AbstraetLárge numbers of thecamoebae are found in the sand and detritus substrate of small streams in the Amazonian terra firme forest. Their rei ative populatlon densities can be determined by searchlng and countlng thecae under a dissecting microscope for a standard time lnterval (constant effort sampling) . The total population per sample can be calculated from a gradual linear depletion during successive such counts In a given sample (remova! trapping). From a series of such regressions the total number of thecamoebae of any sample can be estlmsted from a single, first count. The method depends on specific conditlons with regard to size of area searched, quantity of substrate per area, density of organisms per substrate and performance of the observers . These conditions are generally valld for similar msthodical treatment of any population of sma!l organisms In any type of dense substrate. The linear regresslon of the depletlon effect implies a constant mean probability for any thecamoeba to be found in the speclfied substrate during the specified ti me interval by any of the three observers involved In the study, and this despite the uncontro!lable subjectivity of visual search in a mlcroscopic fíeld .
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