These beds can then be shaped as

needed. The gradient of organic matter size means that decomposers will release nutrients over a

period of decades as well as facilitate soil aeration and moisture retention. A no-till system, hugel beds

also allow for the soil biota to flourish.

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Fig. 3: Cross-section of a typical hugel bed at Riverview.

Swales

Swales are essentially passive rain catchment systems. A trench is dug along the contour line of

an area and the excavated soil is placed on the downslope edge of the trench. They are then planted

with various plants that benefit the gardens by attracting beneficial insects and such. These trenches

retain precipitation that would otherwise run off the property. Swales have a number of positive effects

on an area, namely increased soil moisture, decreased nutrient leeching, and decreased erosion of the

soil.

21

Excavated soil

Overturned turf

Excavated soil

Logs/sticks

Overturned turf

Fine organic debris

~12

3'


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