Book Review
Earthworm Management in Tropical Agroecosystems
by Patrick Lavelle, Lijbert Brussaard and Paul Hendrix (Eds.). CAB International, New York, 1999, xii + 300 pp. ISBN 0-85199-270-6
The importance of earthworms to the soil ecosystem has long been recognized. The great increase in the number of articles dealing with earthworm ecology and their role in soil ecosystems over the past two dec-ades is testimony to their importance to soil fertility and their utility in addressing fundamental questions in ecology. For example, earthworms have been useful in addressing questions about the importance of biodi-versity to ecosystem function and the examination of the ecological basis of species invasions and loss of native species. The explosion in our knowledge of earthworms has called into question some long held assumptions about earthworms, such as the assump-tion that the in¯uences of earthworms are always posi-tive, or that earthworm species can be lumped into a single ecological category. Although our knowledge of earthworms during the past 20 years has greatly expanded, there are still many gaps in our knowledge of the most basic aspects of earthworm ecology and bi-ology, much less their in¯uence on ecosystem proper-ties and their interactions with other soil organisms and plants.
Anyone interested in the ecology and role of earth-worms in terrestrial ecosystems will have no trouble ®nding up-to-date texts that deal broadly with various aspects of earthworm ecology and biogeography. The challenge facing someone who would like to increase their knowledge of earthworm ecology is to decide which text to choose from among the several quality books published in this ®eld in recent years. Each of these books has its own strengths and weaknesses, but all address the broad spectrum of subjects germane to earthworm ecology. These subjects include earthworm systematics and biogeography, the demography of earthworm populations and communities, the role of earthworms in nutrient cycling, organic matter turn-over and soil physical structure, interactions with other soil biota and, ®nally, the management of earthworm populations and their use in composting of organic wastes.
The newly published book Earthworm Management in Tropical Agroecosystems, edited by Patrick Lavelle et al., adds a signi®cant new player to the ®eld of recently published texts addressing this important com-ponent of the soil biota. For one, this book speci®cally addresses tropical earthworms and thus ®lls a void in a ®eld dominated by studies of temperate species and ecosystems. Furthermore, it covers a broad range of topics and is thus useful as a reference text for earth-worms in general, even though its focus is on tropical earthworms and agroecosystems. It contains large amounts of information on basic biology and ecology of tropical earthworm species, and has a strong focus on applied issues, addressing the potential use of earth-worms to improve soil properties and plant growth in agricultural systems.
The book synthesizes research done by the MACROFAUNA network from 1989±96 as part of the activities of the Tropical Soil Biology and Fertility Programme. However, the book is based on over 20 years of preliminary research on the basic biology and ecology of earthworms. It includes extensive inven-tories of over 500 species found in tropical regions, greatly expanding the coverage of these species in a single volume. There are comprehensive studies of over 200 communities of soil macroinvertebrates and earth-worms, including detailed autecological and demo-graphic studies of 26 species found in agroecosystems. These detailed demographic studies were done with an eye toward potential manipulations of earthworm communities in these systems to improve soil fertility. Investigation of the rest of the book turns up the usual suspects: namely, the role of earthworms in soil or-ganic matter dynamics and physical properties, their in¯uence on plant production, and their potential use in soil amelioration and composting of organic wastes. As with any edited volume the chapters vary in terms of quality, comprehensiveness and clarity. However, some of the review material and conceptual diagrams are as good or better than any available.
The ®rst three chapters of the book deal with earth-worm taxonomy, community structure and environ-mental plasticity. A worldwide survey of 151 earthworm species in the humid tropics shows that these species can be divided into four categories based Soil Biology & Biochemistry 32 (2000) 1329±1330
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on whether they are native or exotic and have a broad or narrow range of environmental tolerances. This analysis provides an extremely useful framework for categorizing species for management purposes but also has relevance to conservation of rare, native species with limited environmental plasticity. An analysis of the origin and structure of earthworm communities in the tropics reveals that earthworms comprise 40±90% of the macrofaunal biomass in most tropical ecosys-tems, except for annually cropped systems. Their im-portance to faunal biomass in tropical forests calls into question the prevailing view that other soil groups, such as ants or termites, are more important than earthworms in organic matter processing and soil turnover in the tropics. As in temperate ecosystems, earthworms account for the bulk of the soil animal biomass, except where they are limited by lack of moisture. Interestingly, native species apparently sur-vive better in managed systems of India and Africa than in Mexico and Central America, possibly due to the greater geographical isolation of Central America relative to the other regions. The chapter on environ-mental tolerance of earthworms that have extended distribution helps to explain why these species are so successful and suggests that they may have dierent eects on N mineralization than native endemic species.
The next three chapters of the book deal, in turn, with the eects of earthworms on plant production, soil physical properties and organic matter and nutri-ent dynamics. The chapter on plant production con-tains perhaps the most extensive analysis of the in¯uence of earthworms on plant growth ever underta-ken. Summarizing over 240 results the authors show that most studies report an increase of growth in plant shoots and grain yield in the presence of earthworms and that the greatest increases occur in sandy soils with low organic matter content. This chapter also does an excellent job of summarizing the mechanisms by which earthworms may enhance worm growth, although there remain many uncertainties in this area.
It has long been known that earthworms can have an important in¯uence on soil structure and that their eects depend largely on soil textural properties. How-ever, there are some unique characteristics of tropical soils and the physical properties of casts of some tropi-cal earthworm species that lead to dierent eects of earthworms on soil physical properties than are
observed in temperate ecosystems. In particulate the compact casts of species such as Pontoscolex core-thrurus tend to increase the bulk density of soil (com-pacting species). Other smaller species egest smaller casts, decreasing bulk density (decompacting species). In experiments in cultivated soils it was shown that endogeic worms play a large role in soil structure in kaolinitic soils, an unclear role in smectitic soils and a secondary role to roots in the Vertisols of Martinique. In kaolinitic soils their in¯uence on structure may last for a considerable length of time. As has been shown in other studies, earthworms increase aggregation, and water retention and in®ltration.
The chapter on soil organic matter and nutrient dynamics takes on the dicult question of whether earthworms increase or decrease the storage of soil carbon. While earthworms can increase the breakdown and mineralization of soil organic matter through their feeding activities, they can also increase soil C by depositing organic-enriched casts in which the C is protected from further breakdown and by incorporat-ing surface residues into the soil. The results of the author's experiments in this regard are equivocal. A modeling eort using the CENTURY soil organic matter model shows that C in cultivated systems is in-itially lower in the presence of worms than in their absence but that over time systems with worm achieve a higher long-term equilibrium due to protection of soil C in earthworms casts. Although this modeling exercise has conceptual value we lack the information to validate the model's output and the jury is still out on whether earthworms increase or decrease storage of soil C and under which circumstances they do so.
Although studies of temperate earthworms still dom-inate the scienti®c literature, it can no longer be argued that there is a paucity of information on the ecology of tropical earthworms. Although this book focuses on agroecosystems, it contains much infor-mation on earthworms in other tropical ecosystems, including savannas, pastures and dierent types of for-est. It is clear that had Darwin spent his time observ-ing earthworms in the tropics, instead of the chalky downs of England, he would have come away with the same conclusions concerning the importance of these lowly organized creatures.
P.J. Bohlen
Book Review / Soil Biology & Biochemistry 32 (2000) 1329±1330
