BIODIVERSITY IN AGROECOSYSTEMS - CHAPTER 8 docx

Leakey CONTENTS Introduction Sustainable Production Agroforestry Agroforestry and the Diversification of Agroecosystems Development of Multistrata Agroforests Biodiverse Agroecosystems F

CHAPTER 8

Agroforestry for Biodiversity in

Farming Systems Roger R B Leakey

CONTENTS

Introduction

Sustainable Production

Agroforestry

Agroforestry and the Diversification of Agroecosystems

Development of Multistrata Agroforests

Biodiverse Agroecosystems

Forest Patches, Biogeographical Islands, and Agroforestry

Domestication of Trees for Timber and Nontimber Forest Products

Commercialization

Tree Domestication in Progress

Conclusions

References

Abstract — Agroforestry can be used to diversify and intensify farming systems

through the integration of indigenous trees producing marketable timber and nontim-ber forest products and is described in terms of an agroecological succession, in which climax agroforests are biodiverse, highly productive, and profitable The role

of biodiversity in agroecosystem function is one of the keystones of sustainability Complex agroforests that combine profitability with biodiversity are presented as a model worthy of expansion However, little is known ecologically about how best to integrate agroforestry into the landscape, or to what extent agroforestry can be used

to link forest patches and expand biogeographical islands.

Tree domestication is one way to diversify and intensify agroforestry systems and to make them profitable A wise domestication strategy for indigenous trees will involve the capture and maximization of intraspecific genetic diversity and so benefit both production and the environment.

INTRODUCTION

The numbers of plant and animal species on Earth represents only 0.1% of those that have existed since life appeared on this planet, the other 99.9% being already extinct as a result of five episodes of mass extinction over geological time (Leakey and Lewin, 1996) and the current period of extinctions (30,000/year against a background of 0.25/year) arising from our population growth and lifestyle Land-use changes associated with colonization have been the major caLand-use of these species losses over the last few centuries Agriculture, which has been described as the

“engine of economic growth” because of its powerful role in facilitating and

stim-ulating growth of other sectors of the economy (Mellor et al., 1987), started as a

subsistence activity 8000 years ago The early subsistence systems are generally considered to have been sustainable, while large-scale, capital-intensive, modern agriculture has traded innate sustainability for chemical and other inputs and is characterized by deforestation and a decrease in the overall numbers of associated wild plants and animals to favor the growth of the planted crop Typically, agriculture systems and forestry plantations are monocultures of staple food or tree crops with the almost total disappearance of the biodiversity and spatial complexity of natural ecosystems Characteristically, these monocultures are also based on the few plant species that have been domesticated (Leakey and Tomich, 1998) and which also have a narrow genetic base In recent years, these developments have given rise to concerns about deforestation, the loss of biodiversity, and the sustainability of our lifestyle and, particularly, the crucial food production systems that are essential to prevent famine and malnutrition in the tropics

SUSTAINABLE PRODUCTION

Many of the means to increased productivity and profitability are now perceived

by society as carrying too high a cost in social disruption, human inequity, and environmental degradation The problem in trying to address this is how to define and quantify sustainability Izac and Swift (1994) developed an operational frame-work to assess sustainability based on the premise that “a cropping system is sustainable if it has an acceptable level of production of harvestable yield which shows a non-declining trend from cropping cycle to cropping cycle over the long term.” Their framework is based on the assessment of key ecological and economic parameters at the field, farming system, and village/catchment scales and the concept that a sustainable system never reaches threshold levels of irreversibility and that it achieves a sufficient level of economic efficiency and social welfare One of the

requirements identified by Izac and Swift (1994) is that the by-products (soil and water quality, biological diversity, etc.) of agricultural activities must not disrupt the biological functions of the system to the extent that the capacity of the system to absorb these disruptions is surpassed

Sustainability thus involves a symbiosis between the properties of the ecosystem and the management activities that results in nondeclining and relatively stable outcomes Izac and Swift (1994) consider that the key to this symbiosis lies in the assumed positive relationship of agroecosystem function to biodiversity and com-plexity Biodiversity therefore is a keystone in sustainability, and its loss has been one of the common outcomes of agricultural intensification (Figure 1)

Agroforestry, through the replenishment of soil fertility and the domestication

of indigenous trees producing marketable forest products, has been proposed as one way of diversifying and intensifying agroecosystems in a way that is beneficial to the environment and can maintain and perhaps enhance biodiversity (Sanchez and

