Pollutants distributed in patchy environments may produce
source-sink population behavior in exposed biota. Salminen et
al. [74] specifically tested this hypothesis in Cognettia sphagnetorum,
an enchytraeid worm commonly used to quantify
soil pollution. They found that worms avoided patches of soil
polluted with copper when unpolluted areas were available,
and their movements followed predictions based on densitydependent
mechanisms. As a consequence of this, nonstratified soil sampling could distort estimates of the relation between
population density and average soil pollution.