Interdependence of Food and Natural Resources

2 Interdependence of Food and Natural Resources

David Pimentel, Laura E. Armstrong, Christine A. Flass, Frederic W. Hopf, Ronald B. Landy, and Marcia H. Pimentel College of Agriculture and Life Sciences Cornell University Ithaca, New York

I. II. III. IV.

Introduction World Population Growth Energy Constraints Arable Land—Quality and Quantity

Food and Natural Resources Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

V. Water Constraints VI. Biological Diversity VII. Looking to the Future References

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David Pimentel et al.

I. INTRODUCTION H u m a n s , like other animals, a p p e a r to have an innate drive to convert a m a x i m u m a m o u n t of the e a r t h ' s environmental r e s o u r c e s into t h e m s e l v e s and their progeny. As a result, the escalating h u m a n population is increasing its pressures on natural r e s o u r c e s and t h e r e b y threatening its ability to supply itself with a d e q u a t e a m o u n t s of food, w a t e r , fuel, and other essentials for life (Grant, 1982; Speth et al., 1985). T h e exact quantities of these natural r e s o u r c e s that are still available are not k n o w n , but clearly these r e s o u r c e s are finite. T h e d e v e l o p m e n t and use of new technologies is e x p e c t e d to facilitate the more efficient use of limited natural r e s o u r c e s that are needed to provide food and other human needs. Some claim that technology will provide for the unlimited e c o n o m i c needs of the world population, w h a t e v e r its ultimate size (Simon, 1981 ; W a t t e n b e r g and Zinsmeister, 1984). H o w e v e r , m a n y agricultural scientists disagree and caution that new technologies have limited ability to increase productivity (Jensen, 1978; I. N . O k a , 1985 personal communication). T h e use of finite natural r e s o u r c e s is complicated not only by each society's standard of living, but also by the uneven distribution of essential resources throughout the world (Biswas, 1984). Such constraints emphasize the need to study each major r e s o u r c e and the needs of the h u m a n population as parts of the complex natural s y s t e m , in o r d e r to m a k e reasonable projections and devise sound r e s o u r c e m a n a g e m e n t policies for the future ( U . S . C o n g r e s s , 1984; Biswas and B i s w a s , 1985). This c h a p t e r focuses on the availability and interrelationships that exist a m o n g population size, arable land, water, energy and o t h e r biological resources that function to maintain a quality e n v i r o n m e n t for all h u m a n s .

II. WORLD POPULATION GROWTH T h e major increase in the population growth rate that occurred about 300 to 400 years ago coincided with the discovery and use of stored fossil energy resources such as coal, oil, and g a s . Since t h e n , rapid population growth has closely paralleled the increased use of fossil fuel for agricultural production and improving h u m a n health (Figure 2.1). N e v e r in history have h u m a n s , by their sheer n u m b e r s , so dominated the earth and its r e s o u r c e s . T h e current world population stands at a high of m o r e than five billion ( P R B , 1986). W h a t is m o r e alarming than the n u m b e r s is the 1.7% annual growth r a t e — a rate 1,700 times greater than that of the first t w o million years of h u m a n existence. Such a growth rate adds more than 200,000 people a day to our world population (Salas, 1984). D e m o g r a p h e r s project that the world population will reach 6.1 billion by

2. Interdependence of Food and Natural Resources

1800

1900

2000 Years

33

2100

Figure 2.1 World population, food energy, and fossil energy use (solid lines) and projected trends (dotted lines) for each (Environmental Fund, 1979; Linden, 1980; USBC, 1982; PRB, 1983).