Leakey, 1998; Sanchez et al., 1998) In its Medium Term Plan 1998–2000, the

International Centre for Research in Agroforestry (ICRAF, 1997) foresees that agro-forestry can contribute to human welfare and environmental resilience, with improved systems providing:

1 Tree products that both increase food and nutritional security and generate cash income for poverty alleviation and

2 Services that support and enhance ecosystem function ( Figure 2 ).

The relevant services of trees are those that increase the crop yields (nitrogen fixation, increased soil organic matter, nutrient cycling, soil conservation, etc.), create environmental resilience (niche diversification, food web complexity, carbon seques-tration, reduced greenhouse gas emissions, etc.), and provide social benefits (bound-ary delineation, shade, etc.) Of these, the least is known about the ways in which trees enhance the environment, although the body of information is increasing (see

Ingram, 1990; Swift et al., 1996).

AGROFORESTRY

Agroforestry, where it has been practiced traditionally, such as in the damar agroforests of Sumatra and Jungle Rubber on Kalimantan (Michon and de Foresta, 1996) and in the home gardens of Sri Lanka (Jacob and Alles, 1987), Nigeria (Okafor and Fernandes, 1987), and Tanzania (Fernandes et al 1984), is a mixed and often apparently haphazard polyculture of indigenous trees and crops that form a complex, multistrata system somewhat like a natural forest Interestingly, recent findings in the damar agroforests of Sumatra show that these complex multistrata agroforests contain over 50% of all the regional pool of resident tropical forest birds, most of the mammals, and about 70% of the plants (Table 1) They are also a major source

of resins, fruits, and timber for domestic use and for export Thus, these agroforests are potentially a sustainable resource, a valuable compromise between conservation

Figure 1 The impact of agricultural intensification on an agroecosystem (From Swift, M J.

and Anderson, J M., 1993 Biodiversity and Ecosystem Function, Schulze, E.-D and Mooney, H A., Eds., Springer-Verlag, Berlin With permission.)

Figure 2 The relationship between the two functions of trees and the three goals of

agro-forestry to meet three global challenges (Modified from ICRAF 1997, Int’l Centre for Research in Agroforestry Medium Term Plan 1998–2000 With permission.)

Table 1 Biodiversity in Indonesia Agroforests: Observed Numbers of Species

Primary Forest

Rubber Agroforest

Damar Agroforest

Durian Agroforest

Rubber Plantation

Collembola b

a Thiollay, 1995.

b Deharveng, 1992.

c Sibuea and Herdimansyah, 1993.

d Michon and de Foresta, 1995.

of tropical forest biodiversity and profitable use of natural resources, since in addition to their biodiversity these multistrata damar agroforests in Sumatra are financially attractive

Damar resins are utilized by industries in Indonesia or exported worldwide In

1984, the export market represented one third of the harvested volume, a trade rising

from 250 to 400 t/year between 1972 and 1983 (Michon et al., 1998) In 1994, the

damar production was expected to reach 10,000 t (Dupain, 1994), at a value of U.S

$300 to 400/t Of this trade, 80% is met by the damar agroforests The economic value of the damar trade and its associated activities is of major significance to the villages around Krui In 1993, the profits from damar production were U.S $7.2 million from sales, U.S $2.6 million from added value, and U.S $1.4 million from wages To this is added U.S $0.3 million in profits made by Krui traders (Michon

et al., 1998) This analysis excludes the locally consumed products from these

agroforests, e.g., fruits, vegetables, spices, fuelwood, timber, palm thatching, rattan, bamboo, fibers, as well as paddy rice

With the exception of plantation crops, many farming systems in the tropics, including traditional subsistence swidden farming, are based on mixtures and are frequently haphazard in their configuration and spacing In contrast, monocultures are particularly prevalent in countries with temperate climates It is not clear whether

or not the tendency to complexity and random distribution of the components of farming systems in the tropics is a deliberate attempt by farmers in the tropics to mimic the diversity of natural ecosystems in order to minimize risk

