Agriculture Reference
In-Depth Information
4.1.2.2
Community Structure
Species Richness
Nematode communities may comprise up to 70-100 species in such different ecosystems
as an English pastures (71), a Danish beech-wood (75), a prairie site in Dakota (80) a
Slovakian mixed deciduous forest (92) or wheat-soybean fields in Tennessee (100) (as
reviewed by Petersen and Luxton, 1982 and Bernard, 1992). An absolute maximum of
431 species has been recorded in a tropical forest in Cameroon (Bloemers
et al.,
1997),
Intermediate richnesses of 30-40 species have been recorded in temperate forests (Arpin
et al.,
1986) and minima of 1-16 species are found in tundra soils (Signy Island) (Petersen
and Luxton, 1982).
Functional structure: interbiome comparison
The distribution of trophic groups within nematode populations (
i.e.,
the functional
structure) has been assessed in a variety of environments including tundra, moorlands
and derived grasslands, temperate coniferous, deciduous and mixed forests. There is no
clear pattern of variation in functional structure between these biomes as variations
across biomes and between sites are often less marked than intra-site variation (see,
e.g.,
Schmitt and Norton, 1972). Some trends, however, may be derived from the existing data:
bacterial feeding populations generally represent 30 to 50 % of the total community
(range 12.9 to 72 %) with higher proportions in coniferous forests and crops (
ca,
50 %) than in grasslands and deciduous forests (30-40 %);
fungus-feeding nematodes commonly comprise 20-30 % of total density (range 3.1
to 50 %) with maximum abundances in forests. This may reflect the composition
of microbial communities with, typically, a dominance of bacteria in many grass-
lands and crops; in forests, especially with mor or moder-type humus, fungus-
feeders are more abundant (Arpin
et al.,
1986), commensurate with the fungal
dominance common in microbial communities in those environments.
root-feeding parasitic nematodes are of little importance in forest soils (range 0 to 14
%), but represent 25-40 % on average of grassland communities and up to 83 % in
moorlands. In crops, they comprise 17-33 % of the populations;
predators comprise a few percent of the community; they are nearly absent from
moorlands and coniferous forests; highest values (10 %) have been noted in a
short-grass prairie in Colorado (USA);
miscellaneous feeders have abundances of
ca.
5-25 % with no clear interbiome
differences.
Nematode communities vary at a local scale with changes in the vegetation type, with
drainage and earthworm activities (Dash
et al.,
1980; Yeates, 1981) probably through
their effects on available food resources, bacteria, fungi, algae and fine roots. Tillage also
affects nematode abundance and community trophic structure. In a rye (
Secale cereale
)
field in Georgia (USA), the mean populations of bacterivorous, fungivorous and total
nematodes were greater in a conventional tillage system; in the summer, plant parasitic
and fungivorous nematodes were more abundant in the no-tillage system (Parmerlee and
Alston, 1986).