the year 2000, a p p r o a c h 8.2 billion by 2025 ( U N , 1982), and probably reach 12 billion by 2100. Presently there seems to be no generally accepted way to limit this growth ( N A S , 1975). A large proportion of the world population (e.g., 4 5 % of Africa, 4 0 % of Latin A m e r i c a , and 3 7 % of Asia) is n o w within child-bearing age ( P R B , 1983). This young age structure contributes to a rapid population growth rate for the long t e r m . F o r e x a m p l e , with m o r e than half of the Chinese below the age of twenty y e a r s , even limiting births to o n e child per couple will not stop China's population growth (Coale, 1984). If 70% of the couples have only one child, the current population of slightly more than one billion (Coale, 1984) will still reach 1.2 billion by the year 2000 (Wren, 1982; Z h a o , 1982). Increased e c o n o m i c d e v e l o p m e n t is often cited as a possible solution to slowing birthrates. H o w e v e r , except for J a p a n , K o r e a , T a i w a n , and Singapore (where the e c o n o m y depends on industry and trade), most o v e r p o p u l a t e d c o u n t r i e s h a v e insufficient n a t u r a l r e s o u r c e s to s u p p o r t e c o n o m i c d e v e l o p m e n t similar to t h a t in E u r o p e a n d N o r t h A m e r i c a (Keyfitz, 1976; H a r d i n , 1985). Most of the 183 nations in the world now

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require some food imports from other nations ( F A O , 1982). F u r t h e r m o r e , the "biological carrying c a p a c i t y " of the e c o s y s t e m in m a n y parts of the world already has been severely stressed and in some regions it has been e x c e e d e d ( N A S , 1975). E c o n o m i c growth and prosperity cannot be relied on to control population g r o w t h . Clearly, an urgent need exists for all societies to consider methods to slow the growth of their human population (Reining and Tinker, 1975; S h a r m a , 1981).

III. ENERGY CONSTRAINTS T h e most important factor responsible for rapid population growth and resource destruction has been the availability of cheap fossil energy, which h u m a n s use to manipulate natural s y s t e m s and r e s o u r c e s of the earth (Figures 2.1 and 2.2). This energy use has helped provide effective controls of m a n y h u m a n diseases like malaria, typhoid, and cholera and also increased food production, all of which have contributed significantly to population growth ( N A S , 1971; C E Q , 1980). Most recent increases in crop yields have been achieved by using e n o r m o u s a m o u n t s of fossil energy to supply fertilizers, pesticides, irrigation, and fuel for machinery ( L e a c h , 1976). This has increased the use of the finite resource, fossil fuel, to compensate for degraded land, perhaps not the wisest choice for the future. H o w can food supply and energy e x p e n d i t u r e s be balanced against the n e e d s of the growing world population? Doubling the food supply during the next 25 years would help offset the serious malnourishment that one billion h u m a n s presently e n d u r e ( L a t h a m , 1984), as well as help feed the additional n u m b e r of people. H o w e v e r , such an increase, assuming no land degradation and minimal substitution of labor with mechanization, would require about a four-fold increase in the total a m o u n t of energy

USA 10,410

4,835 India 142

Figure 2.2 Energy consumption rates in kilograms of coal equivalents per capita per year for the United States, United Kingdom, and India (USBC, 1983).

2. Interdependence of Food and Natural Resources

35

e x p e n d e d for food production. Such a large energy input for food production would be necessary to balance the decreasing return of crop output per fertilizer energy input. T h r o u g h o u t the world, energy use for food production continues to grow dramatically. Chinese agriculture provides a striking e x a m p l e of the increased reliance on fossil energy in food production. During the past three d e c a d e s , fossil fuel inputs in Chinese agriculture rose 100-fold and c r o p yields h a v e tripled ( A A C , 1980; Taylor, 1981; L u et al.y 1982). T h e United S t a t e s , like most developed countries, is a heavy energy user. F o r e x a m p l e , 17% of the total energy supply, or a b o u t 1500 liters per person of oil equivalents, is e x p e n d e d on production, processing, distribution, and preparation of food. This c o n t r a s t s with most developing nations that use less than one-tenth this a m o u n t of energy for all their food. In many developing nations crops are still produced by hand, requiring about 1200 hr of labor (Table 2.1). In c o n t r a s t , in the highly mechanized U . S . agriculture c r o p s like corn require only about 10 hr of labor (Table 2.2). T h e energy output to input ratio for the h a n d - p r o d u c e d corn is about 1:10, w h e r e a s in the U . S . m e c h a n i z e d system the ratio is 1:2.2. Using U . S . agricultural technology to feed the current world popu12 lation of 5.0 billion a high protein/calorie diet would require 7.1 x 10 liters/yr of fuel (Pimentel and Hall, 1984). At this rate, assuming petroleum was the only source of energy for food production and all k n o w n r e s e r v e s w e r e used solely for this p u r p o s e , world oil reserves would last a mere 12 y e a r s . Of c o u r s e , not all nations desire the diet typical of the United States. One practical way to increase food supplies with minimal fossil energy inputs and expenditures would be for everyone—especially those living