In contrast to traditional agroforestry, the recent development of agroforestry as

a science by agronomists and foresters has tended to adopt the temperate model and

to plant the tree component in lines, regular patterns, or along the contour of sloping

land (see review by Cooper et al., 1996) This is especially the case in countries

where farm size is large (e.g., Australia), where large areas of countryside are planted

in geometric patterns

Modern agroforestry has also tended to be a set of stand-alone technologies, that together form various land-use systems in which trees are sequentially or simulta-neously integrated with crops and/or livestock (Nair, 1989) Recently, however, it has been suggested that agroforestry practices should be successional phases in the development of a productive and complex agroecosystem, akin to the succession of natural ecosystems (Leakey, 1996) In this way, trees producing different products can be used to fill niches in a mosaic of patches in the landscape, making the system ecologically more stable and biologically more diverse It is anticipated that this diversity would increase with each phase of the agroecological succession Toward this end, current activities at ICRAF are focusing on the development

of agroforestry as “a dynamic, ecologically-based, natural resource management system that, through the integration of trees on farms and in the landscape, diversifies and sustains production for increased social, economic and environmental benefits.” One aspect of this is to determine, through the use of models, the best land-use options for agricultural productivity and biodiversity conservation: the choice

between integration or segregation (van Noordwijk et al., 1995b).

In parallel with these developments in agroforestry there has also been a move

to promote the domestication of indigenous trees, the “Cinderella” trees overlooked

by science (Leakey and Newton, 1994a; Leakey and Jaenicke, 1995; Leakey and Izac, 1996)

Bringing the new ideas about agroforestry and about domestication together provides one with a new paradigm for sustainable land-use development that focuses

on two aspects of biodiversity:

1 Diversifying agroecosystems

2 Capturing and enhancing intraspecific diversity

AGROFORESTRY AND THE DIVERSIFICATION OF

AGROECOSYSTEMS

From past experience, domesticated trees are frequently grown in monocultures, but they could play an important role in species-rich multistrata agroforests (Leakey, 1996b) The development of multistrata systems that include cultivars of domesti-cated trees could increase the profitability of these agroforests Thus this approach could, it seems, go a long way toward the establishment of land uses that will fulfill the needs of rural and urban populations for food and income, while maintaining much of the biological diversity of forests or rehabilitating degraded ecosystems Much research will be needed, however, to achieve this and to demonstrate that productivity and profitability are not necessarily environmentally damaging Evidence already emerging from studies to develop viable alternatives to slash-and-burn agri-culture suggests that the greenhouse gas emissions, especially methane, from areas where sources such as paddy fields are juxtaposed with perennial vegetation are lower

than from areas monocropped with rice (van Noordwijk et al., 1995a) However, the

successful establishment of trees on cleared sites is known to suffer from changes in the populations and species diversity of symbiotic microflora associated with land

clearance (Leakey et al., 1993; Mason and Wilson, 1994), and similar changes

prob-ably occur in the beneficial micro- and mesofauna above- and belowground Evidence exists for the negative effects of site clearance on soil fauna populations (Eggleton

et al., 1995) and for the need to restore them to ensure soil fertility.

A challenge for agroforestry research is to develop economically and socially acceptable land-use systems that function like undisturbed ecosystems and maintain biodiversity Could complex multistrata agroforests, like those of Sumatra, be devel-oped in humid West Africa and in Latin America? The answer is almost certainly yes Indeed, simple indigenous multistrata systems already exist, such as the cocoa farm, and the compound gardens of West Africa (Okafor and Fernandes, 1987), while in the Peruvian Amazon, multistrata agroforests have been found to be an economically attractive system (Table 2)

DEVELOPMENT OF MULTISTRATA AGROFORESTS

There are plenty of tree species that have traditionally provided local people with their daily needs for the full range of nontimber forest products, which could

now be incorporated into agroforestry systems Table 3 illustrates candidate species for West Africa (see also Okafor and Lamb, 1994; Abbiw, 1990) How can these complex agroforests be further developed in the tropics? Probably three things are required First, there is the need to identify the species of commercial importance

of relevance to local markets Second, there is a need to develop an entrepreneurial mentality among the community, who have traditionally been subsistence farmers and hunter–gatherers Third, there is a need to determine how best the tree species may be combined to develop agroforests The growth of the urban market and the absence of jobs in the urban areas may provide the commercial incentive required What is probably missing is the demonstration of what is possible, particularly in the areas near urban markets

With such a wide choice of species for the middle and upper strata of multistrata agroforests, clearly research to determine the best combinations and configurations

is much needed, but also extremely difficult There are, therefore, three research approaches that can be taken:

1 The testing of prototype systems (i.e., best-guess combinations), perhaps aimed at

market needs, and developed with the help of farmers with some experience of compound gardens;

2 Research to test specific hypotheses aimed at the development of some principles regarding the optimal combinations and/or densities of trees in the different strata, which involves both complementarity of species biologically and in terms of labor demands and market opportunities;

