PLS 508 Crop Ecology

Class Notes

Updated Spring 2003

INTRODUCTION

Scope of Ecology

Discuss scope diagram-shows domains of biological disciplines at different spatial and temporal scales. Ecology is primarily concerned with those biological (and biogeochemical) processes that control the functioning of populations, communities, and ecosystems over large spatial (communities to global) and long temporal (days-millenia) scales.

Ecosystem Properties:

• Structure:
Structure refers both to the components of the ecosystem (plants, animals, microbes, soils, carbon and nutrient pools), and also to the links between the components (e.g., food-web structure).
• Function:
Energy capture (primary productivity, yield); energy flow; nutrient cycling; population regulation; stability and resilience; disturbance regime; succession

Crop Growth and Environment

Environment refers both to resources and controls. Resources are materials and energy needed by the crop to grow and produce a yield; controls are the environmental conditions that affect the crops ability to capture resources.

• Resources: CO2, light, water, macronutrients, micronutrients
• Controls: temperature, daylength, clay mineralogy, soil texture, pH, etc.

Global Trends in Land Use, Population, and Crop Yields

As a starting point in this course we will look at some important trends in food production over the last half century. Understanding these trends is critical to defining the needs and directions for future research. Examine the figures provided in class (data are from the United Nations Food and Agriculture Organization) and try to answer the following questions.

Arable land.

What has been the global trend in the total area of arable land?

If tropical forests are being destroyed for agriculture, as commonly discussed in the popular press, why don't we see an increase in the amount of arable land?

Population.

Is there much evidence of a decrease in growth of the world's population?

Where are most of the world's people found?

How do population growth rates in Africa and Europe compare?

Arable land per capita.

What parts of the world are comparately "land rich"? "land poor"?

If populations are increasing while amounts of arable land are more or less static, how can food production be increased?

World grain yields.

Do grain yields show evidence of leveling off?

What characteristics of maize and paddy rice account for their relatively high yields?

What characteristics of millet and sorghum account for their comparatively low yields?

Which of these grains are C3 crops and which are C4 crops? Does photosynthetic pathway correlate with yield? Why or why not?

Do the primary markets for these grains differ?

Total cereal yields.

How were these yield increases accomplished?

Do regional grain yields show any signs of leveling off?

Why are cereal yields comparatively high North American and Europe? Why are they comparatively low in Africa?

Per capita cereal production.

What are the general trends?

How do you think the supply of basic grains is keeping pace with the demand? What determines the demand?

Per capita food requirements.

World grain production averages about 350 kg cap^-1 y^-1; how does this figure relate to basic nutrional needs? This is not an easy question to answer, and estimates of food energy requirements vary.

Lappé et al. (1998) use 2450 kcal cap^-1 d^-1, which is equivalent to about 203 kg grain cap^-1 y^-1

Andrade (1998) gives: minimum annual requirement = 3650 MJ cap^-1 d^-1. which is equivalent to about 198 kg grain cap^-1 y^-1

Earl Ellis (IASC) argues that food security requires production approximately double the nutritional demand.

Penning de Vries et al. (1997) use:

• 490 kg cap^-1 y^-1 grain equivalent for a vegetarian diet,
• 860 kg cap^-1 y^-1 grain equivalent for a moderate diet, and
• 1535 kg cap^-1 y^-1 grain equivalent for an affluent diet.

Not all human food comes directly or indirectly from grain, of course, but these numbers suggest that either yields or harvested acreage would have to increase considerably to raise the standard world diet from a basically "vegetarian" one to an "affluent" one.

The "Green Revolution"

Much of the yield increases achieved by agriculture in the last half of the 20th Century were associated with the "Green Revolution." Yield increases resulted from the synergistic (Evans, 1998) combination of new varieties, irrigation, herbicides, and fertilizers, contributing about equally.

The first dwarf wheat varieties were released in 1962, the first dwarf rice varieties in 1966; dwarf wheat MV (modern varieties) were grown on 25% of land in developing world in 1970, 40% in 1975, 70% in 1998

Dwarfing results in a rise in the harvest index, better response to inputs through reduced lodging. HIs of grains now 50-55%; upper limit estimated at 60% by Evans

"The most likely maximum HI for cereals is between 0.60 and 0.65 because it would be impossible to support more than 65% of the total yield as grain on less than 35% of the overall phytomass ..." (Smil, 1999)

Sinclair & Sheehy (1999, Science 283: 1456-1457) argue that high yield depends on ability of plant to accumulate and store sufficient N. N is stored in leaves, high LAI (leaf area index) is required to provide sufficient storage capacity. Green Revolution MVs have higher leaf angles to provide higher LAI. [Like HI, there is an upper limit to LAI]

The "Next Green Revolution"

What are the barriers to increasing yields:

• Genetic? Many plant scientists claim that the modern plant biotechnology will provide for another green revolution. Most targets for genetic manipulation, however, are not addressed to raising yield levels in developing countries.
• Agronomic/Ecological? "In 1961 the average use by developing countries of [N, P, & K] fertilizers was only 6 kg ha^-1, compared with 45 kg ha^-1 in developed countries. Thirty years later the developing countries used 82 kg ha^-1 compared with 116 in the developed countries, well on the way to closing the gap." (Evans 1998)
• Social/Economic? Many farmers, ability to produce food is limited by their access to credit, insurance, production resources, and markets. This is particularly true in the less developed world.

Trade

Should resource-poor countries simply plan on importing food from developed nations (USA, Canada, Australia, Argentina) with surplus production?

"At present the net export of grain from the more to the less developed countries represents 15% of the former's production of cereals and 9% of the latter's consumption, and is primarily used for animal feed." (Evans, 1998)

The recent per capita decline in grain production reflects a drop in demand from the developing world (inability to pay) and changes in agricultural policies in the developed world (Ehrlich et al., 1995).

Changes in Agriculture 1950-2000

1. Higher Yields

2. Higher annual variability (lower stability) in yield (due to genetic uniformity of crops?)

3. Lower Crop Diversity (increased monoculture, less rotation, less intercropping, etc.)

4. Higher Applications of Fertilizers

5. Higher Applications of Pesticides (incl. Insecticides, herbicides, fungicides, etc.)

6. Improved Seeds (higher harvest index)

7. More Energy Intensive

8. Increased Soil Erosion

9. Decreased Soil Fertility (loss of organic matter, nutrient depletion)

10. Increased Nitrate Leaching

11. Less Effective Pest Control

12. Less Labor Intensive

13. More Subsidized

14. Less Profitable

15. Higher Risks

16. Fewer and Larger Farms (Greater inequity in land ownership)

In this course I hope you will develop an understanding of why these changes have occurred and what are the ecological consequences of these changes. Plant scientists need to come to their own conclusions of how agroecosystems should function in order to contribute to the development--through both basic and applied research--of suitable crops for such agroecosystems.

Strategies for meeting future food demand

L. T. Evans (1998), "Feeding the 10 Billion":

• increase the area of land under cultivation
• increase in yield per hectare per crop [will have to be the main route]
• increase in the number of crops per hectare per year [requires irrigation, fertilizer, short-season varieties]
• displacement of lower yielding crops by higher yielding ones [reduced diversity could have ecological costs]
• reduction of post-harvest losses
• reduced use of crops as feed for animals

Gordon Conway (1997), "The Next Green Revolution", head of Rockefeller Foundation:

• ecological approaches to sustainable agriculture
• participatory breeding
• biotechnology applied to the needs of poor farmers in developing countries

URL: http://ag.arizona.edu/~spmcl/lecturenotes/introduction.html