Table 2.1 Energy Inputs in Corn (Maize) Production in Mexico Using Only M a n p o w e r

Inputs Labor Axe and hoe Seeds

Quantity/ha

kcal/ha

1,144 hr" 16,570 kcal'' 10.4 kg"

624,000 16,570 36,608 677,178

Outputs Corn yield kcal output/kcal input "Lewis (1951). ^Estimated.

1,944 kg'

6,901,200 10.19

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David Pimentel et al. Table 2.2 Energy Inputs per H e c t a r e for Corn Production in the United States"

Inputs Labor Machinery Gasoline Diesel fuel Irrigation Electricity Nitrogen Phosphorus Potassium Lime Seeds Insecticides Herbicides Drying Transportation

Quantity/ha

kcal/ha

10 hr 55 kg 40 liters 75 liters6 2.25 x 10 kcal 35 kwh 152 kg 75 kg 96 kg 426 kg 21 kg 3 kg 8 kg 3,300 kg 300 kg

5,000 1,018,000 400,000 855,000 2,250,000 100,000 3,192,000 473,000 240,000 134,000 520,000 300,000 800,000 660,000 90,000 11,037,000

Outputs Total yield kcal output/kcal input

7,000 kg

24,500,000 2.22

"After Pimentel and W e n (1988).

in the industrial nations—to c o n s u m e less animal protein (Pimentel et al., 1980a). This diet modification would r e d u c e energy e x p e n d i t u r e s and increase food supplies b e c a u s e less edible grain would be fed to livestock to p r o d u c e costly animal protein. T h e average yield from 10 kg of plant protein fed to animals is only 1 kg of animal protein. If the 130 million metric t o n s of grain that are fed yearly to U . S . livestock were c o n s u m e d directly as h u m a n food, about 400 million people—1.7 times larger than U . S . population—could be sustained for one year (Pimentel et al., 1980a). H o w e v e r , dietary patterns and favorite foods are based not only on availability and e c o n o m i c s , but on social, religious and other personal factors as well. F o r these reasons major diet modifications of any kind are difficult to a c h i e v e , especially within a short time span. In view of the heavy drain on fossil fuel supplies, biomass (grain, sugar b a g a s s e , other crop residues, and fuelwood) has b e e n suggested as a major substitute fuel. T o s o m e extent biomass has always b e e n used for fuel, but to use it in the a m o u n t s n e e d e d to spare fossil fuels n e e d s careful analysis. O n e constraint operating against the increased use of biomass for fuel is the a m o u n t of land required to grow it. W h e n the a m o u n t of cropland

2. Interdependence of Food and Natural Resources

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needed to feed o n e p e r s o n and that required to provide b i o m a s s to fuel one average U . S . automobile with ethanol for one year are c o m p a r e d , nine times m o r e cropland is used to fuel the automobile than to feed a person (Pimentel et al., 1984a). Biomass in the form of w o o d has long b e e n a major fuel for h u m a n s . Presently, o v e r half the w o r l d ' s population d e p e n d s on firewood as their primary fuel source (Pimentel et al., 1986). But supplies of firewood are diminishing as an estimated 12 million ha of timber are cut and cleared e a c h y e a r p r i m a r i l y to p r o v i d e m o r e l a n d s for a g r i c u l t u r a l p r o d u c t i o n (Spears and A y e n s u , 1984). As a result, the total a m o u n t of w o o d biomass available in the world p e r p e r s o n has declined 10% during the last 12 years (Brown et al., 1985). A s m o r e food m u s t be p r o d u c e d to maintain the ever-increasing world population, the supply of fuelwood can be e x p e c t e d to diminish. If b i o m a s s u s a g e is i n c r e a s e d , t h e n t h e effect this w o u l d h a v e on available quantities of arable land for food/fiber production must be considered. F u r t h e r the removal of trees and c r o p residues is k n o w n to dec r e a s e soil fertility and facilitate soil erosion. F r o m this, o n e can conclude that biomass r e s o u r c e s are limited in their usefulness as a fossil fuel substitute. Solar energy technology s h o w s s o m e potential for the future. Photovoltaic and solar thermal energy should supply limited a m o u n t s of renewable energy ( E R A B , 1982); h o w e v e r , these techniques do require land and also m a y effect some wildlife (Pimentel et al., 1984b).