3 Use of random mixtures of the species in unstructured combinations, as would probably be developed by farmers.

Research is needed to determine whether or not the apparently random distribution

of trees in many existing examples of multistrata systems is indeed random, or whether farmers from experience grow species in certain combinations Understand-ing this process would be of benefit in assistUnderstand-ing attempts to know why multistrata systems evolve differently under different social and ecological conditions and would therefore help to transfer these systems more effectively to new areas

Table 2 Comparison of Net Present Values and Internal Rates of Returns of

Production Systems in 1985 and 1991 Prices Using a 15-Year Time Horizon in Yurimaguas, Peru

Production Option

Net Present Value (U.S.$ ha –1 )

Internal Rate of Return (%)

Net Present Value (U.S.$ ha –1 )

Internal Rate of Return (%)

Note: Low input and high input continuous cultivation both had negative net present values.

From ICRAF, 1994 Annual Report, Nairobi, Kenya, 1995.

There are a number of options on how to apply these approaches to the devel-opment of multistrata systems:

1 Enrichment planting within logged or degraded forest;

2 Planting in cleared forest land as a perennial tree alternative to slash-and-burn agriculture;

3 Planting under a plantation of either an upper or middle strata species.

Of these options, 1 and 3 have merit environmentally in that the system will be developed more quickly as a demonstration and will not leave the land bare at the establishment phase On the other hand, option 2, the currently most practical from the perspective of smallholders, is probably the most relevant in terms of developing

Table 3 A Sample of the West African Tree/Shrub/Liane Species Appropriate for

Growth in Multistrata Agroforests and for Domestication

Common Names

Mature Height (m)

Canarium schweinfurthii Aiele/Africa canarium/incense tree 45–55

Pentaclethra macrophylla Oil bean tree/Mubala/Ebé 20–30

Ricinodendron heudelotii Groundnut tree/nyangsang/essessang 40–50

From Leakey, R R B., Agroforestry Systems, 1998 With permission.

multistrata systems Such systems are both relevant on good soils of land cleared at the forest margin and on already degraded land, perhaps with proximity to urban markets Because this option is the most relevant practically, it is also probably the one on which the greatest research effort should be concentrated

In all these options, research is especially required to determine the functional groups of species which will do well in the lower strata, where light will be a limiting factor Currently in Southeast Asia, staple food crops are grown beside the multistrata agroforests, because they are light demanding This may be the best arrangement, but it is also possible that new crops, or new cultivars of existing crops, could be integrated into multistrata agroforests

BIODIVERSE AGROECOSYSTEMS

From the biodiversity viewpoint, there is a difference in agroecosystems between

the planned biodiversity and the unplanned, or associated, biodiversity The latter

are all those organisms, above- and belowground, that have found niches to fill among the planted trees and crops The extent to which unplanned biodiversity occurs in different agroecosystems is not well known or understood, although studies have started to address the effects of a broad spectrum of agricultural practices on wildlife populations (McLaughlin and Mineau, 1995; Perfecto and Snelling, 1995)

Swift et al (1996) have, however, drawn four very different scenarios for the

relationships between agricultural intensification and biodiversity (Figure 3), although these may be very scale dependent (e.g., from farm to landscape) and probably also vary depending on the level of biodiversity at the time of planting Thus, the biodiversity associated with an agroforest planted on recently cleared land

at the forest margin, as an alternative to slash-and-burn agriculture, would almost certainly be very different from the same tree/crop mixture planted to rehabilitate already degraded land There is a need for controlled experiments to determine these relationships between intensification and biodiversity In addition there is a need to determine the patterns of diversity in different agroforestry systems and their impli-cations for ecological functioning at different scales There is currently unresolved debate about the functional role of species diversity in ecosystems (Johnson et al., 1996), which makes it difficult to suggest best practices However, it seems that planned biodiversity should aim at maximizing ecosystem processes (nutrient cycling, production of different products, light requirements, etc.) and structural complexity, rather than increasing the number of species per se However, species numbers may also impact on function The few studies in which the diversity of agricultural ecosystems has been manipulated suggest that increases in diversity from 0 to 10 plant species alters ecosystem function, but that there is little effect beyond that point (Schulze and Mooney, 1993)

Species also vary in their importance in different food webs, where, once again, scale is important For example, for a fruit tree species to support a population of monkeys will require a large habitat A small area of fruit trees may therefore avoid monkey damage and so be preferable for production, but without a large area there may not be a market for the crop Thus, ecological and economic factors affecting