Agriculture Reference
In-Depth Information
soil structure, aeration, drainage, and fertility. Before DarwinÔs book was published, earthworms were
commonly considered soil-inhabiting crop pests. His views on the beneficial aspects of earthworms
were supported and expanded subsequently by other contemporary scientists such as Muller (1878),
Urquhart (1887), and many others. The observations Darwin described were so advanced that it was
half a century before many of them were confirmed (see
Chapter 2
, this volume).
EARTHWORM TAXONOMY
Earthworms belong to the order Oligochaeta, which includes more than 8000 species from about
800 genera. Earthworms are common all over the world in natural forests and grasslands as well
as agroecosystems. However, many oligochaetes have an aquatic habit, and there is considerable
controversy over earthworm systematics (see
Chapters 3
to
5
, this volume). Earthworms are found
in most regions of the world, except those with extreme climates, such as deserts and areas that
are under constant snow and ice. Some genera and species of earthworms, particularly those
belonging to the Lumbricidae, are extremely widely distributed and are termed
; often,
when these species are introduced to new areas, they become dominant over the endemic species.
peregrine
This situation applies to parts of the northern United States and Canada, particularly those areas
close to major waterways (see Chapters 3 and 5, this volume).
However, the indigenous earthworm fauna of North America has not been well studied other
than by Gates and Reynolds and earlier workers (
Chapter 4
). Endemic species include those in the
Acanthodrilidae, with its most abundant genus
; members of the Sparganophilidae; and
species in the Megascolecidae, of which the most common genus is
Diplocardia
. There are very few
earthworm taxonomists, which has an impact on earthworm research the world over (see Chapter
4, this volume).
Pheretima
EARTHWORM ECOLOGY
The size of earthworms ranges from a few millimeters to as much as 2 m in length, from 10 mg
to nearly a kilogram in weight, and up to 40 mm in diameter. The record was a specimen believed
to be a
sp. that was 7 m long and 75 mm in diameter (Lungstrm and Reinecke
1969). The larger earthworms are usually found in southern latitudes, such as South America, South
Africa, Southeast Asia, Australia, and New Zealand. No other terrestrial invertebrate has such a
wide range of sizes between the smallest and the largest individuals (Lee 1985)
Populations of earthworms vary greatly in terms of numbers or biomass and diversity. Popu-
lations range from only a few individuals per square meter to more than 1000 per square meter
(Lee 1985; Edwards and Bohlen 1996; Lavelle et al. 1999). The size of populations depends on a
wide range of factors, including soil type, pH, moisture-holding capacity of the soil, rainfall, and
ambient temperatures, but most importantly, on the ready availability of organic matter. This is
Microchaetus
because interactions between organic matter and microorganisms provide food for earthworms.
Earthworm populations in cultivated land usually do not exceed 100 per square meter or 400
per square meter in grassland, and similar populations to those in grassland are usually found in
woodlands, where the availability of organic matter is seldom limiting. Numbers as high as 2000
per square meter have sometimes been recorded, although relatively few earthworms occur in the
more acidic mor soils under coniferous forests. Usually, the largest earthworm populations are
lumbricids, which seem to be able to survive adverse soil and litter conditions much better than
species belonging to many of the other families. The earthworm biomass in most soils exceeds the
biomass of all other soil-inhabiting invertebrates. It has been stated that earthworm biomass in a
pasture may be ten times that of stock animals that graze on it (see
Chapters 6
and
14
, this volume).
The diversity of species of earthworms varies greatly between sites and habitats, and there
often tend to be species associations in different soil types and habitats. Earthworm communities