IV. ARABLE LAND—QUALITY AND QUANTITY Next to sunlight, which provides plant growth, land is the most vital natural resource. In the United States, about 160 million ha of cropland are planted to provide food for 240 million people or 0.67 ha/person ( U S D A , 1983). Considering that m o r e than 2 0 % of U . S . food c r o p s are e x p o r t e d , the arable land planted to feed each person is about 0.5 ha. Worldwide, with arable land resources estimated to be about 1.5 billion ha (Buringh, 1979) and a world population of 5.0 billion, the available arable land p e r person a m o u n t s to only 0.3 ha. According to F A O predictions (1982), the possible net e x p a n s i o n in total world cropland is 3.9 million ha per year. At this rate world cropland will e x p a n d to 2 billion ha by the y e a r 2110. U n d o u b t e d l y , it will be possible to bring s o m e additional land, m u c h of which is considered marginal, into production. Optimistic estimates p r o p o s e that worldwide cropland can b e e x p a n d e d to 3.4 billion h a (Buringh, 1979). H o w e v e r , others conservatively project that worldwide cropland can be e x p a n d e d 2 billion ha, but only with the use of large a m o u n t s

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of energy to m a k e the marginal land productive ( N A S , 1977). If the world population does increase to 12 billion and expands the land base to 2 billion ha, only 0.2 ha or less of cropland would be available p e r p e r s o n (Figure 2.3) and most people would have to c o n s u m e essentially a vegetarian or plant protein diet. With excellent soils, favorable t e m p e r a t u r e and rainfall, h e a v y fertilization, and effective pest control, it is theoretically possible to provide 20 people with a d e q u a t e calories and protein on o n e h e c t a r e of land. H o w ever, with average land, climate and o t h e r factors, it would be optimistic to hope to feed half that n u m b e r . This is especially true considering the current soil erosion crisis that exists throughout the world (Pimentel et al., 1986). In the United S t a t e s , for e x a m p l e , several million h e c t a r e s of marginal land, which is highly susceptible to severe soil erosion, is already being cultivated ( O T A , 1982; Naegeli, 1986). T h e available arable land, water, and energy r e s o u r c e s , as well as the kind of food a particular society desires or can afford, d e t e r m i n e the c r o p s produced in a given region. At p r e s e n t , two-thirds of the people in the world c o n s u m e a primarily vegetarian diet in contrast to industrialized countries w h e r e diets are characteristically high calorie/high animal protein. This latter diet requires large a m o u n t s of land and energy to p r o d u c e . Based on all available data concerning future availability of arable land

1.6

jpita

1.4

Hectares per

u

1.2 1.0 0.8 0.6 0.4 0.2 World 1650

USA 1985

World 2110

Figure 2.3 Arable land per capita in the world in 1650, in the United States, for 1985 and projected for the year 2110 in the world.

