The opposite of reserves: the ecology of agriculture

Agriculture: a vaguely defined interface between human activities and resource cultivation and harvest.

Today consider from ecological principles

Cultivating food resources for human use is the primary purpose of agricultural practices.

How do you promote the population size or renewal rate of a spp?

Increase the amount of limiting resources

Reduce interspecific factors

Competition with other species

Consumption by other species (except humans!)

 

As a practice, agriculture can provide lots of food, although its delivery across the globe is not even

Developed countries take food availability for granted (i.e. food is not a limiting resource... check out the most recent "figures" on obesity in the USA.

The opposite is true in many developing countries

 

Two main challenges define most discussions of agriculture:

Production: Increasing agricultural production (to feed an expanding population)

Distribution: Getting food to hungry people.

 

A third issue: agriculture is a very significant contributor to environmental problems worldwide.

Examples: Air and water pollution, habitat loss, soil degradation, deforestation, introduced species

 

Are current agricultural pracitices environmentally sound?

As populations grow, the amount of arable land declines

Water extraction practices are only delaying problems in some areas (e.g., build up of salts through time)

 

Increasing yield on current land is also difficult because of competition and consumption

Crops have competitors (colonization by undesirable plant species - weeds - compete for light, water, & nutrients.

Crops have consumers other than humans (e.g,. insect herbivores and parasites/diseases).

High input agricultural approaches in developed countries.

Historical shift from local, dispersed, and seasonal farms to centralized, industrial, food processing complexes

Thinking of agricultural systems as ecosystems

Some differences:

1. Diverse communities vs "monoculture". Other plant species are kept out by cultivation practices or (more commonly) by herbicides.

2. genetic homogeneity (hybrid seeds)

3. lack of natural chemical defenses

4. very little nutrient cycling

5. biomass production maintained through high inputs of fossil fuel-derived products: fertilizers, insecticides, herbicides, fuel-run machinery.

Increasing use of fertilizers and pesticides are cause for concerns about sustainability

Modification of current practices:

Soil conservation (a return to more traditional approaches

1. tillage of crop residues into the soil
2. crop rotation, esp with legume crops or else to reduce pests
3. shelterbelt planting to block wind
4. use of organic fertilizers

IPM = integrated pest management... combining biological control with reduced pesticide use.

Biological control methods include:

1. using parasites and predators of pest insects.
2. using sterile males to reduce the population potential of pests.
3. breeding (conventionally or through genetically engineering) crops that are resistant to pests.
4. using pheromone/hormone analogues to disrupt pest reproduction.

 

"Organic" (more "natural") farming approaches

No pesticides (just biological and cultural control), or "natural" pesticides like plant-derived products,

Fertilizers derived from organic material.

Some misconceptions about the use of organic vs. inorganic fertilizers.

No physiological difference to the plant

No nutritional difference to consumer

Why bother?

Organic fertilizers is better for the soil and better for the environment.

Soil is basically 4 things:

mineral particles (weathered rock.... sand, silt, and clay).
organic matter (decomposed and decomposing organisms)
water
air

Organic soils absorb water better, reduces watering, lowers soil erosion

Organic fertilizer releases nutrients slowly; more efficient and less water pollution from nutrient runoff (loss)

Why not more organic farmers?

The problem of selling "blemished" crops

 

Finally applying even more "natural" approaches based on ecological principles: a community approach rather than monoculture and the idea of intercropping.

Two or more crops are grown in the same field simultaneously. This can be more sustainable, but less practical.

An example with beans and corn.

The monoculture model: two large seperate fields with beans in one, corn in the other

The "polyculture" model: beans interspersed with corn

Problems with the latter: hard to farm by machine, but this may be offset by reduced competition (recall intra vs. interspecific ideas)

Additionally, beans are legumes and can fix nitrogen

As a result, harvestable primary productivity may increase via "overyielding"

This occurs roughly half the time in experiments

Another potential advantage of intercropping; reduced loss to pests

An experiment on diversity vs consumption

In a large field, two experimental treatments: monoculture vs diverse background (Root 1973).

The monoculture; rows of collards, widely spaced, cultivated to keep bare soil between plants.

The diverse background: the same number of collard plants in a single row, the same distance apart as previously, but surrounded on both sides by diverse early successional vegetation (many species of mixed herbs and grasses).

Sampling for insect herbivores: calculation of "herbivore load" (insects/plant biomass) = higher herbivore loads in the monoculture

Two explanations for this pattern ( not mutually exclusive):

1. more concentrated resource (the monoculture) = higher herbivore success.

2. more secondary consumers (insect predators and parasites) in the diverse background reduce herbivore populations

Maybe leaving weeds is better?

Think about the role of human consumers...... are you willing to pay more for something because its cultivation is more sustainable?