SOIL ORGANISMS - Soil Ecology - page 259

Bacterial feeders may ingest up to 5000 cells or 6.5 times their own weight
per day. Forty to 60 % of cells are assimilated which results in a growth efficiency of
4-13 % of the consumed material. Overall consumption may be as much as 800 kg
bacteria and the amount of N turned over in the range of 20-130 kg (review by
Coleman et al., 1984a).
Resistance to water-stress
When the soil starts to dry, nematodes may initially migrate. Since they have a limited
capacity for movement, they are unable to avoid desiccation and part of their population
will die. For example, nematodes were virtually eliminated from a fallow soil dried
out to < -6.0 MPa (pF 5.5) (Simons, 1973). However, in most cases, a proportion of
the population will survive since nematodes, as all soil hydrobionts, have developed
a considerable resistance to desiccation (Demeure and Freckman, 1981).
Resistance varies among species and, within populations, among the successive
stages. In some cases, eggs have an unexpectedly low resistance. Adults generally
tolerate desiccation better than juveniles; in a 22 week experiment with alternating dry
and moist periods, 84 % of adults, but only 24 % of juvenile Helicotylenchus sp. sur-
vived. With Tylenchorynchus sp., survival rates dropped to, respectively, 25 and 16 %, and
10 and 0 % with Paratylenchus sp. (Simons, 1973). On the other hand, juveniles of the
phytoparasite Meloidogyne javanica resist starvation better than the adults. In a laboratory
experiment, starved juveniles consumed their body food reserves 7.5 times more slowly
than adults and their motility and infectivity remained unchanged during the 10 weeks
of the experiment (Reversat, 1981b).
In the first stages of desiccation, nematodes may cease activity and coil, thus limiting
subsequent water losses (Demeure et al., 1979). Further resistance is obtained by
anhydrobiosis, encystment or protection in galls, i.e ., specific tissues elaborated by
plants in response to the infection. In the genus Heterodera cysts are the bodies of dead
females whose cuticle has been tanned to form a resistant sac containing the eggs.
Ecological categories
Ecological classifications of nematodes are based on their feeding regimes and they are
divided into the following five categories (Banage, 1966; Wasilewska, 1971): phytophages,
bacterivores, fungivores, predators, and miscellaneous feeders.
Movements and dissemination
Most nematode genera, and even species, have cosmopolitan distributions which indicates
a capacity to live in diverse environments and high vagility. For example, Maggenti
(1961) records Plectus parietinus from Hawaii, continental USA, Antarctica, Australia,
Canada, England, Ireland and the Netherlands.
Cysts and active nematodes are disseminated by wind and circulating waters. At the
scale of a soil profile, nematodes may be attracted by decomposing organic residues
(Griffiths and Caul, 1993), roots (Prot, 1980) and bacteria (Andrew and Nicholas, 1976).
This attraction is mediated by specific chemicals such as ammonium, unidentified
components in root exudates, heat and light (Bilgrami et al., 1985 and diverse authors
quoted by Yeates, 1981). Another efficient medium-scale dissemination process is transport