2. Interdependence of Food and Natural Resources

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and energy, it will not be possible to provide all people of the world with such a high calorie/high animal protein diet. In fact, if population growth continues, diets in industrialized nations will have to be modified to include larger a m o u n t s of plant proteins and less animal protein. The quality of land is as important as the number of hectares available. A s m e n t i o n e d , to bring land of marginal quality into p r o d u c t i o n , w a t e r and energy inputs for fertilizers and/or irrigation are essential. T h e extent of expansion will d e p e n d on the availability of all these r e s o u r c e s . M e a n w h i l e , farmers are using soils that are eroding at an alarming rate (Pimentel et al., 1976; Holdgate et al., 1982; E c k h o l m , 1983). Soil erosion r e d u c e s the natural productivity of soils by removing nutrients and organic matter, and by reducing top soil depth and w a t e r availability (Lai, 1984). Global dimensions of land destruction are a major c o n c e r n . About 3 5 % of the e a r t h ' s land-surface is affected ( M a b b u t t , 1984). T h e natural productivity of m a n y soils has been r e d u c e d 2 5 - 1 0 0 % b e c a u s e of erosion (Langdale et al., 1979; Lai, 1984; M a b b u t t , 1984). In the past, several civilizations including those in M e s o p o t a m i a and G r e e c e failed in part b e c a u s e of the degradation of their agricultural lands (Jacks and W h y t e , 1939; L o w d e r m i l k , 1953; T r o e h et al., 1980). Worldwide, an estimated 6 million ha of agricultural land are irretrievably lost each year b e c a u s e of w a t e r and soil erosion, salinization from irrigation, and other factors ( U N E P , 1980; Dudal, 1981; Kovda, 1983). In addition, every year c r o p productivity on about 20 million ha either has a negative net e c o n o m i c return or is reduced to zero b e c a u s e of p o o r soil quality ( U N E P , 1980). Based on current worldwide soil loss, the results of a recent study for the period 1975-2000 project that rain-fed land degradation will d e p r e s s food production a n o t h e r 15-30% (Shah et al., 1985). Recent surveys in I o w a , which has some of the richest agricultural land in the world, indicate that about half of the original topsoil has been lost (Risser, 1981). U n d e r normal agricultural conditions, the formation of 2.5 c m (1 inch) of soil requires 200-1,000 years ( H u d s o n , 1981; L a r s o n , 1981; M c C o r m a c k et al., 1982; S a m p s o n , 1983; L a i , 1984). F o r t u n a t e l y , so far, increased fertilizers and o t h e r fossil energy inputs have been available to offset the reduced productivity of s o m e U . S . cropland c a u s e d by soil erosion. H o w e v e r , the loss of a b o u t 20 c m of topsoil from a topsoil base of 30 c m requires double the usual energy inputs j u s t to maintain crop yields (Pimentel et al., 1981). In future d e c a d e s these large energy inputs may not be affordable or even available. In developing c o u n t r i e s , the rate of cropland erosion is nearly twice that experienced in the United States (Ingraham, 1975). F o r e x a m p l e , reports indicate that each year, an average of 38 t/ha of soil is lost from over half of India's total land area b e c a u s e of serious erosion ( C S E , 1982).

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Erosion is also a problem in China w h e r e the sediment load in the Yellow River is equivalent to a loss of about 38 t/ha/yr from the 68-million-ha areas of the river's w a t e r s h e d (Robinson, 1981). R e s o u r c e s to offset t h e p r o d u c t i v i t y l o s s e s a r e not a l w a y s readily available or affordable. In some regions, to obtain m o r e land for crop production, forests are being cut and steep slopes are being used for crops. A s vegetation has been r e m o v e d , erosion has intensified. Often flooding in the lowlands has b e c o m e a serious e n v i r o n m e n t a l problem to the extent that, in India for e x a m p l e , losses from flooding h a v e doubled during the last ten years ( U S D A , 1965; S h a r m a , 1981). Certainly, as m o r e marginal land is put into agricultural production, land degradation and flooding will increase ( C E Q , 1980).

