<p>Microarthropod species were classified according to life-history tactics and feeding guilds. Twelve life-history tactics were distinguished, based on well-defined life-history traits like the type of reproduction (thelytoky, arrhenotoky, sexual reproduction), oviposition (semelparity, iteroparity), development, synchronization (of the life cycle with environmental conditions), and dispersal (phoresy, anemochory). Examples are given of the distribution of these tactics among microarthropod species occurring in several biotopes, during decomposition of organic matter, and under several types of disturbance and pollution. Thelytokously reproducing species appeared to have higher numbers at sites with a persistent pollution.<p>Feeding guilds were defined on the basis of gut carbohydrase (cellulase, chitinase, and trehalase) activities. Five species-rich guilds were recognized. In the presence of species able to digest the fungal cell-wall next to cell-contents (called fungivorous and herbo-fungivorous grazers), a higher CO <sub><font size="-2">2</font></sub> evolution during decomposition of pulverized litter was found than in their absence. In the presence of species able to digest cell-contents only (called fungivorous browsers and opportunistic herbofungivores), in such experiments a lower CO <sub><font size="-2">2</font></sub> evolution was found than in their absence.<p>In a simulation model it was tested whether relatively inefficient use of food may be compensated by life-history traits or abilities by which short-term environmental extremes can be overcome. Having the possibility to bridge a relatively long period of food shortage, or withstanding extremes in drought, frost or heat, or having a higher mobility, was indeed shown to result in a better survival of species that use their food relatively inefficiently. This results in the effect that species with different life-history tactics can coexist on the same food sources. Contradictory to the principle of competitive exclusion, species with identical niches may coexist when the mobility of these species is not unlimited.