o suppose that the first colonist will so modify the site that it becomes unsuitable for those species that normally occur early in the succession. The three hypotheses then suggest three different ways in which other species will appear. Model I suggests that early occupants modify the environment so that it becomes more suitable for species that come later in the succession. Model II (the "tolerance" model) suggests that the sequence in which species appear depends solely on their speeds of dispersal and growth. Model III (inhibition) - the species already present makes the environment less suitable for subsequent recruitment of later species. All these hypothesis do not rely on the idea of a community as a sugra-organism but on succession as a sugra-organism but on succession as a process that relies on two factors:1)the probabilities that propagules of different species willbe present and2) the ability of these propagules to survive,develop and reproduce.Now to look at the whole picture, we ask ourselves: "How do we predict the response of a community from a pollutant?" Should we look at one population at a time, or in some holistic approach. Moriarty suggests that some of the currently favored approaches rest on he assumption, often implicit rather than explicit, that communities are sugra-organisms. He (Moriarty) suggests that two topics that should be discussed when dealing with the idea of community response: 1) indicator species2) biological or environmental health may bemisleading.The term indicator species, which is used in the classification of communities (p. 62) is also used in ecotoxicology, with a variety of meanings. At times it indicates the idea that knowledge of one species within a community will indicate the well-being or biological health of the whole community. Moriarty suggests that this seems a reasonable proposition if one accepts the traditional view of community as sugra-organism, but suggests that it is in...