V. WATER CONSTRAINTS Along with sunlight and land, w a t e r is a vital r e s o u r c e for agricultural production. Sufficient rain falls upon most arable agricultural land, but periodic droughts continue to limit yields in s o m e areas of the world. All crops require and transpire massive a m o u n t s of water. F o r e x a m p l e , a corn c r o p that p r o d u c e d 6,500 kg/ha of grain will take u p and transpire about 4.2 million liters of w a t e r p e r h e c t a r e during the growing season (Penman, 1970; Ley ton, 1983). To supply this much water each year, about 10 million liters (100 cm) of rain must fall per hectare and furthermore must be evenly distributed during the year and growing season. A reduction in rainfall of only 5 c m during the growing season r e d u c e s corn production by a b o u t 15% (Finkel, 1983). Decreasing rainfall to 30 c m per season r e d u c e s yields to a b o u t one-fifth of those from areas receiving 100 cm rainfall. Sorghum and wheat require less w a t e r than corn and could be grown instead of it, but these crops p r o d u c e about one-third less grain per hectare than corn ( U S D A , 1984). Irrigation is essential if rainfall c a n n o t be relied upon to supply the moisture needed for c r o p production. Irrigated c r o p production requires the m o v e m e n t of large quantities of water. F o r e x a m p l e , using irrigation to p r o d u c e 1 kg of the following food or fiber p r o d u c t s requires: 1,400 liters of w a t e r for c o r n , 4,700 for rice, and 17,000 for cotton (Ritschard and T s a o , 1978). In the United S t a t e s , agricultural irrigation presently c o n s u m e s 8 3 % of the total 360 billion liters per day that is c o n s u m e d by all sectors of society (Murray and R e e v e s , 1977). Irrigation w a t e r is either piped long distances from rainfed reservoirs or p u m p e d from wells supplied by aquifers. T h e s e large underground storage areas are slowlyYilled by rainfall. Intense and prolonged irrigation stresses aquifers to such an extent that, in s o m e areas of the world, water 4 is being ' m i n e d " and used more quickly than it can be replaced by rainfall

2. Interdependence of Food and Natural Resources

41

( C E Q , 1980). E v e n n o w in the United States w a t e r overdraft e x c e e d s replenishment by at least 2 5 % ( U S W R C , 1979). In addition to its large w a t e r u s a g e , irrigated c r o p p r o d u c t i o n is costly in t e r m s of energy c o n s u m p t i o n . Nearly one-fifth of all energy e x p e n d e d in U . S . agricultural production is used to m o v e irrigation w a t e r ( U S D A , 1974). F o r e x a m p l e , in N e b r a s k a , rainfed corn production requires about 630 liters/ha of oil equivalents, w h e r e a s irrigated corn requires an expenditure of a b o u t three times m o r e energy (Pimentel and B u r g e s s , 1980). P e r i o d s of d r o u g h t , w h e t h e r for a s e a s o n o r e x t e n d i n g o v e r m a n y y e a r s , a l s o influence c r o p p r o d u c t i o n . All c o u n t r i e s e x p e r i e n c e w a t e r shortages at times and this influences agricultural production (Ambroggi, 1980). T h e current severe drought in the Sahel, which e x t e n d s through m u c h of Africa south of the S a h a r a , clearly illustrates the complexity of the problem. A substantial decrease in the normal rainfall results in reduced c r o p s ( H a r e , 1977). T h e effect, h o w e v e r , has been magnified by the large h u m a n population per available arable land as well as the long-time mismanagement of all land resources, including the forest areas (Biswas, 1984). W a t e r r e s o u r c e s a r e b e c o m i n g a g l o b a l p r o b l e m . F r o m 1940-80 worldwide w a t e r use has m o r e than doubled. T h e anticipated growth in world population and agricultural p r o d u c t i o n can be e x p e c t e d to again double w a t e r needs in the coming t w o d e c a d e s (Ambroggi, 1980). By the year 2000 it is estimated that the world agricultural production will c o n s u m e nearly 8 0 % of total w a t e r w i t h d r a w n (Biswas and B i s w a s , 1985). B e c a u s e m o r e w a t e r will be n e e d e d to support agricultural production, both the e x t e n t and location of w a t e r supplies will b e c o m e major constraints on increased c r o p p r o d u c t i o n . Considering that about o n e third of the major world river basins are shared by three or m o r e countries ( C E Q , 1980), water availability is certain to cause competition and conflicts b e t w e e n countries and even within countries (Biswas, 1983). In contrast to limited water supplies is the problem of too much water, a c c o m p a n i e d by rapid w a t e r runoff. Both may c a u s e waterlogging of soils and flooding. Fast w a t e r runoff d e c r e a s e s c r o p yield p e r h e c t a r e by reducing the w a t e r available to the c r o p , by removing soil n u t r i e n t s , and e v e n by washing a w a y the soil and c r o p s t h e m s e l v e s ( O T A , 1982). W h e n forests on slopes are replaced with c r o p s to a u g m e n t food supplies, runoff and soil erosion increase and serious flood d a m a g e o c c u r s to valuable crops and pasture (Beasley, 1972). The economic costs of these losses can be significant. In the United States d a m a g e from sediments and flooding to surrounding areas c a u s e s an estimated $6 billion each year (Clark, 1985). In Bangladesh in 1974, a severe flood diminished the productivity of the soil significantly and r e d u c e d the rice h a r v e s t , which ultimately led to severe food shortages and famine ( B r o w n , 1976). In general, m o s t w a t e r runoff, which brings with it loose soil, has negative effects on agriculture. But w h e r e annual floods can be managed

