In Scotland, the number of wild salmon have been reduced because of uncontrolled deep-sea and coastal netting, by pollution, and by various other threats to the fish's habitat.
Surprisingly, in areas where tidal currents are very weak, juvenile horseshoe crabs are found not to have significant barnacle populations, even though they seldom burrow.
Because each individual is short-lived, the population of an opportunist species is likely to be adversely affected by drought, bad winters, or floods.
Until recently, the main reason for removing dams was to restore salmon runs, but it is now recognized that a more important reason to remove dams is that they are no longer safe.
There were, however, fluctuations in the climatic conditions, with the consequences that wet conditions were followed by dry ones, so that the availability of plants and animals oscillated brusquely.
Attempts have been made to curb this kind tendency to incorporate some measure of quality as well as quantity into the assessment of an applicant's published papers.