Agriculture Reference
In-Depth Information
where these groups are dominant (
e.g.,
the arable land studied by Lagerlöf, 1987 or
the moist African savannas of Lamto, Ivory Coast (Athias, 1976), abundance may be
highly variable with rapid week-to-week variation and a slight but clear seasonal pattern.
Successions
Collembola are clear indicators of ecosystem change in successional situations because
of their large specific and functional diversity (see,
e.g.,
Huhta
et al.,
1969; Bonnet
et al.,
1976; Betsch and Betsch-Pinot, 1983). Recolonisation may be rapid although euedaph-
ic species tend to recolonise more slowly than epiedaphic species (Dunger, 1975).
Recolonisation of reclaimed areas does not appear to proceed evenly; population
aggregates appear at the edges of the colonised area and further split into other such
aggregates which progressively invade the area (Petersen, 1995).
4.2.2
ACARI
4.2.2.1
General Biology
Acari are small Arachnida with an unsegmented posterior parts and, except in the
Prostigmata, no constriction between the anterior and posterior part of the body. The Acari
are subdivided into five main sub-orders based on the location of their stigmata or
respiratory organs: Mesostigmata (= Gamasida), Prostigmata (= Actinedida), Astigmata
(= Acaridida), Cryptostigmata (= Oribatida) and the parasitic Metastigmata (= Ixodida).
In all, they comprise 1200 families and probably as many as 500,000 species of which
many are soil and litter inhabitants. They have chelicerae, primitively forceps-shaped
although in some predatory groups, they have been transformed into claws, cogged tips
(Prostigmata Trombidiforma) or harpoons (Metastigmata) (Kethley, 1990; Krantz and
Ainscough, 1990; Norton, 1990; Philips, 1990) (Figure III.34).
Their morphology is highly variable. Predators generally have long legs and elongated
bodies whereas saprophages are frequently globular with a thick chitinous carapace; some
species have lateral extensions that can close hermetically over the legs, as in the family
Phthiracaridae (Cryptostigmata). Some species have ommatidia (1-5) and all have a variety of
sensory organs whose functions are still poorly understood (Bachelier, 1978). Excluding the
parasitic Metastigmata, their sizes range from 0.1-0.2 mm to 1.5-2 mm and fresh body weight
from 0.25 to 300 µ g (Petersen and Luxton, 1982) equivalent to 0.1-120 µ g in dry weight
As with most mesofauna, soil-dwelling Acari are chiefly epigeic and live in the
leaf-litter and the pore space of the upper 10-15 cm of soil. They have highly diverse
feeding habits and exploit all the resources available in litter and soil using a wide
range of ecological strategies (Vannier, 1985). Three major groups may be distinguished
(Luxton, 1972):
(i) macrophytophages, that mainly occur in the Cryptostigmata and Mesostigmata
Uropodida and which feed on leaf litter;
(ii) microphytophages which feed on bacteria, fungi and algae and may be found in
all the major taxonomic groupings;
(iii) predators,
e.g.,
Mesostigmata or Prostigmata, which feed on small
invertebrates, mainly microarthropods and Enchytraeidae. A number of species prey