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like those characteristic of the Nile River basin, the w a t e r and rich soil sediment benefit the c r o p s growing in the flood plains (Biswas, 1984).

VI. BIOLOGICAL DIVERSITY Almost 9 0 % of h u m a n food c o m e s from j u s t 15 species of plants and eight species of livestock, all of which w e r e d o m e s t i c a t e d from the wild (Mangelsdorf, 1966; M y e r s , 1979). H o w e v e r , high agricultural productivity as well as h u m a n health depend upon the activity of a myriad of natural systems composed of an estimated 10 million species of plants and animals that inhabit the world e c o s y s t e m (Pimentel et al.y 1980b; Ehrlich and Ehrlich, 1981; M y e r s , 1983). W e k n o w that h u m a n s c a n n o t survive with only the p r e s e n c e of their c r o p and livestock species, but exactly how m a n y and which plant and animal species are essential is u n k n o w n . Natural species are eliminated in m a n y w a y s . T h o s e having the greatest impact include clearing land of natural vegetation for agriculture, urban a r e a s , and chemical pollution (pesticides, etc.) of the e n v i r o n m e n t . Of great c o n c e r n is the growing rate of species elimination and the subsequent loss of genetic diversity that h u m a n e n c r o a c h m e n t is causing (Biswas and Biswas, 1985). Predictions are that an estimated o n e million species of plants and animals will be exterminated by the end of this century ( E c k h o l m , 1978; Ehrlich and Ehrlich, 1981). E v e n n o w on the Indian s u b - c o n t i n e n t , 10% of t h e plant s p e c i e s a r e t h r e a t e n e d or e n d a n g e r e d (Sharma, 1981). This high rate of extinction is alarming b e c a u s e it is not k n o w n how many of these organisms may be necessary to maintain food production and other vital h u m a n activities ( M y e r s , 1984, 1985). Natural biota perform m a n y functions essential for agriculture, forestry, and other sectors of the e n v i r o n m e n t . S o m e of t h e s e include providing genetic diversity basic to successful c r o p breeding; recycling vital c h e m i c a l e l e m e n t s s u c h as c a r b o n a n d n i t r o g e n within t h e e c o s y s t e m ; moderating climates; conserving soil and w a t e r ; serving as sources of certain medicines, pigments, and spices; and supplying fish and other wildlife (Myers, 1979, 1984; Pimentel et al, 1980b; Ehrlich and Ehrlich, 1981). In addition, some natural biota like bacteria and p r o t o z o a n s help prevent h u m a n diseases. S o m e prey on and destroy h u m a n p a t h o g e n s , others degrade w a s t e s , while still others r e m o v e toxic pollutants from w a t e r and soil and help buffer the impact of air pollutants (Pimentel et ai, 1980b). The relative impact that a species has in the environment can be judged in part by its biomass per unit area. G r o u p s such as insects, e a r t h w o r m s , p r o t o z o a , bacteria, fungi, and algae average 6400 kg/ha, m o r e than 350 times the average h u m a n biomass of 18 kg/ha in the United S t a t e s . Reducing or exterminating s o m e species groups could seriously disturb the normal or natural functioning of important environmental systems in na-

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ture. For example, soil quality and agricultural productivity jointly depend upon soil biota, organic matter, and the p r e s e n c e of certain inorganic elements (Brady, 1984). E a c h year various soil organisms b r e a k d o w n and degrade a b o u t 20 t of organic m a t t e r per h e c t a r e (Alexander, 1977). This degradation is essential for the release of b o u n d nutrients and their subsequent recycling for further use in the e c o s y s t e m (Golley, 1983). S o m e soil biota help control pests in c r o p s , and others " f i x " a t m o s p h e r i c nitrogen for use by plants (Pimentel et al., 1980b). E a r t h w o r m s , insects, and o t h e r biota o p e n holes, improve w a t e r percolation, and loosen the soil to e n h a n c e new soil formation. B e c a u s e the complex roles of these biota are not clearly u n d e r s t o o d , great care should be t a k e n to p r e v e n t the extinction of any species.

VII. LOOKING TO THE FUTURE T h e earth contains r e s o u r c e s of land, water, energy, and natural biota, which are n e e d e d and used by h u m a n s for their survival. All these resources are interrelated and the availability of one influences the quality, quantity, and usefulness of a n o t h e r . T h u s , fossil energy can be used to increase production on land, but supplies of both land and energy can be stressed in the p r o c e s s . T h e growing h u m a n population, with its needs for food and fuel, is using up important environmental r e s o u r c e s , many of which are finite and c a n n o t be resupplied. Population p r e s s u r e s in some parts of the world already are well a b o v e the capacity of r e s o u r c e s to provide all people living there with a p r o s p e r o u s standard of living (Keyfitz, 1976), and outright shortages of basic r e s o u r c e s now exist in some places (Ehrlich et al., 1977; C E Q , 1980). Soon, if not at present, the people of the world will have to decide w h e t h e r they want a relatively p r o s p e r o u s life style with ample supplies of food, water, and energy for a population of one to two billion or whether they will be satisfied with limited resources and a meager existence because they allowed the world population to grow to ten times this n u m b e r . F u t u r e d e v e l o p m e n t will r e q u i r e b o t h t e c h n o l o g y and n a t u r a l res o u r c e s — t e c h n o l o g y a l o n e is insufficient ( S p e t h et al., 1985). F u t u r e b r e a k t h r o u g h s in science and technology, h o w e v e r , can enable h u m a n s to make more efficient use of land, water, energy, and biological resources to meet their growing n e e d s for food, fiber, and shelter. Ultimately the limitations of natural resources will constrain expansion of crop productivity (Jensen, 1978; I. N . Oka, 1985 personal communication). Meanwhile, immediate conservation of these resources is vital not only to protect them but also to extend their availability for future use. The present degradation of land and w a t e r resources and increased use of chemicals in agriculture

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are a d v e r s e l y affecting natural biota. Maintaining a diversity of natural biota is essential to successful agricultural p r o d u c t i o n , and clearly, the loss of species diversity will m a k e i n c r e a s e d future food p r o d u c t i o n m o r e difficult. R e d u c i n g h u m a n b i r t h r a t e s to limit h u m a n n u m b e r s p r o m i s e s to be the m o s t difficult task e v e r u n d e r t a k e n by h u m a n k i n d . Controlling birthrates can be carried out successfully only if t h e direct c o s t s of having c h i l d r e n i n c r e a s e a n d social p r e s t i g e for small families a l s o i n c r e a s e s (Douglas, 1966; N A S , 1975; H a r r i s , 1977; M a y e r , 1985). Ultimately, e a c h individual must a s s u m e p e r s o n a l responsibility for reducing b i r t h r a t e s . Within each society, these difficult societal changes need to be encouraged in conjunction with scientific and governmental policies that help augment food supplies, i m p r o v e h e a l t h , and p r o t e c t natural r e s o u r c e s .

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